EFFECT OF GRAVITY ON CONTACT-ANGLE - AN EXPERIMENTAL INVESTIGATION

A capillary system is considered that is exposed to a gravitational fi eld 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 b oth places. A previous thermodynamic analysis of such a system, based on the Gibbs model of the interphase and the resulting necessary condi tions for equilibrium, has led to the prediction that the equilibrium contact angle at the upper three-phase line is necessarily smaller tha n 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 a ny line tension effects, this contact angle difference is predicted to exist for a solid surface that is both smooth and homogeneous. The ex perimental 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 conf iguration in which the contact angle is smaller at the upper three-pha se line than at the lower three-phase line not only satisfies the nece ssary conditions for equilibrium, but that this configuration is the s table equilibrium configuration for the system. (C) 1998 American Inst itute of Physics.