The Effect of Solid Surface Roughness on Dynamic Contact Angles in Solid-Liquid-Liquid Systems

The engineering applications of spreading and adhesion phenomena involving fluids on solids are numerous. The adhesive and spreading interactions at the solid-fluid interfaces are well characterized by dynamic contact angles. This study reports on the results of an experimental investigation into the effect of solid surface roughness on dynamic contact angles in solid-liquid-liquid (S-L-L) systems. The experiment involved the use of Wilhelmy Plate apparatus to measure adhesion tension (which is the product of interfacial tension and cosine of the contact angle between the liquid-liquid interface and the solid surface), the DuNuoy tensiometer to measure the liquid-liquid interfacial tension, and a profilometer to characterize the roughness of the solid surfaces used. The components of the solid-liquid-liquid systems studied consisted of: (i) smooth glass, roughened quartz and an actual rock surface for the solid phase, (ii) normal-hexane and deionized water as the two immiscible liquid phases. The dynamic contact angles (advancing and receding angles) of the three-phase (rock-oil-water) system provide essential information about the wettability of petroleum resrvoirs. The wettability of a reservoir is an important parameter that affects oil recovery in primary, secondary, and enhanced recovery operations [1]. Contact angle measurements on smooth surfaces are generally used to characterize reservoir wettability. However pore surfaces within reservoir rocks are essentially rough and hence it is important to determine the effect of such roughness on measured contact angles. There is very little information in the open literature on the effect of surface roughness on dynamic contact angles in S-L-L systems. In the present work, four levels of roughness of solid surfaces of similar mineralogy (quartz and glass) were tested in hexane-deionized water fluid pair. The advancing and receding contact angles measured at ambient conditions were analyzed for wettability effects. It was found that as surface roughness increased, the dynamic contact angles also increased. The wettability of the rock-oil-water system shifted from weakly water-wet for the smooth glass to intermediate-wet for the roughened surface. The general trends observed in our study were found to be in good agreement with other published results. However, the generally held notion of increasing contact angle hysteresis with increasing roughness appears to be incorrect in solid-liquid-liquid systems.