Dynamic wetting in the low capillary number regime

Abstract

An understanding of dynamic wetting is necessary when considering processes in which one fluid is forced to displace another fluid from a solid substrate. This work examines the forced wetting behavior exhibited by a series of solid-liquid systems in the low capillary number regime. In particular, the effects of specific chemical (i.e. acid-base) interactions between the liquid and the solid and of solid surface roughness on dynamic wetting are investigated. Measurement of the dynamic contact angle as a function of the interline velocity is performed using the technique of microtensiometry. The data are compared with empirical correlations in order to test the hypothesis that a universal correlation may be used to describe wetting behavior. The results suggest that two correlation having the same functional form but different constants are necessary to adequately describe wetting behavior above and below a capillary number of the order of 10 −3. No significant differences are observed between the wetting behavior of systems with only dispersive interactions between the liquid and solid and systems with acid-base interactions, implying that the static contact angle adequately accounts for the effect of acid-base interactions. Surface roughness produces noticeable stick-slip effects in the low capillary number regime when the static contact angle is larger than approximately 15°. While these effects produce a large degree of scatter in the dynamic wetting measurements, they do not alter the constants from those found in the case of smooth interline motion.