Contact angle and film jumps

Abstract

THE mechanism of capillary rise in a random sphere packing is such that, depending on the absolute value of the wetting- or contact-angle, θ, at equilibrium either a saturation gradient, widest for θ = 0, is observed, or a sharp front between the saturated and the dry part of the porous medium, above some critical value of θ (ref. 1). We present here another partition in the phenomenological description of capillary rise for systems which have θ>0. There are systems in which θ remains constant and others in which θ decreases during capillary rise. In particular, attention is drawn to an uncommon hypothesis that can explain a decrease in θ. Given the nature of the absorption isotherm in the neighbourhood of saturation, small variations in vapour pressure may induce discontinuous film thickness jumps on a microscopic scale, which may display itself macroscopically as a changing contact angle.