EFFECT OF CONTACT-ANGLE ON CAPILLARY DISPLACEMENT CURVATURES IN PORE THROATS FORMED BY SPHERES

The curvature of an interface in a pore depends upon the shape of the pore and the operative contact angle that the interface makes with the solid surface. Even relatively simple pores formed by the surfaces of equal spheres have a complex shape including nonaxisymmetric cross-se ction and converging-diverging geometry. For such pores, a theory for meniscus behavior has been devised that uses a combination of a theory for meniscus curvature in rods together with the toroidal approximati on of Purcell. The results of the theory show that converging-divergin g geometry tends to compensate for the effect of contact angle. This i s because the position at which the nonzero contact angle meniscus has maximum curvature in a converging-diverging pore is not the narrowest part of the pore throat. Due to this compensation, the effect of cont act angle on maximum meniscus curvatures for drainage is approximately proportional to cos 2/3 theta (rather than the cos theta appropriate for cylindrical tubes). Experiments on pores formed by PTFE spheres us ing partially wetting liquids confirmed the theoretical prediction. Co ntact angle measurements on the PTFE spheres also demonstrated that, b ecause of microscopic surface roughness, receding contact angles (thes e being operative with respect to drainage) on ground surfaces are sig nificantly lower than values for smooth surfaces. (C) 1994 Academic Pr ess, Inc.