DYNAMICS OF INCOMPLETE WETTING ON POROUS MATERIALS

The spreading kinetics of incompletely wetting liquids on porous subst rates, highlighting the competition between composite interface spread ing and liquid infiltration into the porous substrate are considered. The energetic impact of the surface porosity on the advancing of the c ontact line is modeled by using three idealized microtopologies associ ated with the kinetic behavior of the liquid in the pores. The results are used to modify a previously developed hydrodynamic spreading mode l to include the effects of substrate porosity. The results of a previ ously developed model for the depletion of a liquid droplet on a porou s substrate are then used to consider the interaction between the spre ading and depletion processes. It is predicted that, for a wide class of situations, a major portion of the spreading process will generally be completed before liquid drainage becomes significant. A contact an gle hysteresis associated with spreading on a porous substrate is pred icted to occur, with the advancing angle being larger than the recedin g angle; and an analogy is made with spreading on a rough or pitted su bstrate which provides a mechanism for contact angle hysteresis on suc h surfaces. (C) 1994 Academic Press, Inc.