The role of solid substrate on the spreading kinetics of liquid droplet

Classic hydrodynamic wetting theory leads to a linear relationship between spreading speed and the capillary force, being determined only by the surface tension of the liquid and its viscosity. The theory appears in good agreement with results generated from experiments conducted on the spreading of Polydimethylsiloxanes, PDMS on soda-lime glass substrate and fails to account for the behavior of other liquids. The spreading kinetics of three different liquids (PDMS 1000cp, hexadecane and glycerin) was determined on three different solids, namely, soda-lime glass, polymethylmethacrylate (PMMA) and polystyrene (PS), which exhibit different critical wetting energies. The results are summed up in two themes; equilibrial spreading and kinetics. PDMS is found to exhibit complete spreading on all three different solids at similar rate for glass and PS, but at much lower rate on PMMA. Hexadecane, a low surface energy liquid, was noted to exhibit equilibrial wetting that is proportional to the critical wetting energy of the solid substrate. Glycerin, a high energy liquid, was noted to exhibit equilibrial wetting that is inversely proportional to the wetting energy of the solids. The equilibrial spreading was found explicable on basis of the axiom "like wets like". Contributions of the solid substrate to the spreading kinetics are attributed to specific solid-liquid interactions, of dissipative nature, which manifest itself only at the interface. The term "interfacial viscosity" is coined to account for this phenomenon.