TiO₂/PDMS hybrid system for constructing superhydrophobic surfaces of cotton fabrics with resistance to droplet adhesion

Inspired by superhydrophobic phenomena in nature, such as lotus leaves the bionic design of superhydrophobic structures has been extensively studied (Golovin et al., 2017). A superhydrophobic surface is defined as a surface with a contact angle (CA) greater than 150° and a sliding angle (SA) less than 10° (Sun et al., 2022), composed of chemical compositions with low surface free energy and rough topography (Gao et al., 2016). In recent years, researchers have prepared superhydrophobic surfaces on different substrates such as metals, ceramics, wood and cotton fabrics to obtain low-cost, simple preparation and large-scale useable superhydrophobic surfaces that are resistant to high temperatures and corrosion (Liu et al., 2018). The excellent hydrophilicity of cotton fabrics makes them susceptible to staining by contaminants, which seriously affects their function, appearance and application. It is therefore important to give cotton fabrics superhydrophobicity for stain resistance and self-cleaning (Khattab et al., 2020).

In the field of superhydrophobic materials, cotton fabrics are important substrate materials and are widely used in life and production. A large number of hydroxyl groups on the surface make the cotton fabric hydrophilic (Wu et al., 2021). As a fluorine-free polymer, polydimethylsiloxane (PDMS) is considered to be one of the most promising hydrophobic materials due to its low surface energy, good thermal stability, chemical resistance and strong covalent bonding to various substrates (e.g. fabrics) (Gao and Guo, 2017; Yin et al., 2013). Atthi et al. (2020) investigated the microstructure and hydrophobicity of PDMS films and obtained PDMS films with a CA of 120° (Atthi et al., 2020). Zhang et al. (2019) prepared PDMS films by solution mixing and rotational casting and tested a maximum CA of 125° (Zhang et al., 2019). Mazzond et al. (2019) produced hydrophobic cotton fabrics by immersion method using aminomethyl aminopropyl polydimethylsiloxane, which had a static CA of 143° (Mazzon et al., 2019). However, the water repellency of PDMS reported in the literature above is not entirely satisfactory.

In this context, the addition of SiO₂, TiO₂, ZnO, CuO and other micro-nanoparticles to form a rough surface is an important method to improve the superhydrophobic properties of fabrics (Peng et al., 2019. Zhang et al., 2017). Both micro and nano structures can lead to a reduction in effective and adherent area, especially in nanostructures (Ujjain et al., 2016). Nanoparticles such as TiO₂ and PDMS are biocompatible materials (Jokinen et al., 2018. Shiraishi et al., 2009). The hydrophobic finishing agent PDMS@SiO₂ was finished on the surface of cotton fabrics, with a CA of 154° and a SA of 8°, which showed extremely hydrophobic, as well as good friction resistance to friction and washing (Qi et al., 2019). Wu et al. (2021) used TiO₂ complexed bamboo charcoal (BC) and then physically coated the TiO₂-BC composite particles onto cotton fabrics using PDMS in the presence of silane coupling agent. The resulting superhydrophobic fabric was photocatalytic in nature and had broad applications in the manufacture of athletics, outdoor apparel and stain-resistant furniture (Wu et al., 2021). Wang et al. (2022) used silver nanoparticles sprayed onto the surface of cotton fabric to produce a conductive film and then deposited a layer of PDMS onto the film to prepare Ag/PDMS composite films, which had improved superhydrophobic and electromagnetic shielding capabilities (Wang et al., 2022). Jannatun et al. (2020) finished SiO₂ nanoparticles with polyvinyl alcohol by cross-linking on the cotton fabric surface, forming a distinct micro/nanopore structure that exhibited good superhydrophobicity. The surface of the resulting superhydrophobic fabric was further modified by PDMS and the durability against mechanical and chemical interference was significantly enhanced (Jannatun et al., 2020).

In this paper, stearic acid modified nano-TiO₂ (SA-TiO₂) was obtained by modifying TiO₂ nanoparticles with readily available, stable stearic acid. The nanoparticles were changed from hydrophilic to hydrophobic and dispersed in PDMS mixture in a certain proportion to prepare SA-TiO₂/PDMS superhydrophobic suspension. The superhydrophobic suspension was finished onto the surface of cotton fabric by spraying technique to obtain a superhydrophobic coating. Compared to conventional methods like electrodeposition and etching, spraying method is more effective, energy-saving and environmentally beneficial. We have created a fluorine-free, more environmentally friendly superhydrophobic coating. The chemical composition, morphological structure and superhydrophobic properties of SA-TiO₂ nanoparticles and superhydrophobic cotton fabric surfaces were comprehensively characterized using a CA measurement system, scanning electron microscope, Fourier transform infrared spectrometer and energy dispersive spectrometer (EDS) energy spectrometer. The anti-adhesive performance of the fabric was also investigated.

PDMS was purchased from Dow Corning. Titanium dioxide (TiO₂) was purchased from Shaoxing Lijie Chemical Corporation. Anhydrous ethanol and stearic acid (SA), AR, were purchased from Tianjin Yongda Chemical Reagent Co. Sodium carboxymethyl cellulose (CMC), CP, was purchased from Sinopharm Chemical Reagent Co. Potassium dihydrogen phosphate (KH₂PO₄), sodium chloride (NaCl), AR, were purchased from Tianjin Damao Chemical Reagent Factory. Disodium hydrogen phosphate (Na₂HPO₄), AR, was purchased from Tianjin Bodi Chemical Co.

The cotton fabric was cut into 3 cm × 3 cm, soaked in ethanol for 10 min and washed with deionized water, dried in an oven at 80°C. Cotton fabric (yarn count: 35 × 35 tex, density 130 pieces × 70 pieces, twill, Hebei Ningfang Group Co., Ltd, China).

The 100 mL of ethanol and 5 g of TiO₂ nanoparticles were added sequentially to the beaker, stirred well at 20°C, then ultrasonically disperse for 30 min to make the TiO₂ evenly dispersed in the anhydrous ethanol. A certain amount of SA (the ratio of SA to TiO₂ was 1:5, 2:5, 3:5 and 4:5) was added into the mixture. After stirring evenly, the mixture was heated to 70°C and continued stirring for 30 min. Finally, the mixture was stationary at room temperature until stratification occurred. The upper supernatant was poured out and the lower solid-liquid mixture was dried in an oven at 80°C. SA-TiO₂ powder was obtained after milling.

The SA-TiO₂ (2.5 g) was dispersed evenly in 50 mL of ethanol under magnetic stirring. To prepare the SA-TiO₂/PDMS suspension, 3 g of PDMS was added in the ethanol. As a comparison, the same amount of PDMS and 2.5 g of TiO₂ particles were homogeneously dispersed in an equivalent volume of ethanol.

Two type suspensions (PDMS and SA-TiO₂/PDMS) were sprayed at a distance of 15 cm from the fabric for 2 s at an interval of 10 min. Controlled the number of sprays (1–5) and whether tension was applied during spraying. After spraying, TiO₂/PDMS and SA-TiO₂/PDMS coating were produced by drying (80°C, 1 h) and baking (120°C, 1 h).

A JC2000D1 contact angle measuring instrument (Shanghai Zhongchen Technology Co., Ltd. China) was used to test the CA of the SA-TiO₂/PDMS coating. The apparent morphology of the SA-TiO₂ nanoparticles and fabric samples were observed using a scanning electron microscopy (S-4800-I, HITACHI, Japan). The chemical composition of the coatings was examined using a Nicolet iS10 Fourier transform infrared spectrometer (Thermo Fisher, USA). An EDS energy spectrum analyzer (S-4800-I, HITACHI, Japan) was used to analyze the chemical elements on the surface of the raw cotton fabric, TiO₂/PDMS and SA-TiO₂/PDMS coating. The surface tensions of deionized water, simulated blood, dyeing solution, milk, tea liquid and cola were tested using a JYW-200 surface interfacial tensionmeter (Chengde YouteTesting InstrumentsCo., Ltd. China).

The TiO₂ and SA-TiO₂ powder with a mass of 0.5 g was placed into two identical glass bottles containing 10 mL of water, respectively. At various standing durations (0 s, 10 s, 1 min, 2 min, 5 min, 10 min), the settling volume and settling height of TiO₂ powder were observed and recorded.

Anti-adhesion experiments were carried out on superhydrophobic cotton fabrics using deionized water, simulated blood, dyeing solution, milk, tea water and cola. The superhydrophobic fabric was laid flat on a slide, and various droplets of different volumes were dropped vertically on the surface. The slide was slowly tilted from the horizontal direction until the droplets all rolled off the fabric. The volume of the drop and SA were recorded. The adhesion force was calculated using the following equation (1):

m: mass of the droplet, g: gravitational acceleration, 9.8 N/kg; β: angle between the slide and the horizontal plane.

The simulated blood was prepared according to GB 19082-2009 and the formulation was shown in Table 1.

The different droplets, including deionized water, simulated blood, dyeing solution, milk, tea water and cola, with the same volume of 20 μL were dropped onto the surface of the SA-TiO₂/PDMS cotton fabric and their CA and SA were tested. The staining of droplets on the surface was also investigated.

The modification effect of TiO₂ nanoparticles was regulated by varying the amount of stearic acid (1:5, 2:5, 3:5 and 4:5 mass ratio of stearic acid to TiO₂) and the optimum ratio was determined by testing the CA on the surface of the SA-TiO₂/PDMS fabric. The result was shown in Figure 1. When the ratio of stearic acid to TiO₂ was 3:5, the CA on the fabric surface was measured to be 151.33°, showing a superhydrophobic. The CA started to decrease as the stearic acid content increased. Also considering the number of sprays, it was found that the highest CA on the fabric surface was achieved with a 3:5 mass ratio of stearic acid to TiO₂.

The effect of nanoparticle doping and the number of spraying times on the surface CA of SA-TiO₂/PDMS fabric was further determined. As shown in Figure 2(a), with the increase of spraying times, the surface CA of the fabric sprayed directly with PDMS first decreased and then increased, and maintained an increasing trend, with the lowest value appearing at two spraying times. Figure 2(b) showed that the CA on the surface of the fabric increased and then decreased after the addition of nanoparticles. At three sprayings, a maximum value was observed on the surface of the PDMS doped with TiO₂ or SA-TiO₂, with an increase of about 5° compared to the surface CA of the PDMS alone in Figure 2(a). At this point, the maximum CA on the surface of the TiO₂/PDMS fabric and SA-TiO₂/PDMS fabric were 151.33° and 152.03°, respectively. It showed that the modified nanoparticles SA-TiO₂ in the system imparted a better superhydrophobic effect on the surface of cotton fabrics. Orthogonal analysis was performed on the above experiments (Table S4).

The sedimentation effect used to compare the hydrophobic characteristics of TiO₂ nanoparticles before and after modification. The SA-TiO₂ modified with a 3:5 mass ratio of stearic acid to TiO₂ was used as the test object. TiO₂ nanoparticles started to settle as soon as they were applied [Figure S1(a)] and the process gradually finished over time. The settlement was essentially finished when the standing time reached 10 min [Figure S1(f)]. When SA-TiO₂ nanoparticles were added, no settling occurred [Figure S2(a)], and there was almost no settlement even after placing for 10 min. The SA-TiO₂ nanoparticles exhibited hydrophobicity, in contrast to the TiO₂ nanoparticles.

After determining the ratio of stearic acid to TiO₂ as 3:5, as shown in Figure 2(a), the CA on the surface of SA-TiO₂/PDMS was maximum at 3 times of spraying. The effect of spraying times on the CA and SA on the surface of SA-TiO₂/PDMS fabric was further evaluated and the results are shown in Figure S3. It became superhydrophobic surface after two spraying times. The CA increased by about 2° when sprayed three times and started to decrease after four sprays. It may be due to the aggregation of excess SA-TiO₂ in the PDMS solution. To verify the conjecture, the apparent CA (θ *) and the maximum CA (θ ₁) were substituted into the Formula (2), and the proportion (f₂) occupied by air in the composite interface was calculated to be 0.743, 0.758, 0.76, 0.733 and 0.739, respectively. With the increase of spraying numbers, the air pocket space between the fabric surface and the droplet increased first and then decreased. This was due to the aggregation of TiO₂ particles bound to the cotton fabric and the decrease in roughness of the cotton fibers:

Where θ* is the apparent CA on the superhydrophobic surface; f₁ and f₂ refer to the unit apparent volume fraction ratio of water to air on the solid surface, respectively, and f₁+f₂ = 1. θ₁ is the maximum static CA between the liquid and the smooth surface.

From the common characterization of SA and CA in Table S1 and Figure 3, when the modification ratio was 3:5 and the number of sprayings was 3, the SA of the fabric was 7.3°. And the CA reached a maximum of 155.48° and the SA-TiO₂/PDMS fabric exhibited optimum superhydrophobicity.

To investigate the effect of applied tension on the hydrophobicity of the fabric during spraying, the CA and SA of the fabric surface were tested under two conditions, as shown in Figures S3 and S4. The larger the CA and the smaller the SA, the best superhydrophobicity of the fabric. The CA of the superhydrophobic fabric surface obtained with tension applied was significantly greater than that of the surface prepared without tension, with a difference of around 3°. This condition was reflected in a reduction of approximately 2° on SA. This indicated that appropriate tension made the cotton fabric surface more stretchable and flatter, which was conducive to the uniform deposition of the suspension, resulting in a more uniform rough structure on the fabric surface and a better superhydrophobic effect.

The distribution of the nanoparticles on the fabric surface was directly influenced by the dispersibility of suspension, which indirectly affected the hydrophobicity of the fabric. As can be seen from Table S2, the CA of the superhydrophobic surface prepared from the sonically dispersed suspension was approximately 4.15° greater than that of produced without sonication. This was due to the homogenous SA-TiO₂/PDMS system that facilitated uniform distribution on the fabric surface.

SA-TiO₂/PDMS fabric, TiO₂/PDMS fabric and PDMS fabric were placed vertically and flicked 10 times with a glass rod to observe the change in quality before and after flick. As can be seen from Figure S5, The mass of the SA-TiO₂/PDMS fabric remained unchanged after slight flicking, while that of the TiO₂/PDMS fabric only decreased by 0.0064 g, indicating that both the SA-TiO₂/PDMS and PDMS systems had good fastness to binding to the fabric. The compounding of PDMS and TiO₂ promoted the organic-inorganic particles in the hybrid system to form a micro/nano rough structure on the fabric surface. The modification not only improved the dispersion of nanoparticles in the SA-TiO₂/PDMS system and the superhydrophobicity of the SA-TiO₂/PDMS fabric, but also increase the bonding fastness of the SA-TiO₂/PDMS system to the fabric.

As can be seen in Figure 4, the size and shape of TiO₂ particles remained mostly the same before and after the modification. The agglomeration between TiO₂ particles was more visible [Figure 4(d)], but the agglomeration of SA-TiO₂ was minimized and the inter-particle distance significantly increased [Figure 4(b)]. This is mainly attributed to the reduced number of polar hydroxyl groups and growing number of hydrophobic groups after modified, which decreased the surface energy of TiO₂ particles and prevented inter-particle agglomeration. In addition, long polymer molecules coated on the surface enlarged the distance between particles and thus exhibited better dispersibility and hydrophobicity.

Figure 5 revealed the surface morphology of SA-TiO₂ cotton fibers under various spraying durations. After one spraying, there were few SA-TiO₂ nanoparticles on the cotton fibers, which remained smooth locally and the coating was not uniform [Figure 5(a)]. When sprayed for 3 times, the density of SA-TiO₂ nanoparticles on the cotton fabric increased significantly and was uniformly distributed [Figure 5(b)]. It was evident from the magnified image in Figure 5(d), the particles formed a micro-nano structure on the surface of the fibers, indicating that TiO₂ and PDMS were successfully deposited on the fabric surface. When the number of sprays was increased to 5, more nanoparticles were deposited and agglomeration occurred.

Figure 6 showed the SEM images of PDMS fabric and SA-TiO₂/PDMS fabric. The surface of the fabric sprayed with PDMS only [Figure 6(b) and 6(d)] was smoother and covered with only a polymer film-like structure with no rough, while the surface of the fabric sprayed with SA-TiO₂/PDMS [Figure 6(b) and 6(d)] was covered with SA-TiO₂ nanoparticles, making the surface show a rough structure.

The infrared spectra of TiO₂ and SA-TiO₂ were shown in Figure 7, the characteristic absorption peaks of O-H were presented at 3,413.3 cm⁻¹ and 3,417.4 cm⁻¹. This demonstrated the surface of TiO₂ particles contained a large amount of hydrophilic group -OH, and as the -OH groups on the surface of SA-TiO₂ were replaced by stearic acid, the peak strength of SA-TiO₂ was obviously weakened. Strong absorption peaks were found at 2,848.2 cm⁻¹ and 2,916.3 cm⁻¹, with peaks arising from the symmetric and anti-symmetric stretching vibrations of CH₃-, -CH₂-. While the peak at 1,464.3 cm⁻¹ was attributed to the stretching vibration of -CH₂- and CH₃- in the stearic acid. The absorption peak at 1703.1 cm⁻¹ corresponded to the characteristic absorption peak of C = O in the -COOH group of stearic acid. This illustrated that the TiO₂ was modified by stearic acid and the introduction of -CH₂- and CH₃- groups effectively reduced the surface energy of TiO₂. From the IR spectral analysis, it could be concluded that stearic acid was successfully grafted onto the surface of TiO₂ particles and achieved modification.

Table S3 Displayed the surface elemental alterations on the cotton fabric both before and after treatment. the original fabric was made up of the elements C and O with 47.77% percentage of O and 52.23% percentage of C. In addition to C and O, the surface of PDMS fabric contained more Si elements from PDMS, where O element accounted for 46.64%, C for 50.76% and Si for 2.6%. compared to the PDMS fabric, the SA-TiO₂/PDMS fabric showed a higher Ti content on the surface, and C, O, Si and Ti were present in proportions of 36.65%, 52.55%, 4.04% and 6.76%, respectively. The Si and Ti elements were from PDMS and TiO₂ separately. The increase of Si element on the surface was due to the strong binding force between SA-TiO₂ and PDMS, allowing more PDMS to be adsorbed onto the fiber surface.

The resistance of SA-TiO₂/PDMS fabrics to liquid adhesion was investigated by measuring the CA and staining of six kinds of liquid droplets on their surfaces. 20 μL of various droplets were dropped onto the surface, and the SA of each droplet was tested and depicted in Table 2. The SA of deionized water was 6.67°, and by substituting into the equation in 1.5, the adhesion force between the deionized water and the fabric was 22.34 μN. The SA for dye, cola and tea water were 8.33°, 7.67° and 8.43°, with corresponding adhesion forces of 27.9 μN, 25.69 μN and 28.27 μN. The differences in adhesion between dye, cola, tea water and deionized water and fabric were not significant. The SA of the simulated blood and milk were 17.3° and 19.33°, and the adhesion forces were 57.9 μN and 64.74 μN, respectively. The strong adhesion of the two droplets was due to the van der Waals forces between the droplets and fabrics. The experimental results showed that the surface tensions of simulated blood and milk differed greatly from that of water. Their adhesion forces to the SA-TiO₂/PDMS fabric were greater, while the fabric revealed good anti-adhesive properties to cola, dyeing solution, tea water and deionized water.

The surface tensions of deionized water, dye solution, tea water, cola, simulated blood and milk from Table 2 were presented in Figure 8 together with the adhesion forces. As can be observed, the relationship between surface tension and adhesion force was inverse; the higher the surface tension, the lower the adhesion force According to the Formula ΔF_z = ΔF_adh×k, the values for k1, k2, k3, k4 and k5 were, respectively, −1.28, −0.85, −1.20, −0.82 and −0.74. Surface tension was negatively correlated with adhesion force, with k values ranging from −0.8 to −1.2.

Deionized water, tea, dye solution, cola, milk and simulated blood were sequentially dropped onto the SA-TiO₂/PDMS fabric surface. The CA of the different droplets on the fabric surface were measured, as shown in Figure 9. The various droplets dropped on the fabric were all spherical in shape, with no wetting or penetration. The CAs were 155.48°, 151.28°, 152.6° and 152.2°, respectively, which could maintain superhydrophobicity. The droplets were sucked away after placing 1 min, leaving almost no residue on the fabric surface, except for milk and simulated blood with negligible residue (Figure S6). It indicated that the SA-TiO₂/PDMS fabric had good anti-adhesive properties to deionized water, tea water, dyeing solution and cola, while slightly less anti-adhesive to milk and simulated blood.

By spraying of SA-TiO₂ and PDMS onto the cotton fabric, experiments were conducted to create a fluorine-free superhydrophobic cotton fabric with anti-adhesive capacities. Stearic acid was used to modify hydrophilic TiO₂ to hydrophobic as well as improve its dispersibility in the PDMS system. The results showed that the superhydrophobicity of cotton fabrics was enhanced after three sprays of the ultrasonically dispersed SA-TiO₂/PDMS system when tension was applied to the fabric. The SA was 6.67° and the CA was 155.48°. The TiO₂ particles and PDMS was observed on the surface of SA-TiO₂/PDMS fabric with less aggregation and evenly distribution. The tested liquids, including deionized water, tea water, dyeing solution, cola, milk and simulated blood displayed superhydrophobic on the surface of the SA-TiO₂/PDMS fabric, and the surface tension of the liquids exhibited a negative correlation with their adhesion to the fabric. The SA-TiO₂/PDMS fabrics showed good anti-adhesive properties to deionized water, tea, dyeing solution, cola, etc. After 1-min dwell, only trace residues of milk and simulated blood remained on the surface of the fabric, and no wetting or permeation occurred. The superhydrophobic textiles made through this technology showed strong resistant to all common liquids.

This work was supported by Natural Science Foundation of Hebei Province (B2022208014) and Key Project of Hebei Education Department (Grant No. ZD2020112).

The supplementary material for this article can be found online.