Equilibrium and spreading of liquids on solid surfaces

Abstract

Contact angle equilibrium and dynamic spreading of liquids on solid surfaces are reviewed and discussed. The thermodynamic status of the Young equation is critically reviewed. It is redeveloped rigorously and shown to apply locally, even for rough and heterogeneous though rigid and insoluble soolid surfaces, when written in terms of the local interfacial tensions. The line tension approach and the molecular approach to relate the local interfacial tensions to their values far way from the three-phase contact line are critically discussed, and the difficulties with some recent corrections of the young equation are explained. The problem of the shape of the fluid-fluid interface near the contact line is analyzed, and it is shown that a discontinuous transition has to occur between the solid surface and the fluid-fluid interface.Experimental data and theoretical models for dynamic spreading are discussed and compared. Special emphasis is put on the qualitative and quantitative effects of the primary film. The theoretical investigations are analyzed especially regarding their physical content rather than their hydrodynamic details. Three types of situations are distinguished: forced spreading, spontaneous spreading to an equilibrium contact angle and spontaneous spreading when no equilibriium contact angle exists. It is speculated that the primary film forms only in the latter situation.