LIQUID SPREADING

We review the wetting and spreading properties of simple liquids on solid surfaces, putting emphasis on the role of the heterogeneities of the solid surface and on the spreading kinetics. In a situation of partial wetting, the liquid does not spread completely and shows a finite contact angle on a solid surface. The partial wetting behaviour on perfect solid surfaces is well described by classical capillarity. Heterogeneities of the solid surface lead to contact angle hysteresis. In a complete wetting situation, the liquid forms a film on a solid surface with a thickness in the mesoscopic range. The direct long range interaction between liquid and solid described by the so-called disjoining pressure governs the physics of these films. Films of mesoscopic thickness also appear in the spreading kinetics of liquids. These precursor films form ahead of macroscopic advancing liquid fronts. The spreading kinetics is extremely slow.