**2.4 Mechanically tunable surfaces**

As it is mentioned earlier, the surface wettability is mainly governed by chemical composition and structures of the surface. Altering one of these factors would lead to change the surface wetting properties. Most of the external stimulus changes the chemical composition of the substrate; however, changing the surface structures also produces a change in the surface wettability. Various research groups have demonstrated the mechanical stress-responsive tunable wettability using elastic or gel materials [68–75]. Poly(tetrafluoroethylene) PTFE is the most common

#### **Figure 8.**

*(a) The schematic of the light-responsive release system. After irradiation with 365 nm UV light, the surface became wet due to the conformational conversion of spiropyran from the "closed" form to the "open" form (b). The mechanism illustrating the wetting process of surface. Reproduced with permission from [67].*

hydrophobic material, and Teflon tape, made with PTFE crystals, has been widely used to tune the wetting behavior by applying mechanical strain. Zhang et al. demonstrated a change in the density of PTFE crystal by axial extension [68]. They also reported that with an extension of the PTFE crystals to 190%, water contact angle could be tuned between hydrophobic (118°) and superhydrophobic (165°) states. In their next work, using this approach of biaxial extension and unloading, they have shown reversible wettability switch between superhydrophilic and superhydrophobic behavior of the polyamide films with triangular structures [69]. Initially the polyamide film with the triangular structures showed superhydrophobic behavior with a water contact angle of ~151.2°. When the film is extended to 120%, water penetrates through space among the fiber resulting in the complete wetting situation. While unloading, the polyamide film returned to the superhydrophobic state due to the recovery of the surface structures. In addition, poly(dimethylsiloxane) (PDMS) microwrinkle structures also demonstrate tunable wetting behavior upon stretching. Wrinkling approach is very useful as its wavelength and amplitude can be actively tuned to produce various well-defined structures.

Lin et al. have shown the fabrication of superhydrophobic surface, featured with micro and nanoscale roughness, using nanoparticle coating on PDMS wrinkled structures [71]. For the first time, they have shown the fabrication of a superhydrophobic surface by combining microstructured PDMS wrinkles with nanoparticles. **Figure 9** is a schematic illustration of the fabrication of PDMS elastic film with dual scale roughness. Surface topography can be reversibly tuned between complete relaxed state (wrinkle formation) and the complete stretched state (only nano roughness), that is, reversible switching between the dual scale roughness and nanoscale roughness (as shown in **Figure 9f–k**). Using both theoretical and

#### **Figure 9.**

*A schematic illustration of the fabrication of a PDMS film with dual-scale roughness (a–f) and real-time, reversible tunability of its surface topography by mechanical strain (f–k). Inset shows the optical images of a water drop on the surface with dual-scale roughness (right) and nanoscale roughness (left). Reproduced with permission from [71].*

**137**

wettability [78, 81–83].

*Smart Surfaces with Tunable Wettability DOI: http://dx.doi.org/10.5772/intechopen.92426*

**2.5 pH-responsive surfaces**

to pH stimulus.

experimental approaches, they concluded that only micron-scale roughness is not able to attain Cassie-Baxter state. By regulating the surface topography by applying different mechanical strain, they have shown switching between different wetting states from Cassie-Baxter (dual scale roughness) to Wenzel (both micron wrinkle and nanoparticle-coated elastic film). Chung et al. have shown that the drop shape strongly gets affected by the geometrical anisotropy of the sinusoidal grooves as on the isotropic surface contact angle were uniquely defined; however this was not the case for anisotropic surfaces on which drop showed two different contact angles in

In recent years, pH-responsive wetting has gained a lot of attention because of their emerging applications in various fields such as drug delivery and biosensors [76]. It is of particular interest where it is required to change the wetting behavior of various acidic and basic liquids. Xu et al. have shown a novel method to prepare pH-responsive surfaces containing block copolymer thin films of poly(styrene-b-acrylic acid) (PS-b-PAA) and pH-responsive nanostructures composed of cylindrical domains [77]. PS-b-PAA polymer films show different surface morphologies for three different pH regimes. These polymers films swell more rapidly when immersed in high-pH solutions compared to low-pH solutions. They also observed that with the increase in the pH from 2.6 to 9.1, the water contact angle decreased by 30°. The decrease in the contact angle is due to the rear molecular arrangement of PAA chains in response to increase in pH, which results in increasing hydrophilicity [77, 78]. They concluded that wettability can be regulated by controlling the molecular arrangement of PAA chains in response

Uhlmann et al. introduced the idea of surface functionalization for the smart coatings using stimuli-responsive binary polymer brushes containing polymer chains of two different polymers using "grafting from" and "grafting to" approach [79]. The concept of reversible switching can be understood based on the reaction of polymers with different solvents, where a polymer brush of hydrophilic and hydrophobic polymers is treated with nonselective and selective solvents for both the polymers. In a good solvent, due to the dominance of the interchain repulsion, polymer chains show stretched conformation while in a bad solvent, due to strong repulsion between solvent and polymer, chains show coiled and collapsed conformation. This can also be interpreted as; brush structure shows hydrophilic behavior when treated with a selective solvent for polymer A while shows hydrophobic behavior when treated with a selective solvent for polymer B. However, in a nonselective solvent, both polymers show laterally segregate structures. Later Lu et al. reported a system of layer by layer (LbL) hydrogels, composed of amphiphilic polymers, which can undergo a reversible transition in response to pH stimulus. To shed light on the exact wetting state of hydrophobic PaAALbL-coated patterned surface, they measured the CAH (**Figure 10A**) and imaged the behavior of a water drop for different pH values (**Figure 10B**–**E**). For pH < 6, contact angle hysteresis was too large, close to 120° and the drop was sticky to the surface, and did not fall even if the substrate was turned vertically down. Such stickiness behavior with high CAH is realized for the drop in the Wenzel state, and the observed behavior can be accounted for the enhanced contact line pinning by the surface microstructures [80]. Numerous other researchers have also investigated pH-responsive tunable wetting behavior from its fundamental understanding of switchable

the direction parallel and perpendicular to the grooves [70].

*21st Century Surface Science - a Handbook*

hydrophobic material, and Teflon tape, made with PTFE crystals, has been widely used to tune the wetting behavior by applying mechanical strain. Zhang et al. demonstrated a change in the density of PTFE crystal by axial extension [68]. They also reported that with an extension of the PTFE crystals to 190%, water contact angle could be tuned between hydrophobic (118°) and superhydrophobic (165°) states. In their next work, using this approach of biaxial extension and unloading, they have shown reversible wettability switch between superhydrophilic and superhydrophobic behavior of the polyamide films with triangular structures [69]. Initially the polyamide film with the triangular structures showed superhydrophobic behavior with a water contact angle of ~151.2°. When the film is extended to 120%, water penetrates through space among the fiber resulting in the complete wetting situation. While unloading, the polyamide film returned to the superhydrophobic state due to the recovery of the surface structures. In addition, poly(dimethylsiloxane) (PDMS) microwrinkle structures also demonstrate tunable wetting behavior upon stretching. Wrinkling approach is very useful as its wavelength and amplitude can

Lin et al. have shown the fabrication of superhydrophobic surface, featured with micro and nanoscale roughness, using nanoparticle coating on PDMS wrinkled structures [71]. For the first time, they have shown the fabrication of a superhydrophobic surface by combining microstructured PDMS wrinkles with nanoparticles. **Figure 9** is a schematic illustration of the fabrication of PDMS elastic film with dual scale roughness. Surface topography can be reversibly tuned between complete relaxed state (wrinkle formation) and the complete stretched state (only nano roughness), that is, reversible switching between the dual scale roughness and nanoscale roughness (as shown in **Figure 9f–k**). Using both theoretical and

*A schematic illustration of the fabrication of a PDMS film with dual-scale roughness (a–f) and real-time, reversible tunability of its surface topography by mechanical strain (f–k). Inset shows the optical images of a water drop on the surface with dual-scale roughness (right) and nanoscale roughness (left). Reproduced with* 

be actively tuned to produce various well-defined structures.

**136**

**Figure 9.**

*permission from [71].*

experimental approaches, they concluded that only micron-scale roughness is not able to attain Cassie-Baxter state. By regulating the surface topography by applying different mechanical strain, they have shown switching between different wetting states from Cassie-Baxter (dual scale roughness) to Wenzel (both micron wrinkle and nanoparticle-coated elastic film). Chung et al. have shown that the drop shape strongly gets affected by the geometrical anisotropy of the sinusoidal grooves as on the isotropic surface contact angle were uniquely defined; however this was not the case for anisotropic surfaces on which drop showed two different contact angles in the direction parallel and perpendicular to the grooves [70].
