Superhydrophobic and transparent surfaces on cotton fabrics coated with silica nanoparticles for hierarchical roughness

Abstract

Superhydrophobic and transparent surfaces on cotton fabrics have been developed using silica nanomaterials. Initially, trichlorododecylsilane was treated on the silica nanoparticles to lower the surface energy of the fabric. By simply spraying alcohol suspensions containing hydrophobized silica nanoparticles, extremely water repellent coatings were formed on the textile fabrics. The effect of three types of alcohol solvent on the hydrophobicity of the coated cotton fabrics was examined by measuring the surface wettability. The treated cotton textiles in methanol exhibited contact angles higher than 160°, contact angle hysteresis lower than 10°, and good water repellency. It proved to be essential to form hierarchical morphology in achieving superhydrophobicity.

Keywords

superhydrophobicity silica nanoparticles spray-coating hysteresis smart textiles

Cotton fabric is the most widely used natural fiber cloth in textiles today. The fiber is composed of almost pure cellulose of hydrophilic nature. Because the cellulose fiber is a linear polysaccharide polymer with many hydroxyl groups, common cotton fabrics are hydrophilic in nature. If the cotton surface is extremely repellent to water, it would have various commercial, military, and specialty applications, including self-cleaning surfaces,¹ anti-fouling surfaces,² stain-free clothing and spill-resistant protective wear,³ anti-icing,⁴ liquid manipulation,⁵ fluidic drag reduction,⁶ water collection,⁷ selective transportation of microdroplets,^8,9 etc.

The most common parameters of evaluating the wettability of a surface against any liquid are the contact angle and contact angle hysteresis,¹⁰ as well as sliding angle measurements. Based on the contact angle θ of a water droplet, a surface can be classified as superhydrophilic when θ ∼ 0°, hydrophilic when θ < 90°, hydrophobic when θ > 90°, and superhydrophobic when θ ∼ 180°. The second important parameter for characterizing surface wettability, contact angle hysteresis, is defined as the difference between the advancing and the receding contact angles on a solid surface.¹¹ Contact angle hysteresis can arise from molecular interactions between the solid surface and the liquid droplet or from surface anomalies, such as roughness or heterogeneities.¹² When a surface displays very high contact angles (typically θ > 150°) and very low contact angle hysteresis (typically Δθ < 10°) for contacting water, it is considered to be a superhydrophobic surface.

So far, a number of studies have been reported on superhydrophobic surfaces of cotton textiles. However, there are a few literatures that sometimes neglected those two important criteria on this subject. Yu et al.¹³ synthesized a silane coupling agent and silica sol and coated them on cotton fabrics to make the fabric superhydrophobic. However, because the results showed a water contact angle lower than 150°, the coating made by this approach cannot be considered a superhydrophobic surface. Biomimetic superhydrophobic and highly oleophobic cotton textiles were prepared by in situ generation of silica particles with amine groups at the surface, to generate a micro- as well as nano-size surface morphology, followed by hydrophobization with polydimethylsiloxane.¹⁴ The roll-off angles of the cotton textiles that modified this approach were higher than 10°, but the roll-off angle values need to be lower than 10° for a surface to be considered superhydrophobic.

Xue et al.¹⁵ created superhydrophobic cotton fabrics with titania sol and steric and 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane to lower the surface energy. The water contact angle results show sufficiently high values (θ > 160°), but the apparent receding contact angle, contact angle hysteresis, or roll-off angle must be measured and reported to judge whether these surfaces are truly superhydrophobic. Gold particles have been incorporated into cotton fabrics to develop a micro- as well as nano-size surface morphology by Wang et al.,¹⁶ but such expensive materials may limit their practical applications. Recently, superhydrophobic cotton textiles have been achieved by covalently bonding two layers of silica particles (microparticles and nanoparticles) and the incorporation of perfluoroalkyl groups onto the modified surface showing the contact angle to be higher than 150° and roll-off angle to be lower than 10°.¹⁷ However, it is well known that fluorine materials are not environmentally friendly. Some researches about fluorine-free cotton fabrics with superhydrophobic properties have been fabricated by the sonochemistry irradiation method.¹⁸

In this report, extremely water repellent and transparent surfaces were fabricated on cotton fabrics coated with hydrophobized silica nanoparticles by spray-coating. The hydrophobized silica nanoparticles were dispersed in alcohol solvents (1-propanol, ethanol, and methanol) and the effect of the three types of alcohols on the superhydrophobicity of the cotton fabric substrates was investigated. To confirm the superhydrophobicity, the contact angles as well as contact angle hysteresis were measured and compared.

Experimental procedure

Materials

A 25 cm² piece of cotton textile (Taipyung Textile Co.) was cleaned with water and ethanol to remove impurities. Hydrophobic fumed silica nanoparticle (Aerosil), having an average diameter of 16 nm, was used for hierarchical structure development. Trichlorododecylsilane (Aldrich) as a silane coupling agent was used to lower the surface energy of the cotton fabric surface. Toluene, methyl alcohol, ethyl alcohol (Samchun Chemicals), and propyl alcohol (JUNSEI) were used without further purification.

Superhydrophobic treatment

One gram of silica nanoparticles were added to 20 mL of toluene. After dispersion, 1 mL of trichlorododecylsilane, as a silane coupling agent, was dropwise added to the solution. Thereafter, the reaction vessel was stirred for 2 h at room temperature. The purpose of this treatment is the adhesion of dodecyltrichloro groups on the surface of silica nanoparticles, making them hydrophobic. The solution was filtered, dried under vacuum at 323 K for 8 h, and triturated using a pestle and a mortar.

Spray-coating

Trichlorododecylsilane-treated silica nanoparticles (0.3 g) were added to 15 mL of methyl alcohol, ethyl alcohol, and propyl alcohol. The solution was treated using a bath sonicator for 30 min and was stirred vigorously for another 1 h to obtain a stable and homogeneous suspension. The mixtures were manually sprayed over cotton textiles from 20 cm away using an air spray gun (Anest Iwata). The resulting coated samples were dried at room temperature for 24 h.

Scanning electron microscopy and particle size distribution

The surface morphologies of the superhydrophobic treated fabrics were examined by scanning electron microscopy (SEM; JSM-7600F, JEOL). The particle size distributions of methyl alcohol, ethyl alcohol, and propyl alcohol containing the superhydrophobic treated silica nanoparticles were measured using a particle size analyzer (Nanotrac, Microtrac™). The averages of three different measurements of each were recorded.

Contact angle and hysteresis

The wettability of the superhydrophobic-treated cotton textile was evaluated by water contact angles and hysteresis of water droplets on the cotton textiles using a contact angle meter (SmartDrop, Femtofab). For the contact angle measurement, water droplets with a volume of 3 µL were dispensed using a micropipette onto the fabric surface. The average contact angle was obtained by measuring at four different positions of the sample. For the contact angle hysteresis analysis, the advancing and receding angles were obtained using the captive method. While a water droplet of 5.0 µL was suspended to the needle, it spurted out the additional water to 7.5 µL and the advancing contact angle (θ_a) was measured at three different volumes (Figure 1(a)). The receding angle measurement was followed after reducing the water volume to 5.0 µL. The three receding contact angles (θ_b) were measured with the additional suctions to 2.5 µL (Figure 1(b)). The hysteresis was calculated as the difference between the averaged values of three advancing and receding angles, respectively (Δθ = θ_a – θ_b).¹⁹ To identify the transparency and visibility of the silica nanoparticle-coated cotton substrates, pictures of a drop of water on it were taken.

Figure 1.

Schematic view of the captive method for hysteresis measurement.²⁰ (a) Scheme and captured images of the advancing contact angle measurement. (b) Scheme and captured images of the receding contact angle measurement.

Washing durability

To determine the washing durability of the superhydrophobic cotton fabrics treated with different alcohols, the samples were agitated by a magnetic stirrer at 300 rpm for 30 min in the absence and in the presence of the detergent. As a detergent, 1% of Persil® (Henkel) composed of anionic surfactant was used. After washing treatment, the fabrics were dried at room temperature for 24 h. The average values of three different measurements of the contact angles and sliding angles of washed cotton fabrics were recorded. For the sliding angle measurements, a water droplet of 5.0 µL was placed at different positions on the cotton fabrics. The tilting angle of the stage on which the cotton textiles lie was increased by 0.1°, and the angle at which a water droplet rolls off of a 2 cm distance or greater on the cotton fabrics was recorded and referred to as the sliding angle.

Results and discussion

In order to convert the hydrophilic cotton textiles into extremely water repellent hydrophobic fabrics, the trichlorododecylsilane-treated silica nanoparticles were spray-coated on the cotton fabrics using the alcohol solvents. Three types of the alcohol solvent were used to confirm the effect on the surface roughness, the superhydrophobicity of the coated cotton fabrics. As seen in the literature,²¹ the shorter the chain length of the alcohol’s hydrocarbon, the stronger the superhydrophobicity of the coated cotton fabric.

The water droplet on each cotton fabric can be seen in Figure 2. As mentioned in other literatures,^14–17,22 due to the protrudent fibers from the cotton surface it is difficult to determine the baseline of the water droplet, which may in turn lead to underestimation of the contact angles. However, in this study, the SmartDrop® measurement system estimated the rate of change of curvature of the free surfaces, calculated the value of the distorted surfaces, and provided the more accurate liquid droplet information without any loss of contact angles. Based on the red lines that present the real droplets of water on the cotton fabrics (Figure 2), the water contact angles were calculated. In the case of pristine cotton, most of the water droplets were absorbed into the hydrophilic cotton surface. However, droplets of water onto the cotton fabrics coated by hydrophobized silica nanoparticles were not absorbed at all and looks spherical. In addition, as the chain length of the alcohol’s hydrocarbon was shorter, it is getting closer to the perfect sphere and may be more hydrophobic. This image results are well consistent with the average contact angle values of pristine and the cotton fabrics coated with silica nanoparticles in 1-propanol, ethanol, and methanol (Figure 3). The contact angle values are the average ones of the left and the right angles. The contact angles of untreated cotton are about 40°. The average contact angle values of the cotton fabric coated with silica nanoparticles in 1-propanol, ethanol, and methanol are 149.8°, 158°, and 176°, respectively.

Figure 2.

Captured images of the water droplets (3 µL) placed on the cotton fabrics showing the contact angles and the specifications: (a) pristine cotton fabric; (b) cotton fabric coated in 1-propanol; (c) cotton fabric coated in ethanol; (d) cotton fabric coated in methanol. (Color online only.)

Figure 3.

Water contact angles of untreated and cotton fabrics coated with silica nanoparticles in 1-propanol, ethanol, and methanol.

As expected, the pristine cotton fabric shows the hydrophilic property (θ < 90°). However, all of the coated cotton fabrics showed significantly increased water contact angles compared with the uncoated one, meaning that the spray-coatings of the hydrophobized silica nanoparticles converted the hydrophilic cotton textiles into hydrophobic ones. In addition, it is shown that the contact angles coated in ethanol were higher than those in 1-propanol and the contact angles in methanol showed more increased values than those in ethanol. In particular, the contact angles of the cotton fabrics coated in methanol show high contact angles larger than 170°. It can be concluded that the water contact angles increased as the length of the hydrocarbon chain of the alcohol solvent decreased. It is reasonable that hydrophobic silica nanoparticles with dodecyl groups tend to aggregate in the less hydrophobic alcohols.²¹

In addition, the protruding cotton fibers made it difficult to evaluate the roll-off angles, sliding angles, or hysteresis using the conventional methods that measure the values by tilting the stage on which the samples are placed. In particular, in the case of the superhydrophobic surfaces that have extremely high water repellency, the water droplet tends to bounce out easily. In this study, the captive method, in which water droplets are suspended, emits droplets while measuring the advancing contact angles, and sucks them while determining the receding contact angles, was applied to the analysis system and accurate results were obtained.

In Figure 4, it can be seen that the hysteresis is diminished as the length of the hydrocarbon of the alcohol solvent is decreased. In particular, the difference between the advancing and receding angles lower than 10° was obtained only from the methanol solvent (Table 1). The resulting values from the ethanol and the 1-propanol were higher than 10° and the hydrophobic tendency is in good agreement with the contact angle results. Typically, hierarchically structured surfaces have a significantly low solid–liquid contact area and this results in very low contact angle hysteresis.¹¹ Therefore, it can be concluded that the coated cotton fabrics in methanol that showed contact angles higher than 150° and hysteresis lower than 10° can be considered the only superhydrophobic surfaces.

Figure 4.

Advancing and receding contact angles of the cotton fabrics coated with silica nanoparticles in 1-propanol, ethanol, and methanol, measured by the captive method.

Table 1.

The averaged advancing and receding angles, their difference, and hysteresis of cotton fabrics coated with silica nanoparticles in 1-propanol, ethanol, and methanol

Angle (°)	Advancing	Receding	Hysteresis
1-Propanol	148.87	127.82	21.05
Ethanol	156.21	139.74	16.47
Methanol	169.54	161.24	8.30

The wettability of surfaces is governed by two factors. One is the chemical composition of the solid surface and the liquid, and the other is the geometry of the solid surfaces.²³ As the surface energy of the hydrophobized silica nanoparticles would be the same, the type of the solvent would have affected the surface roughness of the cotton fabrics coated with silica nanoparticles. In order to confirm the effect of the alcohol on the surface morphology, SEM images of silica nanoparticle-coated cotton fabric in 1-propanol, ethanol, and methanol were measured (Figure 5). The pristine cotton textile presents a highly textured micro-scale fiber with a typical smooth surface, shown in Figures 5(a) and (b). In low magnification SEM images of treated cotton fabrics, it can be seen that the silica nanoparticles cover the cotton fibers not uniformly and are aggregated partially. In addition, there are the more silica nanoparticle aggregates in methanol than in the others. In high-magnification images, the morphological differences are more clearly visible.

Figure 5.

Scanning electron microscopy images of SiO₂ NPs-covered cotton fabrics. The images on the left are low-magnification ones and those on the right are high-magnification ones. (a) Pristine cotton fibers. (b) Cotton fibers coated in 1-propanol. (c) Cotton fibers coated in ethanol. (d) Cotton fibers coated in methanol.

Generally, superhydrophobicity tends to be exhibited when hydrophobic surfaces have hierarchical roughness, that is, both nanometer- and micrometer-sized roughness .²⁴ As shown in Figure 5, the nano-sized silica nanoparticles are well developed and form rough structures in methanol, whereas the silica nanoparticles in 1-propanol and ethanol are relatively vague. Based on these SEM images, it is reasonable that the hierarchically rough surface was well formed in methanol and it induced the superhydrophobicity of the cotton fabric. In contrast, the silica nanoparticles in ethanol are relatively more aggregated than in 1-propanol and those in methanol are more aggregated than in ethanol, causing the surface of the resulting coatings to be rougher, which is why the contact angles increased with decreasing carbon number and length of solvent. This morphological difference could be attributed to the difference in the aggregation states of the silica nanoparticles with the different type of alcohols.

The particle size distribution results agree well with this assumption and confirm the effect of the alcohol solvent on the surface morphology of silica nanoparticles (Figure 6). It can be seen that the silica nanoparticles tend to form larger aggregates in the less hydrophobic alcohols with the shorter hydrocarbon chains. The average sizes of the silica nanoparticle aggregates were approximately 629 nm in methanol, 486 nm in ethanol, and 412 nm in 1-propanol. Therefore, the hydrophobic silica nanoparticles aggregated more in lower alcohols and that led to the differences in surface roughness, which determined the degree of hydrophobicity of the coated surface of the cotton fabric.

Figure 6.

Particle size distribution of suspension containing the silica nanoparticles using methanol (red line), ethanol (blue line), and 1-propanol (green line). The average particle sizes are 629, 486, and 412 nm, respectively. (Color online only.)

The washing durability of the superhydrophobic-treated cotton textiles in the three different alcohols was analyzed using the water contact angles and sliding angles (Figure 7). The superhydrophobicity of the cotton fabrics after washing treatment was lost. The water contact angles of all the treated fabrics were reduced down below 150°, and the sliding angles show quite high values. The coatings that were realized in this study are not due to chemical interaction but physical bonded states. Therefore, the mechanical durability does not seem very strong. Also, the existence of the detergent somewhat deteriorated the hydrophobicity of the cotton fabrics.

Figure 7.

(a) Water contact angles and (b) sliding angles of the superhydrophobic-treated cotton fabric after washing treatment with and without detergent.

Considering the cotton fabrics as wearable textiles of fashion, the transparency of the superhydrophobic coating on the fabrics is very important.²⁵ The transparency of the spray-coating on the cotton fabrics with methanol containing the hydrophobized silica nanoparticles can be identified in Figure 8. Compared to the untreated cotton textile (Figure 8(a)), it can be concluded that the visibility after the coating processing was not varied. Also, with increasing volume of the water droplets, the coated cotton fabric did not get wet at all and retained a spherical shape (Figure 9).

Figure 8.

Photographs of (a) untreated cotton fabric and (b) coated cotton fabric by silica nanoparticles in methanol on which a water droplet maintained a spherical form. The red square area indicates a region of water absorbed. (Color online only.)

Figure 9.

Photographs of water droplets with the three different volumes, 3, 10, 30 µL, from the left to the right on a cotton fabric coated by silica nanoparticles using methanol: (a) side view; (b) top-down view.

Conclusion

We have successfully obtained extremely water repellent surfaces on cotton fabrics by introducing the hierarchical morphology of both nanometer- and micrometer-sized roughness formed from hydrophobized silica nanoparticles. The coated cotton textiles in methanol exhibited high contact angles, low hysteresis, and good water repellency. The factors that determine the hydrophobicity of the coatings in the different alcohol solvents are the differences in surface roughness and the surface energy of the fabric. To achieve superhydrophobicity, the presence of hierarchical surface topology has proven to be effective. The superhydrophobic and transparent coatings can be fabricated by the spray-coating method. This approach is applicable to a variety of surfaces, including textile fabrics, paper, wood, etc., but the durability issues need to be overcome.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Agency for Defense Development (ADD) and the Defense Acquisition Program Administration (DAPA).

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