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Influence of groove orientation on dropwise condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays
International Communications in Heat and Mass Transfer 112 (2020) 104492
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Influence of groove orientation on dropwise condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays
T
⁎
Qi Penga,b, Li Jiaa, , Yi Dinga, Chao Danga, Liaofei Yina, Xiao Yanb a Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China b Department of Mechanical Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, IL 61801, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Anisotropic wettability Dropwise condensation Hierarchical structures Microgroove arrays Droplet dynamic behavior Heat transfer
This work experimentally investigated vapor condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays to study the influence of groove orientation on the condensed droplet dynamics and heat transfer. This study showed that dropwise condensation on hydrophobic surfaces with microgroove arrays strongly depended on the groove orientation owing to the anisotropic wettability. The suction effect of a large droplet spanning several grooves on the liquid filaments filling the microgrooves was demonstrated on a hydrophobic surface with horizontal microgrooves with the phenomenon disappearing on a hydrophobic surface with vertical microgrooves. Furthermore, the horizontal microgrooves resulted in larger droplet departure sizes, significant deformation and retention of falling droplets, and the elimination of the liquid filaments sliding down the surface. The heat flux on the hydrophobic surface with horizontal microgrooves was 10–30% lower than on the plain hydrophobic surface. However, the groove orientation had less influence on the hierarchical superhydrophobic surface with microgroove arrays. At small surface subcooling, the hierarchical surface was rejuvenated through condensed droplet jumping and jumping-induced sweeping, the dominate droplet behavior transformed to coalesced droplet sweeping and the falling of suspended droplets with increasing surface subcooling. The dominate droplet behavior was little influenced by the groove orientation on the hierarchical superhydrophobic surface. Consequently, the heat flux was increased by 35–107% on both hierarchical superhydrophobic surfaces with microgroove arrays compared to the hydrophobic surface with vertical microgrooves.
1. Introduction Vapor condensation is a common phenomenon in nature [1,2] and is crucial in thermal management [3–5], power generation [6] and airconditioning systems [7] because of the large latent heat release during the phase change, and in water harvesting and desalination systems [8,9] due to its ability to capture vapor from moist air. The improved condensation heat transfer in these systems can significantly reduce energy usage and improve the system economics. The liquid condensate morphologies can be either a continuous liquid film (filmwise condensation) or discrete droplets (dropwise condensation) on a cooling surface depending on the solid-liquid interfacial effects. The heat transfer coefficient for dropwise condensation can be ten times higher than for filmwise condensation [10]. There have been many studies of dropwise condensation on various non-wetting surfaces to explore the
condensation heat transfer enhancement [11–13]. Dropwise condensation requires fast growth and removal of condensed droplets on the functionalized surfaces to expose more dry surface for vapor condensation. Large sessile droplets generate huge thermal conduction resistances that reduce the condensation heat transfer [14]. There have been many recent studies inspired by the structures on lotus leaves, butterfly wings and other natural surfaces using micro/nanostructures to facilitate dropwise condensation by controlling the solid-liquid interfacial effects [15–19]. In particular, an exciting phenomenon has been observed with self-propelled droplet jumping on superhydrophobic surfaces [20–23]. Due to the fast removal of the condensate for a new cycle of droplet nucleation and growth, the selfpropelled droplet jumping has a huge potential for condensation heat transfer enhancement [24–28]. The dropwise condensation can be further improved by tuning the solid-liquid interfacial effects related to
⁎ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (L. Jia).
https://doi.org/10.1016/j.icheatmasstransfer.2020.104492
0735-1933/ © 2020 Elsevier Ltd. All rights reserved.
International Communications in Heat and Mass Transfer 112 (2020) 104492
Q. Peng, et al.
Nomenclature
Greek symbols
T P q k d ΔT h SEM
δ
temperature (K) pressure (Pa) heat flux (W/m2) thermal conductivity (W/m K) the thickness of test sample (m) surface subcooling (K) heat transfer coefficient (W/m2 K) scanning electron microscope
the interval between thermocouples (m)
Subscripts s v Cu n w
saturation vapor copper the serial number of thermocouple wall
2. Experiment
the micro/nanostructures [29,30]. Microgroove structures on surfaces can increase the effective heat transfer area and reduce the droplet slippage resistance, so they have been widely used to enhance dropwise condensation [31–33]. The impact of microgrooved structures on dropwise condensation has been observed in previous studies focusing on the growth and movement of droplets on hydrophobic surfaces with microgroove arrays. These studies have indicated that the droplet growth mechanisms on hydrophobic surfaces with microgroove arrays differ from that on plain hydrophobic surface owing to the microgroove arrays acting as drainage channels for the condensate [34,35]. The energy barriers created by the microgrooves lead to anisotropic wettability on hydrophobic surfaces with microgroove arrays [36,37]. However, the anisotropic wettability leads to undesirable variations in the dropwise condensation which limits the practicality of such surfaces. Recently, hierarchical structured surfaces have attracted interest because of their remarkable ability to affect the droplet dynamics [38–42]. Thus, hierarchical structures provide a possible way to suppress the anisotropic dropwise condensation on surfaces with microgroove arrays. In this work, CuO nanostructures were manufactured on a machined surface with microgroove arrays by chemical oxidation to form a hierarchical structured surface with microgroove arrays. A plain hydrophobic surface and a hydrophobic surface with microgroove arrays were also fabricated for comparisons. The wettabilities of these three kinds of surfaces were then measured and analyzed. The visualization condensation experiments were constructed on all test surface in an enclosed condensing chamber under a vapor pressure of 60 ± 1 kPa. These surfaces were mounted vertically in a condensing chamber during the condensation experiments. The condensed droplet dynamics and the heat transfer on the test samples were then analyzed. Anisotropic dropwise condensation occurred on the hydrophobic surface with the microgroove arrays but not on the hierarchical superhydrophobic surface with microgroove arrays. The heat transfer on the hierarchical superhydrophobic surfaces with the microgroove arrays was much higher than on the hydrophobic surface with microgroove arrays, with the condensation heat transfer even less on a hydrophobic surface with horizontal microgrooves.
2.1. Sample preparation Pure copper (99.9%) was used to manufacture the test surfaces owing to its broad applications in thermal and energy systems. The substrates of all the test samples were 40 mm diameter and 2 mm thick circular copper plates. Fig. 1(a) showed the morphology of the plain copper surface as observed by a 3D optical profiler. The plain copper surface was polished to a roughness of 0.018 μm. Fig. 1(b) showed an SEM image of a flat CuO nanostructured surface. Fig. 1(c) and (d) showed the morphologies of the top surface between the microgrooves and the bottom surface of a microgroove. The rectangular microgroove arrays were 400 μm wide and 400 μm deep at intervals of 300 μm and were manufactured on the polished copper plate by mechanical surface broaching. Fig. 1(e) showed an SEM image of the hierarchical structured surface with the microgroove arrays. The hierarchical structured surface was manufactured by creating CuO nanostructures on a surface with microgroove arrays. The CuO nanostructures were formed through the self-limiting chemical oxidation method. Prior to the chemical oxidation, the polished copper plate with microgroove arrays was cleaned successively with acetone, isopropyl alcohol, ethanol and deionized (DI) water to remove deposited contaminates on the surface. Next, the clean copper plate with the microgroove arrays was immersed in a 2.0 M hydrochloric acid solution to remove the native oxide, then rinsed with deionized water and dried in nitrogen ambient. After that, the surface was dipped into an alkaline solution of NaClO2, NaOH, Na3PO4·12H2O and DI water (3.75: 5: 10: 100 wt%) at 96 °C for 15 min. This created uniform CuO nanostructures on the copper plate with the microgroove arrays. Fig. 1 (f) shows SEM images of the CuO nanostructures. The CuO nanostructures were similar to sharp blades with ~10 nm tips, 200–400 nm widths and ~1 μm heights. These dense nanostructures were expected to increase the nucleation density and regulate the surface-liquid interaction [14]. Finally, the plain copper plate, the microgroove structured surface and the hierarchical structured surface with microgroove arrays were immersed into an ethyl alcohol solution with 2.5 mM n-octadecanethiol at 70 °C for 1 h to make the surface hydrophobic through the formation of self-assembled monolayers of n-octadecanethiol on the surfaces.
Fig. 1. Test sample morphologies and surface characteristics. (a) Surface morphology of the polished copper plate. (b) SEM image of the flat CuO nanostructured surface. (c) The morphology of the top surface between microgrooves. (d) Surface morphology of the bottom surface in a microgroove. (e) SEM image of the hierarchical structured surface with microgroove arrays. (f) Detailed CuO nanostructures on the hierarchical structured surface with the microgroove arrays. 2
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2.2. Experimental system An experimental system with visualization was built to observe the dynamics of the condensed droplets and measure the heat transfer performance [18]. As shown in Fig. 2, the experimental apparatus included a steam generator, a condensing chamber, a vacuum pump, a high-speed camera, an electronic refrigeration subsystem and a data acquisition subsystem. The cylindrical steam generator was filled with deionized water and heated by two cartridge heaters to provide vapor for the condensing chamber. The heating power was manipulated by a voltage regulator to maintain the vapor pressure during the experiments. The connecting pipe was heated to prevent vapor condensation in the pipe. The vapor temperature and pressure in the experimental system were measured by transducers in the steam generator and in the condensing chamber. The vapor condensed on the test surfaces which were mounted vertically on the top of the cylindrical cooling stage at the center of the condensing chamber with the transparent viewing window. The electronic refrigeration subsystem composed of four Peltier elements was attached to the bottom of the cooling stage to regulate the condensing surface temperature. This system provided various surface subcooling by adjusting the power to the Peltier elements. The heat flux during condensation was determined from five thermocouples mounted at 15 mm intervals in the cylindrical cooling stage which was well insulated with Teflon. The high-speed camera recorded the dynamics of the condensed droplets while the data acquisition subsystem measured the temperature and pressure variations during each experiment. Before each experiment, the non-condensable gases in the experimental system were removed by the vacuum pump.
Fig. 3. Comparison of the axial heat fluxes from both ends of the test section.
The condensing surface temperature was obtained as:
Tw = T5 +
2.3. Experimental data reduction
4
∑ n=1
kCu (Tn + 1 − Tn ) δ
q q = ΔT Tv − Tw
(3)
where, ΔT is the surface subcooling which represents the temperature difference between the vapor and condensing surface temperatures and Tv is the vapor temperature.
The heat flux was assuming one-dimensional steady heat conduction through the cooling stage as:
1 4
(2)
Where, T5 is the temperature of the bottom of the test sample and d is the sample thickness. The condensation heat transfer coefficient was determined from Eqs. (1) and (2) as:
h=
q=
q d kCu
2.4. Heat loss analysis
(1)
Where, kCu is the thermal conductivity of copper (398 W/m K), Tn is the temperature of the nth thermocouple with the thermocouple numbers increasing closer to the test surface and δ is the distance between thermocouples (15 mm).
The cylindrical cooling stage was installed in the condensing chamber with only the bottom surface exposed to the atmosphere to allow heat dissipation from the Peltier elements. The heat loss from the test section was determined by comparing the axial heat fluxes at both
Fig. 2. Condensation experimental system. 3
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microgrooves during coalescence, which was attributed to the surface tension, as indicated by the dashed rectangles numbered 2 and 3 in Fig. 5(a). The horizontal microgrooves reduced the deformation of the large droplet by gravity, so the large droplets spanning several grooves had approximatively spherical caps through the action of the surface tension. Unlike on the surface with horizontal microgrooves, the suction effect of large droplets on liquid filaments was not observed on the surface with vertical microgrooves as shown in Fig. 5(b) and the large droplet was stretched into an oblong shape rather than a spherical cap as gravity overcame the surface tension. Meanwhile, as discussed previously [18], the liquid filaments partially filling the grooves refreshed the surface by absorbing the large droplets on the top of adjacent hydrophobic surfaces with microgrooves, as indicated by the dashed ellipses numbered 2 and 3 in Fig. 5(b). Since the horizontal microgrooves hindered drainage of the condensate, the local condensate overflow from the microgrooves and the large droplets spanning several grooves appeared more frequently on the surface with the horizontal microgrooves than on the surface with the vertical microgrooves, which led to more areas being covered by condensate as shown in Fig. 5(a). As a result, the thermal conduction resistance in the condensate increased which reduced the dropwise condensation heat transfer rate. Fig. 6(a) and (b) illustrated the sliding of a large droplet spanning several grooves on hydrophobic surfaces with horizontal and vertical microgrooves. The droplet departure size was about 4.32 mm on the surface with the horizontal microgrooves and about 3.24 mm on the surface with the vertical microgrooves. Since the horizontal microgrooves impeded the sliding of the large droplet driven by gravity, the droplet departure size increased with significant deformation and strong retention of the departure droplet during sliding, which reduced the surface refresh frequency. As shown in Fig. 6(a), after sliding of a large droplet, some liquid filaments remained partially filling the grooves, which led to less area exposed for new vapor nucleation. As a result, the horizontal microgroove arrays reduced the dropwise condensation heat transfer. On the surface with the vertical microgrooves, the liquid filaments above the large droplet spanning several grooves were carried to remove during departure, which facilitated the renewal of the condensing surface [18], as highlighted by the dashed rectangles in Fig. 6(b). The results in Fig. 6 showed that the departure time on the surface with the vertical microgrooves was 324 ms, which was larger than that on the surface with the horizontal microgrooves (108 ms). The departure time was the time from the beginning of a large droplet sliding to the surface being renewed in the field of view. The t = 0 ms was the moment when the large droplet was slightly larger than the departure size and began to slide. The larger departure time on the surface with vertical microgrooves was because the departure time on the surface with vertical microgrooves depended on the length of the liquid filaments carried by departure droplet with longer liquid filaments leading to longer departure times. Although the departure time was longer, the larger areas covered by the liquid filaments above the departing droplet were refreshed for new vapor condensation on the surface with the vertical microgrooves. Generally, the enough long
ends, q1 and q4, as shown in Fig. 3 for all the experimental conditions. q1 was the heat flux near the cooling side calculated from the temperature difference between T1 and T2 while q4 represented the heat flux close to the condensing surface calculated from the temperature difference between T4 and T5. Fig. 3 showed that q1 agreed well with q4 with a maximum difference of 7%. q1 was slightly larger than q4 because some heat was transferred from the high-temperature vapor in the condensing chamber to the cylindrical cooling stage through the side wall. Thus, the results indicated that the heat loss from the test section was no more than 7%. 3. Result and analysis 3.1. Test sample wettability Fig. 4 (a-f) showed the wettabilities of all the test samples for a 3 μL droplet deposited on the surfaces with the contact angles measured by a microgoniometer. Fig. 4(a) and (b) showed that the contact angles on the plain hydrophobic and flat CuO nanostructured superhydrophobic surfaces were 120.8° and 165.7°. The contact angles on non-wetting surfaces with microgroove arrays were classified perpendicular and parallel to the groove orientation. As discussed in previous studies [36,37], the microgrooves restricted the lateral wetting of the deposited droplet which led to anisotropic wettability. The contact angle perpendicular to the groove orientation was generally larger than that parallel to the groove orientation. Similarly, in this work, the contact angles perpendicular to the groove orientations on the hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays were 150.7° and 173.2° as seen in Fig. 4(c) and (e). The contact angles parallel to the groove orientations on the hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays were 119.3° and 137.8° as shown in Fig. 4(d) and (f). 3.2. Influence of groove orientation on dropwise condensation on the hydrophobic surface with microgroove arrays The condensation was visualized on the hydrophobic surfaces with horizontal and vertical microgrooves to investigate the influence of the groove orientation on the condensed droplet dynamics and the heat transfer performance. Fig. 5(a) and (b) showed the formation of a large droplet spanning several grooves on the hydrophobic surfaces with horizontal and vertical microgrooves. The small condensed droplet had a spherical cap on the hydrophobic surface with microgroove arrays which corresponded to the wettability of the plain hydrophobic surface. With the growth and coalescence of condensed droplets in the microgrooves, the condensate partially filled the groove to generate liquid filaments. The local overflow of the liquid filaments in the microgrooves created oblong droplets that coalesced with the liquid filaments in the adjacent grooves, which resulted in the generation of large droplets spanning several grooves. Interestingly, the liquid filaments in the microgrooves were sucked into the large oblong droplets to expose new surface for vapor condensation on the surface with the horizontal
Fig. 4. The wettabilities of all the test samples. (a) Contact angle on the plain hydrophobic surface. (b) Contact angle on the flat CuO nanostructured superhydrophobic surface. (c) Contact angle perpendicular to the groove orientation on the hydrophobic surface with microgroove arrays. (d) Contact angle parallel to the groove orientation on the hydrophobic surface with microgroove arrays. (e) Contact angle perpendicular to the groove orientation on the hierarchical superhydrophobic surface with microgroove arrays. (f) Contact angle parallel to the groove orientation on the hierarchical superhydrophobic surface with microgroove arrays. 4
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Fig. 5. Formation of large droplets on the hydrophobic surfaces with microgroove arrays. (a) Formation of a large droplet spanning several grooves on the surface with horizontal microgrooves at a surface subcooling of 14.1 K. (b) Formation of a large droplet spanning several grooves on the surface with vertical microgrooves at a surface subcooling of 13.5 K. Scale bar: 1 mm.
Fig. 6. Sliding of large droplets spanning several grooves on hydrophobic surfaces with microgroove arrays. (a) Sliding of a large droplet on the surface with horizontal microgrooves at a surface subcooling of 23.4 K. (b) Sliding of a large droplet with some liquid filaments filling the grooves on the surface with vertical microgrooves at a surface subcooling of 23.7 K. Scale bar: 1 mm. 5
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liquid filaments in microgrooves could also slide down on the hydrophobic surface with the vertical microgrooves to accelerate the renewal of condensing surface, as discussed in previously [18], but the phenomenon disappeared on the surface with the horizontal microgrooves. Comparison of the different droplet dynamics on the hydrophobic surfaces with horizontal and vertical microgrooves showed that the dropwise condensation on the hydrophobic surface with microgroove arrays strongly depended on the groove orientation. Since the horizontal microgrooves impeded the droplet movement, the condensation heat transfer would deteriorate on the surface with the horizontal microgrooves than with the vertical microgrooves.
which was consistent with the wettability of the CuO nanostructured superhydrophobic surface. Furthermore, droplets in the microgrooves could grow larger than the microgroove to form deformed droplets instead of liquid filaments partially filling the groove. The deformed droplets would then climb to the tops of the microgrooves driven by the Laplace pressure difference [18]. Fig. 7 showed that coalescence jumping of the condensed droplets happened on the hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays because of the release of excess surface energy during coalescence. The jumping of condensed droplets could be divided into two categories on the hierarchical surfaces due to the effects of the microgroove arrays. One was the traditional self-propelled jumping based on the coalescence of small spherical droplets at small surface subcooling (ΔT < 5 K) such as indicated by the dashed ellipse in Fig. 7(a) and (b). Another was the forced jumping of large deformed droplets in the microgrooves as the coalesce with droplets outside the microgroove, as indicated by the dashed ellipses in Fig. 7(c)-(f). Since the deformed droplets in the microgrooves stored more surface energy than normally spherical droplets [41] and the microgroove confined the lateral deformation of the coalescing droplet, the conversion efficiency of surface
3.3. Influence of groove orientation on dropwise condensation on the hierarchical superhydrophobic surface with microgroove arrays The impact of groove orientation on the condensed droplet dynamics on the hierarchical superhydrophobic surface with microgroove arrays was also analyzed. As indicated in Fig. 7, the small spherical condensed droplets represented the Cassie state or the partially-wetting state with high mobility on the hierarchical superhydrophobic surface
Fig. 7. Coalescence jumping of condensed droplets on the hierarchical superhydrophobic surfaces with microgroove arrays. (a) Self-propelled jumping of small condensed droplets on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 1.6 K. (b) Self-propelled jumping of small condensed droplets on the hierarchical superhydrophobic surface with vertical microgrooves at a surface subcooling of 1.3 K. (c) Forced jumping of large deformed droplets in a microgroove on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 1.6 K. (d) Forced jumping of large deformed droplets in the microgroove on the hierarchical superhydrophobic surface with vertical microgrooves at a surface subcooling of 1.3 K. (e) Forced jumping of large deformed droplets in the microgroove on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 11.9 K. (f) Forced jumping of large deformed droplets in the microgroove on the hierarchical superhydrophobic surface with vertical microgrooves at a surface subcooling of 12.1 K. Scale bar: 500 μm. 6
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droplets, and then the jumping direction of droplet could be changed by impinging upon the edge of microgroove during coalescence, as highlighted by the dashed arrows in Fig. 8(b). Consequently, more surface areas were swept by the jumping droplets. Thus, the jump-induced sweeping was an effective mechanism for accelerating surface renewal and improving dropwise condensation on hierarchical superhydrophobic surfaces with microgroove arrays at small surface subcooling. As the surface subcooling increased, the condensed droplet wetting state changed from the Cassie state or partially-wetting state to the Wenzel state with increasing surface adhesion. As a result, the selfpropelled jumping of the condensed droplets disappeared and the jumpinduced sweeping degraded to coalescence sweeping. Fig. 9(a) and (b) showed coalescence sweeping on the hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays at large surface subcooling. Because of the increased surface adhesion, the out-ofplane component of the momentum of the coalescing droplet was insufficient to enable it to jump off the surface, while the droplet was driven to slip along the surface by the in-plane component of the momentum. Eventually, the slipping droplet would become a large droplet pinned on the surface which would refresh some local areas of the surface through coalescing with other droplets along the way. This coalescence sweeping could facilitate surface renewal for new vapor condensation at large surface subcooling, which would improve the dropwise condensation heat transfer on the hierarchical superhydrophobic surfaces with microgroove arrays.
energy to kinetic energy was improved in the forced jumping mode of large deformed droplets relative to that of traditional self-propelled jumping of small spherical droplets [42]. Therefore, the forced jumping modes of large deformed droplets in microgrooves could be sustained on the hierarchical superhydrophobic surface with microgroove arrays over a wide range of surface subcooling from 0 K to 12 K as indicated in Fig. 7(e) and (f). The differences between traditional self-propelled jumping of small spherical droplets and the forced jumping of large deformed droplets in microgrooves have been discussed previously [42]. The combination of the traditional self-propelled jumping of small spherical droplets and the forced jumping of large deformed droplets in the microgrooves improved the surface renewal for vapor nucleation. In addition to traditional jumping condensation on nanostructured superhydrophobic surfaces [24,26,27], jump-induced sweeping was also observed on the hierarchical superhydrophobic surfaces with microgroove arrays as shown in Fig. 8. The jump-induced sweeping can be explained based on the research of Qu et al. [43] who showed that when the coalescing droplet impinged the edges of microstructures on the superhydrophobic surface, the momentum of the jumping droplet was divided into the in-plane and out-of-plane components. The inplane component of the momentum enabled the droplet to move parallel to the surface, while the out-of-plane component of the momentum prompted the droplet to jump from the surface. The jumping droplet could even move in the direction opposite to gravity such as shown by the dashed ellipse and arrow in Fig. 8(a). Furthermore, in the movement process of jumping droplet, the droplet would coalesce with other
Fig. 8. Jump-induced sweeping on hierarchical superhydrophobic surfaces with microgroove arrays. (a) Jump-induced sweeping on the hierarchical surface with horizontal microgrooves at a surface subcooling of 2.5 K. (b) Jump-induced sweeping on the hierarchical surface with vertical microgrooves at a surface subcooling of 2.3 K. Scale bar: 1 mm. 7
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Fig. 9. Coalescence sweeping on the hierarchical superhydrophobic surfaces with microgroove arrays. (a) Coalescence sweeping on the hierarchical surface with horizontal microgrooves at a surface subcooling of 20.3 K. (b) Coalescence sweeping on the hierarchical surface with vertical microgrooves at a surface subcooling of 21.9 K. Scale bar: 1 mm.
surface with vertical microgrooves because the horizontal microgrooves impeded the droplet movement. Particularly, the droplet departure size on the hierarchical superhydrophobic surface with microgroove arrays was much smaller than on the hydrophobic surface with microgroove arrays. This comparison of the droplet dynamics at various surface subcooling for the hierarchical surfaces with horizontal and vertical microgrooves illuminated that the groove orientation had less influence on the dropwise condensation on the hierarchical superhydrophobic surfaces with microgroove arrays which could be used to eliminate the dropwise condensation heat transfer deterioration on surfaces with microgroove arrays when the grooves were not properly oriented in practical applications.
In addition to the coalescence sweeping, the falling of large droplet by gravity was also a key pathway for renewal of condensing surfaces at large surface subcooling. Fig. 10(a) and (b) showed the falling of large droplets on the hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays at large surface subcooling. Some small droplets remained in the microgrooves on both surfaces after departure of the large droplet, which verified that the large condensed droplet was held on the top of the microgrooves instead of filling the grooves on the hierarchical superhydrophobic surface with microgroove arrays. This illustrated the hierarchical condensation phenomenon that vapor could condense in the microgrooves under large suspended droplets on the hierarchical superhydrophobic surfaces with microgroove arrays. This hierarchical condensation could effectively extend the heat transfer areas for more vapor nucleation and delay flooding condensation, which would increase the condensation heat transfer. Furthermore, the suspended droplet had smaller solid-liquid contact areas and surface adhesion which would reduce the departure size of condensed droplets for faster condensate removal. Although the energy barriers created by the microgrooves hindered lateral wetting of the droplet as discussed with the wettability measurements, the gravity would help large droplets to overcame the energy barriers on vertical surfaces. Meanwhile, because the large droplets were suspended instead of filling the groove, large spherical droplets could rapidly roll off without much deformation or retention on the hierarchical surface with horizontal microgrooves. Interestingly, the falling droplets experienced self-oscillations on the hierarchical superhydrophobic surface with microgroove arrays due to the droplets striding the microgroove structures by coalescence during departure, as shown in Fig. 10(c) and (d). The strong self-oscillations of the falling droplets enhanced the disturbances of the liquid-vapor interface that improved the heat and mass transfer. In addition, Fig. 10(a) and (b) showed that the departure size of the suspended droplets on the hierarchical surface with horizontal microgrooves was about 1.83 mm, which was slightly larger than the departure size of 1.33 mm on the
3.4. Comparison of the condensation heat transfer performance The heat fluxes and condensation heat transfer coefficients on all the test samples including the plain hydrophobic, hydrophobic and hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays were plotted as functions of the surface subcooling in Fig. 11(a) and (b) to further illustrated the impact of groove orientation on the dropwise condensation heat transfer on these test surfaces with microgroove arrays. The results in Fig. 11 showed distinct differences in the condensation heat transfer for the hydrophobic surfaces with horizontal and vertical microgrooves, which again showed that the groove orientation significantly impacted the dropwise condensation heat transfer performance on hydrophobic surfaces with microgroove arrays. The hydrophobic surface with the vertical microgrooves had 20–64% higher heat flux and 21–58% higher heat transfer coefficient than the hydrophobic surface with horizontal microgrooves for surface subcooling from 0 K to 28 K. Although the microgroove arrays increased the heat transfer areas, the horizontal microgrooves hindered the departure of large droplets, increased the departure size of condensed droplets, and eliminated the sliding down of liquid filaments in the microgrooves seen on the hydrophobic surface with vertical 8
International Communications in Heat and Mass Transfer 112 (2020) 104492
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Fig. 10. Falling and strong self-oscillations of suspended droplets on the hierarchical superhydrophobic surfaces with microgroove arrays. (a) Falling of a suspended droplet on the hierarchical surface with horizontal microgrooves at a surface subcooling of 20.3 K. (b) Falling of a suspended droplet on the hierarchical surface with vertical microgrooves at a surface subcooling of 21.9 K. (c) Strong self-oscillation of a falling droplet on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 20.3 K. (d) Strong self-oscillation of a falling droplet on the hierarchical surface with vertical microgrooves at a surface subcooling of 21.9 K. Scale bar: 1 mm.
for surface subcooling from 0 K to 5 K due to the reduction of the jumping droplet condensation. There were only small differences between the condensation heat transfer performance on the hierarchical surfaces with horizontal and vertical microgrooves which further verified that the groove orientation had almost no influence on the dropwise condensation heat transfer on the hierarchical superhydrophobic surfaces with microgroove arrays. The reason was that the dropwise condensation heat transfer on such surfaces was dominated by the droplet dynamics including the jumping of condensed droplets, coalescence sweeping and falling of suspended droplets which did not depend on the groove orientation. These interesting droplet dynamics accelerated the condensate removal and facilitated the vapor
microgrooves which offset the heat transfer enhancement caused by the increased heat transfer areas. Consequently, the condensation heat flux and heat transfer coefficient on the hydrophobic surface with the horizontal microgrooves were even 10–30% and 8–26% lower than those on the plain hydrophobic surface. Fig. 11 further showed that the condensation heat transfer on the hierarchical superhydrophobic surfaces with the microgroove arrays were much better than on the other test surfaces. The maximum condensation heat transfer coefficients were 69.5 kW/m2 on the hierarchical superhydrophobic surface with the vertical microgrooves and 62.4 kW/m2 on the surface with the horizontal microgrooves. Furthermore, the condensation heat transfer coefficient decreased sharply 9
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Fig. 11. Dropwise condensation heat transfer performance on different test surfaces. (a) Heat fluxes and (b) condensation heat transfer coefficients.
condensation heat transfer coefficient than on the hydrophobic surface with the horizontal microgrooves. The condensation heat flux and heat transfer coefficient on the hydrophobic surface with the horizontal microgrooves were even 10–30% and 8–26% lower than on the plain hydrophobic surface. However, the heat flux was 35–107% higher and the condensation heat transfer coefficient was 39–104% higher on the hierarchical superhydrophobic surface with the horizontal microgrooves than on the hydrophobic surface with vertical microgrooves.
condensation on the hierarchical surfaces with microgroove arrays. Consequently, the hierarchical superhydrophobic surfaces with horizontal microgrooves had 35–107% higher heat flux and 39–104% higher heat transfer coefficient than the hydrophobic surface with vertical microgrooves. 4. Conclusions The wettability measurements and visualization of the vapor condensation were used to investigate vapor condensation on hydrophobic and hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays to study the influence of groove orientation on the dynamics of condensed droplets and the condensation heat transfer performance on these surfaces. The main conclusions are:
Declaration of Competing Interest None. Acknowledgments
(1) Anisotropic wettability was observed on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays, the contact angles perpendicular to the groove orientation were larger than those parallel to the groove orientation due to the energy barriers caused by the microgrooves impeding lateral wetting of the droplet. (2) Dropwise condensation on the hydrophobic surface with microgroove arrays was significantly influenced by the groove orientation. The liquid filaments partially filling the grooves were sucked into oblong droplets spanning several grooves on the hydrophobic surface with horizontal microgrooves because of the surface tension. The suction effect was not observed on the surface with vertical microgrooves due to gravity restricting the fluid motion. The horizontal microgrooves hindered the droplet movement which resulted in larger departure sizes, significant shape deformation, retention of falling droplets and the lack of liquid filaments sliding down. (3) The groove orientation had little effect on the dropwise condensation on the hierarchical superhydrophobic surfaces with microgroove arrays because the groove orientation had little effect on the condensed droplet dynamics on such surfaces. At small surface subcooling, jumping of condensed droplets and jump-induced sweeping improved the surface renewal, but the droplet behavior changed to coalescence sweeping and falling of suspended droplets with increasing surface subcooling. The jump-induced sweeping was attributed to the in-plane and out-of-plane components of the jumping droplet momentum. (4) The condensation heat transfer performance on the hydrophobic surfaces with horizontal and vertical microgrooves were significantly different. The hydrophobic surface with the vertical microgrooves had 20–64% higher heat flux and 21–58% higher
This research was supported by the National Key R&D Program of China (2018YFB0604301). The microgoniometer work was conducted at the Energy Transport Research Lab, University of Illinois at Urbana−Champaign, with help from Prof. Nenad Miljkovic. References [1] A.R. Parker, C.R. Lawrence, Water capture by a desert beetle, Nature 414 (2001) 33–34. [2] J. Ju, H. Bai, Y.M. Zheng, T.Y. Zhao, R.C. Fang, L. Jiang, A multi-structural and multi-functional integrated fog collection system in cactus, Nat. Commun. 3 (2012) 1247. [3] J.B. Boreyko, Y. Zhao, C.H. Chen, Planar jumping-drop thermal diodes, Appl. Phys. Lett. 99 (23) (2011) 234105. [4] Q. Lu, L. Jia, Experimental study on rack cooling system based on a pulsating heat pipe, J. Therm. Sci. 25 (1) (2016) 60–67. [5] J. Wang, Y. Li, J. Li, C. Li, Y. Zhang, X. Ning, A gas-atomized spray cooling system integrated with an ejector loop: ejector modeling and thermal performance analysis, Energy Convers. Manag. 180 (2019) 106–118. [6] J.M. Beér, High efficiency electric power generation: the environmental role, Prog. Energy Combust. Sci. 33 (2) (2007) 107–134. [7] Y.X. Yang, L. Jia, Q. Peng, Investigation on condensation heat transfer and pressure drop of R410A during upward flow in vertical smooth and micro-fin tube, Int. J. Heat Mass Transf. 108 (2017) 2293–2302. [8] Y.M. Hou, Y.H. Shang, M. Yu, C.X. Feng, H.Y. Yu, S.H. Yao, Tunable water harvesting surfaces consisting of biphilic nanoscale topography, ACS Nano 12 (11) (2018) 11022–11030. [9] V.G. Reifert, A.I. Sardak, S.V. Grigorenko, V.L. Podbereznyj, Heat exchange at dropwise condensation in heat exchangers of desalination plants, Desalination 74 (1989) 373–382. [10] J.W. Rose, Enhanced condensation heat transfer, JSME Int. J. Series B Fluids Ther. Eng. 49 (3) (2006) 626–635. [11] D.G. Wilkins, L.A. Bromley, S.M. Read, Dropwise and filmwise condensation of water vapor on gold, AICHE J. 19 (1) (1973) 119–123. [12] A. Leipertz, A.P. Fröba, Improvement of condensation heat transfer by surface modifications, Heat Transfer Eng. 29 (4) (2008) 343–356. [13] M.H. Rausch, A.P. Fröba, A. Leipertz, Dropwise condensation heat transfer on ion
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Influence of groove orientation on dropwise condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays
T
⁎
Qi Penga,b, Li Jiaa, , Yi Dinga, Chao Danga, Liaofei Yina, Xiao Yanb a Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China b Department of Mechanical Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, IL 61801, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Anisotropic wettability Dropwise condensation Hierarchical structures Microgroove arrays Droplet dynamic behavior Heat transfer
This work experimentally investigated vapor condensation on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays to study the influence of groove orientation on the condensed droplet dynamics and heat transfer. This study showed that dropwise condensation on hydrophobic surfaces with microgroove arrays strongly depended on the groove orientation owing to the anisotropic wettability. The suction effect of a large droplet spanning several grooves on the liquid filaments filling the microgrooves was demonstrated on a hydrophobic surface with horizontal microgrooves with the phenomenon disappearing on a hydrophobic surface with vertical microgrooves. Furthermore, the horizontal microgrooves resulted in larger droplet departure sizes, significant deformation and retention of falling droplets, and the elimination of the liquid filaments sliding down the surface. The heat flux on the hydrophobic surface with horizontal microgrooves was 10–30% lower than on the plain hydrophobic surface. However, the groove orientation had less influence on the hierarchical superhydrophobic surface with microgroove arrays. At small surface subcooling, the hierarchical surface was rejuvenated through condensed droplet jumping and jumping-induced sweeping, the dominate droplet behavior transformed to coalesced droplet sweeping and the falling of suspended droplets with increasing surface subcooling. The dominate droplet behavior was little influenced by the groove orientation on the hierarchical superhydrophobic surface. Consequently, the heat flux was increased by 35–107% on both hierarchical superhydrophobic surfaces with microgroove arrays compared to the hydrophobic surface with vertical microgrooves.
1. Introduction Vapor condensation is a common phenomenon in nature [1,2] and is crucial in thermal management [3–5], power generation [6] and airconditioning systems [7] because of the large latent heat release during the phase change, and in water harvesting and desalination systems [8,9] due to its ability to capture vapor from moist air. The improved condensation heat transfer in these systems can significantly reduce energy usage and improve the system economics. The liquid condensate morphologies can be either a continuous liquid film (filmwise condensation) or discrete droplets (dropwise condensation) on a cooling surface depending on the solid-liquid interfacial effects. The heat transfer coefficient for dropwise condensation can be ten times higher than for filmwise condensation [10]. There have been many studies of dropwise condensation on various non-wetting surfaces to explore the
condensation heat transfer enhancement [11–13]. Dropwise condensation requires fast growth and removal of condensed droplets on the functionalized surfaces to expose more dry surface for vapor condensation. Large sessile droplets generate huge thermal conduction resistances that reduce the condensation heat transfer [14]. There have been many recent studies inspired by the structures on lotus leaves, butterfly wings and other natural surfaces using micro/nanostructures to facilitate dropwise condensation by controlling the solid-liquid interfacial effects [15–19]. In particular, an exciting phenomenon has been observed with self-propelled droplet jumping on superhydrophobic surfaces [20–23]. Due to the fast removal of the condensate for a new cycle of droplet nucleation and growth, the selfpropelled droplet jumping has a huge potential for condensation heat transfer enhancement [24–28]. The dropwise condensation can be further improved by tuning the solid-liquid interfacial effects related to
⁎ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (L. Jia).
https://doi.org/10.1016/j.icheatmasstransfer.2020.104492
0735-1933/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature
Greek symbols
T P q k d ΔT h SEM
δ
temperature (K) pressure (Pa) heat flux (W/m2) thermal conductivity (W/m K) the thickness of test sample (m) surface subcooling (K) heat transfer coefficient (W/m2 K) scanning electron microscope
the interval between thermocouples (m)
Subscripts s v Cu n w
saturation vapor copper the serial number of thermocouple wall
2. Experiment
the micro/nanostructures [29,30]. Microgroove structures on surfaces can increase the effective heat transfer area and reduce the droplet slippage resistance, so they have been widely used to enhance dropwise condensation [31–33]. The impact of microgrooved structures on dropwise condensation has been observed in previous studies focusing on the growth and movement of droplets on hydrophobic surfaces with microgroove arrays. These studies have indicated that the droplet growth mechanisms on hydrophobic surfaces with microgroove arrays differ from that on plain hydrophobic surface owing to the microgroove arrays acting as drainage channels for the condensate [34,35]. The energy barriers created by the microgrooves lead to anisotropic wettability on hydrophobic surfaces with microgroove arrays [36,37]. However, the anisotropic wettability leads to undesirable variations in the dropwise condensation which limits the practicality of such surfaces. Recently, hierarchical structured surfaces have attracted interest because of their remarkable ability to affect the droplet dynamics [38–42]. Thus, hierarchical structures provide a possible way to suppress the anisotropic dropwise condensation on surfaces with microgroove arrays. In this work, CuO nanostructures were manufactured on a machined surface with microgroove arrays by chemical oxidation to form a hierarchical structured surface with microgroove arrays. A plain hydrophobic surface and a hydrophobic surface with microgroove arrays were also fabricated for comparisons. The wettabilities of these three kinds of surfaces were then measured and analyzed. The visualization condensation experiments were constructed on all test surface in an enclosed condensing chamber under a vapor pressure of 60 ± 1 kPa. These surfaces were mounted vertically in a condensing chamber during the condensation experiments. The condensed droplet dynamics and the heat transfer on the test samples were then analyzed. Anisotropic dropwise condensation occurred on the hydrophobic surface with the microgroove arrays but not on the hierarchical superhydrophobic surface with microgroove arrays. The heat transfer on the hierarchical superhydrophobic surfaces with the microgroove arrays was much higher than on the hydrophobic surface with microgroove arrays, with the condensation heat transfer even less on a hydrophobic surface with horizontal microgrooves.
2.1. Sample preparation Pure copper (99.9%) was used to manufacture the test surfaces owing to its broad applications in thermal and energy systems. The substrates of all the test samples were 40 mm diameter and 2 mm thick circular copper plates. Fig. 1(a) showed the morphology of the plain copper surface as observed by a 3D optical profiler. The plain copper surface was polished to a roughness of 0.018 μm. Fig. 1(b) showed an SEM image of a flat CuO nanostructured surface. Fig. 1(c) and (d) showed the morphologies of the top surface between the microgrooves and the bottom surface of a microgroove. The rectangular microgroove arrays were 400 μm wide and 400 μm deep at intervals of 300 μm and were manufactured on the polished copper plate by mechanical surface broaching. Fig. 1(e) showed an SEM image of the hierarchical structured surface with the microgroove arrays. The hierarchical structured surface was manufactured by creating CuO nanostructures on a surface with microgroove arrays. The CuO nanostructures were formed through the self-limiting chemical oxidation method. Prior to the chemical oxidation, the polished copper plate with microgroove arrays was cleaned successively with acetone, isopropyl alcohol, ethanol and deionized (DI) water to remove deposited contaminates on the surface. Next, the clean copper plate with the microgroove arrays was immersed in a 2.0 M hydrochloric acid solution to remove the native oxide, then rinsed with deionized water and dried in nitrogen ambient. After that, the surface was dipped into an alkaline solution of NaClO2, NaOH, Na3PO4·12H2O and DI water (3.75: 5: 10: 100 wt%) at 96 °C for 15 min. This created uniform CuO nanostructures on the copper plate with the microgroove arrays. Fig. 1 (f) shows SEM images of the CuO nanostructures. The CuO nanostructures were similar to sharp blades with ~10 nm tips, 200–400 nm widths and ~1 μm heights. These dense nanostructures were expected to increase the nucleation density and regulate the surface-liquid interaction [14]. Finally, the plain copper plate, the microgroove structured surface and the hierarchical structured surface with microgroove arrays were immersed into an ethyl alcohol solution with 2.5 mM n-octadecanethiol at 70 °C for 1 h to make the surface hydrophobic through the formation of self-assembled monolayers of n-octadecanethiol on the surfaces.
Fig. 1. Test sample morphologies and surface characteristics. (a) Surface morphology of the polished copper plate. (b) SEM image of the flat CuO nanostructured surface. (c) The morphology of the top surface between microgrooves. (d) Surface morphology of the bottom surface in a microgroove. (e) SEM image of the hierarchical structured surface with microgroove arrays. (f) Detailed CuO nanostructures on the hierarchical structured surface with the microgroove arrays. 2
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2.2. Experimental system An experimental system with visualization was built to observe the dynamics of the condensed droplets and measure the heat transfer performance [18]. As shown in Fig. 2, the experimental apparatus included a steam generator, a condensing chamber, a vacuum pump, a high-speed camera, an electronic refrigeration subsystem and a data acquisition subsystem. The cylindrical steam generator was filled with deionized water and heated by two cartridge heaters to provide vapor for the condensing chamber. The heating power was manipulated by a voltage regulator to maintain the vapor pressure during the experiments. The connecting pipe was heated to prevent vapor condensation in the pipe. The vapor temperature and pressure in the experimental system were measured by transducers in the steam generator and in the condensing chamber. The vapor condensed on the test surfaces which were mounted vertically on the top of the cylindrical cooling stage at the center of the condensing chamber with the transparent viewing window. The electronic refrigeration subsystem composed of four Peltier elements was attached to the bottom of the cooling stage to regulate the condensing surface temperature. This system provided various surface subcooling by adjusting the power to the Peltier elements. The heat flux during condensation was determined from five thermocouples mounted at 15 mm intervals in the cylindrical cooling stage which was well insulated with Teflon. The high-speed camera recorded the dynamics of the condensed droplets while the data acquisition subsystem measured the temperature and pressure variations during each experiment. Before each experiment, the non-condensable gases in the experimental system were removed by the vacuum pump.
Fig. 3. Comparison of the axial heat fluxes from both ends of the test section.
The condensing surface temperature was obtained as:
Tw = T5 +
2.3. Experimental data reduction
4
∑ n=1
kCu (Tn + 1 − Tn ) δ
q q = ΔT Tv − Tw
(3)
where, ΔT is the surface subcooling which represents the temperature difference between the vapor and condensing surface temperatures and Tv is the vapor temperature.
The heat flux was assuming one-dimensional steady heat conduction through the cooling stage as:
1 4
(2)
Where, T5 is the temperature of the bottom of the test sample and d is the sample thickness. The condensation heat transfer coefficient was determined from Eqs. (1) and (2) as:
h=
q=
q d kCu
2.4. Heat loss analysis
(1)
Where, kCu is the thermal conductivity of copper (398 W/m K), Tn is the temperature of the nth thermocouple with the thermocouple numbers increasing closer to the test surface and δ is the distance between thermocouples (15 mm).
The cylindrical cooling stage was installed in the condensing chamber with only the bottom surface exposed to the atmosphere to allow heat dissipation from the Peltier elements. The heat loss from the test section was determined by comparing the axial heat fluxes at both
Fig. 2. Condensation experimental system. 3
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microgrooves during coalescence, which was attributed to the surface tension, as indicated by the dashed rectangles numbered 2 and 3 in Fig. 5(a). The horizontal microgrooves reduced the deformation of the large droplet by gravity, so the large droplets spanning several grooves had approximatively spherical caps through the action of the surface tension. Unlike on the surface with horizontal microgrooves, the suction effect of large droplets on liquid filaments was not observed on the surface with vertical microgrooves as shown in Fig. 5(b) and the large droplet was stretched into an oblong shape rather than a spherical cap as gravity overcame the surface tension. Meanwhile, as discussed previously [18], the liquid filaments partially filling the grooves refreshed the surface by absorbing the large droplets on the top of adjacent hydrophobic surfaces with microgrooves, as indicated by the dashed ellipses numbered 2 and 3 in Fig. 5(b). Since the horizontal microgrooves hindered drainage of the condensate, the local condensate overflow from the microgrooves and the large droplets spanning several grooves appeared more frequently on the surface with the horizontal microgrooves than on the surface with the vertical microgrooves, which led to more areas being covered by condensate as shown in Fig. 5(a). As a result, the thermal conduction resistance in the condensate increased which reduced the dropwise condensation heat transfer rate. Fig. 6(a) and (b) illustrated the sliding of a large droplet spanning several grooves on hydrophobic surfaces with horizontal and vertical microgrooves. The droplet departure size was about 4.32 mm on the surface with the horizontal microgrooves and about 3.24 mm on the surface with the vertical microgrooves. Since the horizontal microgrooves impeded the sliding of the large droplet driven by gravity, the droplet departure size increased with significant deformation and strong retention of the departure droplet during sliding, which reduced the surface refresh frequency. As shown in Fig. 6(a), after sliding of a large droplet, some liquid filaments remained partially filling the grooves, which led to less area exposed for new vapor nucleation. As a result, the horizontal microgroove arrays reduced the dropwise condensation heat transfer. On the surface with the vertical microgrooves, the liquid filaments above the large droplet spanning several grooves were carried to remove during departure, which facilitated the renewal of the condensing surface [18], as highlighted by the dashed rectangles in Fig. 6(b). The results in Fig. 6 showed that the departure time on the surface with the vertical microgrooves was 324 ms, which was larger than that on the surface with the horizontal microgrooves (108 ms). The departure time was the time from the beginning of a large droplet sliding to the surface being renewed in the field of view. The t = 0 ms was the moment when the large droplet was slightly larger than the departure size and began to slide. The larger departure time on the surface with vertical microgrooves was because the departure time on the surface with vertical microgrooves depended on the length of the liquid filaments carried by departure droplet with longer liquid filaments leading to longer departure times. Although the departure time was longer, the larger areas covered by the liquid filaments above the departing droplet were refreshed for new vapor condensation on the surface with the vertical microgrooves. Generally, the enough long
ends, q1 and q4, as shown in Fig. 3 for all the experimental conditions. q1 was the heat flux near the cooling side calculated from the temperature difference between T1 and T2 while q4 represented the heat flux close to the condensing surface calculated from the temperature difference between T4 and T5. Fig. 3 showed that q1 agreed well with q4 with a maximum difference of 7%. q1 was slightly larger than q4 because some heat was transferred from the high-temperature vapor in the condensing chamber to the cylindrical cooling stage through the side wall. Thus, the results indicated that the heat loss from the test section was no more than 7%. 3. Result and analysis 3.1. Test sample wettability Fig. 4 (a-f) showed the wettabilities of all the test samples for a 3 μL droplet deposited on the surfaces with the contact angles measured by a microgoniometer. Fig. 4(a) and (b) showed that the contact angles on the plain hydrophobic and flat CuO nanostructured superhydrophobic surfaces were 120.8° and 165.7°. The contact angles on non-wetting surfaces with microgroove arrays were classified perpendicular and parallel to the groove orientation. As discussed in previous studies [36,37], the microgrooves restricted the lateral wetting of the deposited droplet which led to anisotropic wettability. The contact angle perpendicular to the groove orientation was generally larger than that parallel to the groove orientation. Similarly, in this work, the contact angles perpendicular to the groove orientations on the hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays were 150.7° and 173.2° as seen in Fig. 4(c) and (e). The contact angles parallel to the groove orientations on the hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays were 119.3° and 137.8° as shown in Fig. 4(d) and (f). 3.2. Influence of groove orientation on dropwise condensation on the hydrophobic surface with microgroove arrays The condensation was visualized on the hydrophobic surfaces with horizontal and vertical microgrooves to investigate the influence of the groove orientation on the condensed droplet dynamics and the heat transfer performance. Fig. 5(a) and (b) showed the formation of a large droplet spanning several grooves on the hydrophobic surfaces with horizontal and vertical microgrooves. The small condensed droplet had a spherical cap on the hydrophobic surface with microgroove arrays which corresponded to the wettability of the plain hydrophobic surface. With the growth and coalescence of condensed droplets in the microgrooves, the condensate partially filled the groove to generate liquid filaments. The local overflow of the liquid filaments in the microgrooves created oblong droplets that coalesced with the liquid filaments in the adjacent grooves, which resulted in the generation of large droplets spanning several grooves. Interestingly, the liquid filaments in the microgrooves were sucked into the large oblong droplets to expose new surface for vapor condensation on the surface with the horizontal
Fig. 4. The wettabilities of all the test samples. (a) Contact angle on the plain hydrophobic surface. (b) Contact angle on the flat CuO nanostructured superhydrophobic surface. (c) Contact angle perpendicular to the groove orientation on the hydrophobic surface with microgroove arrays. (d) Contact angle parallel to the groove orientation on the hydrophobic surface with microgroove arrays. (e) Contact angle perpendicular to the groove orientation on the hierarchical superhydrophobic surface with microgroove arrays. (f) Contact angle parallel to the groove orientation on the hierarchical superhydrophobic surface with microgroove arrays. 4
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Fig. 5. Formation of large droplets on the hydrophobic surfaces with microgroove arrays. (a) Formation of a large droplet spanning several grooves on the surface with horizontal microgrooves at a surface subcooling of 14.1 K. (b) Formation of a large droplet spanning several grooves on the surface with vertical microgrooves at a surface subcooling of 13.5 K. Scale bar: 1 mm.
Fig. 6. Sliding of large droplets spanning several grooves on hydrophobic surfaces with microgroove arrays. (a) Sliding of a large droplet on the surface with horizontal microgrooves at a surface subcooling of 23.4 K. (b) Sliding of a large droplet with some liquid filaments filling the grooves on the surface with vertical microgrooves at a surface subcooling of 23.7 K. Scale bar: 1 mm. 5
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liquid filaments in microgrooves could also slide down on the hydrophobic surface with the vertical microgrooves to accelerate the renewal of condensing surface, as discussed in previously [18], but the phenomenon disappeared on the surface with the horizontal microgrooves. Comparison of the different droplet dynamics on the hydrophobic surfaces with horizontal and vertical microgrooves showed that the dropwise condensation on the hydrophobic surface with microgroove arrays strongly depended on the groove orientation. Since the horizontal microgrooves impeded the droplet movement, the condensation heat transfer would deteriorate on the surface with the horizontal microgrooves than with the vertical microgrooves.
which was consistent with the wettability of the CuO nanostructured superhydrophobic surface. Furthermore, droplets in the microgrooves could grow larger than the microgroove to form deformed droplets instead of liquid filaments partially filling the groove. The deformed droplets would then climb to the tops of the microgrooves driven by the Laplace pressure difference [18]. Fig. 7 showed that coalescence jumping of the condensed droplets happened on the hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays because of the release of excess surface energy during coalescence. The jumping of condensed droplets could be divided into two categories on the hierarchical surfaces due to the effects of the microgroove arrays. One was the traditional self-propelled jumping based on the coalescence of small spherical droplets at small surface subcooling (ΔT < 5 K) such as indicated by the dashed ellipse in Fig. 7(a) and (b). Another was the forced jumping of large deformed droplets in the microgrooves as the coalesce with droplets outside the microgroove, as indicated by the dashed ellipses in Fig. 7(c)-(f). Since the deformed droplets in the microgrooves stored more surface energy than normally spherical droplets [41] and the microgroove confined the lateral deformation of the coalescing droplet, the conversion efficiency of surface
3.3. Influence of groove orientation on dropwise condensation on the hierarchical superhydrophobic surface with microgroove arrays The impact of groove orientation on the condensed droplet dynamics on the hierarchical superhydrophobic surface with microgroove arrays was also analyzed. As indicated in Fig. 7, the small spherical condensed droplets represented the Cassie state or the partially-wetting state with high mobility on the hierarchical superhydrophobic surface
Fig. 7. Coalescence jumping of condensed droplets on the hierarchical superhydrophobic surfaces with microgroove arrays. (a) Self-propelled jumping of small condensed droplets on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 1.6 K. (b) Self-propelled jumping of small condensed droplets on the hierarchical superhydrophobic surface with vertical microgrooves at a surface subcooling of 1.3 K. (c) Forced jumping of large deformed droplets in a microgroove on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 1.6 K. (d) Forced jumping of large deformed droplets in the microgroove on the hierarchical superhydrophobic surface with vertical microgrooves at a surface subcooling of 1.3 K. (e) Forced jumping of large deformed droplets in the microgroove on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 11.9 K. (f) Forced jumping of large deformed droplets in the microgroove on the hierarchical superhydrophobic surface with vertical microgrooves at a surface subcooling of 12.1 K. Scale bar: 500 μm. 6
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droplets, and then the jumping direction of droplet could be changed by impinging upon the edge of microgroove during coalescence, as highlighted by the dashed arrows in Fig. 8(b). Consequently, more surface areas were swept by the jumping droplets. Thus, the jump-induced sweeping was an effective mechanism for accelerating surface renewal and improving dropwise condensation on hierarchical superhydrophobic surfaces with microgroove arrays at small surface subcooling. As the surface subcooling increased, the condensed droplet wetting state changed from the Cassie state or partially-wetting state to the Wenzel state with increasing surface adhesion. As a result, the selfpropelled jumping of the condensed droplets disappeared and the jumpinduced sweeping degraded to coalescence sweeping. Fig. 9(a) and (b) showed coalescence sweeping on the hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays at large surface subcooling. Because of the increased surface adhesion, the out-ofplane component of the momentum of the coalescing droplet was insufficient to enable it to jump off the surface, while the droplet was driven to slip along the surface by the in-plane component of the momentum. Eventually, the slipping droplet would become a large droplet pinned on the surface which would refresh some local areas of the surface through coalescing with other droplets along the way. This coalescence sweeping could facilitate surface renewal for new vapor condensation at large surface subcooling, which would improve the dropwise condensation heat transfer on the hierarchical superhydrophobic surfaces with microgroove arrays.
energy to kinetic energy was improved in the forced jumping mode of large deformed droplets relative to that of traditional self-propelled jumping of small spherical droplets [42]. Therefore, the forced jumping modes of large deformed droplets in microgrooves could be sustained on the hierarchical superhydrophobic surface with microgroove arrays over a wide range of surface subcooling from 0 K to 12 K as indicated in Fig. 7(e) and (f). The differences between traditional self-propelled jumping of small spherical droplets and the forced jumping of large deformed droplets in microgrooves have been discussed previously [42]. The combination of the traditional self-propelled jumping of small spherical droplets and the forced jumping of large deformed droplets in the microgrooves improved the surface renewal for vapor nucleation. In addition to traditional jumping condensation on nanostructured superhydrophobic surfaces [24,26,27], jump-induced sweeping was also observed on the hierarchical superhydrophobic surfaces with microgroove arrays as shown in Fig. 8. The jump-induced sweeping can be explained based on the research of Qu et al. [43] who showed that when the coalescing droplet impinged the edges of microstructures on the superhydrophobic surface, the momentum of the jumping droplet was divided into the in-plane and out-of-plane components. The inplane component of the momentum enabled the droplet to move parallel to the surface, while the out-of-plane component of the momentum prompted the droplet to jump from the surface. The jumping droplet could even move in the direction opposite to gravity such as shown by the dashed ellipse and arrow in Fig. 8(a). Furthermore, in the movement process of jumping droplet, the droplet would coalesce with other
Fig. 8. Jump-induced sweeping on hierarchical superhydrophobic surfaces with microgroove arrays. (a) Jump-induced sweeping on the hierarchical surface with horizontal microgrooves at a surface subcooling of 2.5 K. (b) Jump-induced sweeping on the hierarchical surface with vertical microgrooves at a surface subcooling of 2.3 K. Scale bar: 1 mm. 7
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Fig. 9. Coalescence sweeping on the hierarchical superhydrophobic surfaces with microgroove arrays. (a) Coalescence sweeping on the hierarchical surface with horizontal microgrooves at a surface subcooling of 20.3 K. (b) Coalescence sweeping on the hierarchical surface with vertical microgrooves at a surface subcooling of 21.9 K. Scale bar: 1 mm.
surface with vertical microgrooves because the horizontal microgrooves impeded the droplet movement. Particularly, the droplet departure size on the hierarchical superhydrophobic surface with microgroove arrays was much smaller than on the hydrophobic surface with microgroove arrays. This comparison of the droplet dynamics at various surface subcooling for the hierarchical surfaces with horizontal and vertical microgrooves illuminated that the groove orientation had less influence on the dropwise condensation on the hierarchical superhydrophobic surfaces with microgroove arrays which could be used to eliminate the dropwise condensation heat transfer deterioration on surfaces with microgroove arrays when the grooves were not properly oriented in practical applications.
In addition to the coalescence sweeping, the falling of large droplet by gravity was also a key pathway for renewal of condensing surfaces at large surface subcooling. Fig. 10(a) and (b) showed the falling of large droplets on the hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays at large surface subcooling. Some small droplets remained in the microgrooves on both surfaces after departure of the large droplet, which verified that the large condensed droplet was held on the top of the microgrooves instead of filling the grooves on the hierarchical superhydrophobic surface with microgroove arrays. This illustrated the hierarchical condensation phenomenon that vapor could condense in the microgrooves under large suspended droplets on the hierarchical superhydrophobic surfaces with microgroove arrays. This hierarchical condensation could effectively extend the heat transfer areas for more vapor nucleation and delay flooding condensation, which would increase the condensation heat transfer. Furthermore, the suspended droplet had smaller solid-liquid contact areas and surface adhesion which would reduce the departure size of condensed droplets for faster condensate removal. Although the energy barriers created by the microgrooves hindered lateral wetting of the droplet as discussed with the wettability measurements, the gravity would help large droplets to overcame the energy barriers on vertical surfaces. Meanwhile, because the large droplets were suspended instead of filling the groove, large spherical droplets could rapidly roll off without much deformation or retention on the hierarchical surface with horizontal microgrooves. Interestingly, the falling droplets experienced self-oscillations on the hierarchical superhydrophobic surface with microgroove arrays due to the droplets striding the microgroove structures by coalescence during departure, as shown in Fig. 10(c) and (d). The strong self-oscillations of the falling droplets enhanced the disturbances of the liquid-vapor interface that improved the heat and mass transfer. In addition, Fig. 10(a) and (b) showed that the departure size of the suspended droplets on the hierarchical surface with horizontal microgrooves was about 1.83 mm, which was slightly larger than the departure size of 1.33 mm on the
3.4. Comparison of the condensation heat transfer performance The heat fluxes and condensation heat transfer coefficients on all the test samples including the plain hydrophobic, hydrophobic and hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays were plotted as functions of the surface subcooling in Fig. 11(a) and (b) to further illustrated the impact of groove orientation on the dropwise condensation heat transfer on these test surfaces with microgroove arrays. The results in Fig. 11 showed distinct differences in the condensation heat transfer for the hydrophobic surfaces with horizontal and vertical microgrooves, which again showed that the groove orientation significantly impacted the dropwise condensation heat transfer performance on hydrophobic surfaces with microgroove arrays. The hydrophobic surface with the vertical microgrooves had 20–64% higher heat flux and 21–58% higher heat transfer coefficient than the hydrophobic surface with horizontal microgrooves for surface subcooling from 0 K to 28 K. Although the microgroove arrays increased the heat transfer areas, the horizontal microgrooves hindered the departure of large droplets, increased the departure size of condensed droplets, and eliminated the sliding down of liquid filaments in the microgrooves seen on the hydrophobic surface with vertical 8
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Fig. 10. Falling and strong self-oscillations of suspended droplets on the hierarchical superhydrophobic surfaces with microgroove arrays. (a) Falling of a suspended droplet on the hierarchical surface with horizontal microgrooves at a surface subcooling of 20.3 K. (b) Falling of a suspended droplet on the hierarchical surface with vertical microgrooves at a surface subcooling of 21.9 K. (c) Strong self-oscillation of a falling droplet on the hierarchical superhydrophobic surface with horizontal microgrooves at a surface subcooling of 20.3 K. (d) Strong self-oscillation of a falling droplet on the hierarchical surface with vertical microgrooves at a surface subcooling of 21.9 K. Scale bar: 1 mm.
for surface subcooling from 0 K to 5 K due to the reduction of the jumping droplet condensation. There were only small differences between the condensation heat transfer performance on the hierarchical surfaces with horizontal and vertical microgrooves which further verified that the groove orientation had almost no influence on the dropwise condensation heat transfer on the hierarchical superhydrophobic surfaces with microgroove arrays. The reason was that the dropwise condensation heat transfer on such surfaces was dominated by the droplet dynamics including the jumping of condensed droplets, coalescence sweeping and falling of suspended droplets which did not depend on the groove orientation. These interesting droplet dynamics accelerated the condensate removal and facilitated the vapor
microgrooves which offset the heat transfer enhancement caused by the increased heat transfer areas. Consequently, the condensation heat flux and heat transfer coefficient on the hydrophobic surface with the horizontal microgrooves were even 10–30% and 8–26% lower than those on the plain hydrophobic surface. Fig. 11 further showed that the condensation heat transfer on the hierarchical superhydrophobic surfaces with the microgroove arrays were much better than on the other test surfaces. The maximum condensation heat transfer coefficients were 69.5 kW/m2 on the hierarchical superhydrophobic surface with the vertical microgrooves and 62.4 kW/m2 on the surface with the horizontal microgrooves. Furthermore, the condensation heat transfer coefficient decreased sharply 9
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Fig. 11. Dropwise condensation heat transfer performance on different test surfaces. (a) Heat fluxes and (b) condensation heat transfer coefficients.
condensation heat transfer coefficient than on the hydrophobic surface with the horizontal microgrooves. The condensation heat flux and heat transfer coefficient on the hydrophobic surface with the horizontal microgrooves were even 10–30% and 8–26% lower than on the plain hydrophobic surface. However, the heat flux was 35–107% higher and the condensation heat transfer coefficient was 39–104% higher on the hierarchical superhydrophobic surface with the horizontal microgrooves than on the hydrophobic surface with vertical microgrooves.
condensation on the hierarchical surfaces with microgroove arrays. Consequently, the hierarchical superhydrophobic surfaces with horizontal microgrooves had 35–107% higher heat flux and 39–104% higher heat transfer coefficient than the hydrophobic surface with vertical microgrooves. 4. Conclusions The wettability measurements and visualization of the vapor condensation were used to investigate vapor condensation on hydrophobic and hierarchical superhydrophobic surfaces with horizontal and vertical microgroove arrays to study the influence of groove orientation on the dynamics of condensed droplets and the condensation heat transfer performance on these surfaces. The main conclusions are:
Declaration of Competing Interest None. Acknowledgments
(1) Anisotropic wettability was observed on hydrophobic and hierarchical superhydrophobic surfaces with microgroove arrays, the contact angles perpendicular to the groove orientation were larger than those parallel to the groove orientation due to the energy barriers caused by the microgrooves impeding lateral wetting of the droplet. (2) Dropwise condensation on the hydrophobic surface with microgroove arrays was significantly influenced by the groove orientation. The liquid filaments partially filling the grooves were sucked into oblong droplets spanning several grooves on the hydrophobic surface with horizontal microgrooves because of the surface tension. The suction effect was not observed on the surface with vertical microgrooves due to gravity restricting the fluid motion. The horizontal microgrooves hindered the droplet movement which resulted in larger departure sizes, significant shape deformation, retention of falling droplets and the lack of liquid filaments sliding down. (3) The groove orientation had little effect on the dropwise condensation on the hierarchical superhydrophobic surfaces with microgroove arrays because the groove orientation had little effect on the condensed droplet dynamics on such surfaces. At small surface subcooling, jumping of condensed droplets and jump-induced sweeping improved the surface renewal, but the droplet behavior changed to coalescence sweeping and falling of suspended droplets with increasing surface subcooling. The jump-induced sweeping was attributed to the in-plane and out-of-plane components of the jumping droplet momentum. (4) The condensation heat transfer performance on the hydrophobic surfaces with horizontal and vertical microgrooves were significantly different. The hydrophobic surface with the vertical microgrooves had 20–64% higher heat flux and 21–58% higher
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