Superhydrophobic Textiles: Review of Theoretical ...jeffjournal.org/papers/Volume10/V10I4(1) C. Park.pdf · Superhydrophobic Textiles: Review of Theoretical Definitions, Fabrication

Journal of Engineered Fibers and Fabrics 1 http://www.jeffjournal.org Volume 10, Issue 4 – 2015

Superhydrophobic Textiles: Review of Theoretical Definitions, Fabrication and Functional Evaluation

Sohyun Park1, Jooyoun Kim, PhD2, Chung Hee Park, PhD1

1Seoul National University, Seoul KOREA

2Kansas State University, Department of Apparel, Textiles, and Interior Design, Manhattan, KS UNITED STATES

Correspondence to:

Chung Hee Park email: [email protected] ABSTRACT Engineering of superhydrophobic textile surfaces has gained significant scientific and industrial interest for its potential applications in outdoor wear and protective textiles, resulting in many publications especially on theoretical models and fabrication methods. In this review, progress in theoretical definitions to explain the wetting behavior and realization techniques for superhydrophobic textile surfaces is discussed. Firstly, theoretical models from Young, Wenzel, and Cassie-Baxter to the more recent re-entrant angle model are overviewed to understand the design strategy for superhydrophobic surfaces. Secondly, major surface manipulation techniques to produce superhydrophobic textiles were reviewed for: modification of surface energy, addition of surface roughness by depositing or growing nanoparticles either in spherical form or in high aspect ratio, etching by plasma or caustic chemicals. Particular attention is paid to evaluation methods to measure the level of hydrophobicity for superhydrophobic textile surfaces, as a limitation of static water contact angle (WCA) on differentiating superhydrophobic surfaces has been reported elsewhere. The challenges in application of superhydrophobic textiles to clothing materials in terms of comfort properties and durability are discussed with the suggestion of further research opportunities to expand the application. INTRODUCTION Recently, there has been active research in biomimetic technology for developing highly functional materials that mimic nature. In superhydrophobic research, superhydrophobic surfaces refer to surfaces with excellent water repellency with a water contact angle (WCA) exceeding 150° and low contact angle hysteresis (CAH) of less than 10°. The most well-known example is the development of superhydrophobic self-cleaning materials that mimic lotus leaves. The lotus-leaf, which is one of the best known natural

superhydrophobic surfaces, effectively removes impurities, such as mud, with water. It was found that this is because nano-level hydrophobic wax crystals on top of micro-level bumps on lotus-leaf surfaces come together to have strong superhydrophobic attributes [1]. At such surfaces, dirt and soils are loosely attached, and a rolling water drop can easily attach the loosely bonded substances, removing them from the surface, giving self-cleaning effects. Due to this, the phenomenon of self-cleaning resulting from a superhydrophobic surface that does not become wet is called the lotus effect. This surface characteristic is applicable in industries for oil repellency, anti-corrosion, anti-fogging/frosting, anti-bioadhesion, and water-oil separation. Because of this, there has been active research for the past several decades on various methods and materials that propose superhydrophobic and ultra-oil repellency that control wettability for water, oil, and non-polar liquids through the chemical makeup of solid surfaces and designing geometrical surface structures [2-7]. In the past few years, various studies have been focused on the textile applications of such superhydrophobic/superoleophobic characteristics, and textile materials with large WCA and self-cleaning effects have been commercialized [8]. Superhydrophobic textiles can grant not only excellent water repellency and oil resistance, but also active self-cleaning performance, and thus they can be used as high protective clothing textiles and functional outdoor clothing materials [2]. Furthermore, it can reduce the number of launderings thanks to the self-cleaning performance. When the number of launderings is reduced, the performance of the highly functional textiles can be maintained for long times and can lead to the development of environment-friendly materials that can reduce the use of resources and energy needed for laundry.

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Research on superhydrophobic textiles for clothes started relatively later than other fields. Despite reports on various developments, the configuration methods and evaluation methods of other fields are being applied as in textiles, and there are tendencies to focus only on the configuration of high superhydrophobicity without considering the unique features of required performance of clothing textiles. Clothing materials are very closely related to the safety and health of people. Therefore, close studies on the effects of superhydrophobic textiles such as the safety of used materials, environmental responsibility of processing methods, and the functional suitability or durability of developed materials are necessary. In particular, by reviewing the studies on the theoretical development and fabrication techniques for developing superhydrophobic surfaces, adequate design strategies for superhydrophobic textile surfaces will be able to be suggested. Thus this review is intended to overview the progress of theories and engineering techniques to learn about the challenges of superhydrophobic textile applications and to explore the research opportunities to realize practical applications. Also, discussions on evaluation methods would give hint on relevant assessment methods that measure the representative characteristics for the applied uses. MODELS FOR WETTING THEORY Important theories for explaining surface wettability include Young’s model, which shows the thermal-dynamic equilibrium relations of interface energy of surface and water drops on flat surfaces, and Wenzel’s and Cassie-Baxter’s model that explains the wettability of roughened surfaces. Young’s Model Young’s equation assumes that the surface is smooth and explains wettability of liquid drops with the relationship of static contact angle, interfacial tension between solid and vapor phases, and interfacial tension between liquid and vapor phases (Eq. (1)) [9]. At this time, surface wettability is determined by the chemical makeup of the solid; at an equilibrium state where a certain liquid drop is in contact with solid, a solid phase with low surface free energy ( ) would give large interfacial tension ( of the solid surface and liquid drop, and large static contact angle of the liquid phase through Young’s equation. Therefore, for the processing of superhydrophobic textiles, the surface free energy of solids is commonly lowered using water repellant agents. However, even though the surface is treated with very low surface free energy material, such as C9

perfluorocarbon ( , 5~10 dyne/cm), if there’s no roughness developed at the surface, static WCA stops short with superhydrophobicity (WCA was 105-118°) [10].

(1)

FIGURE 1. Young’s model for static contact angle in relation with interfacial tensions of solid/liquid/vapor phases. ( : solid-vapor interfacial tension, : water-solid interfacial tension, water-vapor interfacial tension).

Wenzel & Cassie-Baxter Models Most solid surfaces in daily life have roughened surfaces and violate the assumptions of Young’s model [9]. Therefore, there are limitations in explaining the surface wettability through Young’s equation. According to the study of Wenzel [11] and Cassie-Baxter [12], it explains that on surfaces with roughened areas, counting both surface free energy and surface roughness as critical factors for wettability. The equations of Wenzel and Cassie-Baxter’s theory were compared in Table I. Wenzel [11] assumed complete contact of a liquid drop on a solid surface and stated that the contact angle (ƟW) on the surface is proportional to the contact angle (Ɵe) on flat surfaces and surface roughness factor (r). At this time, the surface roughness factor (r) was displayed as the ratio of the surface area in contact with liquid to the projected surface area [11]. In the Wenzel model, the liquid drop is in complete contact with the solid surface so (r) is always larger than one. Thus, in the case of hydrophobic surfaces with low surface free energy (Ɵe>90), it is explained that as the contact area of the surface and liquid grows, the contact angle also rises. The Cassie-Baxter model [12] assumed the heterogeneous contact where the liquid is not completely in contact with the solid surface, but is simultaneously in contact with the trapped air pocket in surface bumps. At this time, the solid fraction of contact is defined by the contact area proportion of the surface with the liquid droplet compared to the overall projection area. In this model, the apparent contact angle (ƟCB) is defined as the sum of the


contribution of the solid surface and air contact, and explains that when the contact area of the liquid and air is large, the contact angle increases. The implication of Cassie-Baxter’s research is that in order to obtain superhydrophobic surfaces, there are limitations in using the single method that reduces surface free energy, and it is advantageous to implement small-scale surface roughness to enlarge the contact area between liquid and air.

TABLE I. Wenzel and Cassie-Baxter model.

Figure 2 shows the relationship of the water contact angle (Ɵflat) on surfaces without roughness and contact angle (Ɵrough) in surfaces with roughness. The slope can be determined by the contact area surface fraction (Φs) and the surface roughness factor (r). The transformation from the Wenzel state to the Cassie-Baxter state can be checked by the contact point of the two lines through the graph, and such transformation can be induced through control of surface structure [13].

FIGURE 2. Wenzel and Cassie-Baxter models in relation to the surface roughness and static contact angle.

As a natural substance with superhydrophobic and self-cleaning performance, lotus leaves and insect wings are made up of substances with low surface free energy; it has also been reported that they have micro/nano-scale binary structure [1]. Since then, there have been many studies on developing superhydrophobic surfaces with dual-scale surface roughness that mimics their surface structure [8, 14-17]. In such research, nano particles were implemented to artificially change the surface free energy or design the surface roughness by methods such as nano implants or lithography, and the surface with added roughness was analyzed to be superior in achieving superhydrophobicity [18-23]. When Cheng et al., [20] removed all of the nano bumps in dried lotus leaves, WCA dropped from 142° to 126°, and the importance of nano-scale roughness on surfaces was validated as to improve superhydrophobicity and self-cleaning effects. Patankar [18, 19] made a paraffin wax structure with dual scale roughness to confirm that binary structures contributed greatly in enhancing hydrophobicity. In the study, the size ratio of micro and nano-scale protrusions and the distance between the protrusions were analyzed to have an impact on hydrophobicity and self-cleaning effects. Similar to this, Bhushan & Jung [21] produced a superhydrophobic surface adjusting the diameter, height, and distance of bumps and reported that the column distance, by influencing the shut air pocket, has a great effect on water drops either becoming pinned or unwet at the surface, stating that superhydrophobicity can be enhanced by adjusting the distance of rough structures. There are a number of other studies that provide empirical data for the development of superhydrophobic surfaces. When summing these up, it is evident that the size ratio of micro and nano-scale roughness structures, perpendicular and horizontal proportions of surface protrusions, and their geometrical shapes affect superhydrophobicity [15-25].

FIGURE 3. Superhydrophobic difference according to micro/nano-scale surface roughness of HDPE surfaces [22].


Re-Entrant Angle Model The robust superhydrophobic surface displays repellency not only for water, but also for liquid with lower surface tension (under 30 dyne/cm). Furthermore, a superoleophobic surface is defined to have static contact angles greater than 150° with organic liquids such as alkanes, like octane, which have much lower surface tensions than water [24]. Such highly-repellent surfaces would have resistance against various oily solvents and polluted water, having a broader range of application in separation materials, fluid transport, fingerprint resistant surfaces, self-cleaning textiles, protective clothing, etc. To achieve a superoleophobic surface, the surface free energy of the solid must have lower surface tension than oily solvents and the geometrical structure of micro/nano-scale roughness at the surface needs to be optimized [6, 24, 25]. In particular, because there are limitations in modifying the surface free energy of the solid surface to be lower than the surface tension of the oily solvent, there have been efforts to fabricate superoleophobic surfaces by designing the optimal geometrical structure of the surface roughness. Tuteja et al., [6] theoretically explained the conditions for oil-repelling surfaces with the same surface free energy and different surface structures (Figure 4). When the surface protrusion’s geometric angle (ψ) is larger than the contact angle θ that is made by an interface between the liquid and solid protrusion (Figure 4a), the liquid’s re-entrant contact area goes downward and becomes wet as it goes into the Wenzel state. On the other hand, when ψ becomes smaller than θ (Figure 4b), the contact area of the liquid does not make progress and is maintained to go into the Cassie-Baxter state. There have been many studies that claim that superoleophobic surfaces can be effectively configured by adjusting the micro/nano-scale protrusions or roughness to become the ‘re-entrant structure’ through studies related to superoleophobicity [26-28]. Until now, in order to configure oleophobicity in the textile sector, fluorinated compounds with low surface free energy were often used, but in order to configure superoleophobicity, multi-faceted analysis and research on both surface free energy and surface roughness design are needed. Also, there are many limitations in the approach that controls the geometrical shape of rough structures in textile surfaces.

FIGURE 4. Effects of other surface structures of the same surface free energy on the solid-liquid-vapor interface (a)superhydrophobic, (b)superoleophobicity [6]. Dynamic Characteristics of Water-Solid Interface Superhydrophobic surfaces have various applicable features such as self-cleaning, oleophobicity, anti-corrosion, drag reduction, non-adhesion, low surface tension, prevention of snow piling, etc. The criteria for distinguishing superhydrophobic surface characteristic is not only high static contact angle (>150°), but also dynamic contact angle or rolling off angle [29]. In the Cassie-Baxter state, due to the trapped air between the protrusions or rough structures, the adhesion between the solid surface and water is small, thus the interface of the water and solid separates easily and rolls off. In the Wenzel state, the contact area of the solid surface and water is large and so is the amount of energy to separate the water from solid surface. Therefore, at this state it does not roll off, but the water droplet remains on the fabric surface making it easy to permeate. This “roll-off” phenomenon can be measured through the CAH, sliding angle (SA), and shedding angle. The surface that shows low dynamic contact angle represents the high level of hydrophobicity and the self-cleaning effects can appear [1, 30]. The self-cleaning effect, which is a representative feature of superhydrophobic surfaces, is the effect of pollutants, such as dust, on the surface attaching to water rolling off and thus being removed [30]. Barthlott & Neinhuis [1], who studied the effects of superhydrophobic self-cleaning effects of various types of lotus leaves, analyzed the impact factors on self-cleaning effects according to the pollutant particle size and rain intensity. When the pollutant


particle is larger than the micro-level protrusion in the solid surface, it is placed on the upper part of the protrusion resulting in the contact area between the solid surface and particle being very small and thus having very low adsorption energy. At this time, the water rolling off from the surface, gaining the adsorption energy, attaches to the pollutant. In order for the pollutant and water to separate again, a force stronger than the adhesion force between them is necessary. Upon examining the self-cleaning effects according to the strength of artificial rain, water drops with very low kinetic energy, such as fog or dew, were found to have considerably lower self-cleaning effects compared to regular rainfall [1]. Because the kinetic energy of water drops that fall from a certain height gives elastic deformation to surface protrusions and dust, the kinetic energy of raindrops is advantageous in adsorption of dirt. Fog or sprinkles with low kinetic energy cannot give such deformation, thus having low self-cleaning effects. Figure 5 shows the self-cleaning effects from flat and rough surfaces. Self-cleaning effects are manifested when the pollutant particle has a stronger adhesion force with water than its coherence to the solid surface. Therefore, in order to configure the superhydrophobic surface with self-cleaning functions, the surface energy must be lowered and the surface roughness enlarged to weaken the cohesion of the pollutant with the solid surface, while maintaining the water droplet form so that it rolls off easily [8]. However, when the pollutant is chemically superhydrophobic, the adhesion force with the water drop would be low, thus having little self-cleaning effect. Studies on the self-cleaning effects of superhydrophobic surfaces until now have focused on revealing the self-cleaning process. However, additional discussions are also necessary on various factors that affect the self-cleaning effect such as the size and surface structure of pollutant particles, chemical attributes of pollutant particles, and the quantity of water that is rolled off and its falling distance.

FIGURE 5. Effects of solid surface bumps and self-cleaning effects [8] (a) self-cleaning of flat surfaces, (b) self-cleaning of roughened surfaces.

ENGINEERING TECHNIQUES FOR SUPERHYDROPHOBIC TEXTILES Many superhydrophobic textiles with a WCA over 160° and low CAH have been introduced with commercialization efforts in the outdoor wear industry. Early development of superhydrophobic textiles was mostly made by coating the textile surface with a low surface free energy material to lower the surface free energy. Recently, with the emphasis on the importance of micro/nano binary structures, attempts to grant nano-scale roughness on the micro-rough fiber surface were attempted. To achieve this, textile surfaces were coated with nano-particles or processed so that nano structures were formed by self-assembly or by surface etching, while adding post-processing to lower surface free energy have been made. The following are representative superhydrophobic processing technologies.

Modification of Surface Free Energy The method for lowering surface free energy is a basic control for superhydrophobic characteristics and is a very effective approach for generating hydrophobicity easily and at low costs. Textiles use filament groups made up of yarn and form various surface structures according to the yarn’s texturizing processing, weave pattern, density, etc. Textiles form a unique surface roughness by the filament groups and yarns, while adjusting the yarn size and number of filament fibers in a yarn can adjust the roughness at the submicron-scale. As a method for granting


surface roughness using yarn and filament, Gao & McCarthy [31] used a polyethylene terephthalate fabric weaved with 2 μm filaments for dip-pad-dry coatings using siloxane water-repellants and the resulting textile surface showed a WCA of 170° and CAH of under 5°. Compared to general specimens, the number of trapped air pockets in the microfilament was much larger, thus water was able to easily roll off, validating their claim that it was possible to configure superhydrophobic textiles using micro fibers without nano-scale roughness [31].

FIGURE 6. Effects of yarn that make up fabrics on wettability [31]. (a) 1 mm scale woven bundles of 40 ㎛ fibers. (b) 50 ㎛ scale

woven bundles of 2 ㎛ fibers. Due to the unique surface structure of textiles, it is possible to grant highly hydrophobic properties with water-repellent processing agents only. Various water-repellant processes have been introduced since the mid-20th century. Water-repellent/oil-repellent agents can be categorized as pyridine, silicone, and fluorocarbon compound types, and, depending on the chemical composition, the surface free energy of treated fabrics changes and provides wetting resistance against water and/or oil. Among the repellent agents, silicone and fluorocarbon types are most commonly used. Silicone or siloxane types have –O-Si-O- backbones with alkyl groups oriented to the surface, giving hydrophobicity. The processed fabric surface has a surface free energy of about 20 dyne/cm [32, 33], thus obtaining water repellency. Fluorocarbon compounds are normally perfluoroalkylacrylate copolymers with forms that are suitable for fibers as compounds containing perfluoro alkyl groups. As the fluorine containing ester becomes arranged vertically on the fiber surface, the surface free energy becomes lower and, depending on the orientation and distribution of the perfluoroalkyl groups, it obtains surface free energy of about 5-20 dyne/cm [32, 33]. Fluorocarbon type water-repellent agents often show repellency for not only water, but also oil and oily solvents. It has thus

broadened its range of use as it was expected to have oil resistance and soiling resistance. However, as it was found that perfluorooctanoic acid (PFOA) can be potentially cancer-causing and hazardous to the body in the decomposition process of C8 fluorocarbon compounds, there have been restrictions on the use of C8 fluorocarbon processing agents [34].

ENGINEERING TECHNIQUES BY NANO-SCALE SURFACE ROUGHNESS STRUCTURING Superhydrophobic and superoleophobic processing are developing from lowering the surface free energy of fibers to designing micro/nano-scale binary structure surfaces to reduce the contact area of the surface and liquid droplet, using nanoparticles such as silica, TiO2, CNT, ZnO, etc., in various ways. Surface Roughness Formed by Spherical Particles Zhao [38] used the layer by layer (LBL) assembly method on cotton fabrics to give roughness using polyelectrolyte/silica nanoparticle multilayers and post-treating with fluoroalkylsilane to develop superhydrophobic textiles with a sliding angle (SA) of 10° even after ten washes. Wu et al., [39] attached nano particles on cotton, PET, and silk fabrics by dip coating in toluene solution with silica nanoparticles containing long hydrophobic alkyl side-chains to develop superhydrophobic fabrics with less than a 10° shedding angle. The modified fabric was verified to maintain hydrophobicity even after 200 abrasions and washes. This fabrication method was claimed to be a practical method for the commercialization of superhydrophobic textiles [39]. Ramaratnam et al., [40] and Xue et al., [41] chemically combined silica nanoparticles with a fabric surface to create superhydrophobic textiles. This method exhibited excellent adhesion of silica nanoparticles onto a fabric substrate. Athauda & Ozer [42] attached different sized silica nanoparticles (7~40 nm) on the 1st and 2nd layers and developed superhydrophobic fabrics with a hierarchical binary roughness structure. In developing superhydrophobic textile fabrics, TiO2 nano aggregates were also used to create a binary roughness on the fabric surface by a sol-gel method with post treatment using stearic acid and 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTDS) to lower surface energy. The resulting surface showed a high WCA of 160°. Furthermore, TiO2 crystallization and aggregation exhibited UV blocking abilities by scattering light, thus showing the possibility of developing multi-functional fabrics [43]. In another study, ZnO/SiO2 particles were made


with core/shell structures on a PET fabric to give superhydrophobicity with a WCA of 160°. Since ZnO can decompose organic materials when exposed to UV, there are concerns that durability of the ZnO treated fabric itself and the repellent coating would also be degraded in direct sunlight. Accordingly, ZnO nanoparticles with SiO2 could not execute photolysis on the repellent agent hexadecyltrimethhoxysilane (HDTMS), thus the treated fabric maintained hydrophobic functionality under direct sunlight without degrading the coating agent [44].

FIGURE 7. Surface roughness formed by spherical particles (a) Micro/nano-scale roughening formed with different sized nanoparticles [42], (b) ZnO/SiO2 nanoparticle coating process on PET fabrics(left), UV durability of ZnO/SiO2 nanoparticle coating fabrics(right) [44].

Surface Roughness Formed By Pillar Type Particles Instead of using spherical forms of nanoparticles, utilization of nanoparticles with high aspect ratios, such as carbon nanotubes (CNT) and nanofilament type graphene, was also introduced. In a number of studies that developed superhydrophobic textiles using CNT as nanoparticles [45-50], superhydrophobic fabrics with CAH of less than 5°

and high WCA was developed by attaching multi-wall CNT (MWCNT) using simple and economic methods such as coating or dipping. Among them, a method developed by Li et al., [46] exhibited strong chemical adhesion with fabrics with a high WCA of over 145°, even when the treated fabric was immersed in acidic solutions of pH 2-12. Shim [47] proposed a processing method that can apply CNT on fabrics simple and efficiently, where CNT in a repellent agent was deposited on to PET fabrics and gained a WCA of over 160° and shedding angle of less than 10°. However, it did not reach a level that controlled the surface arrangement of CNT and therefore could not develop the surface roughness using the aspect ratio of CNT. Shateri-Khaliabad & Yazdanshenas [50] proposed a superhydrophobic fabric with 7° shedding angle and 163° WCA by dipping the fabric in oxidized graphene dispersed in a solvent and hydrophobized. Graphene is a hydrophobic substance and specimens treated only with graphene showed hydrophobicity with a WCA of 143°; the treated fabric can also be used as an electrical conductive material. CNT and graphene are materials that are receiving a great deal of attention due to large surface area, high durability, high elasticity, and excellent thermal properties. Also, graphene’s high mechanical strength as well as transparency and flexibility draw particular industrial attention. Due to the diverse features of nano carbon particles, it is expected that superhydrophobic textiles made with this would have mutiple functions for a broad range of applications. In-depth research utilizing this carbon material is expected as it has high industrial potential in various sectors such as the clothing, bio-medical, and electronic sectors. Zimmermann et al., [51] used a method for growing silicone nano-filaments by chemical vapor deposition on textiles such as cotton, wool, and polyester to design silicone hairs as nano-scale roughness on top of micro-rough fibers to have binary surface roughness like lotus leaves. As a result, superhydrophobic polyester textiles with excellent abrasion resistance were developed with an SH of 2°; even after abrading 1,000 times, the SH remained at 25° [51].


FIGURE 8. Surface roughness formed by pillar type particles (a) PBA-g-CNT treated fabric surface SEM image and water contact [45]. (b) Silicon nano-filament-treated fabric surface SEM image [51]. ZnO as a nanorod was also utilized to design a binary roughness structure on a cotton fabric using a sol-gel method, and n-dodecyltrimethoxysilane (DTMS) was treated afterwards to lower the surface free energy [52]. This processing technique costs less than other methods, producing a superhydrophobic fabric surface with a WCA of over 161° [52]. Xu & Cai proposed 161° WCA superhydrophobic cotton fabrics by coating ZnO crystals on cotton fabrics and then coating the nanorod developed in a vertical direction with hydrophobic materials. Xu et al., also examined and reported in another study the effects of particle shape of ZnO nanorod and spherical SiO2 nanoparticles on superhydrophobicity [53]. The roll-off angle was influenced by the shape of nano-structure at the surface, giving smaller values when the surface was treated with ZnO nanorods. However, there was no significant difference in static WCA made by the shape of nanoparticles. This phenomenon was explained by the fact that more air gap is maintained between the nanorod structures than the spherical forms, making the adsorption of water drops on the solid surface more difficult and thus making water drops roll-off more easily.

FIGURE 9. SiO2 Nanoparticle and a ZnO Nanobar scheme on fabrics [53]. (a) SiO2 nanoparticles (b) ZnO nanorods Surface Etching Many reports were made on etching the fiber surface to form nano roughening, followed by grafting or physically/chemically attaching compounds with low surface energy to develop superhydrophobic textiles [54-72]. When treating with UV-laser or plasma, chemical bonds of treated surfaces are broken forming radical groups, reactors, etc., resulting in etching and grafting or physical/chemical deposition. In particular, plasma treatment can adjust hydrophilic to hydrophobic wettability according to the types and treating conditions of injected gases, such as oxygen, argon, helium, and fluorine [55, 60]. As this technique can be applied to most polymeric materials, it is being actively used for various attempts in the textile sector [54, 56, 58, 61, 62, 66, 68, 71]. Most studies related to superhydrophobic textiles using plasma treatment often use this technique to make a thin layer of chemical coating with hydrophobic compounds [55]. These applications include microwave plasma to graft oleic acid on cotton fabrics producing superhydrophobic surfaces with a WCA of over 150°, normal pressure plasma treatment to polymerize hexamethyldisiloxane (HMDSO) on cotton fabrics to produce a fabric showing a WCA of over 140°, and received scores exceeding 90 in the AATCC Spray test, as well as 50 grade after five washes [68]. SF6 RF plasma treatment was also utilized on cotton, silk, and PET fabrics to produce hydrophobic surfaces with a WCA between 130-150° [56]. Fluorine compounds are one of the most used in plasma treatment to produce superhydrophobic textiles [58],[64]; in a study by Huang et al., [73] a superhydrophobic surface was obtained through PTFE plasma sputter coating on silk fabrics with 152° WCA and 5° CAH.


Studies related to plasma treatment showed comparable moisture absorption and vapor permeability of treated fabrics as untreated ones, concluding that plasma treatment only affects the fabric surface with no significant impact on intrinsic bulk properties [55, 64, 74].

FIGURE 10. AATCC spray test results on HMDSO plasma treated cotton fabrics [68]. (a)Non-treated cotton fabric, (b) HMDSO plasma treated cotton fabric (t4c2). Meanwhile, studies with respect to the optimization of plasma treatment conditions to grant nano-scale surface etching on fiber surfaces have been reported [75-78]. Atomic Force Microscopy (AFM) was often used; Poletti et al.,[77] used AFM to measure changes of the surface area and surface roughness according to the type of injected gas by summarizing the results of treating air, He, Ar, SF6, CF4 gases with various pressures and voltages on polyester fabrics and compared the etching effects. In another study, a processing method to form nano-scale wrinkles by UV laser was proposed to transform hydrophobic materials with high water and oil resistant performance [78]. By treating with CF4, H2, and He gases on cotton fabrics at atmospheric pressure, the fabric surface turned superhydrophobic where a water drop does not adhere to the specimen surface but rather bounces off [63, 65]. Studies that combined surface roughness with low surface energy include the work of Twardowski et al., [79] that used argon plasma and HMDSO to fabricate superhydrophobic polyester fibers with 150° WCA and the work of Hodak et al., [65] that used RF plasma to etch and polymerize fluorocarbon compound on silk surfaces with a WCA of 140°.

FIGURE 11. AFM image of polyester fiber surface with nano-scale roughness. (a) made by UV laser treating [65]. (b) made by air plasma treating polyester fiber surface, (b) plasma etched polyester fiber[77].

FIGURE 12. Selected time sequence images of water droplet falling on (a) superhydrophobically treated gold film, (b) clean Si wafer, (c) superhydrophobically treated cotton, and (d) untreated cotton. All panels are at 2ms intervals, except the ones with dots between panels. One dot between panels indicates a 4 ms interval. Two dots are at 6 ms intervals [63]. Etching with a plasma technique gives nano-scale roughness on fiber surfaces, being an effective method for engineering surface roughness. In the materials sector, many research results that evenly form nano-pillars of high aspect ratios with plasma etching have been made. Ko et al., [80] used oxygen plasma etching and HMDSO plasma enhanced chemical vapor deposition (PECVD) on carbon fibers to make a surface structure with nano hair in an


aspect ratio of up to 37. They observed the condensation of water drops on fiber surfaces in saturated water vapor environments at 2℃ and water vapor pressure of 5.2 Torr, respectively, and reported that as the aspect ratio of nano-pillar grows, superhydrophobicity can be more effectively displayed. Shin et al., [74] used oxygen plasma etching on polyester nonwovens to create uniform nanohair structures. Treatment conditions for maximizing the linearity of plasma action are important for etching nano-pillars or hairs that are dense and have large aspect ratios; it can be adjusted by gas type, gas flux, in chamber pressure, voltage, and treatment time. For superhydrophobic textiles, there are few studies that define the plasma process parameters to form uniform and dense nano-pillars or hair on textile surfaces. Further research on engineering parameters is expected for developing superhydrophobic textiles using plasma etching. Meanwhile, as an example of fiber surface etching through alkali hydrolysis, Mazrouei-Sebdani & Knoddami [81] produced superhydrophobic fabrics with less than 10° SHA through forming roughness by alkali etching and water-repellant treatment by fluorocarbons.

FIGURE 13. E-SEM image of plasma etching and HMDSO coating treated carbon fiber [80]. (a) pristine, (b) 15 min plasma-treated and 30 s hydrophobic film-coated and (c) 60 min plasma-treated and 30 s hydrophobic film-coated surfaces (Scale bars are 50 nm). (d–f) Schematics regarding the condensation of water with respect to the aspect ratio of nanostructures formed on the CFs.

Electrospinning Another method for creating nano-scale roughness for superhydrophobicity is using nanofibers through electrospinning. Nano-webs that have nanofibers produced through electrospinning are made up of fibers and pores, with a surface with nano-scale surface roughening [82]. Lim et al., [83] heat-treated

the intrinsically hydrophobic polyacrylonitrile (PAN) fiber to make it hydrophilic, then electrospun hydrophobic PAN on top of it to develop a two layer nano-web with asymmetric wettability, one of whose surface was hydrophilic and one having a WCA of over 150° (Figure 14). Similarly, Thorvaldsson et al., [84] electrospun cellulose acetate on top of micro-sized lyocell filaments to produce a fabric with asymmetric wettability. The electrospun web, when made with low surface energy polymers, exhibited superhydrophobic characteristics without adding extra geometrical roughness structures on to the nanoweb [85]. Wang et al., [86] used fluorinated polyurethane with dispersed SiO2 nanoparticles to make an electrospun nanoweb with additional roughness contributed by nanoparticles, and reported a high CA for oil-based solvents. Particularly, the thickness of nanofibers and the porosity (with distance between fibers) was reported to be a major determinant factor influencing not only water repellency but also oil repellency [87]. In the study by Miyauchi et al., [88], electrospun polystyrene was made by mimicking silver ragwort leaf to form a micro-scale web roughness and nano-scale wrinkles by adjusting the volatility of solvents in preparing superhydrophobic webs.

FIGURE 14 Solvent-friendly nano web showing both superhydrophobic and superhydrophilic attributes [83]. COMPARISON OF ENGINEERING TECHNIQUES FOR SUPERHYDROPHOBIC TEXTILES When summing up studies related to development of superhydrophobic textiles, the traditional method of coating water-repellant processing agents for textiles has been put aside, and in its place, various methods


that implement nano-scale roughness for superhydrophobic configuration have been reported. The benefits and disadvantages of the various approaches for using superhydrophobic textiles for clothing were compared. Lowering fiber surface free energy is a simple and effective method for coating compounds with low surface tension. For methods that apply processing agents on textiles, there is dip-coating that coats processing agents on the entire fabric, spin/spray coating that sprays only on the surface, and knife coating and foam coating. Such coating methods can adjust thickness of a coating layer formed on the textile surface relatively easily with the advantage that it can be mass produced without being limited by the type of textile or coating substance. However, after coating, it can affect the physical/chemical properties of the textile, possibly creating a negative influence on clothing comfort. Coating agents can block the pores by forming an impermeable layer resulting in lower air permeability, vapor transmission, and moisture absorption of textiles. This would prevent sweat from being transmitted to the outside of clothes raising the humidity within the clothes to give an unpleasant feeling to the body. Also, when it is cold, vapors can condense inside. Meanwhile, wet-processes, such as a dipping method are being used for a wide array of purposes for the fiber and textile industries, where the coating material is fixed through dipping, drying, and heat-treating and the remaining process residues are removed. For the wet process, resource usage of water and electricity are high, and the need for development of and conversion to environmentally responsible processing methods has been rising. Dry-processes that transform fiber surfaces to low surface free energy include plasma or laser process techniques where a thin film in several nanometers coats the fabric by vaporizing or ionizing compounds that physically or chemically bond with the fiber surface. This process does not block pores or make the other face hydrophobic unlike dipping or spray coating. This is advantageous in maintaining the intrinsic properties of textiles. Processes of such methods have little effect on the intrinsic physical/chemical properties of textiles, and can have little impact on comfort properties of clothes. There is still room for research to achieve economic development by optimizing process parameters in engineered superhydrophobic textiles. Also, more study is needed to learn about the functional sustainability over repeated use, abrasion, and washings.

The textile is made up of yarns composed of several strands of micro-scale filaments that contribute to the micro-scale surface roughness. By adding nano-scale roughness on the textile surface, a micro/nano-scale binary structure is obtained where the fiber aids in creating the superhydrophobic surface. Thus, if nanoparticles are well attached chemically/physically to the fiber surface, a relatively stable micro/nano-scale surface roughness and superhydrophobic functionality will be maintained over repeated use and washing. However, nanoparticles have high van der Waals forces with high specific surface area, making it difficult to be uniformly dispersed in solvents because of aggregation. Therefore, to make the uniform deposits of nanoparticles and enhance the adhesion on a fabric surface, surface modification of nanoparticles as a pre-treatment is essential. In addition, there is a possibility that nanoparticles that dropp out of the fiber can pass through skin cell membranes, casting growing concerns in academic circles on biological effects. In 2013, a Safety Data Sheet (SDS) preparation for manufactured nanomaterials, ISO/TR 13329, was enacted related to the hazardous influence of nano substances. This shows that there is a need for prolonged research on the effects of nanoparticles on the environment and human body. The method of etching the fiber surface instead of adding particles on the fiber surface to form micro/nano-scale surface roughness can be regarded as a more environmentally-responsible process free from controversies over the hazards resulting from dropped out nanoparticles. However, in order to form nano-pillar type surface structures with large aspect ratios, vacuum plasma processing is found to be more effective, becoming a costly process for large-scale manufacturing. Follow-up research on process conditions that allow mass production would be required. Superhydrophobic textile research until now focused on fabrication methods for superhydrophobic functionality, but there was not enough review on the practicality of processing methods for the final use and commercialization of textiles. In future studies, efforts to select developed technologies appropriate to final use are needed. EVALUATION OF SUPERHYDROPHOBIC TEXTILES The evaluation of superhydrophobic textiles are summarized in Table II. Evaluation of superhydrophobicity is made up mainly of WCA, CAH, SA, and SHA, and there are differences in the measurement methodology and resulting values (droplet size, dropping distance, the number of


experiment repetitions, etc.). Evaluations on the wearing comfort that considers the application in clothing materials and the functional durability have only been conducted in a few studies. TABLE II. Evaluation item analysis of superhydrophobic textile research.

Title Journal Evaluation Microwave plasma induced grafting of oleic acid on cotton fabric surfaces

Applied Surface Science /2012

WCA

Preparation and characterizations of PTFE gradient nanostructure on silk fabric

World Scientific / 2007

WCA, CA hysteresis

Superhydrophobic behavior of plasma modified electrospun cellulose nanofiber-coated microfibers

Cellulose / 2012 WCA

Superamphiphilic Janus Fabric

Langmuir / 2012

WCA porosity&pore size distribution

Silicone nanofilaments and their application as superhydrophobic coating

Advanced Materials / 2006

water shedding angle (SHA) Tensile strength Color difference

A simple, one-step approach to durable and robust superhydrophobic textiles

Advanced Functional Materials/ 2008

WCA, Durability(textile friction analyzer[TFA])

Durable hydrophobic textile fabric finishing using silica nanoparticles and mixed silanes

Textile Research Journal / 2009

Durability (laundering, crocking tests)

Artificial lotus leaf structures from assembling carbon nanotubes and their applications in hydrophobic textiles

Journal of Materials Chemistry / 2007

WCA, water absorbability

Functionalization of cotton with carbon nanotubes

Journal of Materials Chemistry / 2008

WCA, Flammability UV-blocking property CNT Wash fastness

Use of atmospheric pressure plasma to confer durable water repellent functionality and antimicrobial functionality on cotton/polyester blend

Surface & Coatings Technology / 2011

WCA

Modification of Low Energy Polymer Surfaces by Immobilization of Fluorinated Carboxylates with Zirconium-Based Coupling Agents

Journal of Applied Polymer Science / 2004

WCA, surface energy, drop penetration time oil repellency test [AATCC 118]

Plasma Treatment of Thermo active Membrane Textiles for Superhydrophobicity

Materials Science / 2012

WCA

Atmospheric pressure plasma polymerization of HMDSO for imparting water repellency CTTN fabric

Textile Research Journal / 2011

Water repellency [AATCC spray test 22-2005], WCA, Air permeability[IS 11056-2006], water vapor transmission[ASTM-E 96(2005)],

Improvement of hydrophobic properties of silk and cotton by hexafluoropropene plasma treatment

Applied Surface Science / 2007

WCA, wet-out time[AATCC] water vapour permeability tensile strength[HKS-5]

Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification


WCA

Superhydrophobic cotton fabrics prepared by sol-gel coating of TiO2 and surface hydrophobization

Science of Technology and Advanced Material/ 2008

WCA, UV-Shielding properties

Superhydrophobic surfaces on cotton textiles by complex coating of silica nanoparticles and hydrophobization

Thin Solid Films / 2009

WCA, thermo gravimetric analysis

Mimic nature, beyond nature: facile synthesis of durable superhydrophobic textiles using organosilanes

Royal Society of Chemistry / 2013

Water shedding angle(SHA), Abstraction test laundering test, oil/water separation

Superhydrophobic cotton fabric fabricated by electrostatic assembly of silica nanoparticles and its remarkable buoyancy


WCA CA hysteresis, washing durability

WCA, which is one of the most representative evaluation methods for measuring hydrophobicity [29], is a simple measurement technique, but has limitations in that there can be experimental errors due to the effects of gravity caused by the size of the liquid and irregular baseline of the textile surface. When the measured lighting, contrast, lens focus, and contact base-line of water and surface were changed in the studies of Zimmermann et al., [51], WCA of the same surface was shown to vary by more than 10°. Furthermore, the instantaneous energy produced when a water drop contacts a textile surfaces depends on the load amount and load height, and affects WCA; a smaller drop tends to generate higher


WCA. However, there are as yet no testing specifications on WCA measurements that can minimize such testing discrepancies. Also, in the case of superhydrophobic surfaces with high WCA values, it is difficult to discriminate the level of hydrophobicity. Error can also occur on the angle for slanting the plate for CAH, SA, and SHA, and it is difficult to not only obtain clear images for the roll-off moment, but also to accurately set the base-line due to the surface features unique to fabrics. As a method to more easily measure CAH, captive drop methods are used where automated equipment is needed to adjust the fluctuation of the water with high-resolution camera settings that can film over 20 frames per second. For consistent measurement, concrete evaluation standards that define appropriate load height, load amount, and measurement processes for evaluating the wetting characteristics and the level of hydrophobicity of textiles are needed. In order to resolve such issues, the shedding angle (SHA) measurement method was drawn up as a more convenient measurement to differentiate the level of hydrophobicity for superhydrophobic surfaces. This measures the 2 cm-rolling angle of water drops when 12.5㎕ of water is dropped at 1 cm on a specimen (Figure 14). SHA was used in many superhydrophobic fabric studies as an efficient evaluation method for superhydrophobic textiles [89].

FIGURE 15. (a) Measurement errors that can occur when measuring WCA [89]. (b) Shedding angle (SHA) measurement method [89]. WCA and CAH measurement is an evaluation method focusing on superhydrophobicity of surfaces while excluding the effects of external factors such as

the quantity of water or falling energy. However, in order for the developed superhydrophobic textiles to have applications for clothing materials recognized, it is necessary to examine water-repellency in real life. Standard evaluation methods are often used to determine the level of water-repellency; such standards are the spray test (AATCC22-2005 Water Repellency: Spray Test, ISO4920, KS K 0590), rain test (AATCC35-2006 water resistance: rain test) and drop penetration test (AATCC42-2007 water resistance: impact penetration test) [90]. As superhydrophobic textiles are a relatively new pioneered research sector, there are limitations to determining the characteristics of superhydro-phobicity, and further study is needed on developing measurement methods for superhydrophobic functionality, self-cleaning performance, and oil repellency performance for textile materials.

NEW RESEARCH FIELD FOR SUPERHYDROPHOBIC TEXTILE RESEARCH Recent studies on superhydrophobic textiles are making multi-faceted evaluations considering use as clothing materials focusing on the aforementioned limitations and problems, and there are attempts being made to develop multi-functional superhydrophobic textiles that go beyond just configuring superhydrophobicity. There are studies that have examined the effects of superhydrophobic processing methods on the comfort properties required for clothing textiles such as air permeability, vapor transmission, and moisture absorption. When water repellent agents were foam-coated on one face of a cotton fabric [91], the treated fabric maintained softness and sweat absorption property at adequate levels, which are advantages for hydrophilic cotton fabrics. The resulting fabric exhibited asymmetric wettability with one surface being superhydrophobic and the other side being hydrophilic. Compared to the non-treated fabric in vapor transmission, the fabric that received double side water-repellent treatment had 87% of the water vapor transmission rate of the untreated fabric, while fabrics that received a one-sided water-repellent treatment showed a 94-99% water vapor transmission rate compared to the untreated sample. When the superhydrophobic side came into contact with the ambient environment, it showed a 94% water vapor transmission rate compared to non-treated fabrics, and when facing the water, it showed a 99% water vapor transmission rate. This was thought to be due to the difference of hydrophilic/hydrophobic layers affecting the transmission rate and amount of vapor. In other words, when vapors transmit into the hydrophilic layer, the vapor moves from the


hydrophilic layer to the hydrophobic layer to be pushed outside, and this behavior is opposite to the regular movement tendencies of water and moisture. Meanwhile, when vapors permeate into the hydrophobic layer first, there is little absorption occurring and vapor passes through the hydrophilic layer and is absorbed in the upper hydrophilic layer. When this hydrophilic layer comes into contact with outside air, the moisture is easily evaporated. This study also confirmed laundry durability and validated the effectiveness of the single-sided foam coating method through multi-faceted evaluation for development of superhydrophobic textiles that displayed asymmetrical wettability [91]. A plasma treatment method was also evaluated for maintaining comfort properties for air, vapor, and moisture, configuring superhydrophobicity without changing the original characteristics of the fabric too much [55, 64, 74]. Despite several studies that evaluated such comfort properties, there are not enough studies for the influence of superhydrophobic treatment on moisture and heat transfer mechanisms. In addition, configuration of multi-functional textile materials is being made by adding other functions to superhydrophobicity in textiles. Examples include use of silver with antimicrobial effects [92, 93] or graphene with electric conductivity and applying it on fabrics in a nanoparticle form to configure fabrics with not only superhydrophobicity, but also other functionalities including antimicrobial [92, 93], electrical conductivity [93], or UV protection [92]. CONCLUSION Studies on superhydrophobic textiles experienced rapid growth in a short period of time due to high industrial and academic value. Also, superhydrophobic textiles made with some commercialized technologies are being sold in the market. However, in the course of applying the development and evaluations of related studies focusing on superhydrophobic surface development, there are not enough discussions on problems that limit the use of superhydrophobic clothing materials such as bio-suitability, clothing comfort, and functional durability. In particular, basic safety verification on nano-materials used for the introduction of water-repellant processing materials and surface roughening have not yet been made. Therefore, sufficient verification on the use of clothing materials as processing materials must be made. In addition, there are problems that basic performance for clothing such as durability of functions and convenience of management has reduced quality compared to demands after

superhydrophobic processing. Thus, there are limitations in terms of functions, processing, and economic aspects as clothing materials for use in actual life. Furthermore, superhydrophobic evaluation methods do not consider the unique characteristics of textiles and are conducted focusing on WCA measurements. Therefore, it is necessary for future studies to deal with the effectiveness of superhydrophobic processing methods that can satisfy not only the basic functional performance, but also the comfort properties such as moisture management, vapor transmission and air permeability, bio-toxicity, safety, and functional durability over repeated use and wash for use as clothing materials. Furthermore, it is necessary to develop an evaluation method that enables the differentiation of the level of superhydrophobicity other than WCA, studying the effect of liquid drop size and dropping height. Development of evaluation methods for the self-cleaning effects of superhydrophobic fabrics would also be another area of study, considering the properties of contaminants. There are efforts being made in various sectors for sustainable development amidst the recent dangers of environmental destruction. Superhydrophobic textiles are thought to contribute to sustainable textile development by reducing the use of water and energy, and by possibly extending use life resulting from extended functional durability with a reduced number of washings. Therefore, positive ripple effects on the environment and society can be expected when superhydrophobic textiles are commercialized. Accordingly, if the limitations are overcome for commercialization, multi-functional superhydro-phobic textiles may be able to further create sustainable economic value. ACKNOWLEDGEMENT This research was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (2011-0014765) and Korea Textile Trade Association. REFERENCE [1] Barthlott, W. and Neinhuis, C.; Purity of the

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AUTHORS’ ADDRESSES Sohyun Park Jooyoun Kim, PhD Chung Hee Park, PhD Seoul National University 222 Dong, 323 Ho Seoul 151-742 KOREA

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