Wetting

Wetting is the tendency of a liquid to spread on or adhere to a solid surface. Wettability describes how well a liquid wets a surface and is typically quantified by the contact angle formed at the junction of liquid, solid, and air. If a liquid spreads completely, the surface is said to be wetted; if droplets form beads, wetting is poor.

The contact angle is measured at the intersection of the three phases and provides a macroscopic indicator of surface-liquid affinity. Young's equation expresses the balance of interfacial tensions: gamma_sv = gamma_sl + gamma_lv cos theta, where gamma_sv is the solid–vapor tension, gamma_sl is the solid–liquid tension, gamma_lv is the liquid–vapor tension, and theta is the contact angle. Small contact angles indicate good wetting (often associated with hydrophilic surfaces), while large angles indicate poor wetting (often hydrophobic surfaces).

In real systems, surface roughness and chemical heterogeneity cause contact-angle hysteresis, leading to different advancing and receding angles as a liquid spreads or retracts. The Wenzel model describes how roughness amplifies intrinsic wetting, whereas the Cassie–Baxter model describes surfaces with trapped air pockets that reduce solid–liquid contact. Dynamic wetting concerns the spreading rate and how surface features influence it.

Measurement techniques include the sessile drop method and the Wilhelmy plate method. Wetting is a key consideration in coatings, painting, printing, lubrication, soil science, and biological processes, influencing adhesion, spreading, and transport phenomena.

Commonly, hydrophilic surfaces have contact angles below 90 degrees, while hydrophobic surfaces exceed 90 degrees; superhydrophobic surfaces can reach angles above about 150 degrees.