sessiledrop

A sessile drop is a liquid droplet resting on a solid substrate and in contact with a surrounding immiscible phase, typically air. The droplet’s edge forms a three-phase contact line at the junction of solid, liquid, and vapor, and the shape reflects the balance of interfacial tensions and gravity. It is a common configuration in surface science and wetting studies.

The equilibrium contact angle of a sessile drop is described by Young's equation: gamma_sv - gamma_sl = gamma_lv cos theta, where gamma_sv is the solid–vapor interfacial tension, gamma_sl is the solid–liquid interfacial tension, gamma_lv is the liquid–vapor interfacial tension, and theta is the static contact angle. This angle characterizes the wettability of the surface by the liquid.

Drop shape is governed by the competition between surface tension and gravity. The dimensionless Bond number Bo = rho g L^2 / gamma_lv, with rho the liquid density, g gravity, L a characteristic length, governs the deviation from a spherical cap. For small drops (Bo much less than 1), gravity is negligible and the drop is well approximated by a spherical cap; larger drops flatten as gravity becomes significant.

Surface roughness and chemical heterogeneity can lead to contact angle hysteresis, where advancing and receding angles differ, affecting droplet mobility and stabilization on a surface.

Measurement methods include static sessile drop profiling to extract the contact angle, as well as dynamic approaches such as tilting plate tests to determine advancing and receding angles. More advanced analyses fit the entire drop profile to the Young–Laplace equation to determine interfacial tensions and surface energies.

Applications span characterization of surface energy and wetting behavior, coatings and printing, microfluidics, biomaterials, and adhesion studies.