Effects of surface roughness on wettability

The effects of surface roughness on wettability have been studied by many researchers using surface roughness factors such as the Wenzel roughness factor and so on. However, these Factors depend on each other which makes a precise discussion difficult. We adopted two kinds of model, a hemispherical close-packed model and a hemiround-rods close-packed model. Using these models, the changes in height roughness, namely the radius, can be varied without changing the Wenzel roughness factors. This paper analyzed the influence of surface height roughness on wettability using the sessile drop method with nonwetting paraffin!water systems. For the hemispherical close-packed models, the effect of height roughness on wetting can be explained by the change in the radius of curvature, R, of the liquid in trapped air pockets at the solid/liquid interface. When the height roughness, h, is more than 125 mu m, R is constant at 50 mu m, but when h is less than 125 mu m, R is conjectured to be a function of the h, theta and gamma(LV) values. From these discussions, we define the wettable flat surface and the atomistic flat surface. In the case of the hemiround-rods close-packed models, we find that the contact angles, measured from the direction parallel to the rods, resemble the advancing and receding angles of contact angle hysteresis. (C) 1998 Acta Metallurgica Inc.