Drop penetration into a thin porous medium

Abstract

A thermodynamic analysis of the penetration of a drop into a thin porous medium is presented. It is shown that the thinness of the porous medium and the finite size of the drop play an important role in the phenomenology of penetration. In particular, a new equilibrium state is revealed in relationship with basal penetration, which is the situation where the liquid penetrates just beneath the base of the drop. The basal penetration equilibrium may be stable or metastable, depending on the parameters of the system. "Phase diagrams" are presented and show the regions of complete penetration, basal penetration, and no penetration as a function of the true contact angle and the thickness of the porous medium relative to the drop size. The existence of the basal penetration equilibrium is shown to be sensitive to the relative thickness of the medium. For very thin media, it may entirely replace the mode of complete penetration. In addition, a saddle point in the free energy surface of the system, which corresponds to the initial stage of penetration, is revealed. This observation implies that initially the driving force for penetration is low and the process is expected to be slow. © 1988.