THERMODYNAMIC ASPECTS OF CONTACT-ANGLE HYSTERESIS

Theoretical models regarding the thermodynamic aspects of contact angl e hysteresis are reviewed. General thermodynamic considerations are br iefly summarized in order to emphasize the local nature of the thermod ynamic equilibrium condition regarding the contact angle. General cons iderations are also presented concerning the phenomenon of hysteresis, explaining the relationship between multiplicity of metastable equili brium states, branching, and hysteresis. Models of various degrees of complexity are discussed, and their contribution to improving the unde rstanding of contact angle hysteresis is presented. It is pointed out that in most models there is no consideration of the effect of varying the external constraint, such as the drop volume or the depth of a pl ate inside a liquid. It is stressed that calculating metastable contac t angles for a system under a given constraint may not be sufficient. In order to understand contact angle hysteresis and use it to advantag e for characterizing surfaces, the process of measuring hysteresis, i. e. the process of adjustment of the system to systematic variations in the external constraint, must be modelled. Possible effects of the dr op volume on apparent contact angles are indicated for a simple model of heterogenous smooth surfaces.