Cassie's law

Cassie's law estimates the apparent contact angle of a liquid on a composite surface, such as solid patches separated by trapped air.

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Cassie–Baxter equation: An equation for apparent contact angle on a surface where a liquid contacts multiple materials or material and air. This commonly used form makes Cassie's area-weighted relation explicit.
Wetting: The tendency of a liquid to maintain contact with or spread across a solid surface. Cassie's law describes one way surface structure changes wetting behavior.
Superhydrophobicity: A state of extreme water repellency, commonly characterized by contact angles above 150 degrees and low droplet adhesion. Cassie's regime is a standard explanation for high apparent water angles on textured surfaces.
Cassie-to-Wenzel transition: A transition from a composite wetting state with trapped air to complete liquid penetration of surface texture. It marks a failure mode for Cassie's assumed air-supported state.
Contact angle: The angle at which a liquid–vapor interface meets a solid surface, measured through the liquid. Cassie's law predicts this angle as an apparent quantity.
Interfacial tension: The energy cost per unit area of an interface between two phases, including liquid and vapor. Interfacial tensions determine the equilibrium angle beneath Cassie's apparent-angle estimate.
Lotus effect: Water repellency and self-cleaning on lotus leaves, produced by waxy chemistry and hierarchical surface texture. The leaf's texture can sustain droplets on solid tips and air pockets.
Impaled state: A wetting state in which liquid penetrates partway into a textured surface while air remains trapped elsewhere. Partial penetration complicates the simple solid-and-air area fractions in Cassie's law.
Young's equation: A force-balance relation connecting equilibrium contact angle to solid–liquid, solid–vapor, and liquid–vapor interfacial tensions. It supplies the intrinsic contact angle used for each solid region in Cassie's relation.
Contact line: The boundary where solid, liquid, and vapor meet. Pinning of this boundary can make measured angles depart from Cassie's ideal relation.