Wettability

Wettability is the tendency of a liquid to maintain contact with a solid surface, determined by the balance of interfacial tensions at the solid–liquid–vapor interface. It is commonly quantified by the contact angle, the angle formed between the liquid–solid interface and the tangent to the liquid surface at the contact line. A small contact angle indicates high wettability, while a large angle indicates poor wettability. In water-based systems, surfaces with contact angles below about 90 degrees are considered hydrophilic, whereas angles above 90 degrees are hydrophobic; angles above about 150 degrees are described as superhydrophobic when accompanied by low contact angle hysteresis.

The classical description for an ideal smooth, homogeneous surface is Young’s equation: cos theta = (gamma_sv - gamma_sl)/gamma_lv, where gamma_xy are interfacial tensions. Real surfaces are rough and chemically heterogeneous, so Wenzel’s model (rough surfaces) and Cassie–Baxter model (composite or porous surfaces) modify the effective contact angle. Contact angle hysteresis, the difference between advancing and receding angles, reflects surface roughness, chemical heterogeneity, and contact-line pinning.

Wettability is influenced by surface energy and chemistry, roughness, cleanliness, temperature, and time. Contamination, aging, or environmental exposure can alter wettability by changing the surface energy. Measurement methods include the sessile-drop method, where a droplet is placed on a surface, the Wilhelmy plate method, and captive-bubble techniques; both advancing and receding angles may be reported.

Applications span coatings and adhesion, printing, oil recovery, microfluidics, and biomedical devices. Engineering wettability enables control of liquid spreading, wetting and dewetting dynamics, and capillary-driven transport in diverse materials.