Cotton Fabrics with Single-Faced Superhydrophobicity - Langmuir

Nov 27, 2012 - This article reports on the fabrication of cotton fabrics with single-faced superhydrophobicity using a simple foam finishing process. ...
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Cotton Fabrics with Single-Faced Superhydrophobicity Yuyang Liu,† J. H. Xin,‡ and Chang-Hwan Choi*,† †

Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong SAR

‡

S Supporting Information *

ABSTRACT: This article reports on the fabrication of cotton fabrics with singlefaced superhydrophobicity using a simple foam finishing process. Unlike most commonly reported superhydrophobic fabrics, the fabrics developed in this study exhibit asymmetric wettability on their two faces: one face showing superhydrophobic behavior (highly nonwetting or water-repellent characteristics) and the other face retaining the inherent hydrophilic nature of cotton. The superhydrophobic face exhibits a low contact angle hysteresis of θa/θr = 151°/144° (θa, advancing contact angle; θr, receding contact angle), which enables water drops to roll off the surface easily so as to endow the surface with well-known self-cleaning properties. The untreated hydrophilic face preserves its water-absorbing capability, resulting in 44% of the water-absorbing capacity compared to that of the original cotton samples with both sides untreated (hydrophilic). The single-faced superhydrophobic fabrics also retain moisture transmissibility that is as good as that of the original untreated cotton fabrics. They also show robust washing fastness with the chemical cross-linking process of hydrophobic fluoropolymer to fabric fibers. Fabric materials with such asymmetric or gradient wettability will be of great use in many applications such as unidirectional liquid transporting, moisture management, microfluidic systems, desalination of seawater, flow management in fuel cells, and water/oil separation.

1. INTRODUCTION Cotton is one of the most favorable fabric materials for many of our clothes and textile products because it is soft, comfortable, breathable, causes little skin irritation, and biodegradable.1 In addition, cotton has porous structures with superior water and moisture absorption ability that adapt themselves to a variety of uses such as absorbent material in medical and healthcare applications, surgical cotton, fascias, hemostatic cotton, diapers, and hospital sheets. However, the super-water-absorbing nature of cotton fabrics also allows them to be easily stained and dirtied; they can be wetted or contaminated by liquids such as water, beverages, blood, and even bacteria and germs, which is undesired in their use as clothes, particularly in clinic and hospital textiles. Thus, in recent years, great efforts have been made in the modification of cotton, such as making it superhydrophobic with self-cleaning and stain-repellent properties.2 Nowadays, imparting superhydrophobicity to fabrics is a well-established technology. However, in most commercial products or laboratory-fabricated superhydrophobic textile samples, the whole fabric matrix is typically hydrophobized, which seriously compromises the functionality of cotton as a comfortable, soft, sweat-absorbing textile material. Fabrics with asymmetric surface wettability (e.g., one surface with superhydrophobic wettability and the other with its original hydrophilic property) are of great interest in textile engineering, as the superhydrophobic surface can repel water, oil, and even bacteria and germs, providing self-cleaning protection, whereas the hydrophilic surface can preserve the comfortable, breath© XXXX American Chemical Society

able, soft, low-skin-irritating nature of cotton fabrics. In addition, fibrous materials with asymmetric or gradient wettability can allow the directional transportation of liquids within the fibrous matrix, which also has great potential in biological, medical, and healthcare applications. Therefore, the development of methods that allow us to producing fabrics with single-faced superhydrophobicity has been of great interest in recent years.3 In this article, we demonstrate a simple, scalable process that enables us to prepare cotton fabrics with asymmetric and gradient wettability. We also characterize the surface morphology, chemical composition, wettability (for both droplets and underwater), water absorbability, moisture transmissibility, and laundry fastness of single-faced superhydrophobic cotton fabrics for future potential applications.

2. FABRICATION SCHEME Superhydrophobic surfaces have drawn a lot of interest in both academia and industry because of their great potential for selfcleaning coatings, 4 low-friction surfaces,5−7 microfluidic devices,8 anti-icing coatings,9 condensation,10 and templates for self-assembly processes.11,12 One of the promising applications of such superhydrophobic surfaces in textiles is for water- and stain-repellent self-cleaning fabrics.2 SuperReceived: February 8, 2012 Revised: November 21, 2012

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Figure 1. Schematic of the fabrication of single-faced superhydrophobicity on cotton fabric through the foam finishing process. (i) Preparation of a fluoropolymer emulsion foam composite and transfer of the foam composite onto a cotton fabric surface. (ii) Spreading the fluoropolymer emulsion foam composite on a fabric surface with a blade-coater and pressing the foam composite to enter the cavities between fibers and create the gradient of concentration (as indicated by the color changing from red to gray). (iii) The foam collapses and leaves a fluoropolymer coating gradient on the surfaces of the cotton fibers (as indicated by the color changing from red to gray). (iv) Drying and curing the fluoropolymer layer to form a hydrophobic coating on the surfaces of cotton fibers.

surface roughness of textile fibers. However, the major drawback of the nanoparticle-built roughness is the poor bonding between nanoparticles and fibers. The nanoparticles are prone to detach from the fibers and have potential risks of causing serious health problems.25,26 To improve the bonding between nanoparticles and textile fibers, many methods were explored. For example, either the nanoparticles or the textile fibers were modified with polymers or reactive groups to fix the nanoparticles firmly on textile fibers.27 Some recent new achievements allow us to grow or graft nanostructures directly on the surfaces of textile fibers in order to obtain durable superhydrophobicity on textiles.28,29 However, it is still challenging to obtain single-face-hydrophobized fabrics by using such a simple coating method with hydrophobic chemicals. The development of an efficient method to deposit the hydrophobic chemical coatings precisely on one surface of the fabric but leave the other surface unaffected is necessary. Fabrics with asymmetric or gradient wettability can be achieved via either the physical configuration30 or chemical finishing process.3 The physical configuration usually involves the technique of weaving the fabric in the form of multilayered structures with fibers that have different wettabilities (e.g., the outer layer of the fabric is made of hydrophobic fibers such as polyester, and the inner layer of the fabric is woven with hydrophilic fibers such as cotton30). The fabrication of multilayer structured fabrics, however, requires the use of special fiber yarns and complicated weaving procedures. Another way to prepare fabrics with single-sided hydrophobic properties is to use digital inkjet deposition. For example, Ali et al. reported the fabrication of single-sided hydrophobic fabrics by depositing onto fabric substrates the formulated inks incorporating fluorocarbon chemistry through inkjet printing.31 The inkjet printing process is efficient at selectively depositing hydrophobic materials onto a single side of the fabric. However, it is a slow serial process, so it may not be practical to prepare the single-sided hydrophobic fabrics over a large area with high throughput. Recently, fabrics with asymmetric wettability have been reported using one-step chemical finishing processes.32,33

hydrophobic surfaces with very high contact angles (e.g., greater than 150°) and low contact angle hysteresis (typically less than 15°) can cause water and even oil drops to roll off the surface, carry away the dirt with them, and leave little or no residue on the surface, begetting self-cleaning surfaces.13 To date, many methods have been developed to prepare superhydrophobic coatings/surfaces on fabric substrates with cheaper and simpler means to achieve high water contact angles and low contact angle hysteresis.13 In addition to cheaper and easier fabrication procedures, the robustness and durability of the coatings/structures and the safety of the hydrophobic treatment are the central concerns associated with superhydrophobic fabric materials. Superhydrophobic surfaces can easily be achieved on fabric substrates because the textile fiber itself and the woven textures afford multiscale microstructures on the fabric substrate.2,13 The diameters of most textile fibers are around 10−25 μm, and the textures of woven fabrics can substantially offer additional multiscale roughness. With only a hydrophobic finish, the fabrics can entrap air on the surfaces and make them superhydrophobic to possess the desirable wetting properties of high contact angles and low contact hysteresis.2,13−15 Modifying textiles with hydrophobic chemicals to make the surfaces of textile fibers superhydrophobic and leave the fabric permeable to vapor and air is a well-established technology developed in the early 1940s.13−15 For example, a patent published in 1945 disclosed a method for the hydrophobization of paper and fabric substrates with hydrophobic silanes.14 Following this method, Gao and McCarthy15 successfully fabricated artificial lotus leaf effects on both regular fiber-woven polyester fabrics and microfiber-woven polyester fabrics. To promote the superhydrophobic effects on fabrics, a variety of chemical and physical procedures have also been developed, especially to enhance the surface roughness of textile fibers. The deposition of nanoparticles of various materials, such as carbon nanotubes,16,17 gold,18 At/Pt aggregates,19,20 silica,21,22 ZnO nanorods,23 and copper structures24 was successfully demonstrated on textile fibers, showing the improvement of B

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being superhydrophobic and the other surface remaining hydrophilic. The key principle in the foam finishing process is the use of air as a “solvent” to dilute the high-concentration aqueous solution under high-speed shearing. Under high-speed shearing, air bubbles are trapped in the aqueous solution. With the aid of a foam-forming agent and foam stabilizer, the air−aqueous composite foam can remain stable for a short period of time (e.g., several minutes to hours), allowing the transfer and spreading of the foam composite on a fabric surface. Because of the introduction of air into the system, the contents of water and the finishing chemical are significantly reduced. For example, in a traditional wet padding finishing process, the liquor content is around 60−80%. However, the liquor content for a foam finishing process is around 20−40%, which is 2 to 3 times less than that of the wet padding finishing process.36 In addition to the reduction of the liquid content, the foam finishing process also ensures a uniform distribution of chemicals. Another advantage of foam finishing is that it enables us to use the air content to control the viscosity of the foam composite and therefore control the permeability of the chemical through the fabric matrix in a vertical direction.36

In the methods, TiO2 nanoparticle-based coatings were used to alter the surface wettability of fabrics under UV irradiation. TiO2 nanomaterials have been known to show a transition between hydrophobic and hydrophilic states triggered by UV irradiation.32,33 When sun or UV light illuminates the open surface of the TiO2 nanoparticle-coated cotton fibers, the face of the fabric exhibits a superhydrophilic property and the shaded back remains hydrophobic.32,33 Such a novel wetting property makes use of TiO2 nanoparticle-based coatings on cotton fibers that are attractive for tuning the gradient of the surface wettability of the fabrics. However, they may still cause serious health problems because of their imitate contact with human skin and their photocatalytic activity. Under UV or sunlight irradiation, TiO2 nanocrystals generate active free radicals that can degrade the textile polymers and cause adverse effects to skin. Another concern is that in order to tune the surface wettability of the TiO2 nanoparticle-based coatings it usually takes several days to keep the surfaces in the dark and recover their superhydrophobicity,32,33 which is not practical in real applications. In comparison to TiO2 nanoparticle-based surfaces, the treatment of textiles with fluoropolymers can offer a durable superhydrophobic effect with better wearing comfort and washing durability.34,35 If fluoropolymers can be precisely delivered and grafted onto only one surface of cotton fabrics through a conventional finishing process such as a wet chemical padding process, then this would provide a simple and scalable way to produce cotton fabrics with single-faced superhydrophobicity. However, because of the porous nature and super-water-absorbing capability of cotton fabrics, it is very difficult to provide them with direct asymmetric wettability using such a traditional wet chemical padding process. Alternatively, a foam finishing process is regarded as a more suitable process for treating porous substrates. It uses foamed chemicals with very low wet pickups, which have been employed to fabricate single-faced functions on textile and porous substrates.36 Thus, in this study, the foam finishing process has been used to develop single-faced superhydrophobic cotton fabrics. Figure 1 shows the schematic of the foam finishing process. First, fluoropolymer composite foam is prepared and transferred to a cotton fabric (i). Then, the fluoropolymer composite foam is applied to the fabric surface with a blade-coater (ii). The bladecoater can uniformly spread the fluoropolymer composite on the surface and help the fluoropolymer composite to enter the cavities between fibers. Because of its high viscosity (1800− 2200 mPa·s), the fluoropolymer composite foam cannot penetrate the matrix of the fabric deeply but forms a gradient concentration within the fabric matrix. The viscosity of the fluoropolymer composite foam is critical to controlling the penetration of the hydrophobic composite into the fabric matrix. With the proper viscosity, the hydrophobic foam composite can fully wet the top surface of the cotton fabric but partially wet the inner cotton fibers of the fabric matrix and barely the bottom fibers (as indicated by the gradient of the color used in Figure 1(ii)). Then, the foam composites break quickly and leave wet coating layers on the cotton fiber surface (iii). After the drying and curing process (iv), cotton fibers with gradient hydrophobicity are obtained within the cotton fabric, as indicated by the gradient of color used in Figure 1(iv) (from hydrophobic to hydrophilic). Because the bottom surface of the cotton fabric is uncoated, it preserves the original hydrophilicity of the cotton fabric. As a result, the resultant cotton fabric possesses asymmetric wettability on its two faces, with one face

3. EXPERIMENTAL SECTION 3.1. Foam Finishing Process. The cotton fabric used in this study is a white cotton fabric (15 tex, knit with a weight density of 203 g/ m2) commonly used for garment manufacturing. The fluoropolymer emulsion was purchased from AGC Chemicals America, Inc. (Exton, PA) under the trademark AG-710 (nonionic, 30% solid content by weight). To prepare for fluoropolymer emulsion foams with controlled viscosity, an associated thickener was custom made.37−39 First, equimolar hexadecanol and 2,4-diisocyanatotoluene were mixed at 40 °C and stirred for 30−60 min. Second, poly(ethylene glycol) with an average molecular weight of around 10 000 was added to the mixture obtained in the first step. The mixture was then heated to 120 °C and stirring for 5 h at 120 °C. The resultant product was dissolved in an acetone/water solution with a concentration of 10% (by weight) for further use.37 Then, the composite foam was prepared by mixing 10% of the fluoropolymer emulsion, 1% of the sodium carboxymethyl cellulose (CMC, average Mw 90K, Sigma) thickener, and 1−5% of the associated thickener under high-speed shearing. To make effective single-faced superhydrophobic cotton fabrics, the mixture was sheared at a speed of 6000 rpm for 20 min to achieve a viscosity of around 1800−2200 mPa·s. The foam composite was then transferred to the cotton fabric and spread on the fabric surface with a blade coater. The cotton fabric was dried at 100 °C in an oven for 2 min and then cured at 150 °C in an oven for 1 min. The treated cotton fabric was rinsed with hot water (60 °C) to remove thickener residues. The fabric was further dried at 120 °C in an oven for 2 min. 3.2. Measurement of Surface Morphology and Chemical Composition. Microstructures of the textile surfaces and the chemical composition were investigated by using scanning electron microscopy (SEM, Zeiss 982 FE-SEM) and energy-dispersive X-ray spectroscopy (EDS, Oxford ISIS), respectively. 3.3. Measurement of Contact Angles. Contact angles were measured with a contact angle measurement system (DSA10-MK2, Krüss GmbH, Germany) at room temperature (23 °C). Deionized water with a droplet volume of 10 μL was used. Average contact angle values were obtained by measuring three different positions on the same sample. Contact angle hysteresis was measured using a volume expansion/reduction method. 3.4. Measurement of Water-Absorption Ability. The water absorption abilities of untreated cotton, single-face-hydrophobized cotton, and fully hydrophobized cotton fabric were measured. Each fabric sample (10 g) was dipped into pure water and kept in water for 10 min to allow water absorption. Then the fabric sample was taken out of the water and hung on a clotheshorse for 10 min for the C

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superfluous water to flow away from the fabrics. Then, the mass of fabrics with absorbed water was measured. The water absorption ability was calculated according to the following equation

w% =

wa − w0 × 100% w0

(1)

where w% is the water absorption ability of the fabrics, wa is the mass of the fabric with absorbed water, and w0 is the mass of the dried fabric samples (10 g). 3.5. Measurement of Vapor Transmission. The vapor transmission rate of the prepared fabric samples was measured using an ASTM E-96 (open cup test) method. In this test, a fabric sample is placed tightly over a cup of water. The rate of water vapor that passes through the fabric is determined by measuring the mass loss of water in the cup over 24 h. The vapor transmission rate of the single-facetreated fabric was measured in two ways. In the first way, the fabric sample was placed tightly over a cup of water with the hydrophilic side facing the water. The second way is to place the hydrophobic side so as to face the water. Untreated cotton fabric and fully treated superhydrophobic cotton fabric were used as control samples to compare with the single-face-treated fabric. 3.6. Measurement of Laundering Durability. The washing fastness was evaluated following AATCC test method 61-1996, test no. 2A using an AATCC Standard Instrumental Atlas Launder-Ometer LEF. One wash in Atlas Launder-Ometer LEF is equivalent to five home machine launderings according to the AATCC test method. The fastness of the fluorocarbon polymer coating on the superhydrophobic face of the cotton was determined by measuring the contact angle reductions after 0, 5, 10, 20, and 30 repeated wash cycles. To improve the washing fastness of the fluorocarbon polymer coating, a crosslinking agent (NBP-75, a commercially available product from Meisei Chemical Works, Japan, and MgCl2 as a catalyst) was used in the foam composite to bond the hydrophobic polymer to cotton fibers chemically.

Figure 2. Effects of foam viscosity on (a) the wet pickup and (b) the wettability of the single-face-hydrophobized cotton fabric sample.

penetrate the fabric matrix in a vertical direction and fully wet the fabrics. If the viscosity of the fluoropolymer composite foam was higher than 2200 mPa·s, then the fluoropolymer composite tended to wet only part of the cotton fabric surface and could not offer a stable superhydrophobic coating on the surface. 4.2. Surface Morphology and Chemical Composition. Figure 3 shows the SEM images and the EDS spectrum of the hydrophilic and hydrophobic sides of the single-face-hydrophobized cotton fiber (made at a viscosity of 2100 mPa·s), respectively. From the SEM images, no significant morphological difference was observed between the hydrophilic (Figure 3a) and hydrophobic (Figure 3b) cotton fibers. However, the EDX spectrum indicates the difference in the chemical composition of the surfaces. Compared to the hydrophilic surface (Figure 3b), the hydrophobized surface (Figure 3b) shows a pronounced peak for fluorine (F), which is attributed to the fluorocarbon polymer coating on the superhydrophobic surface. The EDX analysis proves that the foam finishing process with well-regulated viscosity is an effective way to deliver and deposit hydrophobic chemicals precisely on confined cotton fiber surfaces with no significant change in the structural morphology of the fibers. 4.3. Characterizations of Single-Faced Superhydrophobicity. The surface wettability of the single-faced superhydrophobic cotton fabric was first evaluated by observing the spreading behaviors of water droplets on the treated and untreated cotton fabric surfaces, respectively (Figure 4). Figure 4a shows water droplets (0.1 mL for each drop stained with blue ink for better visualization) placed on both faces of the cotton fabric. On the hydrophobized surface, a water droplet does not spread out but forms a drying bead with a high contact angle (∼150°). However, on the untreated original hydrophilic surface, water wets and spreads quickly on the fabric surface.

4. RESULTS AND DISCUSSION 4.1. Influence of Foam Viscosity on Wet Pickup and Surface Wettability of Cotton. To prepare cotton fabric with asymmetric or gradient wettability, the key is to make the hydrophobic chemicals fully coat the top layer of the fabric, partially coat the middle layer, and hardly coat the bottom layer. As addressed earlier, one of the most important advantages of a foam finishing process over a conventional wet chemical padding finish is that it allows control of the permeability of the chemical through the fabric matrix in the vertical direction by adjusting the viscosity of the foam composite. To determine the optimum viscosity for singleface treatment, fluoropolymer composite foams with different viscosities were prepared and then applied to only a single side of the cotton fabric surfaces under constant coater pressure. The wet pickups of samples were recorded, and then the samples were dried and curried. The wetting behaviors on the coated and uncoated surfaces were evaluated on the basis of contact angle measurements. Figure 2a shows the effect of viscosity on the wet pickup of the single-face-treated samples. For a single-face treatment, low wet pickup (typically lower than 30%) is desirable. The result shows that the fluoropolymer composite foams with a viscosity of around 2100 mPa·s can offer a wet-pickup rate lower than 30%. Figure 2b further shows the influence of foam viscosity on the wettability of the coated and uncoated surfaces. It reveals that fluoropolymer composite foams with viscosity ranging from 1800 to 2200 mPa·s can provide the most pronounced wetting gradient (i.e., the treated surface being hydrophobic but the untreated side being hydrophilic). When the viscosity of the fluoropolymer composite foam was lower than 1800 mPa·s, it tended to D

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Figure 3. SEM images and EDX spectrum of the (a, b) hydrophilic and (c, d) hydrophobic sides of the single-faced superhydrophobic cotton fabrics.

Figure 4. Optical pictures demonstrating the asymmetric wettability of the cotton fabrics. (a) Water droplets (stained with blue ink for clear indication) sitting on a superhydrophobic face form “dry beads” on the fabric surface, and water droplets placed on a hydrophilic surface are completely wet and leave water marks. (b) Top: Water (ink) stains form on a hydrophilic surface. Bottom: After flipping the hydrophilic side, we placed a water droplet on a hydrophobized back side. There is no ink stain seen on the superhydrophobic side, indicating that water cannot pass through the fabric from the hydrophilic side to the superhydrophobic back side. It shows that the superhydrophobic face retains its water repellency even though the opposite hydrophilic side is wetted with water.

Then, it is fully absorbed by the fabric, leaving a large ink spot on the fabric. In Figure 4b, a water droplet was placed on the hydrophilic side first, and then the fabric was cut with scissors and flipped so that the hydrophobized back side could be exposed. A water droplet was also placed on the back side to compare the wettability of the same location of the two opposite sides directly. It shows that the water droplet wets and spreads out easily on the hydrophilic side whereas it does not on the superhydrophobic side. Furthermore, there is no ink stain observed on the hydrophobic face, indicating that water absorbed on the hydrophilic side cannot pass through the fabric to the hydrophobic face. This result directly demonstrates that the cotton fabrics developed by the foam finishing process exhibit heterogeneous wetting properties on its two surfaces: one surface showing superhydrophobic behavior and the other

surface showing hydrophilic properties. Even when the hydrophobic back side is fully covered with droplets, the hydrophobic face of the cotton fabric remains superhydrophobic. (A video is available in the Supporting Information.) The surface wettability of the treated cotton fabrics was further studied by measuring the contact angle, contact angle hysteresis, and roll-off angle of a sessile droplet of water on their surfaces. Figure 5a presents the profile of a water droplet (droplet size, 10 μL) sitting on the superhydrophobic face of cotton fabric. The apparent contact angle of the water droplet on the hydrophobized face is ∼150°. The measured advancing contact angle (θa) and receding contact angle (θr) on the hydrophobized surface were 151 ± 2 and 144 ± 3°, respectively, resulting in a contact angle hysteresis (θa − θr) of 7°. Figure 5b shows the tilting test for the measurement of a E

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Figure 6. Change in the apparent contact angle of a water droplet on the superhydrophobic face over time (measured under 95% RH). It shows no significant change in the contact angle and droplet volume, suggesting the stable superhydrophobicity of the hydrophobized face of the treated cotton fabric.

action takes place through the fabric so that the water droplet cannot propagate from the hydrophobic face to the hydrophilic. The results presented in Figures 3−6 clearly show that the foam finishing process proposed in Figure 1 is effective at creating asymmetric wettability on the two surfaces of fabric substrates. They also confirm the results of the EDS spectrum in which the cotton fibers on the superhydrophobic side have been successfully coated with the hydrophobic fluoropolymer whereas the other side has not been treated and maintains its hydrophilicity. 4.4. Water Absorption Ability. The asymmetric wettability of the treated cotton fabrics was also tested under water. Figure 7a shows the single-face-hydrophobized cotton fabric

Figure 5. Profiles of water droplets on (a, b) a superhydrophobic face and (c, d) a hydrophilic face of the treated cotton fabric. (a) Measurement of an apparent contact angle (∼150°) on a leveled superhydrophobic face. (b) Measurement of an inclination angle (∼12°) for roll off. (c) Measurement of an apparent contact angle (∼30°) on a leveled hydrophilic face. (d) The water droplet placed on the hydrophilic face was adsorbed into the fabrics in a few seconds, showing no profile on the surface.

roll-off angle. Given a tilting angle of ∼12°, the water droplet started to roll off the fabric surface. In comparison, Figure 5c,d presents the profile of water droplets on a hydrophilic face. When first placed on the hydrophilic face, a water droplet initially had a contact angle of ∼30° (Figure 5c). After a few seconds, the water droplet spread out and was totally absorbed into the fabric (Figure 5d). This demonstrates that the singleface-hydrophobized cotton fabric can still act as a waterabsorbing material because of the hydrophilic face on the opposite side. Fabric with such asymmetric wettability on its two faces is of great significance in producing clothes with both sweat-absorbing and water-repellent properties. In previous reports,32,33 similar asymmetric wettability was demonstrated using a one-step chemical finishing process based on TiO2 nanoparticle-based coatings. However, the superhydrophobic wettability on the hydrophobized face was not stable in the studies, so the water droplets tended to move through the fabric from the hydrophobic face to the hydrophilic side automatically as a result of capillary effects.40−42 Capillary action is the primary force that drives the water moving along and through the fabric matrix. Within a fabric with homogeneous fiber wettability, the capillary action is dependent on the surface tension of water and the interfacial force between textile fibers and water. When the adhesion force between the fiber surface and water is greater than the surface tension of water, capillary motion occurs. The fabrics developed in this study, however, show stable water-repellent properties on the hydrophobic face. Figure 6 shows the profile of a water droplet (10 μL) sitting on the hydrophobic surface of the treated cotton. The water contact angle remained constant (∼150°), and the size of the droplet did not decrease significantly for nearly 20 min. This indicates that no capillary

Figure 7. (a) Optical image of the single-face-hydrophobized cotton fabric immersed in water. (b) Water absorption ability of (i) untreated cotton fabric, (ii) single-face-hydrophobized cotton fabric, and (iii) fully hydrophobized (both faces) cotton fabric.

immersed in water. The hydrophilic side shows the capillary rise, causing the water to “climb” up along the hydrophilic fabric surface because of the high surface energy. In contrast, on the superhydrophobic side, the water descends along the fabric surface.43 An air bubble layer is clearly seen to be attached to the superhydrophobic side of the fabric immersed in water. When taking the fabric out of the water, we found that the F

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hydrophobized cotton fabric. Then, column d represents the vapor transmission rate of the single-face-hydrophobized sample, where the hydrophobic side faces down to liquid water. In that case, the measured vapor transmission rate was 1963 ± 23 g/m2/d, which is ∼99% of that of the untreated control sample. Compared to case c, where the hydrophobic side faces up to open air, the vapor transmission rate in case d, where the hydrophobic side faces down to liquid water, was slightly greater (∼5%). It is speculated that such a difference is attributed to the different transport rates and tendencies of the vapor through the hydrophilic and hydrophobic layers of the cotton fabrics, respectively. When a hydrophilic side faces liquid water, the water vapor is relatively easy to absorb into the porous hydrophilic fibers as moisture. Then, the moisture has to be delivered from the hydrophilic side to the hydrophobic side in order for the absorbed moisture to evaporate out into the air. However, it is thermodynamically unfavorable because of the adverse capillary effects from the hydrophilic surface to the hydrophobic surface. In comparison, when the hydrophobic side faces down to liquid water, the vapor cannot be efficiently absorbed as moisture by the hydrophobic cotton fibers. However, it can still diffuse through the hydrophobic layer as a vapor phase and then be absorbed as moisture by the porous hydrophilic fibers on the top layer that is open to the atmosphere to facilitate evaporation. Moisture transmission through fabrics is critical to controlling the thermophysiological comfort of the human body, which is realized by perspiring in both vapor and liquid forms.40,41 The current result demonstrates that the treatment of cotton fabrics with a fluoropolymer hydrophobic coating via a foam finishing process does not make a significant difference in the vapor transmission rate of the cotton fabrics, especially for the single-face-hydrophobized fabric. Despite a slight difference, the result also shows that the facing direction of the hydrophobized side does not cause a significant difference in vapor transmission. Then, along with the good moisture transmissibility, single-side-hydrophobized cotton fabrics can provide good wearing comfort because the hydrophilic side is in contact with the human body and the superhydrophobic side faces the atmosphere for effective self-cleaning protection. 4.6. Laundering Durability. The washing fastness of the fluorocarbon polymer coating on the cotton fibers was determined by measuring the regression of a water droplet contact angle after 0, 5, 10, 20, and 30 repeated wash cycles (Figure 9). The result shows that the apparent contact angle on the hydrophobic face does not change much until five wash cycles have been completed. However, after five cycles, it gradually decreases with an increasing number of laundering cycles. After 10 washing cycles, the contact angle decreased from ∼153 to ∼144° and the water droplet rolled off the fabric surface at a tilting angle of 30°. After 20 wash cycles, the contact angle decreased to ∼132° and the droplet became pinned and sticky on the fabric surface. To improve the washing fastness, a cross-linking agent (NBP-75, Meisei Chemical Works, Japan) and MgCl2 as a catalyst were used in the foam composite. The cross-linking agent and catalyst help to bond the fluorocarbon hydrophobic polymer to cotton fibers chemically. NBP-75 is an isocyanatebased (−NCO) cross-linking agent that is commonly used to improve the water washing fastness of fluoropolymer coatings on fabrics. On the basis of the information provided by the manufacturer (Meisei Chemical Works, Japan), NBP-75 is a hydrophobic-group-blocked isocyanate.44,45 The cross-

hydrophilic surface has been fully wetted but the hydrophobic side of the fabric remains dry. Figure 7b shows the water absorption ability of the single-face-hydrophobized cotton (ii) compared to that of untreated (i) and fully hydrophobized (iii) cotton fabrics. The initial weight of the original untreated cotton fabric was 10 g. The measured weight of the untreated cotton fabric after the immersion test was 28 g, indicating good water absorption ability (i.e., w% = 180%, column (i)). In comparison, the measured weight of the single-face-hydrophobized cotton fabric after the immersion test was 18 g, showing a significantly lower absorption capability (i.e., w% = 80%, column (ii)) than that of the untreated cotton fabric (w% = 180%, column (i)). The single-face-hydrophobized cotton fabric results in a reduction of the amount of absorbed water from 18 to 8 g, which is ∼44% of that of the untreated cotton fabrics. In the case of the fully (both sides) hydrophobized cotton fabric (iii), no water absorption was measured effectively. Compared to the untreated and fully hydrophobized samples, the single-face-hydrophobized cotton fabric shows medium-level water-absorbing ability. It suggests that water is absorbed only by one side of the hydrophilic surface whereas the other surface of superhydrophobic wettability precludes water from absorbing into the fabric. 4.5. Vapor Transmissibility. Although the single-facehydrophobized cotton fabric effectively blocks the transport of liquid water, it is still permeable to air and water vapor. Figure 8

Figure 8. Water vapor transmission rate of cotton fabrics with different surface wettabilities: (a) untreated cotton as a control; (b) cotton fabric with the hydrophobization of both sides; (c) cotton fabric with single-faced superhydrophobicity with the hydrophilic surface facing down to liquid water; and (d) cotton fabric with single-faced superhydrophobicity with the superhydrophobic surface facing down to liquid water.

shows the vapor transmission rate of different fabric samples. The value of the vapor transmission rate is the average one based on multiple tests. All tests were performed at 23 °C and 50% RH for 1 day. Column (a) represents the vapor transmission rate of the untreated cotton fabric, which was 1976 ± 33 g/m2/d. The relatively high value suggests that the original cotton fabric has good permeability to air and water vapor because the cotton fabric acts as a buffer layer to absorb and evaporate the vapor continuously. Column (b) represents the vapor transmission rate of the fully hydrophobized (both sides) cotton fabric, which was 1728 ± 27 g/m2/d. This value is ∼87% of that of the untreated cotton fabric. Column c corresponds to the vapor transmission rate of the single-facehydrophobized cotton fabric, where the hydrophobic side faces ambient air. In this case, the measured vapor transmission rate was 1866 ± 11 g/m2/d, which is ∼94% of that of the untreated control sample. This value is ∼7% higher than that of the fully G

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original untreated cotton fabrics, the fabric with single-sided superhydrophobicity also shows good vapor transmissibility with no significant degradation. With customized chemical treatment in the foam finishing process for the promotion of adhesion between hydrophobic coating materials and fabric fibers, the single-faced superhydrophobic fabrics also show robust laundry fastness. Fabric material with such asymmetric and tailored wettability will be of great significance in textile, medical, and industrial applications, such as microfluidic systems, desalination of seawater, flow management in fuel cells, and water/oil separation.

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ASSOCIATED CONTENT

S Supporting Information *

Video of the characterization of the wettability of the cotton fabric with single-faced superhydrophobicity. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Washing fastness of the fluoropolymer hydrophobic coatings of single-face-hydrophobized cotton fabrics.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. linking mechanism is based on the reaction of the isocyanate functional group (−NCO) with a hydroxyl (−OH) or carboxyl (−COOH) group on cotton fabric and a fluoropolymer coating. At high temperature (>80 °C), the blocked −NCO group can be released to cross-link the fluoropolymer (containing both hydroxyl and carboxyl groups) to cotton fibers (containing a hydroxyl group). The advantage of using NBP-75 is that it can significantly improve the washing fastness of the superhydrophobic coatings on fabrics with little influence on the water repelling properties of the treated fabrics. The measured contact angle and roll-off angle of a water droplet on the cross-linked fabric were ∼153 and ∼13°, respectively, showing no significant difference compared to those of un-cross-linked fabric (∼150 and ∼12°, respectively). As also shown in Figure 9, the cross-linking agent effectively improved the washing fastness of the fluorocarbon polymer coating on the cotton fiber. Even after 20 washing cycles the contact angle on the hydrophobic surface did not change significantly (∼148°) and the droplet rolled off the fabric surface at a tilting angle of 15°. This suggests that the foam finishing process can produce robust hydrophobic coatings with customized chemical treatments.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Chou at the Stevens Institute of Technology for his help with EDX analysis. REFERENCES

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5. SUMMARY AND CONCLUSIONS A new type of functional cotton fabric with single-faced superhydrophobicity was achieved through a simple foam finishing process. Unlike most other superhydrophobic fabrics, the fabrics developed in this study exhibit asymmetric wettability on its two faces: one face shows superhydrophobic behavior and the other retains the inherent hydrophilic nature of cotton. The foam finishing process does not alter the texture and surface morphology of the cotton fibers. With the wellregulated viscosity of the foam solution, the hydrophobized face of the cotton fabrics exhibits stable ultrawater-repellent characteristics such as a high contact angle and low contact angle hysteresis. Such superhydrophobic wettability enables water drops to roll off the surface easily, which is essential for self-cleaning textiles. Meanwhile, the opposite untreated hydrophilic face of cotton preserves its good water-absorbing capability such that the single-faced superhydrophobic fabric has about half of the water-absorbing capacity of the original (both sides hydrophilic) cotton samples. Compared to the H

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