Superwettability of 0D materials

Powder Technology 312 (2017) 103–112

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Powder Technology journal homepage: www.elsevier.com/locate/powtec

Review

Superwettability of 0D materials Bin Su a, Lei Jiang b,c,⁎, Xuchuan Jiang a, Aibing Yu a,⁎⁎ a b c

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China School of Chemistry and Environment, Beihang University, Beijing 100191, PR China

a r t i c l e

i n f o

Article history: Received 2 September 2016 Received in revised form 24 January 2017 Accepted 2 February 2017 Available online 8 February 2017 Keywords: Superwettability Superhydrophobic Superhydrophilic Particles Powders

a b s t r a c t Engineering the wettability of zero-dimensional (0D) materials (particles, powders and agglomerates) is a key issue in applications such as dissolution, dispersion, granulation, coating, drying and so on. In the past, the superwettability of 3-dimensional (3D), 2-dimensional (2D) and 1-dimensional (1D) materials has been intensively studied. However, the superwettability of 0D materials has received limited attention. It is often investigated as a “collective effect” contributed by many closely packed particles, rather than individual ones. This perspective article reviews the current state of understanding in this area. The definition of superwettable 0D materials is first discussed. Some fundamental rules for understanding 0D superwettability are proposed. Then, methods for investigating the superwettable state of 0D materials are briefly introduced, and the potential applications of superwettable 0D materials are described. Finally, the future development of superwettable 0D materials is discussed. © 2017 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamental rules for understanding superwettability of 0D materials . . . . . 2.1. Interaction between liquid and 0D materials at an atomic/molecular level 2.2. Definition of two superwetting states . . . . . . . . . . . . . . . . 2.3. Construction of sufficient roughness to generate a superwetting state . . 3. Potential methods for investigating superwettability of 0D materials. . . . . . 3.1. Sessile drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Floating particle method . . . . . . . . . . . . . . . . . . . . . . 3.3. Environmental scanning electron microscopy . . . . . . . . . . . . . 3.4. Atomic force microscopy . . . . . . . . . . . . . . . . . . . . . . 4. Promising application of superwettable 0D materials . . . . . . . . . . . . . 4.1. On-demand assembly towards functional structures . . . . . . . . . . 4.2. Super liquid marbles . . . . . . . . . . . . . . . . . . . . . . . . 4.3. High-performance superwettable coatings . . . . . . . . . . . . . . 4.4. Biocompatible interfaces . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Correspondence to: L. Jiang, Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (L. Jiang), [email protected] (A. Yu).

http://dx.doi.org/10.1016/j.powtec.2017.02.002 0032-5910/© 2017 Elsevier B.V. All rights reserved.

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1. Introduction Wettability can be regarded as the ability of a liquid to make contact with a solid surface, and is mainly decided by the intermolecular interactions between these two phases [1–3]. Superwettability is the

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extreme state of wettability, indicating that a solid surface can totally repel or attract a targeted liquid. This state has been reported a century ago by Ollivier [4,5], who found a super-anti-wetting composite surface consisting of soot, lycopodium powder and arsenic trioxide. With continuous research in this filed, especially in the recent two decades, a family of superwettability states, including superhydrophobicity, superhydrophilicity, superoleophobicity, superoleophilicity, superaerophobicity and superaerophilicity, has been proposed, where the solid, liquid and environmental media can be freely chosen to form diverse superwettable states [6–8]. Focusing on the solid part (Fig. 1a–c), 3D superwettability state (such as sponges [9], bulks [10– 12], and aerogels [13]), 2D state (such as surfaces [14–18], interfaces [19], films [20], and coatings [21]), and 1D state (such as fibers [22– 24] and nanochannels [25–28]) have gained tremendous interests among researchers and scientists of both academic and industrial backgrounds, and as a result, investigations on such states have been intensively made in the past. However, studies on the superwettability of 0D materials (such as particles, powders, and agglomerates) are few. According to the ISI Web of Science, the search with the keywords “superhydrophobic or superhydrophobicity” and “particle or powder” only results in a few references, mainly related to the fabrication of

“superhydrophobic particles” [29–32]. However, the as-prepared 0D materials have been pressed into compact forms such as disc, plate, beam and wafer for the contact angle (CA) measurements. The air pockets that are trapped in the voids in the compressed bulk have assisted 0D materials surrounding the voids to fight the permeation of a liquid into the bulk. In this case, even the measured CA value is higher than 150°, which exhibits only the “collective effects” of numerous 0D materials (Fig. 1d), rather than an individual 0D material effect. Thus, the concept of “superhydrophobic particles” is not proper, leading to some misunderstanding for further research. There is a need for an objective and theoretical base for research in this area. In this perspective article, we will first describe the challenge in the study of superwettable 0D materials, mainly focusing on the aspects of the superwettability of an individual fine/hierarchical 0D material (Fig. 1e). Then, the fundamental rules for fabricating these liquidrepellent/absorbent 0D materials will be introduced. It is pointed out that because of the size limitation of 0D materials, some rules valid for macroscopic superwettable 3D/2D/1D materials are not applicable to such 0D materials. Then, some methods recently proposed for observing superwettable states of individual 0D material are summarized. It is emphasized that to observe the superwetting phenomenon, the size of 0D

Fig. 1. a) Optical photograph of superhydrophilic (left) and superhydrophobic (right) 3D sponges. For easy observation, water droplets were colored with 0.3 M methyl orange. Adapted with permission [9]. Copyright 2012, Wiley-VCH. b) Water drop profile for superhydrophilic and superhydrophobic PNIPAAm-modified 2D silicon substrates stimulated by thermal changes. Adapted with permission [17]. Copyright 2004, Wiley-VCH. c) Water drop profile for hydrophilic (top) and hydrophobic (bottom) spindle-knots on the artificial fibers. Adapted with permission [22]. Copyright 2012, Wiley-VCH. d) The superwettability of 0D materials was considered as the “collective effect” contributed by numerous closely packed particles, rather than the individual ones. e) The superwettability of an individual fine/hierarchical 0D material.

B. Su et al. / Powder Technology 312 (2017) 103–112

materials and liquids should be comparable. On this basis, we describe several potential applications of superwettable 0D materials, such as highly-ordered assembly of colloids, and super-liquid marbles, anti-friction superhydrophobic coatings, and biocompatible interfaces. Finally, we discuss future developments in superwettable 0D materials. 2. Fundamental rules for understanding superwettability of 0D materials Chemical compositions and surface roughness are two key factors that dominate the wettability of solid surfaces at the macroscopic scale [1–3]. However, the sizes of 0D materials are commonly ultrasmall (from several nanometers to hundreds of micrometers). Thus, some rules in the field of macroscopic superwettable 3D/2D/1D materials are not applicable, and the study of superwettable 0D materials requires the assistance of computational modeling and the creative use of novel characterization techniques. 2.1. Interaction between liquid and 0D materials at an atomic/molecular level Young's model [33] indicates that solid surfaces having a CA N 90° with water can generally be defined as hydrophobic (Fig. 2a). However, Volger et al. [34] pointed out that a CA of 65°, rather than 90°, is the watershed that divides hydrophobicity from hydrophilicity. Following this concept, the CAs of materials in the fabrication of superwettable surfaces should be tested first. For example, solid materials with intrinsic water CA N 65° can easily become superhydrophobic when optimized by introducing enough surface roughness [35] (Fig. 2b for Cassie' model). However, there is a problem in the development of superwettable 0D materials, because the sizes of 0D materials are much smaller (from several nanometers to hundreds of micrometers) compared to a liquid droplet (several millimeters) used in the traditional sessile drop CA measurements. Thus, it is difficult to place a liquid droplet upon an individual powder particle and observe its apparent CA value. Even with the assistance of environmental scanning electron microscopy (ESEM) used in a high humidity environment, the

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resolution of the images of ESEM is on the micrometer scale. In this case, we should first discuss the effective size region of 0D materials for CA measurement (Fig. 2c). Because of the intrinsic limitation of optical microscopy, ESEM and other facilities [36,37], the proposed range of 0D materials for CA testing is around ten to hundreds of micrometers (Fig. 2c). Powders with sizes in this range can be tested by a sessile drop, ESEM or atomic force microscope (AFM), which will be discussed in detail in Section 3. On the other hand, measuring the intrinsic wettability of 0D materials for particle sizes smaller than a few micrometers raises a new question: how to measure the CA or obtain useful information for intermolecular interactions between solid and liquid phases? Although routine experimental techniques (microscopy and spectroscopy) cannot solve such problems, theoretical methods can provide an alternative way to do so. Simulations based on computational modeling [38–40] have gradually emerged as a powerful tool in research and development activities, particularly with the rapid development of computer technology and new simulation techniques. This tool not only allows us to understand and predict nanoscale phenomena but also to increase the pace of discoveries in all scientific disciplines. It can also reduce the cost of development and commercialization of novel technologies and materials. Different theoretical methods, such as Density Function Theory (DFT) [41,42] and Molecular Dynamics (MD) [43,44] methods, at different time and length scales, can be applied for understanding the intermolecular interactions between solid and liquid phases at the nanoscale. The computational modeling techniques can provide information that is normally difficult to observe, such as: the breaking off of liquid molecules from their surrounding liquid molecules; displacement of vapor molecules adsorbed at a 0D material surface; and adherance of liquids to a 0D material surface by forming bonds with molecules belonging to the solid (Fig. 3). In this case, the attractive forces can be calculated theoretically, providing a standard to validate the fabrication of superwetting 0D materials. Similarly, superanti-wetting 0D materials can be predicted when the solid–liquid adhesive forces are weaker than both the liquid cohesive and solid/gas adhesive forces. Computational modeling and simulation provide a

Fig. 2. Traditional wettability models are not applicable to the study of superwettability upon the 0D materials. Classical a) Young's and b) Cassie-Baxter's models to exhibit the interaction among the liquid, solid and vapor (air) at the macro-scale. c) The proposed effective size region of 0D materials for the CA measurement. The proper size range of 0D materials for CA test is around ten to hundreds of micrometers.

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Fig. 3. Different theoretical methods, such as Density Function Theory (DFT) and Molecular Dynamics (MD) methods at different time and length scales, can be applied for understanding the intermolecular interactions between the solid/liquid two phases at the nanoscale. The computational modeling and simulation methods provide an effective way to observe the primary bonds involving chemical bonds (ionic, covalent or metallic), the secondary bonds referred to as hydrogen and the van der Waals bonds, and their resulting attractive/repulsive forces at the nanoscale solid/liquid interfaces.

cost effective way to observe the primary bonds involving chemical bonds (such as ionic, covalent or metallic), the secondary bonds referred to as hydrogen and the van der Waals bonds, and their resulting attractive (or repulsive) forces at the nanoscale solid-liquid interfaces.

the cavities of the microstructure of a textured powder surface, and the liquid thin layer formed is continuous and stable, this state can be defined as superhydrophilicity of the 0D materials. The powder surface may exhibit a relatively high adhesion due to the total permeation of water. Alternatively, superhydrophobic state might exist in at least two models when considering the competitive interactions among the atoms (or molecules) of the solid, liquid and vapor. For a hierarchical 0D material, the air pockets trapped on its textured surfaces are able to support the water droplet, allowing a discontinuous nanoscale Three-phase Contact Line (TCL). As a result, the adhesion of the powder surface can be reduced greatly, leading to a nano-Cassie state which enables the droplet to easily roll off. For a fine/smooth 0D material, the tested water droplet can partly wet the powder surface since the air pockets could be trapped only in a certain regions of the powder surface, indicating a dynamically metastable superhydrophobic state between the Cassie and Wenzel states. As a result, the water droplet can roll off, yet leaving a liquid tail linked with the powder. Besides the case of water, definition of two opposite superwetting states can also be built for diverse liquids or in different environmental media, following the above rules. The intermolecular interactions (attractive/repulsive forces) between solid and liquid phases determine the superwettable trend, while the environmental media pockets (such as air, water or other organic liquids) dominate their adhesion behaviors. Since the adhesion of a liquid to a solid interface under oil or water is smaller than that in air, the definitions of these superwettable states are easier; however, further studies are still required.

2.2. Definition of two superwetting states 2.3. Construction of sufficient roughness to generate a superwetting state It is widely accepted that solid surfaces with a static CA N 150° can be defined as superhydrophobic materials while those with a static CA b 10° within 1 s can be regarded as superhydrophilic ones [1–3]. Unlike the materials in the above cases, 0D materials are physically small in their sizes. Thus, the definition of superwettability of 0D materials may be different from their higher-dimensional counterparts. For example, the proposed CA testing procedure for 0D materials can be performed as follows. A size-comparable water droplet is carefully held above one target 0D material and released (Fig. 4). If water can fully fill up

In 1936, Wenzel et al. [45] introduced the surface roughness factor, r, to improve the Young's equation and highlight the importance of surface roughness in optimizing the apparent CA. That is given by cos θ
¼ r  cos θe

ð1Þ

where θ* is the apparent CA on a rough solid surface; r is the ratio of the actual surface area of the rough surface to the geometric surface area; and θe is the CA on an ideally flat solid surface. In 1944, Cassie and Baxter [46] proposed another equation to describe CA hysteresis for composite interfaces with various heterogeneity: cos θ
¼ f 1  cos θ1 þ f 2  cos θ2

ð2Þ

where θ* is the apparent CA on the composite interface; θ1 and θ2 are the CAs on the ideal flat solid surface 1 and surface 2, respectively; and ƒ1 and ƒ2 are the calculated apparent surface area ratios of surfaces 1 and 2, respectively (where f 1 + f 2 = 1). This equation can be simplified to describe the rough solid surfaces consisting of solid matters and air, given by: cos θ
¼ f 1  cosθ1 −ð1− f 1 Þ

Fig. 4. Definition of two superwetting states of 0D materials. A size-comparable water droplet is carefully held above one targeted 0D material and released. (Top) If water can fully fill up the cavities of the microstructure of a textured powder surface, this state can be defined as the superhydrophilicity of the 0D materials. (Bottom) Superhydrophobic state may exist as at least two models. For a hierarchical 0D material, the air pockets trapped on its textured surfaces are able to support the water droplet, leading a nanoCassie state which makes the droplet to easily roll off. However, a fine 0D material will create a dynamically metastable superhydrophobic state between the Cassie and Wenzel states. As a result, the water droplet will roll off but leave a liquid tail linked with the powder.

ð3Þ

where θ* is the apparent CA on the rough surfaces. This equation offers the possibility of building an extreme hydrophobic state by providing enough air pockets in (1 − ƒ1) regions on the hydrophobic solid surfaces (intrinsic CA, θ1, N65°). Accordingly, the apparent CA (θ*) can increase greatly, allowing an optimized surface wettability, such as superhydrophobicity in air. It should be noted that liquid will fully wet the textured solid surface in Wenzel's state, allowing superhydrophilicity in air when the intrinsic CA (θ1) b 65°. This roughness optimized effect can benefit diverse superwettable states besides those in a solid/water/air system. For example, the following relationship can be deduced [1–3]: cos θ
¼

γT−A  cos θ1 −γE−A  cos θ2 γ T−E

ð4Þ

B. Su et al. / Powder Technology 312 (2017) 103–112

where θ* is the apparent CA of the target matter (liquid or gas) on the rough surfaces in the environmental media (liquid or gas); γT-A, γE-A, and γT-E, are the interface tensions for target-matter/air, environmental-media/air and the targeted-matter/environmentalmedia interfaces, respectively; and θ1 and θ2 are the CA of the target matter in air and CA of environmental media in air, respectively. In summary, introducing surface roughness can provide pockets of environmental media the ability to fight against the target matter. Further, the degree of surface roughness should be carefully chosen. Taking the superhydrophobicity as an example, Chandler et al. [47] has theoretically confirmed that the characteristic length of hydrophobic interaction among water molecules is about 100 nm, indicating the importance of building nanostructures b100 nm to combat water permeation. This concept has been experimentally confirmed by Koch et al. [48] and Robin et al. [49]. For 0D materials with repulsive solid/liquid interaction (intrinsic CA, θ1, N 65°), introducing a nanostructure on micro-powders can generate a “nano-Cassie state” to repel water. Accordingly, superhydrophobic 0D materials can be fabricated. For 0D materials with attractive solid/liquid interaction (intrinsic CA, θ1 b 65°), water droplets can fully wet the powder, leading to the superhydrophilic 0D materials. This “roughness optimized effect” can be applicable to the 0D materials with sizes larger than several micrometers. For a powder with particle sizes of several nanometers order, defining superwettability is an open question, and finding an answer to this question will require development of new theories and experimental characterization techniques. 3. Potential methods for investigating superwettability of 0D materials In the early studies [29–32,36,37], the as-prepared powders were usually molded into tablets or compact forms by exerting a pressure (~ 70–700 MPa) in order to directly determine the equilibrium CA on the testable surfaces. However, numerous air pockets trapped in pores of the resulting compressed bulk will support the liquid. Therefore, the measured apparent CA value is a result of the contributions from the “collective effects” of numerous 0D materials and air pockets (Fig. 1d), and does not indicate the intrinsic wettability of an individual particle. Furthermore, other disadvantages exist in this method of measurement. The first one comes from considerable adhesion between the target liquid and powders. The probing liquid could penetrate the porous bulk, making it difficult to measure the apparent CA. For example, Saleh et al. [36] reported that penetrating liquid could “swell” powders into liquid body, yielding continuously changing surface tension and an unstable droplet shape. Therefore, the measured apparent CA may not accurately represent the wettability of a powder. On the other hand, the physically applied pressure also causes issues during the CA measurements. Buckton and Newton [50] reported that the CA values were greatly affected by the applied pressure. A higher compaction pressure would have led to a reduced porosity, which indicates that fewer air pockets may be trapped in the compactions to fight the water penetration. Therefore, CA measurements based on pressed compactions are not accurate enough to characterize the wettability of 0D materials. 3.1. Sessile drop Sessile drop method is an easy to perform, being a straightforward and accurate method for direct observation and measurement of the apparent CAs of solid surfaces (Fig. 5a). According to this method, a liquid drop with a controlled volume is placed on one powder particle surface and photographed [36]. The CAs are determined by measuring the point of TCL after drawing the drop profile by image projection. During the last decade, various high performance digital recording systems have been developed. With improved imaging analysis capabilities, accurate CA measurements can be done by the microscopic method. Due to its

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simplicity and high reproducibility, this technique attracts more attention compared to other techniques with complex operations. It should be noted that the required liquid droplet and powder particle sizes are hundreds of micrometers to a few millimeters, depending on the operation capacity and equipment limitations. In addition, the accuracy of CA value can reach ± 2° when the apparent CA N 20° [51]. In testing the superwettable 0D materials, it is expected that a drop will either fully wet or totally jump off a particle (see Fig. 4). 3.2. Floating particle method In 1979, Good [52] proposed a strategy called as a floating particle method, which is also known as the particle immersion depth method, to determine the CA of individual fine particles (Fig. 5b). The floating mechanism is based on the static equilibrium of an object at the gas/liquid interface, which is controlled by the balance between solid/liquid surface tension force and the gravitational force of the powder particle. These two forces on an ideal smooth spherical particle can be calculated by the following equation:   ðρL −ρV Þ  g  R2 ρ −ρV −ð1− cos ϕÞ2  4 S 6γLV ρL −ρV z  ð2 þ cos ϕÞ−3   sin2 ϕÞ ¼ 0 R

sin θ  sinðθ þ ϕÞ þ

ð5Þ

where R is the radius of the particle; z is the immersion depth; ρS, ρL and ρV are solid, liquid and air densities, respectively; γLV is the water-air interface tension; and θ, and ϕ can be found from Fig. 5b. By measuring the immersion depth z, the apparent CA or θ, can be calculated from Eq. (5), which indicates that this method is a straightforward one. Unfortunately, accurate measurement of the immersion depth heavily depends on: particle size (the larger the better), the proficiency of the operator, and the turbulent motion on the water surface. In some cases, the immersion depth is quite difficult to observe. In addition, only an ideal spherical particle can be used in this method, because the deduction of the mathematical equation is based on that assumption. It can be predicted that superhydrophobic 0D materials will yield a nearly zero immersion depth while superhydrophilic counterparts will be totally immersed in water (i.e., z ≈ 2R). 3.3. Environmental scanning electron microscopy The environmental scanning electron microscope (ESEM) is a special type of SEM that allows the collection of electron micrograph specimens even if they are “wet”, non-conductive, or both, by creating a gaseous environment in the specimen chamber [53]. Owing to the specialized electron detectors and special design of the systems, ESEM can transfer the electron beam through the high vacuums in the gun region to specimen chamber, which is at a high pressure. The sample observation can be performed under a middle-level vacuum (up to 50 Torrs towards low magnifications), and in a humid environment of which pressure, temperature and humidity are controllable. Thus, it is a unique instrument designed for the purpose of imaging the droplet condensation on 0D materials. The general operation procedure of ESEM is that the pressure of the water vapor in the specimen chamber is first adjusted, followed by a reduction of temperature to saturation condition (100% RH). As a result, many water droplets can condense on the sample in the chamber. By recording ESEM images of the condensed water droplets on the 0D material surface, CA values can be collected by using a high performance digital recording system and imaging analysis (Fig. 5c). The ESEM technique provides a direct observation method for collecting water CA on 0D materials at the microscale, which is especially accurate in the measurement of high CA values [36]. When such measurements are made on superwettable 0D materials, it is expected that condensed drops will either totally cover a particle or jump off it.

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Fig. 5. Diverse characterization strategies towards measuring/observing the superwettable states of individual 0D material. Representative schematic illustrations of: a) sessile drop, b) floating particle, c) environmental scanning electron microscopy (ESEM), and d) atomic force microscopy (AFM).

Besides the advantage of direct observations, there are neither damages to test samples nor special needs when preparing them. These are the advantages of the ESEM technique. However, the precise measurement of low CAs, especially the swelling behavior of droplets on porous powders, is still a challenging issue in the ESEM technique. 3.4. Atomic force microscopy Atomic force microscopy (AFM) is a kind of scanning probe microscopy with a resolution as small as a nanometer, which is 1000 times greater than the optical diffraction limit. By gathering information during the process of a mechanical probe coming into contact with a sample surface, the forces and morphologic images are recorded. Preuss and Butt [54] reported the use of an underwater AFM to determine the CA of a fine powder. In a standard procedure, a powder particle is fixed to the end of the cantilever, which then move in the liquid medium towards an air bubble (Fig. 5d). By carefully moving the cantilever, its deflection as well as the bubble position is recorded, allowing the curves of force-versus-position and converted force-versusseparation to be determined. Thus, the CA value of the powder can be calculated based on the force-versus-position results. Notably, this technique can only be applied to smooth and ideal spherical particles, similar to the floating particle method. 4. Promising application of superwettable 0D materials Due to their extreme liquid-repellent/absorbent properties, controllable adhesion and other promising properties, 0D materials with superwettability have attracted strong interests of scientists and engineers with different backgrounds. Several potential applications have resulted, as discussed below.

4.1. On-demand assembly towards functional structures Periodic arrangement of monodispersive nanoscale building blocks, such as nanospheres, nanocubes, and latex spheres, have attracted considerable interest from researchers due to their unique optical/electrical manipulable properties [55] as well as potential applications in novel optical devices [56], protective coloration [57], humidity sensors [58] and multi-functional superwettable surfaces [59,60]. Self-assembly strategy has been proved to be the most feasible yet low-cost route to generate 2D and 3D colloidal crystals [61]. Nanoscale building blocks are dispersed in a liquid where each block keeps a relative large gap with others. With the evaporation of the solvent, solvophobic interactions play a key role in attracting the aggregation of nanoscale building blocks, known as the poor solvent effect. When two colloids come into contact with each other, surface overlapping will happen, allowing a reduction in the total surface energy of the two colloids to happen. Following the continuous solvophobic interactions among all the colloids, a closely packed arrangement will appear with advanced optical/electrical manipulable properties. Since the arrangement of 0D materials is dominated by the solvophobic interactions, tailoring their surface wettability can significantly improve the final assembly performance and related functionality. It can be predicted that superhydrophobic 0D materials can exhibit a considerable solvophobic interaction while their superhydrophilic counterpart shows nearly a negligible one. The resulting assemblies will be different, as has been partly confirmed by a pioneering research reported by Wang et al. [62] (Fig. 6b). In their study, they employed a spherical colloid consisting of a hydrophobic polystyrene core and a hydrophilic poly(acrylic acid) (PAA) shell. This binary design allows a switchable wettability of colloid surfaces by tailoring the orientation of the hydrophilic PAA segments. In this case, solvophobic interactions among the spherical colloids are tunable. It is reported that the regular

B. Su et al. / Powder Technology 312 (2017) 103–112

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Fig. 6. Potential applications of superwettable 0D materials. a) Schematic illustrations of superhydrophilic/superhydrophobic 0D materials. Owing to their unique surface properties as well as small sizes, they can find promising application in guiding highly-ordered assembly of colloids, and fabricating unique liquid marbles, anti-friction superhydrophobic coatings, and biocompatible interfaces. b) Typical SEM image of highly ordered arrangement of colloidal spheres with hydrophilic outside layers. Adapted with permission [62]. Copyright 2007, Wiley-VCH. c) Diverse liquid/gas marbles models stabilized by functional powders. Adapted with permission [69]. Copyright 2007, Nature Publishing Group. d) Digital images of the sandpaper abrasion test for superhydrophobic coatings. Right part is the dependence of water CA on the mechanical abrasion cycles. Adapted with permission [76]. Copyright 2015, AAAS. e) ESEM image of cancer cells captured upon powder surfaces consisting of sea-urchin-shaped irons. Adapted with permission [79]. Copyright 2014, Wiley-VCH.

arrangements of hydrophobic colloids require a assembly temperature as high as or above 80 °C, while hydrophilic ones just need a lower assembly temperature (~20 °C). This result indicates that an extra energy barrier exists in the hydrophobic colloid case compared to the hydrophilic ones. To overcome the increased solvophobic interactions, external energy inputs are necessary. It should be noted that the as-prepared colloids are smooth under the SEM observations, indicating that they may not show extreme superwetting states. Thus, it can be expected that superhydrophilic 0D materials will be easier to assemble due to their nearly negligible solvophobic interactions. This concept should be further investigated and confirmed in future studies.

polytetrafluoroethylene particles (CA N 65°) or hydrophilic graphite and carbon black (CA b 65°), while the liquid can be selected from a wide range of liquids, such as from water and water solutions to glycerol and ionic liquids [64]. Owing to their non-stickiness and ability to connect with the atmosphere, which are also known as the breathing properties, liquid marbles can be employed as smart capsules [65], miniature chemical reactors [66], bio-incubators [67], electrostatic-based applications [68] and others [69]. In an ideal situation, one liquid droplet is covered by a monolayer of powder particles. Assuming that the particles are spherical with an identical radius Rp, the ratio of liquid–air to solid–air interfacial areas on the marbles can be calculated, as given below [70]:

4.2. Super liquid marbles Liquid marbles are liquid (or air) droplets covered by micro/ nano-scale powders in an environment of air (or liquid) [63], exhibiting extremely low adhesion and friction when placed on a solid substrate (Fig. 6c). Particles that cover the liquid can be hydrophobic

ALA 1− cos θ ¼ 2 ASA

ð6Þ

where θ is the liquid CA of powder surfaces; and ALA and ASA are the liquid–air and solid–air interfacial areas, respectively. According to this

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equation, the surface free energy on attachment of a particle to the liquid–air interface is reduced, indicating that this attachment is energetically favored. Recent studies [63–70] have used hydrophobic or hydrophilic powders to form the liquid marbles. When the solid powders are endowed with superwettability, the ratio of ALA to ASA, for example, will significantly increase when using superhydrophobic powders (θ N 150°) to cover one water droplet. As a result, the channels (ALA) for breathing the air are increased, which may benefit cell culture and related biological fields. Arbatan et al. [71,72] found that liquid marbles possess the ability to circumvent the problem of cell adhesion onto the cell culture dish by providing a 3D volume and a maximum potential contact for cell aggregation. This strategy can be used as platforms for the incubation of embryoid bodies (EBs) from embryonic stem cells (ES cells). It is believed that the participation of superwettable 0D materials will favor this field in the future. 4.3. High-performance superwettable coatings The research on superhydrophobic surfaces has gained a rapid momentum from the late 1990s due to their dramatic application potential in self-cleaning anti-icing surfaces, drag-reduction, and heat transfer enhancement [1–28]. As mentioned in Section 2, chemical composition and surface roughness are two essential features for superhydrophobicity. However, these two aspects are greatly susceptible to mechanical wear [73]. The physical damage can create a loss of liquid repellency, leading to a poor mechanical durability which has prevented their applications in practice in the recent two decades. A strategy of “polymer plus hydrophobic particles” [74,75] has been proposed to fabricate superhydrophobic surfaces in an effective, lowcost and easy-to-manipulate manner. Generally, hydrophobic nanoparticles are mixed with the polymer solution, and then coated on solid substrates. With the rapid evaporation of the solvent, the hydrophobic nanoparticles become aggregated, yielding a 3D hierarchical roughness. Since the density of nanoparticles is rather low, they are inclined to float onto the outside layer of the composites, indicating a nonpolar surface chemical composition. Accordingly, superhydrophobic coatings can be fabricated with high water-repellent properties. However, the asprepared 3D hierarchical roughness is fragile and easy to get damaged, leaving the structure-free polymer layers. To solve this problem, very recently, Lu et al. [76] reported an improved “polymer plus hydrophobic particles” strategy. In their study, two kinds of TiO2 nanoparticles with different size ranges (~ 60 to 200 nm and ~ 21 nm) were used to construct surface roughness. Perfluorooctyltriethoxysilane took charge in two roles: one role was to cover the TiO2 nanoparticles so as to convert them into ultrahydrophobic particles, and the other was to link the particle hierarchical structures with the substrates. Owing to this advanced design, the asprepared coatings show superhydrophobic properties on a series of supports, such as rigid steel, glass and flexible cotton paper. The important advancement of this study is the improvement of the resistance to mechanical abrasion (Fig. 6d). A glass substrate coated with the composite of polymer and nanoparticles was placed face-down on a sandpaper and moved a distance of 10 cm to guarantee the surface has been abraded longitudinally. Even after 40 cycles of this abrasion process, the water CA remained between 156° and 168°, indicating that superhydrophobicity did not deteriorate by mechanical abrasion. This unique resistance to friction can be attributed to ultra-hydrophobic, yet high-density particles. In the early researches [74,75], low-density hydrophobic nanoparticles were inclined to float on the outside of the composite layer and become aggregated into hierarchical roughness. As a result, mechanical abrasion will peel off the top layer of polymer/ nanoparticle composites, resulting in a loss of both the low-surfaceenergy nanoparticles and roughness. In the study of Lu et al. [76], covered by perfluorooctyltriethoxysilane, TiO2 nanoparticles were ultra-hydrophobic yet high in density, allowing their uniform dispersion in the composite. Thus, the low-surface-energy nanoparticles and

roughness could be retained even after the composite top layers were peeled off. However, the wettability of perfluorooctyltriethoxysilane modified TiO2 nanoparticles has not been carefully investigated. It can be predicted that the resistance towards mechanical abrasion of superhydrophobic composites will be further improved by the use of superwettable 0D materials, such as superhydrophobic nanoparticles. Since each nanoscale building blocks can repel water completely, the composites can exhibit superhydrophobicity even after severe damages have happened due to considerable mechanical abrasion. 4.4. Biocompatible interfaces The interactions between cells and materials highly depend on the surface wettability [77,78], and this dependency has been taken into account by scientists in designing surfaces of medical implants. Meng et al. [79] have reported that superhydrophilic surfaces can effectively capture circulating tumor cells (Fig. 6e). In their study, spherical iron particles have been heated at 400 °C, yielding a rough sea-urchin-like shape. To endow the iron powder with superhydrophilicity, the surface chemical composition has been improved by coating the iron powder with a nanoscale SiO2 thin-layer. Furthermore, anti-EpCAM, which is a cytophilic molecule antibody, was covered to increase the chance of powder surfaces to be in contact with cells. Owing to the sufficiently large roughness of superhydrophilic powder surfaces, the topographic interactions between cancer cells and rough powder surfaces have been considerably improved, giving a cell capture efficiency of 49.7–58.3%. Besides cells, aerobic bacteria such as Saccharomycetes can also be favored by the control of surface wettability. Generally, Saccharomycetes should breathe enough oxygen to allow their daily activities. However, this common requirement is generally restricted by their living environment (e.g. under water). Interestingly, Lei et al. [80] found that the production rate of aerobic bacteria has been improved by more than 100-fold when the bottom feeding plates were replaced by superhydrophobic ones. The participation of superhydrophobic feeding plates allows a continuous gas input contributed by their atmospherically connected nature. Thus, microbes are able to gain more oxygen than from hydrophilic surfaces alone, resulting in an improvement in their biochemical activities. This pioneering study may open up a new avenue to develop industrial biochemical applications. Superwettable 0D materials may find their promising contributions in bio-chemical field not only in building a feeding environment but also in making it possible physically bond/interact with microorganisms. 5. Conclusions In this perspective article, we have discussed three related fundamental topics: definition of superwettability of a 0D material, fabrication of superwettability materials, and developments in academic research and industrial applications. We point out that when defining the superwettability of 0D materials, one should focus on the individual effects of particles, rather than the “collective effect” contributed by many closely packed 0D materials. Fundamental rules for utilizing and investigating the superwettable properties of individual 0D materials have been recommended. Recent progress in measuring/observing the superwettable states of individual 0D materials is summarized. We have also discussed potential applications of superwettable 0D materials by representative examples. Superwettability of 0D materials indeed represents an important research area, academically and industrially. We have also highlighted several challenges in the investigation of superwettable 0D materials. First, it is difficult to observe superwettable 0D materials. Compared to macroscopic materials, the sizes of 0D materials are commonly ultra-small. Consequently, conventional macroscopic theories and techniques may not be applicable to 0D materials; new computational theories and experimental tools/equipment are necessary for the characterization of 0D materials. Secondly, the

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Table 1 Advantages/drawbacks of four techniques towards CA testing of superwettable 0D materials.

Sessile drop Floating particle method Environmental scanning electron microscopy (ESEM) Atomic force microscopy (AFM)

Recommended size

Ideally smooth 0D material?

Operation

N Hundreds of micrometers N Hundreds of micrometers N Several micrometers N Several micrometers

No Yes No Yes

Easy Difficult to observe Easy Difficult to manipulate

quantification of wettability of individual 0D material is a prerequisite. There are a few potential methods for this purpose, including sessile drop, floating particle, ESEM and AFM. Table 1 lists their relative advantages and drawbacks. To be able to quantify the wettability by optical techniques such as sessile drop and floating particle methods, particle size should be large enough (more than hundreds of micrometers). For small particles, one should consider using ESEM or AFM. However, at this stage of development, the manipulation of an individual 0D material by these techniques requires highly skillful operations. Moreover, the superwettability of 0D materials strongly depends on their size and morphological properties. Therefore, a reliable CA has to be obtained from many repeated tests. Finally, it is a difficult task to quantitatively assess the performance of superwettable 0D materials against their normally-hydrophobic/hydrophilic counterparts. Superwettable 0D materials need to be studied in a more systematic way in order to realize their potential applications in areas such as human health, green industry and many other sectors.

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