Nanostructured superhydrophobic silk fabric fabricated using the ion beam method

Abstract

Superhydrophobic silk fabric surfaces with high-aspect-ratio nanostructures were fabricated using ion beam treatment. The ion beam irradiated silk fabrics were characterized for wettability, as well as other physical properties unique to silk. The nanostructures were produced in various configurations, ranging from columnar to hairy shapes, on the silk fibers through anisotropic etching using oxygen ion beam treatment. Subsequent hydrophobic coating on the nanostructured, superhydrophobic silk fiber surfaces was achieved with an increase of the static contact angle from 0° for the pristine hydrophilic silk fabric to 170° for the superhydrophobic silk fabric and with a decrease of shedding angle by less than 5°, which is sufficient roll-off a water droplet to the silk fabric surface. Because the ion beam-treated side of silk fabric becomes superhydrophobic, while the opposite side, or body contacting side, remains pristine or superhydrophilic, an extreme asymmetric wettability can be achieved in the silk fabric, which improves its breathability by improving moisture transmittance through the fabric from the body to the outer surface. The luster and color of silk fabric before and after ion beam irradiation were found to exhibit no significant degradation. The breaking load of the silk fabric after the ion beam treatment was assessed to be mechanically durable in comparison to that of the pristine fabric. Therefore, after introducing the superhydrophobic property via the ion beam treatment, the silk fabric maintained its primary advantages, as a result, the range of its applications can expand into breathable self-cleaning clothing textiles, such as neckties, blouses and dresses.

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1. Introduction

Silk is a natural protein fabric having prominent characteristics of luster, handle, toughness and breathability, which broaden its applications for various fields, such as textiles, cosmetics, membrane materials, and medical biomaterials.^1,2 However, being composed of degradable proteins, wearable textiles made of silk have disadvantages, such as staining and poor water resistance. In particular, the mechanical properties of silk fibers submerged in water decrease because water weakens hydrogen bonds and reduces the van der Waals force between the chain segments in the amorphous phase of silk.³ Furthermore, surface cleaning should be performed with dry-cleaning solvents, such as perchloroethylene, which has environmental disadvantages because of its toxicity.⁴ Therefore, it is important for silk fabric to overcome staining and water-related issues to enable its use in applications for which washing with water is required.

For developing the hydrophobicity of silk, water-repellent finishing has been adopted, which can be implemented using various methods. One of the methods is padding or impregnation process, which is a typical way to improve textile hydrophobicity. This method, however, has several problems such as the decrease of vapor permeability, which causes discomforts while wearing. Coating finishing is also used to improve the property of hydrophobicity on silk, but it exhibits low durability, making this method inappropriate for practical use. Another method, chemical grafting, is not always environmentally friendly and may change the mechanical properties of silk.⁵ In addition, due to the reduction in unique advantages of silk, such as luster, textile and vapor permeability, a chemical-based finishing process to convert silk into usable materials may be inapplicable for luxurious silk.

Due to the mechanical weakness of silk in wet condition, silk needs to have water resistance and superhydrophobicity. The methods to induce superhydrophobicity on the surfaces can be divided into three categories:⁶ top-down approaches (nano-imprinting,⁷ template-based technique⁸ and e-beam^9,10 or plasma treatment¹¹), bottom-up approaches (layer-by-layer [LBL] deposition,¹² hydrogen bonding¹³ and colloidal assemblies¹⁴) and a combination of both top-down and bottom-up approaches (casting of polymer solution and phase separation¹⁵ and electro-spinning with chemical vapor deposition¹⁶). Among those methods developed, the plasma or ion beam treatment has been widely used to produce a hydrophobic surface on polymers and textile materials, resulting in a static contact angle of over 150° and a contact angle hysteresis of less than 10°. This process is known to be a rapid and environmentally friendly method that does not interfere with the bulk properties and can treat one side of a textile.^17,18 For example, Li et al.¹⁹ developed a hydrophobic habutai silk exhibiting the static contact angle of 123° and a wet-out time of over 60 min using a hexafluoropropene (C₃F₆) plasma treatment. Several studies also suggested that the plasma treatment would improve hydrophobicity, as well as reduce the contact angle hysteresis to a value as low as 5° on various textile materials, such as silk or polypropylene.^20–22 However, no reports have yet provided superhydrophobic silk fabrics.

It is known that moisture transmittance through a fabric is a main factor to enable comfort in the clothing climate, which could be controlled by asymmetric wettability, i.e., obtaining one side as a hydrophobic fabric while other or opposite side as hydrophilic. A fabric with the asymmetric wettability exhibits unidirectional water flow, i.e., water, oil, bacteria and germs are repelled from one surface with self-cleaning property, while water vapor is absorbed on the other side, resulting in a comfortable, breathable and less skin-irritating fabric. Thus, the surface with asymmetric wettability is an interesting and important component of apparel applications.²³ In recent years, the development of the asymmetric or unidirectional water permeability surface has significantly progressed. Kong et al.²⁴ reported that after irradiating solar or UV light onto a piece of cotton coated with TiO₂, the surface of the cotton piece exhibited superhydrophobicity because of the photo-induced phenomenon of TiO_2, while the opposite side of the cotton preserved its hydrophobicity with 140° of static contact angle. Lim et al.²⁵ demonstrated extreme asymmetric wettability on a fabric formed by electrospinning polyacrylonitrile on the hydrolyzed polyacrylonitrile electrospun web. For the fabric exhibiting the asymmetric wettability, water repellency or easy cleaning is exhibited on its outer surface, whereas the unique properties of clothing comfort are maintained on its inner surface. In addition to the asymmetric property, the superhydrophobic silk fabrics could be used for a broader range of applications, e.g., use in rainy weather or being easily managed by reducing the number of washings required.

In this study, we introduced a novel method to fabricate the nanostructured superhydrophobic silk fabrics using the ion beam treatment of oxygen etching and subsequent hydrophobic deposition. The nanostructured silk fabrics were characterized by measuring the static contact angle, the shedding angle, the water repellency for various liquids that could get stained on the silk fabrics in daily life (see Fig. 1a), and the unidirectional water absorption. Chemical analysis was performed, focusing on the change in chemical composition of silk before and after the ion beam treatment. In addition, we examined the unique silk properties by measuring the luster and color values of the nanostructured silk fabric. The breaking load of silk fabric was carefully measured before and after pouring water onto the surface of pristine and superhydrophobic silk fabric.

Fig. 1 (a) A photographic image of grape juice (purple), water droplet (dyed violet), coffee (brown) and milk (white) on the silk surfaces (left) before and (right) after superhydrophobic ion beam treatment. Here, the superhydrophobic silk fabric surface was etched by the oxygen ion beam for 5 min and subsequently coated with the F-DLC coating for 30 s on the right half of the fabric. (b) A SEM image of the superhydrophobic silk fabric etched for 5 min (magnified to 500×). (c–g) SEM images of the silk fibers irradiated by the oxygen ion beam irradiation for different exposure durations: (c) pristine (d) 1 min, (e) 2 min, (f) 5 min and (g) 10 min (magnified to 1000× for the upper images and 40 000× for the lower images).

2. Experimental

Fabrication of superhydrophobic silk

Silk fabric (#615 ISO Silk adjacent fabric [ISO 105/F06], Test Fabrics Co. USA) was used for this study. The silk fabric was Plain Habutai silk fabric consisting of three doubling yarns (warp count-30.2-3d, weft count-20.2-3d), fabric count of 53 × 36 cm⁻¹ and weight of 60.277 g m⁻². To prepare the superhydrophobic surface, the ion beam method was adopted, which involved the initial ion beam etching step to form nanostructures on the silk fabrics and the subsequent ion beam deposition step to make the nanostructured surfaces superhydrophobic. O₂ gas was first introduced into the anode of the linear ion source to produce oxygen ions. The distance between ion source and substrate was maintained at 150 mm. The gas flow rate was set at 3.5 sccm and the total gas pressure was stabilized at 35–50 mTorr. The ion beam power and substrate bias voltage were 100 W and −400 V, respectively. The duration of the oxygen ion beam irradiation was varied from 0 to 10 min, resulting in various nanopatterns, ranging from dot to hair-like structures. A mixture of C₂H₂ and CF₄ gases with a ratio of 4 to 16 sccm was then used for ion beam deposition of a fluorinated diamond-like carbon (F-DLC) film onto the nanostructured silk fabrics for 30 s with a deposition rate of 44 nm min⁻¹ to deposit a low-surface energy coating. The surface energy of the F-DLC coatings showed 31.57 mN m⁻¹, which resulted in the previously measured static contact angle of 81.3 ± 0.3° on a flat stainless steel disc surface.²⁶ The working pressure was stabilized at 150 mTorr. The ion discharge power for ion beam and bias voltage was applied under the same conditions as those of the oxygen ion beam irradiation.

SEM, XPS analysis and water behavior measurement

The nanostructures on silk fabric were imaged using a scanning electron microscope (SEM, Nova NanoSEM 200, FEI, USA). Before the SEM observation, a 10 nm thick Pt film was coated onto the substrates to avoid electron charging on the silk fabric surface. The electron accelerating voltage of the SEM was 10 kV. The aspect ratio, defined as the ratio of height over width, of nanostructures was estimated on the silk fiber by direct measurement in SEM images magnified at 40 000× with a tilting angle of 30°. The width and height was measured with the Image J program. The chemical composition of pristine and superhydrophobic silk fabric was analyzed using X-ray photoelectron spectroscopy (XPS, Axis-HSI instrument, Kratos Inc., USA). The hemispherical analyzer equipment was equipped with an Mg/Al dual anode X-ray source used in Mg K_α mode with a pass energy of 20 eV and an energy resolution of 0.1 eV. The photoelectron spectrum was recorded with the X-ray source operating at 150 W (10 mA × 15 kV).

The wetting behavior of each sample was evaluated by performing the measurements of static contact angle, the shedding angle and the water droplet bouncing. To determine the static contact angle of deionized (DI) water, 3.40 ± 0.3 μl of DI water was deposited onto the silk surface, and the static contact angle was measured using a contact angle goniometer (Theta Lite, Attension, Finland). The shedding angle was measured using the same equipment used for the static contact angle measurement. We used the shedding angle methodology developed by Jan Zimmermann.²⁷ The shedding angle was determined by the measured angle at which a water droplet with 12.5 ± 0.1 μl in volume rolled off over 2 cm. The distance between the syringe tip and the sample surface was fixed at 10 mm.

The rain resistance of each silk fabric sample was tested by a water droplet impacting experiment (droplet bouncing movie) that simulates a raindrop from sky. By releasing the water droplet onto the surface from a syringe tip at the height of 70 mm from the fabric surface, the bouncing or penetration through the silk fabric surface was recorded. The bouncing movie on both the pristine and superhydrophobic silk fabric was recorded using a high-speed motion camera (Motion Pro® HS-4, Redlake Imaging, USA) with a frame rate of 500 fps (pristine silk) and 1000 fps (superhydrophobic silk).

Analysis of the luster and color change

The luster property of silk fabrics before and after ion beam irradiation was determined using a goniophotometer (GP-200, Murakami Color Research Laboratory, Japan). One fixed light source with spot size of 5 mm illuminated one point of the substrate at 30°. The incident light was detected by a detector arranged at an angle ranging from −75° to +90° at every 0.1°. Because some research studies reported that plasma treatment on a surface affects the surface yellowing,²⁸ we also assessed the color value, which was measured according to the values of L* (whiteness vs. darkness), a* (red vs. green) and b* (yellow vs. blue) using a spectrophotometer (CM-2600d, Konica Minolta, Japan).²⁹

Measurement of the mechanical property

The mechanical property of the silk fabric before and after ion beam irradiation was measured to monitor the breaking load with and without pouring distilled water onto each of the silk surfaces. The load-extension curve was obtained by using a Universal Testing Machine (Instron-5543, Instron, USA). The experiment was performed according to the ASTM D 5035 strip method. All the substrates were prepared with a width of 35 mm and a length of 160 mm. On a square glass dish (185 mm × 185 mm), each substrate was taped around the four edges at intervals of 5 mm. 150 ml of DI water was then poured over the glass dish until the surface of substrate was completely immersed, and the immersion was maintained for 30 s. The mechanical test for the breaking load was performed after discarding water and successively cutting the four edges attached by a tape. A clamp interval was configured with 76 mm; 10 N of load and 100 mm min⁻¹ of tension speed were applied until the sample broke. Three samples with and without the ion beam etching and deposition were investigated under the same loading and speed conditions.

3. Results and discussion

The wettability of various liquid droplets was demonstrated on the silk fabric, for which half of the region was changed to the superhydrophobic surface, while the other half remained as the water-absorbable and hydrophilic surface. Several droplets of liquids, grape juice (purple), water droplet (dyed violet), coffee (brown) and milk (white), were gently placed onto the silk fabric. As shown in Fig. 1a, after the placement of each of the liquid droplets, the pristine silk fabric immediately absorbed all of the liquids, became stained with its dye colors and even bent because of the swelling due to its hydrophilic nature. While, each of the liquid droplets retained a spherical shape when placed onto the superhydrophobic silk fabric, which can repel the droplets as well as roll away the liquid easily.

Nanostructured silk

The surfaces of the silk fibers before and after the oxygen ion beam irradiation, varying from 0 to 10 min, were characterized, as shown in Fig. 1b–g. With an increase of the oxygen ion beam irradiation on the silk fibers, the surface patterns were observed to evolve into various configurations, such as dots, pillars and hairs. As shown in the geometries of the nanostructures in the insets of Fig. 1b–g, an aspect ratio (defined as the proportional relationship between width and height) of the nanostructures linearly increased with the duration of ion beam exposure. At the initial stage of 1 min and 2 min of the oxygen ion beam irradiation, the surface structures on the silk fibers appeared as rippled and embossed structures, of which the aspect ratio is approximately 1. After 5 min of the oxygen ion beam irradiation, the aspect ratio increased distinctively up to approximately 10. It was reported that the nanostructures on polymeric surfaces were formed by a selective etching mechanism with the co-deposition of hard inhibitors, which originated from the metallic reactor walls or chamber.³⁰ The metallic elements were co-deposited onto the target surface during the ion beam irradiation of the sample surface, where the regions were covered by hard metallic elements. The etching inhibitors are barely etched, while the other regions with no metallic elements are rapidly etched due to the high rate of reaction between the oxygen ion beam and carbon-based polymeric surfaces. The reactive oxygen ions break the polymer C–C backbone and produce volatile species, such as CO or CO_2. Consequently, with the increase of the ion beam irradiation duration, the aspect ratio increased and the surface became rough and textured, such as those shown in Fig. 1b–g.³⁰

XPS result analysis before and after ion beam deposition

To explore the surface property of the silk fabrics before and after the oxygen ion beam irradiation and hydrophobic deposition, the chemical analysis of the pristine and superhydrophobic silk fabrics was performed, as shown in Fig. 2. A survey spectra for the pristine silk fabric revealed the spectral peaks for C1s, O1s and N1s, which is similar to reported peaks for silk fabrics.¹⁷ However, an F1s spectral peak was only detected for the superhydrophobic silk fabric, whereas relatively low signals for other peaks were detected due to the F-DLC coating over the nanostructured fabrics (Fig. 2). Therefore, we can conclude that the F-DLC was coated on the superhydrophobic silk fabric surfaces from the peaks of C1s and F1s.³¹

Fig. 2 XPS spectra in survey scans for the pristine (blue line) and the superhydrophobic silk fabric (exposed to oxygen ion beam etching for 10 min and F-DLC coating for 30 s, red line).

Static contact angle and shedding angle

Fig. 3a shows the hierarchical structures having nanostructures on the silk woven fabric, on which the static contact angle and the shedding angle of water droplets were measured. After the formation of nanostructures on silk fabrics by oxygen ion beam irradiation and subsequent deposition of the thin film of low-surface energy material, the hydrophobicity was observed to significantly increase from near zero for the pure silk fabric to 170° for the nanostructured silk fabric, as shown in Fig. 3b. Note that the pristine silk fabric itself has a hierarchical roughness in two different scales: the fiber itself of 19 μm scale and the fiber bundle on the 2.57 × 1.86 mm per inch scale. In addition, the oxygen ion beam irradiation fabricated a nanoscale roughness on each fiber surface with nanostructures in the range of 10–50 nm in width and 10–500 nm in height. In brief, due to at least three different scales in roughness (Fig. 3a), the nanostructured silk fabric easily became superhydrophobic with the addition of the low-surface energy coating measuring approximately 22 nm in thickness. The pristine silk fabric surface has the static contact angle of near zero, as shown in Fig. 3b, which clearly indicates its water absorbing nature due to the hydrophilic characteristic of the protein forming the silk fibers. After the deposition of the low-surface energy material, the static contact angle rapidly increased with the exposure duration of oxygen ion beam irradiation. The surface became superhydrophobic after 5 min of the oxygen ion beam exposure. Moreover, the shedding angle was dramatically reduced from 85°, which is the maximum available measurement for shedding angle, to below 10° after 2 min of the oxygen ion beam exposure. As the static contact angle increased to 170°, the shedding angle significantly decreased to approximately zero for the superhydrophobic surface with 10 min etching, as shown in Fig. 3b. Therefore, these results imply that the ion beam treatment for distributing nanostructures on the silk fabric can create a super-water-repellent surface.

Fig. 3 (a) SEM images of nanostructures formed on a silk fabric exposed by ion beam treatment. The inset in (a) shows micro-scale woven structures of the silk fabric. The scale bars are 1 μm and 400 μm. (b) Static contact angle (black line) and shedding angle (blue line) of a water droplet measured after 30 s of hydrophobic coating with various ion beam durations.

Asymmetric wettability and water-drop impact behavior

We investigated the water droplet impact behavior of the silk fabric exhibiting the asymmetric wetting condition of the superhydrophobic silk surface (up-side of the silk fabric) and the pristine silk surface (down-side). One of advantages of the dry etching method is its ability to perform the superhydrophobic treatment only on one side (or up-side) of the silk fabric, producing the static contact angle of 165°, as shown in Fig. 4. The down-side of the superhydrophobic silk fabric retained its original superhydrophilic and water absorbable nature, showing the absorption of a water droplet within 14 s. It is well-known that garments made of silk fabrics having single-side superhydrophobicity are expected to exhibit breathable and comfortable properties as well as self-cleaning performance.²³ Detailed analysis regarding the silk properties were performed in terms of the luster and color of silk fabric before and after the ion beam irradiation.

Fig. 4 Absorption behavior of the water droplet on the up-side (nanostructured, superhydrophobic surface) and down-side (pristine surface) of the silk fabric lapsed from 1 to 14 s.

We also examined the durability of the superhydrophobic fabric with asymmetric wettability by immersing it underwater (Fig. 5a). It was found that after immersing the fabric underwater, the superhydrophobic surface showed high water contact angle of more than 160° (Fig. 5b), while the opposite hydrophilic surface remained hydrophilic, but the water contact angle was about 15°. Note that the hydrophilic side of the fabric with asymmetric wetting did not absorb water immediately due to the saturation of water into the silk fabric due to underwater immersing.

Fig. 5 (a) Optical images of the silk fabric with asymmetric wetting immersed in DI water. (b) The ion beam treated surface and (c) untreated surface of the same fabric maintaining superhydrophobicity and hydrophilicity, respectively.

The water droplets impacting experiment was performed to simulate the water resistance of a raindrop on the silk fabric with and without the superhydrophobic treatment.

Representative snapshot images of water droplets falling on the pristine and the superhydrophobic silk fabric surfaces are sequentially presented in Fig. 6. A water droplet was immediately absorbed on the pristine silk surface, due to the hydrophilic nature of silk fibers, as shown in Fig. 6a. In contrast, as shown in Fig. 6b, a water droplet bounced on the superhydrophobic silk fabric surface, leaving no residue of water, indicating the high durability of water repellency. This result is induced by the hierarchical structures and lower surface energy of the superhydrophobic fabric (see Fig. 3b). Therefore, it can be considered that clothes produced by the superhydrophobic silk are more resistant to water impact than pristine silk fabric clothes. The static contact angle was not changed even after stretching (upto 40% of strain rate) and bending (up to 180° of bending angle) (Fig. S1 and S2†). It is expected that the nanostructures formed on the fiber surface may not be significantly damaged under the macroscopic deformation such as stretching and bending.

Fig. 6 Bouncing behavior of water droplets on the (a) pristine and (b) superhydrophobic silk fabric.

Measurements of the luster and color values

The luster and color change of samples before and after the ion beam treatment were quantified. Because the luster is a major characteristic of silk, we investigated the change of luster after the superhydrophobic treatment. The main factors changing luster property were considered to be the nanostructures induced by the oxygen ion beam etching, as well as F-DLC coating.

First, the effect of nanostructuring, performed by the ion beam etching, on luster was examined; the silk fabrics after the exposure to oxygen ion beam from 0 (or pristine) to 10 min were examined, as shown in Fig. 7a. With the increase of the ion beam exposure duration, the maximum value of luster decreased before and after the F-DLC coating; reductions of only 7% and 2%, respectively. Note that in comparison to the luster for the etched silk fabrics without the F-DLC coating, the etched silk fabrics after the F-DLC coating increase the luster property. This may be caused due to the reduction of nanoscale roughness on the etched silk fabric by the coating of F-DLC film, which results in the relative increase in the reflectance compared to the etched silk fabric surface without the F-DLC deposition. Second, the effect of F-DLC coating on luster was explored by varying the F-DLC deposition duration. The deposition duration of F-DLC at the fixed etching duration of 5 min was varied from 0 to 60 s, which corresponds to the deposition of no coating film to the deposition of film with 35 nm thickness. With the increase of the deposition duration, the luster was measured to decrease only by 4%, as shown in Fig. 7b. Therefore, compared to the pristine silk fabric, the luster values measured on the superhydrophobic silk fabrics exhibited no significant difference for the etching duration of less than 10 min and for the deposition of F-DLC of less than 60 s. Based on these results, it can be concluded that the superhydrophobic treatment on the silk fabric would not alter the luster properties considerably.

Fig. 7 (a and b) Plots of the average values of luster at every angle of the silk fabrics: (a) various etching duration from 0 to 10 min on the silk fabrics with and without the F-DLC coating for 30 s; (b) various F-DLC coating duration from 0 to 60 s on the silk fabrics with and without the oxygen ion beam etching. The oxygen ion beam treatment was performed for 5 min. (c–f) Optical images considering the color change of various silk fabrics. (g and h) Plots of the color changing of the silk surfaces: (g) various etching duration from 0 to 10 min with and without the F-DLC coating for 30 s and (h) various coating duration from 0 to 60 s with and without the oxygen etching for 5 min. (L*: darkness index, a*: redness index, b*: yellowing index; NC: no coating, C: coating for 30 s, NE: no etching, E: etching for 5 min).

Fig. 7c–f indicate that the color is affected by the F-DLC deposition. The F-DLC coated silk fabric surfaces became relatively darker and yellower than the pristine and ion beam-etched silk surface. The color values of the silk fabrics were investigated by verifying the alteration before and after the oxygen ion beam irradiation and F-DLC deposition (Fig. 7g and h). With oxygen ion beam irradiation from only 0 to 10 min, the values of L* (darkness index) and b* (yellowing index) on the silk surfaces were found not to change (Fig. 7g). Therefore, within the ion beam irradiation condition chosen in this work, the color changing values remained unchanged. In the case of the silk fabric surfaces with the oxygen ion beam irradiation from 0 to 10 min and subsequent deposition of F-DLC film for 30 s, the change for L* and b* was slightly increased. For the silk fabric deposited only with the F-DLC deposition, the b* significantly increased from 1.17 to 8.46, and the L* decreased with respect to the various F-DLC coating duration (Fig. 7h). Regardless of the ion beam etching, the color of silk fabric with the F-DLC coating was changed to dark and yellow. As shown in Fig. 2, after F-DLC coating, the silk fabric surface was covered by an F-incorporated amorphous carbon layer, which turns the interference color yellower, depending on the thickness coated on the surface.²⁸ Therefore, the covered carbon layer may interfere with the natural color of silk by making it yellower and darker.

Mechanical change before and after pouring water on fabric surfaces

Natural silk has the characteristic of decrease in the breaking load for increase in the extension due to water wetting. Thus, we measured the dry and wet breaking load of the silk fabrics by performing the mechanical property measurements before and after the water immersion. As shown in Fig. 8a, the wet breaking load of the pristine silk decreased to 184 N from 247 N for the dry breaking load. However, the wet breaking load of the superhydrophobic silk fabric was almost the same as the dry breaking load. In comparison to the pristine silk fabric with the water immersion, the reduction in the mechanical property of the superhydrophobic fabric was not much significant. The result can be explained by the fact that because of the non-wetting condition of the Cassie–Baxter state with low pressure³² on the superhydrophobic silk surface, the hierarchical structure holds air between the nanostructures, which does not allow water droplets to get absorbed through the silk fabric, as illustrated in Fig. 8b (right). However, the pristine silk fabric became weak due to breaking load because the water deposited on the surface was fully absorbed through the silk fabric and could break the hydrogen bonds in the amorphous region of silk fibers, as illustrated in Fig. 8b (left).³ In brief, the superhydrophobic silk fabrics obtained by the ion beam treatment can enable silk to sustain its mechanical properties in dry or wet conditions.

Fig. 8 (a) Load-extension curves of the various silk fabrics with and without water immersion. (b) Illustration of the side view of the silk fabrics after water immersion: (left) pristine silk fabric, (right) superhydrophobic silk fabric.

4. Conclusions

In this study, we fabricated the superhydrophobic silk fabric with a static contact angle of 170° and a shedding angle of less than 5°, which has high-aspect-ratio nanostructures formed by the selective etching mechanism using the oxygen ion beam irradiation. Because our two-step manufacturing process, i.e., nanostructuring and subsequent deposition of low-energy materials, can create superhydrophobicity on one side of the silk fabric, while the other side of the silk fabric remains intact as hydrophilic and water absorbing, our method can modify silk fabric to exhibit extremely asymmetric wettability. Such asymmetric wettability improves the moisture transmittance through the fabric or enhances the unidirectional water flow from the skin to the outer surface. After the superhydrophobic treatment, the unique properties of silk such as the luster and color did not change significantly. In addition, the wet breaking load of the superhydrophobic silk fabrics was not reduced and, rather, it was similar to the dry breaking load due to its non-wetting superhydrophobic nature. Thus, it is expected that the production of superhydrophobic silk fabric without the loss of the unique silk properties would be widely applicable for breathable self-cleaning clothing textiles, such as neckties, blouses and dresses.

Acknowledgements

This work was partially supported by the Global Excellent Technology Innovation R&D Program by the Ministry of Knowledge Economy (MKE), a project no. 10040003 funded by the MKE, and this research was supported by the SRC/ERC program of MOST/KOSEF (R11-2005-065) and National Research Foundation of Korea (NRF) Grant funded by the Korean Government (2011-0014765).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05938g

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