Effect of gravity on contact angle: An experimental investigation

Abstract

A capillary system is considered that is exposed to a gravitational field and contains a single component fluid. A portion of the liquid is above and a portion below the vapor. A three-phase line is formed at both places. A previous thermodynamic analysis of such a system, based on the Gibbs model of the interphase and the resulting necessary conditions for equilibrium, has led to the prediction that the equilibrium contact angle at the upper three-phase line is necessarily smaller than that at the lower three-phase line, and to a method that can be used to predict the contact angle at the lower interface for a given value of the contact angle at the upper interface. Even in the absence of any line tension effects, this contact angle difference is predicted to exist for a solid surface that is both smooth and homogeneous. The experimental investigation reported herein supports this prediction and shows that if the system is displaced from this configuration that the system returns to the same configuration. This suggests that the configuration in which the contact angle is smaller at the upper three-phase line than at the lower three-phase line not only satisfies the necessary conditions for equilibrium, but that this configuration is the stable equilibrium configuration for the system.