The influence of wettability and critical pore-throat size ratio on snap—off

Abstract

Experiments under quasistatic conditions show that the aspect ratio (pore-to-throat effective diameter ratio) necessary for snap-off in throats is ∼ 1.5 when advancing contact angles ( θ A) are equal to zero and increases only slightly, to 1.75, when θ A is equal to 55°. Above ∼70°, snap-off in throats does not occur in systems with pressure equilibrium between pores and throats. Three critical contact angles are recognized for two types of interfaces. Advancing ( θ A) and receding ( θ R) angles for convex interfaces and θ S, the angle measured for selloidal interfaces at the moment of instability when snap-off occurs. Capillary pressure, disjoining pressure, thin-film thickness, and contact angles are interrelated phenomena and, for a given solid and fluid pair, θ A, θ R and θ S may be defined at different pressures and are not equal but are points on a common curve, the equation for which is defined. As θ A increases from 0° to 70° we estimate that θ S increases from 30° to 77°. The difference between θ S and θ A decreases with increase in θ A. The Young-Laplace equation assumes constancy of contact angle and must be applied with caution since contact angle is a function of capillary pressure as well as of interface type.