A COMPUTER-MODEL FOR WETTING HYSTERESIS .1. A VIRTUAL WETTING BALANCE

The modified Wilhelmy plate method is widely used to determine the wet tability of solid surfaces by probe liquids. This is accomplished by m onitoring the wetting force experienced by a solid suspended from a mi crobalance during immersion into and emersion from the fluid. In the a bsence of surface roughness, fluctuations in force observed as the liq uid slides over the solid and hysteresis between the advancing and rec eding modes of measurement are caused by surface chemical heterogeneit y. An independent analysis based upon the fundamental rules of stick-s lip behavior of the moving contact line is developed to explain these effects quantitatively and to predict the hysteresis loop for a known surface. The extent of hysteresis and the amplitude and frequency of f orce fluctuations are shown to depend upon the nature of the heterogen eity and its spatial distribution. A detailed description of the model is given, followed by results of computer simulations performed on mo del surfaces. The results agree well with previous experimental work.