HOW DO LIQUID DROPS SPREAD ON SOLIDS?

The coating of a solid with a liquid and the displacement of oil in a porous medium by water are processes that involve the motion of a contact line, the three-phase line common to three materials. In the above illustrations the contact line involves liquid, gas and solid and liquid, liquid and solid, respectively. The realization that effective slip occurs near the contact line allows one to solve problems involving mutual displacement. One such prototype problem involves the spreading of a liquid drop on a solid, the spreading occurring spontaneously upon the placement of the drop on the solid. Such a model was posed by Greenspan for the creeping flow of a thin, Newtonian liquid on a smooth solid. In the present paper, we shall reexamine this model in order to probe more deeply into the roles of viscous and surface tension forces and the wetting characteristics of the solid. We shall discuss two distinct contributions to the spreading characteristics, which we call 'capillary push' and 'contact-line pull'. Since many applications of spreading studies involve the coating by polymer liquids, we shall extend our study to viscoelastic liquids and identify which non-Newtonian properties dominate in the spreading flows of interest.