**1. Introduction**

There are numerous examples in our daily life where we encounter liquid-solid interaction, for example, wall paints, cookwares, cleaning textiles, raindrops on glass surfaces of automobiles, houses, solar panels, to name a few. In all these examples, drops of different liquids interact with given solid surfaces. The macroscopic behavior of liquids on such solid surfaces is actually governed by the microscopic molecular interactions between the liquid and solid molecules, which in scientific terms is described as the wetting behavior of solid surfaces. **Figure 1** shows the schematics of liquid drops on homogeneous and heterogeneous surfaces describing their final wetting behavior. **Figure 1a** shows Young's wetting state of a liquid drop sitting on a homogeneous surface where the surface wettability is described in terms of Young's contact angle (θY), which can be derived by balancing various interfacial forces (γ's) as given by Eq. (1) where *S*, *L,* and *V* represent solid, liquid, and vapor phases respectively [1]. cos θ*Y* = \_

$$\mathbf{Cost}\Theta\_Y = \frac{\mathbf{y}\_{SV} - \mathbf{y}\_{SL}}{\mathbf{y}\_{LV}}\tag{1}$$

**Figure 1.**

*Schematics of different wetting states of a liquid drop on a homogeneous surface: (a) Young's state and heterogeneous surface; (b) Wenzel and (c) Cassie-Baxter states.*

On heterogeneous surfaces, as shown in **Figure 1b, c**, the final wetting behavior is different compared to homogeneous surfaces and is described by Wenzel and Cassie-Baxter contact angles as given by Eq. (2) [2, 3]:

$$
\cos \theta\_W = r \cos \theta\_Y \text{ and } \cos \theta\_{CB} = f \cos \theta\_Y \text{ + (} f \text{ -1} \text{)}\tag{2}
$$

where *r* is the roughness factor, and *f* is the area fraction of the solid-liquid interface.

In many cases, maximum interaction of liquid-solid is required, for example, washing textiles, wall paints etc., whereas there are other cases where minimum liquid-solid interaction is desirable, for example, solar panel, nonstick cookware, and glass windshields. Therefore, it becomes essential to manipulate liquid-solid interaction or wetting behavior as per the requirement. Alternative to the conventional approach, where the wetting behavior is manipulated by varying the surface topography or chemistry, continuously and reversibly varying the wettability based on some external energy source has recently gained huge popularity. This method has many advantages over the conventional method due to its nondestructive nature. Various researchers and engineers have investigated many such external energy sources or stimuli (e.g., electric field, temperature, radiation, mechanical strain, pH, magnetic field, etc.), which can be efficiently used to manipulate the wetting behavior of given solid surfaces continuously as well as reversibly. In this chapter, we provide a comprehensive summary of most of these techniques with the fundamental principle behind them as well as their potential applications.
