The microscopic origin of droplet line tension

TL;DR

This paper investigates the microscopic origins of droplet line tension, revealing how gravitational effects and interfacial tension variations influence droplet behavior across scales, explaining experimental variability.

Contribution

It introduces a new mechanism for line tension arising from gravitational and pressure effects, unifying diverse experimental observations.

Findings

Line tension varies with droplet size, wettability, and initial volume fraction.

Sign of line tension depends on surface and volume conditions.

Results agree with experimental and simulation data across scales.

Abstract

The size dependence of the equilibrium droplet contact angle is governed by line tension. In this work, we identify a contribution to line tension arising from gravitational effects and pressure-induced changes in volume-fraction-dependent interfacial tensions within an adsorption layer. This mechanism addresses a multiscale problem of line tension in droplets ranging from nanometric to millimetric sizes that change sign and span several orders of magnitude, in agreement with experimental and simulation results. The sign of the apparent line tension is controlled by surface wettability, the initial volume fraction in the adsorption layer, and the droplet size, which also strongly influences its magnitude. Our results provide a unified physical interpretation of the experimentally observed variability in both the sign and magnitude of line tensions.