**2.5 pH-responsive surfaces**

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 to pH stimulus.

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 wettability [78, 81–83].

#### **Figure 10.**

*(A) Advancing and receding CAs of PEAALbL on a micro-patterned substrate with a square array of cylindrical pillar structures. Optical images show water droplets sticking to the substrate at pH 3, suggesting strong pinning. Inset in the upper right corner represents advancing and receding CAs of water droplets on the unpatterned substrate. (B–E) Cryo-SEM images of a water droplet on a PEAALbL-coated micropillarpatterned substrate at (B, C) pH 3 and (D, E) pH 8. Reproduced with permission from [80].*

#### **2.6 Other stimuli**

Some of the recent work also demonstrates using magnetic field and solvents as external stimuli to manipulate the wettability for given fluids. Smart surfaces that respond to magnetic field have been demonstrated by numerous research groups in the last decade [84, 85]. Grigoryev et al. fabricated a microstructured surface with reentrant geometry composed of Ni micronails, which shows a reversible transition from superomniphobic to omniphilic wetting state in response to the external magnetic field [85]. Cheng et al. also showed the reversible wetting transitions of the microdroplet consisting of superparamagnetic Fe3O4 nanoparticles [84]. They have shown that the wettability can be reversibly switched between the Cassie state and the Wenzel state on the microstructured silicon substrate and the transition between two wetting states can be controlled by both concentration of nanoparticles and the intensity of the magnetic field. On the other hand, solvent as an external stimulus is also feasible to tune the wetting behavior as solvent-responsive surfaces are affected by the surrounding medium and their wettability change is governed by the change in their interfacial energy caused by the rearrangement of the molecular chains in response to a solvent [81, 86–88]. Minko et al. demonstrated a novel route to fabricate two-level structured polymer brushes, and the surface wettability could be reversibly controlled by exposing the polymer brush surfaces to the solvent, which is selective to one of the polymers. Surface morphology and surface properties change when exposed to different solvents, which is caused by the interchange between vertical and linear phase segregation of the polymers [81].

#### **3. Conclusions**

To conclude, smart surfaces with tunable wettability based on various external stimulus prove to be useful candidates in different applications where the continuous and reversible tuning of surface wettability is important. These surfaces exploit the additional energy gained from the external stimulus to change their wetting behavior as per the requirement. It has been demonstrated that the surface wettability can be varied from hydrophobic to hydrophilic on smooth surfaces and from

**139**

**Author details**

Meenaxi Sharma and Krishnacharya Khare\*

provided the original work is properly cited.

\*Address all correspondence to: kcharya@iitk.ac.in

Department of Physics, Indian Institute of Technology Kanpur, Kanpur, India

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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

application.

**Acknowledgements**

**Conflict of interest**

The authors declare no conflict of interest.

superhydrophobic to superhydrophobic on patterned surfaces. Many such external stimuli are available to alter the surface wettability because each technique has few advantages and disadvantages over others. As described in different sections, almost complete understanding about most of the techniques has been established. Of course, there are some limiting cases where the scientific understanding is not completely clear and is an active area of research currently. Hence, depending upon the feasibility and available resources, users can choose a suitable stimulus for their

The authors acknowledge fruitful discussion with Reeta Pant and Subhash Singha, which was particularly useful for the section "**Photoresponsive surfaces**". *Smart Surfaces with Tunable Wettability DOI: http://dx.doi.org/10.5772/intechopen.92426*

superhydrophobic to superhydrophobic on patterned surfaces. Many such external stimuli are available to alter the surface wettability because each technique has few advantages and disadvantages over others. As described in different sections, almost complete understanding about most of the techniques has been established. Of course, there are some limiting cases where the scientific understanding is not completely clear and is an active area of research currently. Hence, depending upon the feasibility and available resources, users can choose a suitable stimulus for their application.
