Managing water on heat transfer surfaces: A critical review of techniques to modify surface wettability for applications with condensation or evaporation

Applied Energy 222 (2018) 967–992

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Managing water on heat transfer surfaces: A critical review of techniques to modify surface wettability for applications with condensation or evaporation M. Edalatpoura,b, L. Liuc, A.M. Jacobid, K.F. Eidb, A.D. Sommersa,

T

⁎

a

Department of Mechanical and Manufacturing Engineering, Miami University, Oxford, OH 45056, USA Applied Nano- and Micro-Physics Lab, Department of Physics, Miami University, Oxford, OH 45056, USA c A. Leon Linton Department of Mechanical Engineering, Lawrence Technological University, Southfield, MI 48075, USA d Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Surface wettability Gradient surfaces Hydrophilic Hydrophobic Energy systems HVAC&R applications

Most materials of practical interest are neither completely wetting nor completely non-wetting. “Surface wettability” then refers to the degree that a surface is hydrophilic (i.e. water-loving) or hydrophobic (i.e. waterfearing). Through careful design, it is possible to alter the natural wettability of a surface to be more waterloving or water-fearing. This is principally achieved by modifying the surface chemistry and/or surface roughness. In some cases, modifying the surface may bring operational benefit or advantage. For example, aluminum and copper (which are used in the construction of heat exchangers) tend to retain water in application, which can degrade performance. Modifying the surface however to be superhydrophilic can help to spread out the condensate, reduce the air-side pressure drop, and facilitate drainage. Moreover, by creating a wettability pattern or gradient, it is possible to predetermine the initiating sites for condensation on a surface as well as facilitate droplet motion and/or control the water droplet movement path. In the first part of this review, the current state of the art of surface wettability modification and control techniques are presented, which includes topographical manipulation, chemical modification, as well as methods for creating gradient surfaces and patterned wettability. In the second part of this review, possible applications and the potential impact of these methodologies in energy systems are discussed with a special focus on heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems and components.

1. Introduction It is well-known that the condensation of a vapor on a surface depends largely on the surface characteristics. By manipulating either the surface chemistry or roughness, the wetting behavior can be favorably altered. For example, the application of a coating to a surface changes the underlying chemistry of that surface (and therefore its wettability) because the interfacial free energies related to the droplet contact angle are altered. In the same way, the micro-scale roughness of a surface can also be used to significantly affect the wetting behavior and/or motion of water droplets on a surface by increasing or decreasing the solid/ liquid contact area [1,2]. The topic of “surface wettability” has attracted considerable interest over the past few years. In fact, the number of publications in this research area has seen a significant and sustained increase from 1992 to

2016 according to the Science Citation Index (SCI) source (see Fig. 1). It is also important to point out that this plot only shows journal publications in the areas of engineering and materials science; it does not include conference proceedings. Thus, the actual number of publications is considerably higher. Because of the breadth of the encompassing literature, this review will be divided into the following sections—(1) homogeneous surface wettability manipulation (chemical and topographical), (2) variable and patterned surface wettability, (3) applications in energy systems, and (4) operational challenges and current limitations (i.e. fouling, longevity, adhesion, etc.). Over the past decade, many researchers have attempted to use surface wettability modification in different types of energy systems. For example, surface wettability modification has been used in aircooled evaporators as a means of enhancing condensate drainage and improving the overall energy efficiency of these systems. Heat

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Corresponding author. E-mail addresses: [email protected] (M. Edalatpour), [email protected] (L. Liu), [email protected] (A.M. Jacobi), [email protected] (K.F. Eid), [email protected] (A.D. Sommers). https://doi.org/10.1016/j.apenergy.2018.03.178 Received 20 December 2017; Received in revised form 3 March 2018; Accepted 30 March 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

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1800

Number of published papers

1500

Source: ISI Web of Science Database: Science Citation Index-Expanded Research Areas: Engineering and Material Science

condensate retention, which can degrade the overall performance of the heat exchanger. More specifically, this retention of water on the heat exchanger is problematic because it increases the air-side core pressure drop, creates the possibility of corrosion, provides a site for biological activity, and perhaps most importantly can reduce the air-side heat transfer rate. In air conditioning systems, water that does not drain from the evaporator will eventually return to the air as vapor and therefore, must be recondensed which increases the latent load of the system. As a result, the heat exchanger is often oversized and/or a higher rate of refrigerant flow is required to attain the same sensible cooling and coefficient of performance (COP). Moreover, the retention of water on the heat transfer surface not only decreases the rate of heat transfer, but it also provides a site for biological growth and activity that can be detrimental to human health [6–8]. In refrigeration systems, due to the periodic requirement for defrosting (and thus downtime), refrigerator evaporators tend to be rather inefficient. Furthermore, heat exchanger fin spacing is often quite large in these systems to mitigate frost blockage, and thus convective heat transfer coefficients are typically low which further reduce their energy efficiency. Thus, the management and control of water droplets on heat-transfer and air-handling surfaces is vital to the overall energy efficiency, functionality, and maintenance of these systems. Wettability manipulation has also been widely used in other energy systems such as the aerospace, automotive, renewable energy, and biomedical industries where water retention techniques have proved helpful in mitigating surface drag, improving wing de-icing, manufacturing more intelligent lab-on-a-chip devices, etc. The purpose of this review then is to present the current state of the art in surface wettability manipulation and control, which includes a discussion of homogeneous surface wettability manipulation (Section 2) as well as gradient surfaces and patterned wettability (Section 3). Following that, the application of these methodologies and their potential impact in energy systems are discussed with a special focus on HVAC&R systems (Sections 4.1 and 4.2), although applications in other systems are also briefly addressed (Section 4.3). The review then finishes with some basic discussion about the challenges of using these approaches and future work (Section 5). Although other review articles exist [9–13], this review provides both a summary of recent developments in the field as well as the fundamental science behind surface wettability modification and gradient surfaces. The information presented here has been systematically gathered and critically analyzed in the context of their potential application in energy systems including HVAC&R systems. In this way, this review seeks to fill an important gap in the field by examining the current state of the art in terms of both recent developments and future opportunities.

1470

1200

939 900

584

600

288

300

67

109

138

1996

2000

0 1992

2004

2008

2012

2016

Year Fig. 1. Plot showing the rapid increase in “surface wettability” research in recent years according to the Science Citation Index-Expanded (SCI).

Tubular heat exchangers Plate-fin heat exchangers

Plate heat exchangers

Classification of heat exchangers Tube-fin heat exchangers

Regenerative heat exchangers Printedcircuit heat exchangers

Fig. 2. Classification of heat exchangers with respect to their construction type [4].

2. Homogeneous surface wettability manipulation Although the earliest observations of wetting can be attributed to Galileo Galilei in 1612, he did not formally recognize the concept of surface tension (i.e. surface wettability) when he observed an ebony wood chip floating slightly below the surface of a water bath [14,15]. Rather, it was Thomas Young, an English physicist, whose equation and work revived discussions on the topic of surface wettability in 1805. His well-known equation in its contemporary form (see Fig. 3a) can be written as:

exchangers are important to the overall efficiency, cost, and compactness of many thermal management and energy systems including solar heating applications and heating, ventilation, air-conditioning and refrigeration (HVAC&R) systems. According to a 2015 Department of Energy study, HVAC&R systems are responsible for nearly 30% of all the energy used in U.S. commercial and residential buildings and are the single largest energy end-use in buildings, consuming approximately 14 quads (or, 1.5 × 1019 J) of primary energy annually [3]. Heat exchangers are devices responsible for the transfer of heat between two or more fluids (liquid or gas) by a combination of conduction, internal/ external convection, and/or radiation. Various types of classifications have been introduced for categorizing heat exchangers; however, a popular one is based on their construction type (see Fig. 2) [4,5]. It is important to also point out that aluminum, copper, and stainless steel which are widely used in the construction of heat exchangers are naturally hydrophilic. This affinity for water makes it difficult for these materials to drain water effectively, which in turn leads to increased

γsv = γsl + γlv cosθ

(1)

in which γ is the surface tension (or surface free energy), θ is the static contact angle (CA), and the subscripts s, v, and l denote solid, vapor, and liquid, respectively [16]. By proposing this physical relationship, he is widely regarded as the father of scientific research on wetting and contact angles. The contact angle property shown in Fig. 3(a) is generally determined by the attraction of the droplet molecules towards the surface (adsorption force) and the attraction of the droplet molecules towards 968

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Fig. 4. Theoretical and experimental contact angles versus water droplet size on polytetrafluoroethylene (PTFE) spherical surfaces with two different radii of curvature R [23].

are often significantly more complicated to implement and can be more restrictive and/or limited in their use. Although Young’s equation (Eq. (1)) is independent of the water droplet size, surface chemistry, and surface roughness, Drelich et al. [22] later demonstrated that the static contact angle increases with an increase in the water droplet size on solid flat surfaces. In this work, the static contact angle of “larger” water droplets (< 2 µL) was measured and found it to be between 90 and 93°; while for smaller water droplets (0.024 µL), it was approximately 55°. The same observation was also made by Wu et al. [23] who performed a similar experiment on solid spherical surfaces (i.e. convex and concave). In this study, Wu et al. measured the static contact angle for several different water droplet volumes (3–11 µL) on curved surfaces having different radii of curvature (R). Their experimental data and theoretical predictions are shown in Fig. 4. These results not only show that the water droplet contact angle increases as the droplet volume increases, but it also decreases slightly as the radius of curvature R increases. In addition to static contact angles, dynamic contact angles are also often measured using a contact angle goniometer when characterizing the overall wettability of a surface. For these tests, a water droplet is injected onto the surface, and then the advancing (or receding) contact angle is measured by increasing (or decreasing) the volume of the water droplet until the maximum (or minimum) volume is achieved without a change in the droplet contact area. Typically, after performing these measurements, the contact angle hysteresis is reported for the surface. The contact angle hysteresis, which is the difference between the advancing and receding contact angles (a term initially introduced in 1923 [24]), can be stated as follows:

Fig. 3. Schematic of (a) Young’s equation, (b) Wenzel model of wetting, (c) Cassie-Baxter model of wetting, (d) high contact angle hysteresis (θadv−θrec ), and (e) low contact angle hysteresis [17].

Table 1 Surface tension values for different liquids [18]. Liquid

Temp. (°C)

γ (mN m−1)

Water Methanol Ethanol Isopropanol 1-Propanol 1-Butanol 2-Butanol Acetone Glycerol Mercury Acetic acid Diethyl ether n-Hexane n-Octane

25 25 25 20 25 25 25 25 20 15 20 20 20 20

71.99 22.07 21.97 21.7 23.32 24.93 23.46 23.46 63.0 487 27.6 17.0 18.4 21.8

one another (cohesive force). Thus, a droplet will not readily wet a surface when the cohesive force is more dominant than the adhesive force (which is true on hydrophobic surfaces) [18]. The liquid/vapor surface tension which depends on temperature can be more easily measured (see Table 1), whereas the solid/vapor and solid/liquid surface tensions are not often readily known. Thus, measuring the static contact angle as described in Eq. (1) is a simple and popular method of determining the overall wettability of a material. Although the tangent line method is perhaps the most commonly used, a number of different procedures have been proposed for more accurately measuring the static contact angle including [19–21]:

θH = θA−θR

(2)

H = cosθA−cosθR

(3)

Surfaces with low contact angle hysteresis generally retain less water than those with high contact angle hysteresis (see Fig. 3). 2.1. Topographical modification In 1936, Wenzel [25] postulated that physically roughening both metallic and non-metallic surfaces (from aluminum to glass) amplifies the wetting behavior of the materials (a fact that was not previously recognized). During his experimentation, Wenzel observed that roughness makes a hydrophilic surface (wetting surface) more hydrophilic, and a hydrophobic surface (non-wetting surface) more hydrophobic (see Fig. 3b). His finding thus provided an initial framework for future developments in surface engineering and surface science

• Half-angle algorithm, • Axisymmetric drop shape analysis (ADSA), and • Minimum Gibbs free energy. While each of these methods has its own merits and can potentially increase the overall accuracy of the contact angle measurement, they 969

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Fig. 6. Effect of surface roughness on the water droplet contact angle for an aluminum alloy (Al 2024) coated with either PDMSVT or C9F20 [27].

Fig. 5. Effect of surface roughness on roughened copper surfaces [20].

applications. More specifically, he proposed an empirical relationship by which the droplet static contact angle and a surface roughness factor are connected, namely:

cosθ∗ = r × cosθ

surface roughness, their research suggests that an optimum roughness value may exist for which the static contact angle is maximized. In other words, the level of surface roughness can be optimized for making a surface more hydrophobic. It was also found that coating an Al surface with C9F20 yielded a stronger non-wetting behavior than coating it with PDMSVT. In 1998, Wolansky and Marmur [28] proposed a correction to the Young equation (Eq. (1)) which included the effects of line tension to broaden its applicability and improve its accuracy on rough surfaces, namely:

(4)

∗

where θ is the new apparent contact angle on the roughened surface, θ is the contact angle on a smooth surface, and r is the roughness factor calculated such that:

actual surface area projected surface area

r=

(5)

In a paper on roughened copper (Cu) surfaces, Gu et al. [20] examined the effect of surface roughness on the static contact angle of water droplets placed on both hydrophilic smooth and roughened Cu surfaces. The roughened Cu surfaces were prepared using different sandpaper grit sizes— i.e. 600 (coarse), 1500, and 2000 (fine). They repeated each experiment ten times and then averaged their results. Their measurements are plotted in Fig. 5. They demonstrated that the water contact angle decreases as the roughness factor increases (thereby corroborating the Wenzel model). They also reported that the degree of repeatability of the measurements became more difficult as the roughness factor (r) of the surface increased. This is reflected in the larger error bars for the 600 and 1500 grit roughened surfaces. After the introduction of Wenzel’s model [25], Cassie and Baxter [26] reported in 1944 that the Wenzel relation (Eq. (4)) was unable in some cases to correctly predict the static contact angle of droplets on rough porous surfaces. As a result, they modified the Wenzel relation and introduced another formula (which is well-known today) which can be used to predict the new apparent contact angle on rough porous surfaces containing trapped air pockets such that (see Fig. 3c):

cosθ∗ = f1 cosθ1 + f2 cosθ2

cosθ∗ = cosθ−

where

area in contact with droplet projected surface area

(7)

f2 =

area in contact with air beneath the droplet projected surface area

(8)

⎜

⎟

(9)

in which τ is the line tension, rd is the radius of the contact line, rs = 1 + |∇s|2 , and s(x,y) is a function which maps the surface profile. The level of the correction then depends on both the (a) geodesic curvature of the contact line, and (b) the directional derivative of the line tension. Wolansky and Murmur [28] point out that if the line tension can be ignored, then the actual contact angle on the surface equals the Young contact angle; however, if the line tension is significant, then the actual contact angle on a rough surface will differ somewhat from the Young contact angle. Building upon the work of Wenzel [25], Bikerman [29] studied the wetting behavior of grooved/ridged surfaces. He reported that roughness affects the wetting behavior according to both (a) how much a surface is grooved/ridged, and (b) the condition of the water droplet static contact angle on the original surface. He then summarized the behavior of water droplets along grooved/ridged surfaces in terms of the hysteresis and the droplet contact angle as shown in Table 2. According to these experiments, he reported that when the hysteresis is sufficiently small (see Figs. 3d and 3e), a water droplet will spread spontaneously along the surface. Although numerous researchers have experimentally advocated for the veracity and utility of both the original Wenzel and Cassie-Baxter relations [30–33], Gao and McCarthy [34] claimed that these equations have certain limitations. For example, they reported that both the Wenzel and Cassie-Baxter equations can be used only to the extent that the structure of the contact area reflects the ground state energies of the contact lines and the transition states between them. Building upon this, Choi et al. [35] reported that the original form of the Cassie-Baxter relation (Eq. (6)) is unable to predict the advancing and receding contact angles (contact angle hysteresis) for non-wetting droplet formation on textured surfaces. Thus, they recommended instead the use of a slightly different set of relations for predicting the advancing and receding contact angles by modifying the original form of the Cassie-

(6)

f1 =

1 ⎛τ dτ ⎞ + rs σl ⎝ rd drd ⎠

where θ1 represents the contact angle of the smooth solid surface and θ2 represents the contact angle for air (i.e. 180°). (Note: If f2 is set equal to zero, the Cassie-Baxter equation reduces to the Wenzel relation.) In research performed on Al surfaces, Guo et al. [27] measured the static contact angle of water droplets on roughened surfaces which were either chemically coated with poly(dimethysiloxane) vinyl terminated (PDMSVT) or perfluorononane (C9F20) (see Fig. 6). Although the surface wettability generally decreased with an increase in the 970

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fabricating a non-wetting antibacterial, Cu-based coated surface using nanoparticle-incorporated polydimethylsiloxane (PDMS) by an aerosol assisted chemical vapor deposition technique (AACVD). After characterizing the surface using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX), Ozkan et al. measured both the water droplet contact angle and the resistance of the modified film against different bacterial activity, namely Escherichia coli and Staphylococcus aureus. They reported that chemically coating the surface leads to water contact angles as high as 155° ± 2°, while without the AACVD method, water contact angles as high as only 111° ± 5° could be attained. They also reported that this chemical approach was able to prevent the adhesion of both types of bacteria to the surface, which in turn would reduce the incidence of infection. Li et al. [39] fabricated a superhydrophobic surface by taking advantage of both physical etching (via HF acid) and chemical film deposition (fluoropolymer) on both 304 and 316-grade stainless steel surfaces (which are widely used in shell-and-tube heat exchangers) and measured the water droplet contact angle on such surfaces. They reported that after film deposition, the maximum water droplet contact angles on the etched 304 and 316 surfaces were 159 ± 2.8° and 146 ± 3.2°, respectively. After chemical passivation of the surfaces in nitric acid to restore the intrinsic corrosion resistance of these surfaces, static contact angles as high as 157.3° and 134.9° were reported on the etched 304 and 316 surfaces, respectively. Although durable non-wetting (superhydrophobic) surfaces have been developed, Golovin et al. [40] demonstrated that most non-wetting surfaces produced by chemical modification can eventually become mechanically damaged. To address this shortcoming, they reported the fabrication of superhydrophobic Al surfaces with extremely high mechanical durability using fluorinated polyurethane elastomer (FPU) and 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (F-POSS). Some of their surfaces were observed to even recover their non-wetting behavior over a wide range of mechanical treatments—namely, abrasion, scratching, burning, plasma-cleaning, flattening, sonication, and/or chemical attack. The most significant findings from this literature survey in terms of contact angle hysteresis, static, advancing, and/or receding contact angles on Cu, Al, stainless steel, and Si (a popular research-based surface) are summarized below in Table 3.

Table 2 Observations of water droplets on grooved/ridged surfaces by Bikerman [29]. Surface

Hysteresis

Contact Angle

Observation

Grooved

Small

Acute (i.e. θ < 90°)

Droplet would spread along the grooves and θ would be small.

Right (i.e. 90°)

Angle would not be affected.

Obtuse (i.e. θ > 90°)

Droplet spreads and θ would become more obtuse.

Small

≈ 0°

The droplet would spread over surfaces of any kind and degree of roughness.

Not Too Small

Acute (i.e. θ < 90°)

The spreading or contraction of the droplet would be obstructed.

Ridged

Right (i.e. 90°) Obtuse (i.e. θ > 90°) Hybrid

–

–

The behavior of the droplet is difficult to predict.

Baxter relation such that ∗ cosθadv = rϕ ϕd,adv cosθ1 + (1−ϕd,adv )cosθ2

(10)

∗ cosθrec = rϕ ϕd,rec cosθ1 + (1−ϕd,rec )cosθ2

(11)

where rϕ is the roughness factor (same as r in Eq. (4)), and ϕd is the surface texture parameter calculated by using a small differential term between the solid and air regions. 2.2. Chemical modification Apart from the use of physical roughness, the use of various chemical techniques can also be beneficial (even necessary) to make a metallic surface either more wetting (hydrophilic) or non-wetting (hydrophobic). In fact, achieving water droplet contact angles which approach 180° typically requires the use of chemical coatings. Although research on chemical surface modifications has a long history, just like that of physically roughening surfaces, the utility and widespread application of such techniques in producing non-wetting surfaces were not recognized for a long time. In 1946, Zisman [36] published the first systematic preparation procedure of a monomolecular surfactant layer by adsorption onto a clean metallic surface (i.e. self-assembled monolayer). As illustrated in Fig. 6, the use of chemical coatings (such as PDMSVT and C9F20) in combination with multi-scale hierarchical roughness (see Fig. 7) traps air pockets near the solid surface. This allows a Cassie-Baxter-like state (see Fig. 3c) to be generated on metallic surfaces like Al, Cu, and stainless steel which are otherwise naturally hydrophilic. Chemical modification of metallic surfaces can be executed using a multiplicity of approaches, namely through the use of self-assembled monolayers (SAMs), surface coatings, and also surfactants [13,17,27,37]. Although chemical coatings can be employed to reduce the surface energy of metals which is ideal for producing a non-wetting condition, they often lose their effectiveness and/or durability over time. Therefore, they frequently need to be reapplied or otherwise “rejuvenated”. This is particularly true in HVAC&R applications due to the thermal cycling, thermal fatigue, and large temperature gradients often experienced in these systems. Ozkan et al. [38] reported an effective yet simple method of

3. Variable and patterned surface wettability 3.1. Hybrid and biphilic surfaces As was discussed earlier, water droplets will only drain from vertical surfaces if the CA hysteresis is sufficiently small and the droplet size is sufficiently large. This is true not only of homogeneous surfaces but also biphilic, patterned, and gradient surfaces. Biphilic is a term used to describe a surface which combines both hydrophilic and hydrophobic regions. For example, Moumen et al. [66] observed that liquid droplets with volumes less than 13 nL (0.25 mm in footprint radius) would not spontaneously roll off on surfaces possessing a gradient prepared by two various silanes (C12H25Cl3Si and C10H21Cl3Si). Consequently, the necessity of understanding the “critical droplet size” on different metallic and non-metallic surfaces with and without surface wettability variation was highlighted. The so-called “critical droplet” refers to a droplet whose size is sufficiently large that it rolls off a tilted surface freely. Since understanding the shape and size of the critical droplet offers insight into the ultimate goal (i.e. spontaneous droplet motion and improved droplet drainage), some researchers have concentrated their investigations into characterizing the critical droplet size and shape which would result in such motion. In one of the earliest demonstrations, Goodwin et al. [68] measured the critical droplet size on a variety of solid surfaces tilted vertically and reported that a direct relation exists between the Bond number, Bo = (ρgD 2sinα /γ ) (or the 971

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Fig. 7. The SEM images of (a) microstructure, (a.1) and nanostructures, (a.2 and a.3) hybrid hierarchical surface, (b) micropillars containing nanostructures, (b.1) intact mode of the nanospikes on micropillars (θ = 167 ± 3°, θH = 6 ± 3°), (b.2) intact mode of the nanopillars on micropillars (θ = 168 ± 2°, θH = 4 ± 2°), (b.3) inking mode of the nanospikes on micropillars (θ = 162 ± 4°, θH = 9 ± 4°), (b.4) inking mode of the nanopillars on micropillars (θ = 168 ± 2°, θH = 10 ± 5°) [32].

be preserved with increasing wet/dry cycles. This study also reported that the existence of fin press oil can provide a temporary wet pressure drop reduction benefit; however, the surfactant in the oil (which provides this benefit) is easily washed off. Another potential strategy for incorporation in heat exchangers is the use of patterned wettability— namely, the creation of surface regions having hydrophilic behavior (< 90°) and regions having hydrophobic behavior (> 90°). This can be accomplished using both chemical and topographical-based approaches. In a paper focused on the harvesting of water, Al-Khayat et al. [75] mimicked the back structure of a beetle called Physosterna cribripes and fabricated three different patterned polymer surfaces (nano, micro, and macro) containing both topographical and chemical modifications on copper tubes. The surfaces prepared by dewetting of thin films of poly-4-vinylpyridine (P4VP) on top of polystyrene films (PS) films, upon solvent annealing (to facilitate water roll-off), consist of raised hydrophilic bumps on a hydrophobic background. They demonstrated that the rate of water condensation ameliorated by 57% on the hydrophilic bumps with hydrophobic background than that of plain hydrophobic surfaces. According to Fig. 9, their experimental results showed that the water collection efficiencies of the patterned surfaces are more favorable than that of the bare polystyrene films (PS) especially at the higher temperature difference test condition (i.e. ΔT = 10 °C). Alwazzan et al. [76] fabricated patterned copper tubes consisting of two different wettability regions in an alternating stripe-patterned arrangement (i.e. hybrid surface). They demonstrated that all of the patterned surfaces (see Fig. 10) experienced a substantial improvement in the condensation heat transfer coefficient and heat flux as compared to the filmwise condensation case. Interestingly, some of the tested cases even outperformed surfaces with complete dropwise condensation. They also found that the β-regions, which were more hydrophobic in their study, mainly served as droplet nucleation sites with rapid droplet mobility, whereas the α-regions facilitated droplet removal and acted as drainage paths. They reported that the optimum (β/α) ratio was 2 with β and α-regions of 0.6 mm and 0.3 mm, respectively. At this optimum ratio, the heat transfer performance was 480% and 180% higher than that of complete filmwise and dropwise condensation, respectively. Garrod et al. [77] used a simple two-step plasmachemical methodology and prepared a variety of hydrophilic and hydrophobic patterned surfaces. They observed that although purely superhydrophobic

Weber number) and the critical liquid droplet size. This was done as part of a larger effort to study droplet motion on a rotating disk. As part of this work, Goodwin et al. [68] also showed that the critical droplet size scales proportionally with the contact angle hysteresis (i.e. θA–θR). In another study, Briscoe and Galvin [69] explored the critical condition of sessile and pendant water droplets sliding down solid surfaces. They reported that the critical inclination angle for sliding is proportional to V−2/3 for sessile droplets and V−1 for pendant droplets, where V stands for the droplet volume. In a more comprehensive work, contemporary researchers [70–72] reported that the local contact angle of small water droplets on tilted surfaces can be described using a thirddegree polynomial involving the azimuthal angle. The azimuthal contact angle variation on vertical and inclined flat surfaces was modeled as:

cosθmax −cosθmin ⎞ 3 ⎛ cosθmax −cosθmin ⎞ 2 cosθ = 2 ⎛ ϕ −3 ϕ + cosθmax π3 π2 ⎝ ⎠ ⎝ ⎠

(12)

Later, Sommers and Jacobi [73] showed that Eq. (12) is only valid for water droplets to the extent that the surface is smooth and homogeneous. On micro-grooved aluminum surfaces, which can exhibit a significant reduction in the critical droplet size (see Fig. 8), they recommended the use of another correlation for predicting the azimuthal contact angle variation, namely:

cosθadv−cosθrec ⎞ 3 cosθadv + cosθrec−2cosθmax ⎞ 2 cosθ = ⎛ cos ϕ + ⎛ cos ϕ 2 2 ⎝ ⎠ ⎝ ⎠ + cosθmax (13) As was demonstrated in the previous study [73], a simple grooved surface design (i.e. topographical modification) can be quite effective in improving water drainage and increasing the hydrophobicity of the surface. In a study focused instead on homogeneous chemical surface modification by Hong and Webb [74], hydrophilic coatings were examined for possible use on three different heat exchanger fin surface geometries (wavy, lanced, and louver) under both dry and wet test conditions. Both the air-side heat transfer and pressure drop were measured for the different heat exchangers. They concluded that for the wet tests, the hydrophilic coating applied to the louver and wavy fin reduced the wet pressure drop to 45% and 15%, respectively, for a frontal air velocity of 2.5 m/s without decreasing the wet sensible heat transfer coefficient. The authors acknowledged however that these test results could not establish that this wet pressure drop reduction would 972

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Table 3 Summary of surface wettability modification techniques found in the literature for various metallic substrates using chemical coating and/or physical roughness. Summary

Surface Material

Researcher

Modification Type C

P

Outcome

Ref.

Kashaninejad et al.

Si

–

X

⎧ θR ≈ 118−126° ⎨ ⎩ θA ≈ 140−144°

[33]

Gao & McCarthy

Si

X

X

θR = 81–134°, θA = 117–168° (smooth field) θR = 11–111°, θA = 33–120° (hydrophobic field) θR = 82–132°, θA = 116–168° (rough field)

[34]

Hong et al.

Cu & Al

–

X

Cu surface {θθ == 3953−−7282°° coarse coarse Al surface

[41]

Xiu et al.

Si

X

X

⎧ θR = 142.6−158.6° ⎨ ⎩ θA = 163.1−167.6°

[42]

Extrand

Si

X

X

θR < 2° θA = 7 ± 3°

[43]

Raj et al.

Si

X

⎧ θA = 20−38° SiO2−based θA = 44−58° photoresist−based ⎨ ⎩ θA = 34−53° pattern−based

[44]

Cieśliński & Krygier

Stainless Steel

X

–

θ ≈ 80°

[45]

Guo et al.

Al

X

X

θ ≈ 160–162.5° pH = 1 θ ≈ 155–168° pH = 14

[46]

Guo et al.

Cu

X

–

θ ≈ 160° (PDMSVT coating)

[47]

Shirtcliffe et al.

Cu

X

X

θ ≈ 136 ( ± 3°) electrodeposited Cu θ ≈ 160 ( ± 3°) “chocolate chip cookie” texture

[48]

Ren et al.

Al

X

X

θ < 5° (Al), θ = 166° (STA), θ ≈ 120° (monolayer), θ < 5° (PEI)

[49]

Shirtcliffe et al.

Cu

X

X

θ ≈ 155°, θH = 22° (rough tops & pits) θ ≈ 152°, θH = 37° (smooth tops & rough pits)

[50]

Wan et al.

Al

–

X

θmax = 155.98°

[51]

Sun et al.

Cu

X

–

θmax = 162°

[52]

Ou et al.

Cu

X

–

θmax = 152 ( ± 1.5)°

[53]

Lu et al.

Al

X

–

θmax ≈ 170°

[54]

Meng et al.

Cu

X

–

θmax ≈ 151°

[55]

Hu et al.

Cu

X

–

θmax ≈ 160 ( ± 2)° θmax ≈ 157 ( ± 2)° (after 5 months)

[56]

Huang & Leu

Cu

X

–

θmax ≈ 151.69 ( ± 5.11)°

[57]

Wang & Zhang

Cu

X

–

θmax ≈ 165 ( ± 2)°

[58]

Rahman & Jacobi

Al

–

X

θ = 50.84–149.09° θH = 42.8–62.4°

[59]

Li et al.

Al

X

–

θmax ≈ 90-137° (unmodified) θmax ≈ 96-152° (with ODT)

[60]

Chen et al.

Al

X

–

θ = 134 ( ± 1.1)°-154.8 ( ± 1.6)°

[61]

Wu et al.

Stainless Steel

X

–

θmax ≈ 166.3°

[62]

Safaee et al.

Cu

X

–

θmax ≈ 156°

[63]

Qian & Shen

Cu & Al

X

–

θ = 156°, θA = 158°, θR = 153° (Al) θ = 153°, θA = 155°, θR = 145° (Cu)

[64]

Huang et al.

Cu

X

–

θmax ≈ 153 ( ± 2)°

[65]

NOTE: C = Chemical; P = Physical; θA = advancing angle; θR = receding angle; θH = hysteresis. 973

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hydrophobic patterned surfaces. The maximum condensation rate for the tested surfaces was measured and found to be approximately 22 g h−1. Since their experiments however also revealed poor water drainage capability from the hydrophilic surfaces, they theorized and subsequently tested the use of a thin hydrophilic lane at the lower portion of the surface (surrounded by hydrophobic area) to ameliorate the rate of drainage. They reported that the length and width of this hydrophilic drainage path should be optimized. Thickett et al. [79] prepared a micropatterned, dewetted surface modified using both chemical and topographical features to capture water droplets due to humidified air. The coatings consisted of raised hydrophilic bumps on hydrophobic background. They reported that at an airflow rate of 9.8 L min−1, the dewetted surface could collect 3.4 ± 0.2 L m−2h−1 which was approximately 50% more than both the PS and P4VP-coated Si surfaces. Varagnolo et al. [80] experimentally investigated the dynamics behavior of sliding water droplets on periodic, stripe-shaped hydrophilic and hydrophobic pattern surfaces. They observed that the motion of water droplets was similar to ‘stick-slip’, while the average speed of the nonlinear motion was smaller than that of uniform surfaces (due to the pinning-depinning transition of the contact line). As a result, this behavior of water droplets on patterned surfaces shows that the passive control of water droplets on patterned surfaces by chemically tailoring is possible. In Wong et al. [81], simple micropatterned surfaces having hydrophilic bumps on a hydrophobic background were designed using chemical and topographical contrast to harvest water from the atmosphere. They observed that using PHPMA polymer on these surfaces resulted in more rapid nucleation, water droplet growth, and ultimately droplet coalescence. Using a combination of colloidal lithography and inclined reactive ion etching (RIE), Xue et al. [82] devised a new versatile method of creating a geometric gradient in silicon using well-defined and ordered micro/nano spheres and cones. They reported that as the water droplet contact angle increased, the contact angle hysteresis and subsequently the necessary sliding angle for water to drain from these surfaces decreased. In related work, Zhang et al. [83] devised a facile one-step method (mask-free) for producing large-scale superhydrophilic micropatterns on a superhydrophobic background inspired by the Stenocara beetle’s ability to collect water droplets. In fabricating these surfaces, they used mussel-inspired ink possessing an optimized solution of dopamine in the creation of the superhydrophobic regions, while the superhydrophilic regions were achieved by the formation of polydopamine via in-situ polymerization. They reported that all three patterned surfaces showed a desirable water-retention operation. It was also shown that one of the surfaces (whose polydopamine-patterned superhydrophobic regions consisted of a pattern size of about 500 µm and a pattern separation distance of about 1000 µm) could capture approximately 61.8 g cm−2 h−1, which was just over two times and four times higher than the water collection rate on the superhydrophilic and uniformly superhydrophobic surfaces, respectively. They observed that during the water collection process, tiny water droplets would spread over the superhydrophilic surface and quickly form thin films of water (Fig. 11a); while on the superhydrophobic surface, they would coalesce in a more arbitrary manner and then roll off when the critical droplet size was reached (Fig. 11b). Perhaps more importantly, these tiny water droplets would spontaneously and preferentially travel toward the polydopamine-modified superhydrophilic regions (due to the existence of these wettability differences) and form larger unstable droplets on these micropatterned regions due to coalescence which enabled them to roll off (and be removed) more easily. Zhang et al. [84] employed TiO2 surfaces and UV light through a photomask for achieving a superhydrophobic-superhydrophilic, stripeshaped patterned surface to collect water droplets. They reported that water droplets wetted the superhydrophobic regions isotropically with a circular area in the absence of the stripes. In the presence of the stripes, however, they tended to not only wet the regions anisotropically, but they also elongated in a direction parallel to the stripes

Fig. 8. Comparison of the critical water droplet size on baseline and microgrooved Al surfaces [73].

Fig. 9. Water collection efficiency of the PS, nanopatterned, micropatterned, and macropatterned surface coatings on Cu tubes at φ = 95 ± 3.6%, Tamb = 22.1 ± 1.0 °C, ΔT= 3 °C (blue), 5 °C (red), and 10 °C (green) during a span of 6 h [75]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

surfaces exhibited better capability to collect water droplets than that of hydrophilic surfaces, both purely superhydrophobic and hydrophilic surfaces did not collect considerable volumes of water. On the other hand, utilization of patterned hydrophilic-hydrophobic surfaces demonstrated noticeable water collection capability. The maximum mass of water collected was found to be approximately 0.8 g cm−2h−1 for a sample with plasma-deposited hydrophilic spots arranged in an array on a superhydrophobic CF4 plasma-fluorinated polybutadiene substrate. The optimal hydrophilic spot size/center-to-center distance was measured and found to be 500 µm/1000 µm. Beyond this threshold of approximately 500 µm, the water collection rate was observed to decrease with increasing hydrophilic spot size. In other research inspired by the Litoria Caerulea (green tree frog), Lee et al. [78] fabricated multiple surfaces containing both uniform and non-uniform wettability using chemical modifications to analyze their water collection and drainage characteristics. Unlike Garrod et al. [77], they concluded that uniformly hydrophilic and superhydrophilic surfaces collected higher rates of water, as compared to uniformly hydrophobic and hydrophilic/ 974

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Fig. 10. Condensation image of water harvesting on a patterned Cu tube (α = less hydrophobic region, β = hydrophobic region) [76].

and created static contact angles as large as ∼ 138° normal to the stripes. For a 50 µm wide stripe patterned surface cooled down to 5 °C at 60% relative humidity, they observed that tiny water droplets selectively condensed in the superhydrophilic stripes to initially form a water droplet and then a water column in that order. Zheng et al. [85] combined lithography and electrochemical etching processes to fabricate low adhesive superhydrophobic Al surfaces with “sticky” hydrophobic patterns (tracks). They reported that different adhesive forces could be achieved by controlling the electrochemical etching time. Their results show that the sliding angle decreased as the volume of the water droplet increased on the surface. Moreover, the sliding angle perpendicular to the track was found to be larger than the sliding angle parallel to the track, and the difference became more pronounced as the track width increased. Their results also revealed that the sliding of droplets at different tilt angles could be controlled by designing the hydrophobic tracks with different widths. Zhu et al. [86] created three different patterned surfaces decorated with circular pillar arrays (h = 100 μm,d = 50 μm) where the pillar intervals (25, 50, or 100 µm) were achieved through standard photolitography and reactive-ion etching processes. After rendering the pillars superhydrophobic using a thin photoresist layer of AZ-9260, they observed significant variations in the wetting behavior of water droplets on the surfaces due to coalescence. They reported that on the surface with the smallest pillar interval (25 μm), water droplets reduced their distance slower than droplets on the surface whose pillar interval was the largest (100 μm). The spreading of the merged water droplet (daughter droplet) however was the strongest on this surface, as compared to the larger pillar interval surfaces. They also monitored the behavior of approaching droplets where one droplet was 5, 7, or 9 µL and the other droplet was 5 µL. They found that when the volume of the approaching droplet increased, its movement became faster, yet it needed to travel a longer distance to be coalesced. Yang et al. [87] milled smooth hydrophilic structures including micro dots, lines, and circle grooves on superhydrophobic Al alloy surfaces to study the adhesion behavior of water droplets on such surfaces. They observed that the milled dots showed considerable sliding resistance due to high water adhesion, whereas water droplets could slide more easily and precisely along their route on the circular groove pattern due to anisotropic adhesion. They also suggested that since their approach was simple, rapid, and a top-down micromilling process, it could be extended to other materials.

Numerous experiments have demonstrated that although hydrophobic surfaces promote the rate of water collection often resulting in higher heat transfer coefficients, they suffer from a lower overall rate of condensation as compared to hydrophilic surfaces. As a result, Chatterjee et al. [88] created several different patterned and unpatterned Cu surfaces with distinct sizes and shapes (i.e. completely hydrophilic, completely hydrophobic, island-pattern, tree-pattern) to try and take advantage of both these hydrophilic and hydrophobic surface characteristics (see Fig. 12). They observed that depending on the type of the pattern on the surface, the condensation heat transfer coefficients were either higher or lower than that of the completely hydrophobic surface. In addition, they reported that the heat trasfer coefficient depended primarily on the condensate droplet size and the frequency of departure of the droplets from their nucleation sites. The highest heat transfer coeficients (which were approximately 3.5 times higher than that of the hydrophilic surface) were measured for the modified condensation surface containing the hydrophilic island pattern with a diameter of 0.25 mm. To remove water droplets condensed on the surfaces of heat exchangers in order to improve the heat transfer performance, Lei et al. [89] propounded a novel approach. They fabricated square-shaped pillars to create rough superhydrophobic patterned surfaces by means of polydimethylsiloxane and photolithography techniques and studied the dynamic behavior of water droplets on such surfaces. They demonstrated that water droplets could bounce off the surfaces by adjusting the frequency and amplitude of the vertical displacement. They also showed that at a certain water droplet size, the vibration amplitude at the resonant frequency was noticeably smaller than the necessary detaching amplitudes of neighboring frequencies. The frequency which had the water droplets bounce off the surface with the lowest external vibration was measured to be 70 Hz. Peng et al. [90] experimentally investigated the maximum radius, droplet size distribution, and the optimum pattern size (i.e. ratio of hydrophobic to hydrophilic regions) on various copper surfaces. Their results showed that as the width of the hydrophobic region increased, the maximum droplet radius also increased, whereas the droplet population density decreased. They also found that the heat transfer coefficient on patterned surfaces decreases as the hydrophilic region got wider. They showed that the optimum hydrophobic region width was approximately 0.55 mm while its corresponding optimum maximum droplet radius was about 0.25 mm. An 975

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Fig. 11. Water harvesting process on (a) superhydrophilic surface, (b) superhydrophobic surface, (c) patterned surface with 500 µm polydopamine patterns (x) and 1000 µm separation (y), (d) patterned surface with x = 200 µm and y = 400 µm, (e) patterned surface with x = 200 µm and y = 100 µm [83].

patterned surfaces in comparison to the post-patterned surfaces due to smaller pinning of water droplets on grates (see Fig. 13). Furthermore, they realized that despite the similar surface wettability and roughness factor, the water harvesting performance of the grates-patterned surface was approximately 75 (mg × mHz). Remarkably, that was better than that of post surfaces, while the Cassie-Baxter state grate featured the best water harvesting performance during experiments. (Note: This harvesting performance metric was calculated by multiplying the average droplet mass and falling frequency together. The average falling droplet mass varied from 20 to 60 mg, while the average falling frequency varied from 0.75 to 3.5 mHz across all tested surfaces.) In other research, Seo et al. [92] tailored Cu tubes using chemical oxidation of the Cu followed by a low surface energy coating of silane to make the surface superhydrophobic and useful for water harvesting applications under fogging and dewing conditions. They reported that on the bare and superhydrophilic Cu tubes, filmwise condensation was observed, while on the hydrophobic and superhydrophobic surfaces dropwise condensation was seen. The maximum measured dew harvesting on the test tubes (approximately 3.5 g over 90 min) belonged to the hydrophilic surface due to its higher nucleation rate. Wang et al. [93] proposed a facile method to fabricate a bioinspired hydrophilicsuperhydrophobic patterned surface for large-scale highly efficient water harvesting applications. Copper meshes (i.e. gauzes) were first

Fig. 12. Patterned surfaces (a) island-pattern, (b) tree-pattern [88].

enhancement of approximately 23% in the heat transfer coefficient was measured on these patterned surfaces. Seo et al. [91] studied the water harvesting performance of superhydrophobic surfaces with two different patterns (posts and grates) for both Wenzel and Cassie-Baxter states by the alteration of structural parameters to determine the effect of the microscopic surface geometry on average mass of collected water and water harvesting falling frequency. They observed that the harvesting falling frequency was considerably higher on the grates976

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Fig. 13. Time-lapse images of water condensation on post and grate patterned surfaces in Wenzel (W) and Cassie-Baxter (C) states [91].

droplet on a surface. If the hysteresis is too large, it is difficult to generate a surface tension force that is large enough to generate motion. The creation of such a surface can be achieved using a variety of approaches; however, all of them can generally be classified under two broad categories, namely:

calcined in an oven to form an oxide layer and then modified using PFDT to make them hydrophobic. Afterwards, they were thermally pressed into polystyrene sheets (PS) to produce a hydrophilic–superhydrophobic patterned composite surface. The maximum water collection rate was measured to be 159 (mg cm−2 h−1). Zhu et al. [94] created a superhydrophilic Cu surface with two superhydrophobic circular patterns via a simple and rapid route. The maximum water collected by the surface was measured and found to be 1316.9 (mg cm−2 h−1). They also reported that this amount of water was successfully captured when the experiment was repeated ten times. Table 4 contains a summary of the important outcomes from these works and others.

• Passive methods: surface tension gradients due to physical roughness and/or chemical modification (see Fig. 14). • Active methods: temperature gradients (thermo-capillary), electrostatic gradients (electro-capillary), elastic surfaces, and/or periodic oscillation of surface.

Research has proven that in addition to the aforementioned techniques (i.e. passive and active), using a combination of self-guiding tracks, coalescence, vibration due to noise, and/or magnetic fields (in the case of ferrofluids) can also aid in the movement of droplets on surfaces. Historically, the possibility of spontaneous water droplet motion due to a surface tension gradient was pioneered by James Thompson, the brother of Lord Kelvin, in 1855 who first coined the term “tears of wine” to describe the motion of droplets on a wine glass [104]. Next, in 1865, the Italian physicist Carlo Marangoni studied these “tears of wine” as part of his doctoral dissertation [105]. Due to the significance of his findings, the “tears of wine” phenomenon came to be known as the “Marangoni effect” [106]. Following additional research in this field, Hardy in 1919 observed the spontaneous spread of liquid droplets on a clean solid, which had been previously placed in a chamber through which a rapid current of dry air was flown for 30 min [107]. Elwing et al. [108] later reported a new, creative method by which solid surfaces (from Si to glass) could become hydrophobic at one end and hydrophilic at the other end using xylene and Cl2(CH3)2Si. The driving force, Fsx , which is responsible for the spontaneous motion of a water droplet on a surface enhanced by a gradient was later formulated by Moumen et al. [66] as:

3.2. Surface tension gradients According to Newton’s first law [103], a net force is needed to move an object across a horizontal surface. This of course also holds for condensate droplets in HVAC&R applications. The Navier-Stokes equations for fluid motion can be written as:

⎯ ∂ → T → + ⎯→ (ρ u ) + ∇ ·(ρ→→ u u ) = −∇P + ∇ ·[μ (∇→ u + ∇→ u )] + ρg F (14) ∂t ⎯→ ⎯ where the term F represents external body forces. Thus, in the absence of air flow on a horizontal surface (where the gravitational force, Fg = 0 in the x-direction), some other force is needed to achieve droplet motion. One such possibility is the creation of a net surface tension force where Fs = −γ D

∫0

π

cosθ cosϕdϕ

(15)

This can be accomplished by varying the underlying surface wettability in an intentional manner such that cos θ is varied underneath the droplet from one end to the other. Such a surface is called a “gradient surface”. Moreover, as was explained in the previous section, the results of the Bikerman study [29] show that low contact angle hysteresis is also generally a requirement for the spontaneous spreading of a water

Fsx = 2γR 977

∫0

π /2

{cos(θ )f −cos(θ )r }cosϕdϕ

(16)

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Table 4 Summary of key outcomes from the literature with regards to the behavior of water droplets on patterned wettability surfaces. Summary

Surface Material

Researcher

Pattern Type C

T

Outcome

Ref.

MWCa = 2.78 (g cm−2 h−1) (5–pointed star-shaped pattern)

Bai et al. Al-Khayat et al.

Glass Cu

X X

– X

Garrod et al.

Si

X

–

152° < θa < 154° 151° < θr < 152°

[77]

Lee et al.

Si

X

–

⎧ θ = 162° super hydrophobic ⎪ θ = 105° moderate hydrophobic ⎨ θ = 3° hydrophilic ⎪ θ = 1° super hydrophilic ⎩

[78]

Thickett et al.

Si

X

X

⎧ θ = 91.3 ± 1.8° PS θ = 36 ± 2.7° P 4 VP ⎨ ⎩ θ = 90.2 ± 1.9° Dewetted

[79]

Wong et al.

Si

X

X

[81]

Zhang et al.

Si

X

–

Zheng et al.

Al

X

–

Zhu et al.

Si

X

–

Yang et al. Chatterjee et al.

Al Cu

X X

X –

Lei et al. Peng et al.

Si Cu

X X

– X

Zhu et al. Narhe & Beysens Hou et al.

Cu Si Si

X X X

– –

Lee et al.

–

X

–

Seo et al. Tenjimbayashi et al. Yoon et al.

Si – Si

X X X

– X

Zorba et al.

Si

X

–

Yao et al.

Si

X

–

⎧ θ < 89 ± 1° PS ⎨ ⎩ 44 ± 3° < θ < 57 ± 2° Patterned θ = 157° Sliding angle < 1° θ = 160° perpendicular to the 0.5 mm track θ = 110° parallel to the 0.5 mm track 120.8° < θ < 121.9° no pillars 140.5° < θ < 165.1° with pillars Parallel and perpendicular water droplet contact angles on patterned surfaces are discussed Condensation heat transfer coefficient of the patterned surface was either higher or lower than the completely hydrophobic one depending on the pattern θ = 148.5 ± 3.5° 120.4° < θ < 121.3° hydrophobic region 51.5° < θ < 53.2° hydrophilic region 155.2° < θ < 159.5° θ = 138° θ = 161 ± 4° MWC ≈ 10+7 (µm2) in 5 min θ = 159 ± 4° MWC = 0.094 (g h−1) Guiding tracks ability of water droplet (θ > 150° ) Dynamic analysis of the water droplet (73.59° < θ < 96.93° ) adhesion on the patterned surfaces Demonstration of annealing impact on dewetting from hexagon, honeycomb, and circular-shaped pattern surfaces Achievement of 66° < θ < 130° by tailoring the surface micro and nanometer scale via femtosecond laser irradiation θ = 112.1°

a

⎧ θ = 81 ⎪ θ = 85 ⎨ θ = 88 ⎪ θ = 45 ⎩

± ± ± ±

1° 2° 3° 1°

nanopattern micropattern macropattern flat P 4 VP

[67] [75]

[83] [85] [86] [87] [88] [89] [90] [94] [95] [96] [97] [98] [99] [100] [101] [102]

Maximum water collection; C = Chemical; T = Topographical.

where γ is the surface tension of the water droplet, R is the radius of the water droplet, θf and θr are the equilibrium contact angles at the front and rear, respectively, and ϕ is the azimuthal (or polar) angle. For a wedge-shaped wettability gradient (see Fig. 15), it can be shown that

this net force is:

Fsx = 2γR {cosθphilic−cosθphobic }[sinϕB−sinϕF ]

(17)

where θphilic is the contact angle on the hydrophilic region and θphobic is y

y

x

x

height

Chemistry Gradient

Roughness Gradient

Fig. 14. Examples of different surface tension gradient designs involving (a) chemical stripes, and (b) variable spaced channels. 978

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In other work, Qiang et al. [117] observed the motion of water droplets on a horizontal gradient surface composed of both hydrophilic and hydrophobic regions which was prepared using a SAM (C12H25Cl3Si) via the chemical vapor deposition (CVD) technique. Although the motion of the water droplet from the hydrophobic to the hydrophilic region was described as “squirming” in some cases (i.e. alternately spreading and shrinking), a maximum travel distance and peak velocity of 3 mm and 40 mm/s, respectively, were measured. In a similar procedure to [117], Zhu et al. [118] fabricated a gradient surface by means of SAM (C12H25Cl3Si) and CVD techniques to determine the behavior of water droplets when the gradient surface was both horizontal and tilted (30°). They reported that the velocity and travel distance of water droplets on the gradient surface directly rise with an increase in the water droplet size. The maximum velocity of a water droplet (2 µL) reached 42 mm/s on the horizontal surface, whereas it declined to 18 mm/s on the inclined gradient surface due to the fact that the gravitational potential energy is generally a resistance to the motion. They also recorded that the maximum travel distance for a water droplet placed on the horizontal gradient surface was approximately 2.9 mm, while on the inclined surface, it was about 2.8 mm. In an experiment performed by Ito et al. [119], they prepared a gradient surface with a SAM (CH3(CH2)17Si(OCH3)3) and reported that the mean water droplet (2 µL) velocity and displacement on the surface were between 0.5 and 6 mm/s and 3 and 4 mm, respectively. They also demonstrated that the contact angle hysteresis of water droplets always remained less than 10° during the spread over the surface. Alheshibri et al. [37] devised a simple technique for inducing water droplets to move on a hydrophilic aluminum surface possessing a hydrophobic copper background using a triangular wedge-shaped gradient (see Fig. 15). The surface gradient was fabricated by means of physical vapor deposition (PVD) of copper films on mirror-finished aluminum plates through thermal deposition. After coating the gradient surface with a self-assembled monolayer (SAM), they observed the spontaneous spread of water droplets on both horizontal and inclined gradient surfaces. Travel distance up to 2–3 cm was reported over a range of droplet sizes. They also demonstrated that for smaller wedge angles, significantly longer water droplet travel distances were possible (even though the spreading speed was lower due to the decreased Fsx values which accompany smaller wedge angles (see Eq. (17))). Khoo et al. [120] fabricated a chemically-patterned nanotextured wedge-shaped gradient on an Al surface by which subnanoliter water droplets were induced to move spontaneously (similar to Fig. 15). The surface was reported to have two distinct wetting properties and low hysteresis, while the experiment was carried out for a wide range of water droplet sizes and inclination angles. The maximum water droplet speed increased more than three times and reached 0.5 m/s when the wedge angle altered from 4° to 8°. They also demonstrated that when the inclination angle increased from 40° to 180°, the velocity of the water droplet (2 µL) diminished from 122 to 34 mm/s. In another similar study, Zheng et al. [121] demonstrated that water droplets could be driven into motion on a wedge-shaped superhydrophobic Cu surface on which a layer of poly-(dimethylsiloxane) (PDMS) oil was laid. Their results supported the previous conclusion— namely, as the wedge angle decreases, the water droplet (5 µL) travel distance increases, but the droplet velocity decreases [37,122]. A maximum travel distance and velocity of up to 15.8 mm and 14.0 mm/s were reported. In other work, Han et al. [123] presented a novel method called “space limited plasma oxidation” to create a self-assembled monolayer (chemical) gradient on a solid surface to precipitate the movement of small water droplets. Various scales (between 1 and 18 mm) and ranges of contact angle variation (up to 100°) were considered in their research. The maximum droplet travel distance and velocity up to ∼2.5 mm and 0.64 mm/s were recorded. In a paper comparing different gradient shapes and patterns, Zhang and Han [124] scrutinized the self-running behavior of water droplets on both horizontal and inclined surfaces possessing different types of

Fig. 15. A triangular wedge-shaped surface tension gradient produces a net capillary force toward the more hydrophilic region [37].

the contact angle on the hydrophobic region. In this equation, ϕF represents the azimuthal angle to the first boundary at the front of the droplet, and ϕB represents the azimuthal angle to the second boundary at the back of the droplet. According to Eq. (17), for a given droplet size, larger wedge angles, ψ, (see Fig. 15) will produce larger overall surface tension driving forces. Higher droplet velocities are generally realized in this case, but also shorter overall travel distances. The magnitude of the driving force is also shown here to be proportional to the contact angle difference between the two regions. (i.e. A larger surface wettability contrast means a larger driving force.) The movement of water droplets uphill was first demonstrated by Chaudhury and Whitesides in 1992 [109]; however, the theoretical prediction as to its possibility was initially suggested by Greenspan back in 1978 [110]. Choi and Newby [111] later reported a simpler, yet reproducible method for producing micro-scaled gradient surfaces noting that the creation of the surface tension gradient used by [108–109] to induce the movement of water droplets is difficult since it generates a considerable amount of organic waste and requires a wellcontrolled deposition condition. In another work, Suda and Yamada [112] followed the fabrication procedure proposed by [109] and created a solid surface to facilitate water droplets to run uphill toward the more wettable region by exposing the surface to a diffusing decyltrichlorosilane vapor (Cl3Si(CH2)9CH3). They observed that the velocity of the water droplet on the surface decreases exponentially with increasing inclination angle (from horizontal to vertical). At the same time, the velocity was observed to increase exponentially with the driving force generated by the surface tension gradient. Their results revealed that a small change in Fsx (i.e. from 1.0 to 1.5) could significantly augment the droplet spreading velocity by up to tenfold. Droplet velocities (∼2 µL) in the range of 0.1–10 mm/s were measured. Later, Daniel et al. [113] observed the movement of small water droplets (0.1–0.3 mm) on a Si surface with a radial surface tension gradient. Droplet velocities of 0.15–1.5 m/s were observed. The authors concluded that this increase in velocity (over earlier studies) was due to the coalescence of droplets and propounded that this phenomenon had implications for enhancing heat transfer in heat exchangers. Building on this earlier work, Wasan et al. [114] reported that by some modifications in the size of water droplets, droplet velocities as fast as 5 m/s under the same surface tension gradient could be successfully achievable. In other research performed by Daniel and Chaudhury [115], they reported that when a droplet (1–2 µL) is placed on a surface containing a gradient, it quickly spreads but a velocity of only up to 1–2 mm/s can be achieved. This low velocity due to the surface gradient could be further increased however by applying a periodic force to the water droplet since such a force will increase the Fsx (see Eq. (16)). As a result, they reported that the achievement of droplet velocities up to 5–10 mm/s were possible provided that such a periodic external force was applied. They also demonstrated that increasing the droplet size would result in achieving a higher velocity. The authors also reported that in some cases the droplet initially did not move due to large wetting hysteresis (i.e. ∼20°). The problem of sessile droplets with a low tendency for spreading on a surface has also been reported by other researchers [66,116]. 979

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Table 5 Mean velocities and maximum travel distances of water droplets on various gradient geometries [124]. Gradient Shape

Trapezia Triangle Rectangular Meniscus

Horizontal (θ = 0°)

Tilted (θ = 25°)

V (cm/s)

dmax (mm)

V (cm/s)

dmax (mm)

10.0 ± 0.1 6.7 ± 0.4 2.4 ± 0.4 13.1 ± 0.5

24.5 24.0 26.0 16.3

3.8 ± 0.2 2.3 ± 0.5 0 6.4 ± 0.5

24.5 ± 0.5 24.0 ± 0.5 0 14.0 ± 2.5

± ± ± ±

0.5 0.5 0.5 2.0

Fig. 16. Water droplet driven by a topography-based gradient under disturbance and vibration on a microstructured hydrophobic surface [128].

In similar but earlier research, Shastry et al. [129] ascertained that water droplets could be propelled down using energy supplied by mechanical vibration on chemically homogeneous microstructured surfaces. To produce several texture gradients on the surface, a regular two-dimensional array of square pillars whose dimensions varied spatially across the surface ranging from 4 to 8 mm was designed. A travel distance of 6.24 mm along the gradient was measured. Sun et al. [130] prepared several different topography-based wettability gradients (i.e. linear, radial, cubic micropillars, etc.) with an excimer laser followed by a fluoroalkylsilane surface treatment. They observed that water droplets could spontaneously move towards designed routes of the direction of gradients. Having said that, however, Sommers et al. [131] later also demonstrated that water droplets (4–10 µL) could spread spontaneously on a laser-etched micro-grooved gradient Cu surface without the need for chemical coating, unlike earlier research which used SAM [128,129]. Droplet motion on laser-etched micro-grooved Cu gradient surfaces solely relied on the underlying topography-based gradient, which consisted of variably-spaced 100 µm deep microchannels. Travel distances between 0.5 and 1.5 mm were observed. The authors also suggested that this research could aid in reducing water retention on the metallic fins used in heat exchangers in HVAC&R systems.

gradient shapes, namely trapezia, triangle, rectangular, and meniscus. They concluded that three independent parameters play prominent roles in the self-running motion of the water droplets: (a) low contact angle hysteresis of the surface materials, (b) existence of a large surface wettability difference, and (c) the space limitation of the shape-gradient transportation area (for example, the existence of a desirable wedgeangle for the gradient displayed in Fig. 15). The summary of their experimental measurements are tabulated in Table 5. As shown here, the gradient shape which produces the largest mean water droplet velocity on a surface does not necessarily result in the longest travel distance. They concluded that for the horizontal surface, it is more preferable to use a rectangular-shaped gradient to attain the maximum water droplet displacement, while on the inclined surface (25°), it is more desirable to create a trapezia-shaped gradient. The lower droplet velocity on the rectangular-shaped gradient was reportedly due to the large space limitation of the transportation area which affects the elongating stage of the self-running process. It is important to point out here that most attempts to model droplet velocities and travel distances on surface tension gradients rely on a force-based analysis, which tends to be significantly oversimplified. Such analysis should only be used for qualitative purposes, despite its widespread use and popularity among researchers in the field. For example, the droplet base contour shape is often assumed to be circular (as shown in Eqs. (16) and (17), when in actuality it often spreads out to a highly non-circular shape during the travel process. Viscosity, Laplace pressure, and surface tension effects are other examples of corrections that should be taken into account for a more rigorous analysis of the droplet’s motion [125–127]. A more accurate understanding and prediction of the behavior of water droplets on gradient surfaces however can be obtained using a thermodynamics-based approach. Even the simpler tasks of rigorously deriving the Young, Wenzel and CassieBaxter equations should unquestionably (and more appropriately) use thermodynamic arguments based on the total surface free energy.

3.4. Droplet movement through coalescence and evaporation To mitigate the problems associated with the attachment of sessile water droplets to heat transfer surfaces (i.e. heat transfer reduction, etc.), Boreyko and Chen [132] demonstrated that for superhydrophobic surfaces containing micropillars (if used as a heat transfer surface), water droplets can be removed spontaneously without any external forces from the surface using droplet coalescence. Coalescence is a physical process whereby two or more droplets merge to form a single, larger droplet often referred to as the “daughter droplet” (see Fig. 17). Early observations of this natural phenomenon were noted by Bengham and Saweris [134] back in 1938. In the current work, Boreyko and Chen [132] created a superhydrophobic surface composed of twotier carbon nanotube roughness coated with hexadecanethiol and reported that water droplet jumping velocities up to 1 m/s were possible by the coalescence process. They also reported that a discrepancy exists between the experimental and theoretical prediction of the critical diameter for droplet jumping and removal. The self-propelled droplet motion is powered by the surface energy released upon droplet coalescence. The resulting jumping velocity of a merged spherical droplet can be formulated as shown below assuming all the released surface energy is transmitted to the translational kinetic energy:

3.3. Droplet actuation techniques In addition to the surface gradient methods that have been reported thus far by which water droplets can be induced to spread/slide on surfaces, different types of water droplet transportation mechanisms have also been proposed. In a unique study, Lv and Hao [128] successfully fabricated a microstructure hydrophobic surface on which the area fraction of the surface is constant, while the scales of the micropillars are monotonically altered. On such a surface, water droplets were able to move unidirectionally under both disturbance and vibration with the aid of a SAM from the small scale to the large scale as shown in Fig. 16. In this geometry, the driving force in the x-direction (spanwise direction) can be approximated by:

Fsx = (3V )2/3π 1/3

vi =

(1−cosθ∗)

fτ ∂S S 2 (1−cosθ∗)1/3 (2 + cosθ∗)2/3 ∂x

(18)

6σ ⎡ r12 + r22−(r13 + r23)2/3 ⎤ ⎥ ρ ⎢ r13 + r23 ⎣ ⎦

(19)

where ρ is the density and r1 and r2 denote the two droplet radii participating in the coalescence event. In another study, Kim et al. [135] reported that although the coalescence of water droplets on the heat transfer surfaces has a desirable impact on the enhancement of heat transfer and the prevention of icing, the velocity of the coalescence

in which τ is the line tension, f is the area fraction, S is the shapedependent roughness scale. When the frequency and amplitude of the oscillator were approximately 80 Hz and 0.75 mm, a travel distance of up to a few millimeters was observed for a 20 µL water droplet. 980

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Fig. 17. Regional map between surface wettability and droplet coalescence. Region I represents small contact angle (CA) but large contact angle hysteresis (CAH). Region II represents small CA, small CAH. Region III represents large CA, large CAH, and Region IV represents large CA, small CAH [133].

of two water droplets is low since it is constrained by the internal fluid dynamics. Thus, to break the two-droplet speed limit, they experimentally investigated multi-droplet coalescence on a copper oxide nanostructured superhydrophobic surface for a wide range of water droplet radii (5–50 µm). Their results showed that a 50% increase in the modest jumping speed would contribute to a 40% enhancement in the heat transfer performance. The expansion of Eq. (19) for multi-droplet coalescence can be stated as: n

vi =

n

2 3 6σ ⎡ ∑i = 1 ri −(∑i = 1 ri ) n ⎢ ρ ∑i = 1 ri3 ⎣

2/3

⎤ ⎥ ⎦

that the center of mass of a water droplet can move either in or against the gradient direction, but the direction depends on the configuration of the micropillars. Their theoretical model shown in Fig. 19 reveals that the water droplets always move in the direction of the wettability gradient (under the evaporation mechanism) in Regions I and III, whereas the droplet moves against the wettability gradient in Region II where φL > φR and UL < UR . Here, the subscripts of L and R denote left and right, while φ and U stand for the solid fraction of the surface and the energy barrier per unit length of the contact line which is calculated as:

U = sφ [γlv−(γsl−γsv )]

(20)

(21)

In another recent study involving evaporation-induced spontaneous water droplet motion on a periodically compliant surface, Liu et al. [139] reported that when the size of a water droplet becomes small enough, it can finally overcome inertia by a marginal perturbation and erratically travel on the surface. The experiment was carried out on a wide range of water droplet sizes (from a few µm to more than 0.6 mm), with a maximum travel distance and velocity of less than 0.5 mm and 100 µm/s, respectively, being measured due to the evaporation/drying process. They also reported that this self-running motion heavily depends on the water droplet size. Other significant observations and achievements which are relevant to this review have been gathered and summarized in Table 6.

Since the relationship between sessile water droplet contact angle and hysteresis is essential in the comprehension of the coalescence behavior, Chu et al. [133] devised an experiment composed of sessile water droplets (2 µL) and four different conditions for the water droplet static contact angle and contact angle hysteresis on various Al surfaces (see Fig. 17). They determined that when the contact angle is large whereas the contact angle hysteresis is small (Region IV), the selfpropelled droplet motion is easily triggered. In another study focused on Al surfaces and droplet coalescence, Parin et al. [136] reported a maximum heat transfer coefficient on a nano-textured superhydrophobic surface (i.e. ∼52 kW/m2 K) that was approximately four times larger than the untreated hydrophilic Al surface. The jumping droplet phenomenon was also observed randomly on the surface. Feng et al. [137] carried out an experiment to characterize the ability of coalescence-induced spontaneous water droplet motions on superhydrophobic surfaces made out of Cu with two types of roughness (sisal-like nanoribbon and defoliation-like nanosheet structures) (see Fig. 18). They demonstrated that only the superhydrophobic Cu surfaces with the sisal-like nanoribbon structures experienced the spontaneous motion of condensate droplets, as compared to the other surface type. It is believed that the sisal-like nanoribbon structures on the Cu surface provide for an easier, smoother transition to the Cassie state for merged water droplets throughout the coalescence process. In related research, Xu et al. [138] found that the motion of water droplets can also be facilitated by the evaporation mechanism on various wettability gradient surfaces composed of non-uniformly distributed cylindrical micropillars similar to Fig. 16. They ascertained

4. Applications in energy systems 4.1. Applications in air-conditioning systems In air-conditioning systems, the evaporator normally operates with the air-handling surface temperature below the dew-point temperature of the conditioned air. Therefore, moisture condenses and accumulates on the surface of the heat exchanger. Depending on the wettability of the air-side surface, water condensation on a heat exchanger can take the form of a dropwise mode, a filmwise mode, or as a mixed mode. The different condensing mode, together with the heat exchanger geometry play a key role in the overall retention behavior and largely affect the thermal-hydraulic performance of the heat exchanger. Many surface treatment methods have been applied or attempted on 981

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Fig. 18. SEM images of copper surface with (a, b) sisal-like nanoribbon (θ = 167.5°), and (c, d) defoliation-like nanosheet structures (θ = 161.5°) [137].

techniques yielded good surface wettability. In a related paper, Min et al. [163] measured contact angle data for several different available commercial coatings and investigated their long-term wetting characteristics on heat exchangers. In a follow-up work, Min and Webb [164] reported contact angles on aluminum, copper, and two commercial coatings on aluminum. They also measured the condensate retention on these surfaces and found out that the mass of retained condensate per unit area was approximately a function of the receding contact angle. In 2002, Kim et al. [165] described a method of using a DC discharge plasma of a reactive gas to deposit a cross-linked polymer surface coating on a heat exchanger. This deposition technique results in hydrophilic functional groups (i.e. CeO, C]O, (C]O)eC, etc.) forming on top of the coating and thus creates a highly wettable surface. Unlike an oxygen plasma treatment [166,167], Kim et al. [165] reported that the surface wettability of the treated specimens did not change significantly with increasing number of wet/dry cycles. Along with these advances in surface treatment technology over the past decade, the effects of fin surface wettability on the thermal-hydraulic performance of a heat exchanger have also started to attract the attention of researchers. In 1987, Mimaki [168] coated a group of round-tube-plain-fin heat exchangers with an organic film, which reduced the water droplet contact angle on the fin surface from around 70° to approximately 10°. This surface treatment had no impact on the air-side heat transfer coefficient, but due to the flattened water droplet shapes on this highly wettable surface, the air-side pressure drop across the heat exchanger was reduced by 40–50% as compared to the uncoated one. Wang and Chang [169] and Hong and Webb [170] later followed this study by examining different fin geometries including plain, louvered, wavy, and lanced fins. The impact of the fin coating on the heat-transfer performance was also found to be very small, while the air-side pressure drop reduction varied from zero to 50% depending on the fin type. Thus, the overall findings of these studies (especially the potential impact on the air-side pressure drop) were similar. In Wang and Chang [169], a hydrophilic Cosmer coating was used which involved the successive immersion of assembled heat exchangers into a

Fig. 19. Model prediction of the directional droplet evaporation proposed by [138].

full-scale heat exchangers, contributing in the production of uniformly hydrophilic fin surfaces for the objective of reducing air-side condensate retention. In 2000, Hong and Webb [162] categorized and documented the types of coating and treatment methods on aluminum fin materials, and also described their wetting mechanisms. In this article, they introduced five commercial coatings (Sama, SP411, Polygreen, Hypercore, Aqua-Shed) and provided their long-term wettability data. They also suggested several alternative surface treatments including aluminum oxidization, thermal etching, and unidirectional micro roughness (via brushing) on the fin stock, but not all of these 982

983

– Si/Au Si/PDMS Sand Teflon Glass Graphite Si Cu/stainless steel Si Cu Al Glass Coated Au – Si & Al Si – Au PDMS Si Glass – Glass

Surface Material

NOTE: C = Chemical; T = Topographical.

Bennes et al. Wang et al. Mettu and Chaudhury Nilsson and Rothstein Bai et al. Feng et al. Hiltl and Boker Hou et al. Paradisanos et al. Huang and Leu Zhang et al. Lv et al. Luo et al. Wu et al. Langley and Sharp Zielke and Szymczyk Fang et al. Yu et al. Jopp et al. Zielke et al. Sandre et al. Richard and Quéré Decker and Garoff

Researcher

Summary

– X X – X X X X – X X – X – – X – X X X – – X

C

X X – – – – X – – X – X X – X – – – X X X

–

T

Gradient Type

Vibration Surface gradient Vibration Coalescence Surface gradient Surface gradient + vibration Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient + vibration Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Surface gradient Vibration

Type

Droplets motion with respect to volume due to low frequency vibration (30 kHz) Switching water contact angle from < 5° to 164° using LbL deposition Surfaces either with a negligible or high hysteresis in not conductive to any movement Three distinct coalescence regime were found as a function of Bond number and impact angle Water collecting behavior of hydrophilic surfaces Control of droplet transport either by gradient or vibration Spontaneous spread of water droplets motion on wrinkle surface in parallel and perpendicular conditions Controlling the movement of droplets by force analysis and the magnitude of a wettable gradient Patterning the surface with femtosecond laser led to achievement of 505 mm/s velocity Introducing a new technique for mass production of surface wettability Using two-step process or PEM deposition and counter ion exchange to reduce wettability Introducing curvature gradient to accelerate both micro and nano-droplets on heat transfer surfaces Modulation of contact angle hysteresis by stretching/contracting a droplet on oblique surface Self-propulsion of water droplets against the wettability gradients The amplitude and wavelength of sinusoidal surface wrinkles depend on Al film thickness Increment of velocity as a result of rise in droplet size and steepness of gradient Motion and tunable wettability achieved by manipulation of free energy barriers Proposing a method to fabricate a large-scale superhydrophobic to superhydrophilic gradient with various gradient slopes Observation of transition from groove-filling and Wenzel state to non-filling of surface grooves and Cassie-Baxter state No relationship exists between water droplet size and velocity Spontaneous motion achieved by structuring the surface and applying electric field The smaller the droplet, the faster it rolls off from a superhydrophobic surface Effect of vibration amplitude on significant contact angle hysteresis

Outcome

Table 6 Summary of outcomes available in the literature for spontaneous motion of water droplets on various surfaces with different techniques.

[140] [141] [142] [143] [67] [144] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161]

Ref.

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984

2009

Liu and Jacobi [174]

2012

2007

Ma et al. [172]

Kim and Lee [179]

2002

Kim et al. [165]

2017

2002

Wang et al. [171]

Hu et al. [178]

2002

Shin and Ha [173]

2018

2000

Min et al. [163]

Hu et al. [177]

1999

Hong and Webb [170]

2011

1987 1998

Mimaki [168] Wang and Chang [169]

Min et al. [176]

Year

Author

Round

N/A

N/A

Round

Round

Round

Round

Round

Round

Round Rounda Rounda Round

Round Round

Tube type

Louver

Metal Foam (copper/aluminum)

Metal Foam (copper)

Smooth Wavy (Copper Fins)

Slit

Wavy (1, 2, 3)

Plain Parallel louver (3b) Parallel louver (4) Plain (6) Louver (9, 11) Wavy (2) Lanced (4) Louver (6) Wavy (4) Wavy (5) Plain/Slant edge (A1) Plain/Discrete (B1) Plain (C1) Slit (2) Plain (5) Plain (7) Slit

Fin type

Tested specimens

Table 7 Effects of air-side surface wettability on heat-exchanger performance.

1.59

5–40 PPI

5–40 PPI

1.5

1.1

1.2, 1.5, 1.7

1.4

5.5 8.5 5.5 1.28 1.23 1.78

1.5, 1.7, 2.0 1.4 1.4 1.4 1.7, 1.2 2.1 2.1 1.5 2.0

Fin spacing (Fs/mm)

N/A

θA ∼ 42°, θR ∼ 0° θA ∼ 68°, θR ∼ 5° θA ∼ 65°, θR ∼ 0°

(a) Alkali oxidation (30 min) (b) Alkali oxidation (5 min) (c) Stearic Acid SAM (d) n-dodecyl mercaptan SAM Hydrophilic Hydrophobic Dual Coated Fin

1-dodecanethiol SAM coating

NaOH/K2S2O8 mixed solution (2–30 min)

Plasma treatment

Plasma polymerization Organic resin

Organic

No effect

∼10°

N/A

No Effect

θA ∼ 15°, θR ∼ 7° θA ∼ 28°, θR ∼ 13° θA ∼ 148°, θR ∼ 130° θA ∼ 164°, θR ∼ 149°

∼3° ∼130° Both

Hydrophilic: Slightly higher ΔP under frosting but lower under wet conditions

1–95% increase (approx. no effect at 30% RH) N/A

N/A

−2.5% to +5% capacity (depending on RH)

Slight decrease

5–34% increase

∼30° ∼50° ∼110° = 40–71°, = 0–8°

θA ∼ 161.2°, θR ∼ 138.6°

θA θA θA θA θR

Up to 44% reduction 35% increase for θA ∼110°

25% reductione

Up to 40% reduction

15% reduction No effect 40–50% reduction 9% reductione 12% reductione N/A

40–50% reduction 15–40% reduction

Pressure dropd

Both ± observed

N/A

θA ∼ 20°, θR ∼ 0° 10–20°

Up to 20% degradation

< 10°

N/A

No effect No effect

∼10° N/A (Hydrophilic)

Organic film Organic film Cosmer dipping Cosmer dipping Organic film Aqua-Shed Aqua-Shed Sama ZN AQ Plasma polymerization

Heat transfer coefficientd

θ (deg)

Coatingc

Wet-surface results

(continued on next page)

Hydrophilic: Least amount of retained water after defrosting

45–66% lower water retention for the 5–20 PPI hydrophobic copper foam vs. Liu and Jacobi [175] heat exchanger

20–35% reduction for θA ∼ 30°; 25% increase in retention for θA∼110° Low RH: No effect Med RH: 1.67 vs. 1.54 (increase)f High RH: 3.50 vs. 3.18 (increase)f Increased quantity in 40 PPI foam vs. 20 PPI foam

N/A

N/A

75% reduction 57% reduction 66% reduction N/A

N/A

N/A

N/A N/A

Water Retention MpA (g/m2)d

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Applied Energy 222 (2018) 967–992 Smallest for the hydrophilic units; Higher retention observed for the hydrophobic units at lower fin pitch

chemical solution consisting of 23% ethylene glycol monobutyl ether, 9% acrylic resin, 0.7% NH3, and 67.3% water. In related work, Min et al. [163] investigated the long-term effectiveness of two coating types. After 1000 wet/dry cycles, a 9% and 12% reduction in core pressure drop were reported for the ZN and AQ coatings studied. Unlike the previous work, Wang et al. [171] found that the hydrophilic treatment of fin surfaces can cause up to a 20% degradation in the heat transfer performance, and Ma et al. [172] observed both an increase and decrease in heat transfer coefficient for a group of round-tube, wavy-finned heat exchangers after hydrophilic coating. Ma et al. [172] reported that hcoated/huncoated decreased as the relative humidity of the incoming air increased; however, no further explanation of these results was provided in the article. The heat exchangers tested in the above works were all relatively compact, with fin spacings between 1.2 and 2.1 mm (0.047–0.083 in). As compared to investigations on the thermal–hydraulic performance, less data have been reported on condensate retention on a heat exchanger with regards to the impact of fin surface wettability. A few exceptions however should be noted. Shin and Ha [173] measured the amount of water hold-up on hydrophilic heat exchangers with three different fin shapes (plain fin, plain fin with a slanted end, and discrete plain fins). Their work showed that a 57–75% reduction in condensate retention could be achieved by improving the overall fin surface wettability. In 2009, Liu and Jacobi [174] reported real-time retention data for a group of round-tube, slit-fin heat exchangers with identical geometry but varying surface wettability. The specimens that were studied had a tight fin spacing Fs ∼ 1.1 mm (0.043 in). In this work, the reduction in condensate retention after hydrophilic treatment was recorded to be 20–35%. In the same paper, Liu and Jacobi also reported experiments on a specimen treated to manifest high advancing contact angles for water (θA ∼ 110°). Because the specimen exhibited hydrophobic characteristics, droplets beaded up and blocked the airflow passage during wet-surface operating conditions. A 35% increase in airside pressure drop was recorded as compared to the untreated baseline specimen, and a 25% increase in the steady-state water retention data was also reported. The overall veracity of the dip testing method was established in an earlier work by the authors in which 22 heat exchanger specimens were tested with different configurations and surface wettabilities [175]. The major findings of this section related to the aim of this paper are summarized in Table 7. The table provides detailed information on the HEX specimen geometry, types of coating applied, wettability data, as well as the air-side heat transfer coefficient and core pressure drop (or j and f factors) of the specimen compared to its uncoated counterpart, if available. It can be seen that although the extent is influenced by the fin geometry and HEX compactness, a reduction of air pressure drop is consistently observed for heat exchangers after hydrophilic treatment, which means less energy consumption is required by the fan. Long-term investigation also revealed that this advantage persisted after a large number of wet and dry cycles in the system (Min et al. [163], Kim et al. [165]). 4.2. Applications in refrigeration systems In refrigeration applications, surface temperatures are below zero degrees Celsius. Thus, as the water vapor in the surrounding air comes in contact with the surface through mass transfer, frost growth occurs. This frost layer in turn adversely affects the performance of the evaporator due to the increase in the pressure drop in the frosted air passage and the reduction in the overall thermal efficiency. The overall performance of the refrigeration unit continues to decline with increasing frost deposition until a certain threshold is reached, at which time the system must be defrosted in order to improve its performance. Defrosting of course is an inherently inefficient process since a lot of energy is required during this stage for frost removal. Different surface wettabilities however can be used to alter the growing frost structure

g

f

e

d

c

b

a

Micro-finned tubes. Specimen # as shown in the original publication. Please refer to the original article for details. Compared to the uncoated counterpart. Results after 1000 wet/dry cycles. Collected condensate (kg/h). Fins per inch.

N/A Hydrophobic unit (16 FPI) showed least reduction in heat transfer due to frost retardation effect ∼3° ∼130° Both Hydrophilic Hydrophobic Dual Coated Fin 1.81 (14 FPI)g 1.59 (16 FPI)g 1.41 (18 FPI)g 2013 Kim and Lee [180]

Round

Louver

Heat transfer coefficientd θ (deg) Coating Fin spacing (Fs/mm) Fin type Tube type

Year Author

Table 7 (continued)

Tested specimens

c

Wet-surface results

Pressure dropd

Water Retention MpA (g/m2)d

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retarding and defrosting methods for air source heat pumps from 2000 to 2017 with specific areas being mentioned for further research including control strategy optimization and new methods and materials. The need for new hydrophobic materials with good durability were specifically referenced in this context. In summary, even though heat exchanger performance data under different wettability conditions are generally pretty sparse in the literature and some results might even be in disagreement, it has been clearly shown that by changing the surface chemistry and/or topography, the frosting behavior and heat transfer performance can be affected. More testing with different heat exchanger geometries, fin densities, operating conditions, etc. should be conducted in this area. Implementing surface wettability patterns and/or gradients in heat exchangers however to control the location and distribution of the frosting as well as the properties of the frost layer (i.e. density, thermal conductivity, etc.) seems promising. 4.3. Applications in other energy systems Other energy applications also stand to benefit from the incorporation of surface wettability modification. One such example is condensation heat transfer in falling film condensers, tube bundles, and shell-and-tube heat exchangers. In an experimental study by Miljkovic et al. [192], silanized CuO superhydrophobic tube surfaces were fabricated through a simple process to examine the impact of both water droplet coalescence and ejection (i.e. “jumping”) on the overall heat transfer performance. (Researchers have shown recently that the coalescence of small droplets on a superhydrophobic surface result in coalescence-induced droplet ejection or “jumping” against gravity. The jumping results from the release of excess surface energy during the coalescence event.) According to their results shown in Fig. 21, jumping droplet condensation on the superhydrophobic surfaces provided a 30% higher heat transfer coefficient (92 ± 12 kW/m2 K) when compared with dropwise condensation on the smooth Cu tube. Meanwhile, flooding of the surface with water droplets led to a 40% heat transfer coefficient reduction as compared with dropwise condensation on the smooth Cu tube (44 ± 6 kW/m2 K). The overall heat transfer coefficient data of their study were measured using U = q″/ΔTLMTD . In this case, the underlying wettability (i.e. nanostructured surface) helps to reduce the droplet adhesion force by minimizing the solid fraction. In this way, it aids in the breaking away of the coalesced droplet from the surface such that it accelerates and is then subsequently ejected perpendicular to the surface. Because small droplets can transfer heat more efficiently than large droplets, this unique droplet behavior offers the possibility of further enhancing condensation heat transfer over conventional dropwise condensation by increasing the time-averaged density of small droplets on the surface. Other relevant data and findings pertaining to condensation heat transfer enhancement are summarized below in Table 8. Other energy systems which stand to benefit from the incorporation of surface wettability modification include fluid pumping systems and desalination systems. Several researchers have shown that there is a reduction in the frictional pressure drop associated with flows through tubes or channels containing superhydrophobic surfaces [202,203]. More noteworthy, this reduction in frictional drag has been observed in both laminar and turbulent flows [204,205]. The slip length associated with these flows has also been measured, with slip lengths greater than 150 μm being observed [206,207]. Surface wettability modification has also been utilized in water purification and fresh water production. In these systems, using hydrophilic membranes helps to decrease the fouling inside the membranes, while using hydrophobic membranes results in a better separation between the liquid and gas phases [208]. Through a simple coating of a mixture of silica/alumina nanoparticles on a ceramic membrane, Huang et al. [209] observed that a water contact angle as high as 158° was possible under various thermal and salinity conditions.

Fig. 20. Frost growth on treated aluminum surfaces with different dynamic contact angles [181].

and/or properties, which can significantly delay the defrost schedule and also increase water removal efficiency during the defrost period. Shin et al. [181] performed an experimental study aimed at characterizing the frost structure on three different hydrophilic surfaces. Aluminum surfaces treated with lacquer and plasma polymerization, with advancing contact angles (i.e. θA) of 88°, 55°, and 23° were all tested. The frost deposition time was 240 min while the air velocity and surface temperature Ts were 1.57 m/s and –22 °C, respectively. Images of the frost layer at different stages of the frost growth process are compared in Fig. 20. In Yoshiyuki and Akilo [182], the heat exchanger surface was treated with a special coating made of hydrophobic resin and inorganic particles. It was found that the compressor operating time (between defrost processes) could be essentially doubled for the same cooling performance. On the other hand, Ryu and Lee [183] also manipulated the surface wettability of fin-and-tube heat exchangers and tested them under frosting conditions and came to a slightly different conclusion. They found out that the frost thickness on the hydrophilic heat exchanger was thinner and yielded a lower air-side pressure drop (even though the accumulated frost mass was larger, suggesting that the frost deposited on the hydrophilic surface was much denser as compared to the frost on a conventional surface). However, as the relative humidity increased, the thickness of the frost layer decreased the capability for further frost deposition. In related work on flat plates, Sommers et al. [7,184] also found that thinner, denser frost layers tend to form on hydrophilic surfaces, while thicker fluffier frost layers tend to form on hydrophobic surfaces. Because of this potential impact of the surface wettability on frost properties, a new semi-empirical correlation for predicting frost density on flat plates was proposed. Another interesting outcome associated with surface wettability manipulation is the possibility of “delayed frosting” on superhydrophobic surfaces, which has been reported by several different researchers [185–189]. Frosting delays as long as 55 min have been measured [188]. It has been suggested that these delays are due to the large quantity of trapped air which resides near the surface of superhydrophobic materials (i.e. Cassie-Baxter state). This air in turn acts as an insulating layer and results in a decrease in droplet surface contact area [189]. It is important to point out that these experiments were performed on flat plates, not full-scale heat exchangers. Nonetheless, the potential here for application in refrigeration systems is intriguing. A recent review article by Kim et al. [190] summarizes other recent frosting work on hydrophobic and superhydrophobic surfaces, and a review by Mengjie et al. [191] summarizes recent developments in frost 986

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Fig. 21. (a) Experimental steady state log mean temperature difference between water to vapor as a function of overall surface heat flux for silanized CuO superhydrophobic tube surface experiencing jumping condensation, filmwise, dropwise, and flooded. (b) Experimental and theoretical steady state condensation coefficient as a function of saturated vapor pressure for silanized CuO superhydrophobic tube surface experiencing jumping condensation, filmwise, dropwise, and flooded [192].

Table 8 Impact of various superhydrophobic and hybrid (hydrophobic/hydrophilic) coatings on the condensation heat transfer performance of cylindrical tubes. Examples of real applications involving droplet coalescence and the “jumping droplet” phenomenon. Author

Experimental Details Specimen

Pattern

Specifications

Maximum θstatic

HTCa of the coated tube

HTC of the CFMb

HTC of the CDMc

HTC of the uncoated tube

Cu tube

Hydrophobic-hydrophilic parallel stripe hybrid Hydrophobic

Ld = 12 cm De = 6.35 mm Ld = 18 cm De = 18.5 mm –

90 ± 2°

∼17.7 kW/ m2 K –

∼47.2 kW/ m2 K –

–

∼114°

85 kW/m2 K (max) subcooling = 9 °C 76.34 kW/m2 K

78°

∼0.09 kW/m2 K

–

–

∼0.08 kW/m2 K

–

161.2 ± 2°

∼0.12 kW/m2 K

–

–

∼0.10 kW/m2 K

D = 25.4 mm

115°

34 kW/m2 K (island) 22.6 kW/m2 K (tree)

–

–

–

Hybrid Superhydrophobic

– –

161 ± 4° –

1.532 ± 0.272 kW/m2 K 52 kW/m2 K

– –

0.965 kW/m2 K –

– ∼14 kW/m2 K

Hydrophobic

L = 20 nm D ∼ 3 nm L = 3.5 cm Wf = 2.5 cm tg = 3 mm –

139.2 ± 3.5°

∼67 kW/m2 K

–

–

∼20 kW/m2 K

125°

0.03 kW/m2 K

–

–

–

–

∼60 ± 20 kW/m2 K

–

–

L = 22.86 cm D ∼ 1.588 cm

–

141 kW/m2 K (Cu) 144 kW/m2 K (Cu-Ni)

10 kW/m2 K (Al)

–

∼15 ± 9 kW/ m2 K 35 kW/m2 K (Gold)

–

35 kW/m2 K

4.6 ± 0.4 kW/ m2 K

–

∼5 kW/m2 K

159 ± 2°

∼60 kW/m2 K

–

–

–

–

92 ± 12 kW/m2 K

19 ± 1.1 kW/ m2 K

75 ± 15 kW/ m2 K

–

Alwazzan et al. [76] Rajkumar et al. [193] Ghosh et al. [194]

Al surface

Mahapatra et al. [195]

Al surface

Chatterjee et al. [88]

Circular Cu surface Si surface Al surface Cu wires

Hou et al. [96] Parin et al. [136] Wen et al. [196]

Cu tube

Yang et al. [197]

Cu plates

Preston et al. [198] Das et al. [199]

Cu tube

Paxson et al. [200]

Torresin et al. [201]

Miljkovic et al. [192] a b c d e f g

Outcomes

Cu; Al; Cu-Ni tubes Al surface and Cu tube Cu tube

Superhydrophobichydrophilic wettability surface Superhydrophobichydrophilic wettability surface Tree or island (hydrophobic-hydrophilic)

Hybrid; hydrophilic; hydrophobic Hydrophobic Hydrophobic

Hydrophobic L = 2 mm W = 2 mm D=– Superhydrophobic

58.56 kW/m2 K

L = 3.8 cm D = 2 cm Cu tube

Superhydrophobic

L = 13.1 cm D = 6.35 mm

Heat transfer coefficient. Complete filmwise mode. Complete dropwise mode. Length. Diameter. Width. Thickness.

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Table 9 Thermal-mechanical properties of several metals popular in the construction of heat exchangers [212]. Properties

Density (g/cc)

Yield Strength, MPa

Young's Modulus, GPa

Ultimate Strength, GPa

Brinell Hardness, MPa

Thermal Expansion Coefficient, (10−6/°C)

Cu Al Si Stainless Steel Brass

8.91 2.63–2.80 2.33 7.92 8.47–8.74

70–265 95–500 – 260–520 70–435

120 70–75 130–188 190 105–120

0.22–0.39 0.11–0.57 5–7 0.655–0.86 0.27–0.585

235–878 160–550 – – 192–202

16.9 23–23.9 2.56 17.3 18.7–20.9

hardness and adhesion using Scotch tape to investigate the durability and stability of the original film covering the surface in critical conditions. After ten separate attempts of pressing and peeling off the Scotch tape, optical microscopy tests indicated neither damage nor detachment to the thin superhydrophobic film. They also demonstrated that at the end of the adhesion tests, the static contact angle of water droplets remained at approximately 160° and showed only some negligible variations. In the micro-hardness test, they applied forces (1–4 N) to the film layer. At the end of this test, although the variation in the water droplet contact angles was more pronounced (> 5°) in comparison to the adhesion tests, the static contact angle leveled off at nearly 158°. Moreover, after two months of exposure of the surface to air, they reported that the surface retained an acceptable condition and the contact angle only demonstrated a marginal decrease. Chen et al. [216] described a composite coating composed of methyl silicone resin and a superhydrophobic silica sol that featured excellent superhydrophobicity and robust mechanical durability. They reported that such a coating could be sprayed, dipped, rolled, and applied to a variety of materials (including metals) making it useful for large-scale applications such as heat exchangers used in HVAC&R systems. To check the mechanical durability of the superhydrophobic coating, it underwent various standard tests such as finger-wipe, knife-scratch, and finally 50 abrasion cycles with sandpaper. Their results demonstrated that only small changes in the static contact angle and sliding angle took place after 50 cycles of mechanical abrasion. The contact angle decreased from 166° to 155°, whereas the sliding angle increased from nearly 1° to 13°. Having assessed the robustness of the composite coating on aluminum surfaces according to ASTM D3363 and ASTM D2794 standards, they concluded that the coating on aluminum could endure a 9H pencil hardness which was far more than comparable surfaces reported before. During ASTM D2794 test, no cracking or exfoliation was observed on the coated aluminum surface when a 1 kg weight hammer was released from a height of 100 cm. Li et al. [217] developed a weather-resistant superhydrophobic coating for metallic surfaces that could be easily constructed in large scale. They also analyzed its stability and durability under various conditions, namely acidic/alkaline, heat/cool, corrosion resistance, and accelerated aging treatment tests. The water droplet static contact angle and sliding angle on the coating were reported to be 158.6 ± 1° and 3.8 ± 0.2°, respectively, before implementing any tests. The influence of the pH value and temperature on the durability of the coating was then tested. The authors reported that after a 1 h heat/cooling treatment, the surface still exhibited large contact angles (i.e. 146° at −10 °C and 154° at 90 °C). During the corrosion test, the water contact angle showed marginal fluctuations, but in the end it leveled off at nearly 155°. Li et al. [218] introduced a facile spraying method for the fabrication of mechanically robust, self-healing superhydrophobic coatings possessing PAH-SPEEK complexes, PAA, and healing agents with low surface energy. The resultant coatings could automatically and repetitively restore their superhydrophobicity after damage in a humid environment. They reported that using the superhydrophobic coating composed of dual healing agents was better than that of single healing agent. The water contact angle and sliding angle were reported to remain at more than 150° and approximately 15°. In Su et al. [219], the overall stability and durability of superhydrophobic copper surfaces under mechanical abrasion and corrosion were studied.

They reported that their superhydrophobic membrane was able to maintain a water permeation rate of 29.3 L m−2 h−1. In another study on hydrophilic membranes, Yu et al. [210] examined modified thin-film composite membranes (41.3 ± 0.5° < θ < 43.9 ± 0.5°) and demonstrated that although less permeability was obtained using the hydrophilic membranes (5.04–6.02 ± 0.16 L m−2 h−1 bar−1) as compared to the unmodified membrane (6.48 ± 0.20 L m−2 h−1 bar−1), the antifouling characteristics of the hydrophilic membrane were consistently superior. Moreover, they concluded that after simply cleaning the hydrophilic membrane with water, the permeation rate of the hydrophilic membrane was largely recovered, whereas the permeation rate of the uncoated membrane was not recovered. Wang et al. [211] studied the impact of a hybrid surface wettability on the performance of a sodium alginate (SA) membrane for ethanol dehydration. Hydrophilic sulfonate groups and aromatic hydrophobic polycyclic hydrocarbons were used to produce the hybrid wettability. The maximum permeation flux produced by their hybrid membrane was 1879 ± 80 g m−2h−1 with an accompanying optimal separation factor of 1913 ± 69 when separating a 90 wt% ethanol aqueous solution. This corresponded to an enhancement of 150% and 400%, respectively, over the conventional membrane in their study. 5. Operational challenges and limitations According to the previous sections, high static contact angles and small contact angle hysteresis are two intrinsic characteristics of superhydrophobic/hydrophobic surfaces. However, other research available in the literature has shown that most of these superhydrophobic/ hydrophobic surfaces suffer from poor mechanical durability, and their long-term longevity highly depends on where and how they get used. They can often lose their effectiveness over time and are also vulnerable to abrasion, scratching, corrosion, and eventually fouling which are common in heat exchanger applications. In this section, the operational challenges and longevity of highly non-wetting surfaces will be discussed. To begin, the thermal–mechanical properties of some common metals are summarized in Table 9. Ta et al. [213] designed topographical-based patterns on brass surfaces by means of direct laser texturing and measured the contact angle of water droplets (∼5µL) to determine the longevity and the efficacy of surface wettability over a certain period of time while the surfaces were tilted by two marginal inclination angles (i.e. 1.1° and 1.3°). They observed that as time passes, the static contact angle of water droplets noticeably rises from approximately 28–30° to 150–154°. Rausch et al. [214] investigated the longevity of dropwise condensation on polished and unpolished aluminum surfaces (Al 3003 and Al 6951) with an average surface finish of 0.15 µm created by ion beam implantation. They reported that the reproducibility and stability of the surfaces for heat exchanger applications could last for 8 months. They also found that aluminum alloys and their germane compositions were highly sensitive to the implementation parameters (heat transfer coefficient, dropwise versus filmwise condensation, etc.). Barthwal et al. [215] developed a simple fabrication method to make an aluminum plate possessing both micro- and nanoscale structures superhydrophobic by a combination of simple chemical etching and anodization. They carried out some standard mechanical tests, namely 988

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away from known pinning sites on a heat exchanger (i.e. tubes, collars, fin slits, etc.). Similarly, “jumping droplet” condensation has been shown to increase the condensation heat transfer rate versus traditional dropwise condensation by ejecting droplets during the coalescence process and increasing the overall density of smaller droplets present on the heat transfer surface. Surface wettability modification has also shown the potential for reducing frictional pressure drop and flow drag in channels due to the presence of superhydrophobic surfaces. Surface modification techniques have also demonstrated the ability to reduce fouling and improve the permeability of membranes used in reverse osmosis, dehydration, and water purification systems. In summary then, surface wettability modification can be used to increase the heat transfer coefficient, decrease drag, reduce water retention, delay frost formation, and/or control the movement of water in conventional energy systems. More research is needed first to advance the science and determine the most optimal designs and configurations. In particular, experimental testing of prototype systems is needed as well as longer-term durability studies. Currently, there are very few studies on the long-term effectiveness of chemical coatings as well as micro-scale and nano-scale roughness modifications to dust accumulation, hydrocarbon contamination, UV exposure, etc. Studies concerned with the testing of full-scale systems (possessing the modified wettability) across multiple disciplines are also quite sparse in the open literature. What are the challenges when scaling up these technologies? If one considers, however, the recent advances made in surface fabrication (i.e. laser micromachining, additive manufacturing, etc.) as well as new emerging coating technologies, it is clear that the incorporation and use of surface wettability modification in energy systems for performance enhancement has a bright and promising future.

In this work, Su et al. [219] proposed a novel method of creating superhydrophobic surfaces in which the water droplet contact angle and sliding angle were 162 ± 1° and 3 ± 0.5°, respectively. After performing some basic mechanical tests such as hardness, abrasion and corrosion resistance, they reported that the method could be applied to heat exchanger applications since the surfaces largely retained their initial hydrophobic characteristics (i.e. high water droplet contact angle and low sliding angle). Wang et al. [220] proposed a combination of a spraying process and a chemical grafting technique to restore the hydrophobicity of aluminum surfaces after being damaged under mild healing conditions. They reported that chemically bonded healing agents are better than physically absorbed ones in terms of their ability to restore superhydrophobicity to the damaged surface. In this work, the healing agent (POTS) was stored in a “chloroplast-like” structure on the surface by both chemical bonding and physical absorption, which reportedly led to better durability of the coating and improved drag reduction performance, which could make it useful in a wide range of applications including heat exchangers. Zhu et al. [221] examined the superhydrophobicity of a metal/polymer composite surface prepared by hot pressing, Ag deposition, and surface fluorination techniques. It was shown that the metal/polymer surface could endure 10 abrasion cycles with the water droplet contact angle (∼160°) remaining in the superhydrophobic range and the sliding angle (∼15°) still remaining fairly low. 6. Conclusions Historically, interest in application-driven surface wettability modification has been tempered by the limitations of chemical coatings, their longevity and effectiveness, cost, and the inability to translate labbased strategies to larger scales. It has been shown, however, through improvements in manufacturing and recent advances in our understanding of surfaces and interfaces that more robust approaches do exist for application in energy systems including the HVAC&R industries. Homogeneous surface wettability modification (via chemistry or roughness) can be used to render a heat transfer surface more hydrophilic or more hydrophobic. This has the potential for certain operational gains– namely, improved condensate drainage, decreased condensate bridging, reduced air-side pressure drop, improved dehumidification and/or defrosting, frost property modification, enhanced air-side heat transfer, etc. Chemical modifications are often more cost-effective due to the nature of their implementation (i.e. dip coating, chemical vapor deposition, etc.); whereas roughness modification can be more enduring and robust. For example, topographical modification has been shown to stand up better to thermal cycling, frosting and icing, and the wet-dry nature of most heat exchangers that are used in this field. By tailoring the wettability according to the geometry and application, patterned surface wettability has the potential of increasing condensate drainage in known regions of difficulty, especially in compact heat exchangers. Patterned wettability might also be used as part of an overall frost mitigation strategy in refrigeration systems to grow frost more strategically on the heat transfer surface and/or expedite the defrosting process thereby shortening the defrost time interval. Superhydrophobic surfaces have also shown recent promise in their ability to delay the onset of frosting as compared to more traditional surfaces which has the potential to benefit many different fields—i.e. aerospace (wing de-icing), automotive (windshields), domestic refrigeration (heat exchanger design), buildings (self-cleaning windows), etc. Surface tension gradients (which constitute a type of patterned wettability) involve systematically varying the underlying chemistry or roughness in order to generate a small net surface tension force. This net force, which has been shown to move droplets uphill a short distance in a laboratory setting, could potentially be used to increase droplet coalescence on the heat transfer surface or actuate droplets

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