Understanding the wetting of solids by liquids is essential in many biological, medical and industrial processes. Examples range from artificial tissue engineering to practical applications like biofouling, non-fogging coatings, fog harvesting or self-cleaning textiles. Surfaces with high liquid repellency, often inspired by nature, have promising applications.

The wetting behaviour of an ideal, flat and chemically homogeneous surface can be described by the Young equation. This equation can be derived by balancing the interfacial forces at a three-phase contact line:¹ 

Here γ_L, γ_S and γ_SL are the liquid–gas, solid–gas and solid–liquid interfacial tensions. In reality, the surface is never completely flat and chemically homogeneous. To account for this non-ideality, we call the contact angle θ that is measured on a flat surface the material contact angle, instead of the Young contact angle. θ can be estimated by eye or quantified by optical microscopy (Figure 8.1). Depending on the value of θ one distinguishes between hydrophilic (θ < 90°, γ_S > γ_SL) and hydrophobic (θ > 90°, γ_S < γ_SL) surfaces (Figure 8.1a and b). For γ_S = γ_SL + γ_L the liquid completely wets the surface (θ = 0°), whereas for γ_S < γ_SL + γ_L a finite contact angle is formed. A hydrophilic surface is also called a high energy surface. The surface energy can be lowered by coating a surface, for example with fluorinated molecules. In that case, contact angles of about 120° have been reported on a smooth homogeneous surface.^2,3