| Abstract: | Wetting hysteresis, or the variability of contact angle, is recognized to be caused by several phenomena. In particular, we consider it to be due to heterogeneity of the solid surface in contact with the liquid. Results describing the deformation of an initially straight triple line in the proximity of an isolated, small, energetic inhomogeneity are summarized. The theory is extended to describe the behaviour of a wetting front near a circular, high-energy (i.e. corresponding to zero intrinsic contact angle) flaw of dimensions comparable to those of the triple line distortion. A simple model is proposed to explain the breakaway of the isthmus of liquid connecting the flaw to the bulk liquid. Separation time is found to be proportional to -5, where 0 is the intrinsic contact angle of the solid/ liquid system. The behaviour of a triple line on a solid possessing a randomly distributed population of identical, small, circular flaws is considered. Contact angle hysteresis can thus be explained, as can the noise often observed in a (dynamic) Wilhelmy plate experiment. A simple statistical model is proposed in which random force fluctuations related to a moving triple line are shown to correspond to the Poisson process of probability theory. |
