EFFECT OF GRAVITY ON CONTACT-ANGLE - A THEORETICAL INVESTIGATION

Using the Gibbs description of an interphase, the necessary conditions for equilibrium bf a closed, two-phase fluid system in the presence o f gravity are the Laplace and Young equations and a condition on the c hemical potentials. The last condition has been neglected in all previ ous examinations of contact angles in a gravitational field. After int roducing explicit expressions for the chemical potentials, we find tha t the condition on the chemical potentials can be used to determine th e pressure profile within the system. In a ''two-interface'' system in which a liquid phase is both above and below a vapor phase and the va por phase forms a solid-vapor interphase in one region, the pressure p rofile in the liquid phases is the same as it would;have been if the v apor phase were not there; thus in a gravitational field, the pressure is smaller in the liquid phase above the vapor phase than it is in th e liquid phase below the vapor phase. This results in the contact angl e at the upper three-phase line necessarily being smaller than that at the lower three-phase line. This difference in contact angles is conv entionally referred teas contact angle hysteresis; however, we show th at it is simply an equilibrium property of a capillary system in a gra vitational held. The contact angle difference predicted to exist in th e presence of gravity does not violate the Young equation, but the You ng equation does impose a restriction on the equilibrium adsorption is otherms at the solid-vapor and solid-liquid interfaces. (C) 1998 Ameri can Institute of Physics.