An experimental study of some effects of solid surface roughness on wetting

An experimental study was made of the influence of surface roughness on the equilibrium spreading of liquid on solids, employing well-characterized rough surface and a well-defined technique of attaining drop equilibrium. The surfaces were prepared under practical conditions for use, e.g. in adhesion, i.e. without rigorous purification or attempting to eliminate anisotropy or micro-heterogeneities in surface free energy. Two limiting cases of surface roughness were examined: spiral grooves yielding a stick-jumps contact line movement which agreed reasonably well with the theory for a concentric groove model; and radial grooves on which the advancing contact angle agreed with Wenzel's classical equation. On the forms of roughness such as parallel grooves, hexagonal sinusoidal grooves, bead-blasted and composite surface , stick-jump movements become less evident as capillary channeling increased, so that spreading accordingly varied between these limiting cases. Wetting hysteresis occurred on all surfaces, exceeding that predicted theoretically from the roughness measurements made by surface profilometry. Scanning electron microscopic observation suggest that the enhanced hysteresis may have resulted from added complications introduced by microscopic roughness features such as sharp edges which are known to inhibit movement of the contact line.