Equilibrium Contact Angle at the Wetted Substrate

TL;DR

This paper introduces a new model for steady-state contact angles of liquid droplets on substrates, linking microscopic intermolecular forces to macroscopic contact angle measurements, independent of droplet size.

Contribution

The model uniquely relates contact angles to disjoining pressure parameters, providing a simple formula applicable to both small and large angles, and suggests experimental validation for parameter estimation.

Findings

Contact angle is independent of droplet size for large droplets.

A simple formula relates contact angle to surface tension and disjoining pressure.

Model enables constraining disjoining pressure parameters through experiments.

Abstract

We construct a novel model for the steady-state contact angles of liquid droplets at the wetted substrate. The non-removable, thin liquid film covering the substrate is governed by the intermolecular forces between molecules of liquid and solid, which we describe using the standard disjoining pressure approximation. Balancing the disjoining pressure against the surface tension we find the smooth shape of the surface of the liquid. We show that we can extract an effective contact angle from the region where the film and the droplet meet. Crucially, we find that for large droplets the contact angle is independent of the droplet size. Instead, the contact angle is determined by the surface tension and the disjoining pressure parameters through a simple formula that works for both small and large contact angles. We suggest that comparing predictions of our model to experimentally measured contact angles will enable constraining the parameters of the disjoining pressure models.