A SIMPLE ANALYSIS OF LOCAL WETTING HYSTERESIS ON A WILHELMY PLATE

Wetting measurements involving evaluation of the contact angle at the triple line of a solid/liquid/fluid system constitute a valuable means of characterizing solid surfaces. In principle, a given three-phase system possesses a unique contact angle value but, in practice, hysteresis effects often lead to a range of accessible values. Although this inherent range renders estimation of the mean value more difficult, it is suggested here that knowledge of the intrinsic magnitudes and spatial distribution of heterogeneities on the solid leading to hysteresis may be used to characterize the surface more completely. The Wilhelmy plate technique used in dynamic mode (at low speed) effectively leads to a scan of the solid because of the moving triple line. Slight oscillations or ripples on the average measured force are attributed to locally changing populations of weak heterogeneities. A simple mathematical analysis of the behaviour is presented and applied to model arrays of inhomogeneities. Although the models are too simple for direct application to most real surfaces, it is believed that the basic ideas may be refined for use on practical systems.