Dynamic contact angles. Part 8.—Impact spreading of water drops in air and aqueous solutions of surface active agents in vapour on rough paraffin wax surfaces

Abstract

Previous studies of dynamic contact angles involving the impaction of water drops and aqueous solution drops on smooth paraffin wax surfaces have been extended to paraffin wax with roughened surfaces in both random and controlled forms. It is found that the topography of the solid surface imposes additional effects on the basic drop shapes of impaction, mainly via an impact shock wave. The surface roughness increases the dynamic contact angle during the spreading sequence but not during the retraction sequence. The rate of attainment of the maximum spreading contact angle during impaction of aqueous aliphatic n-alcohol solutions varies with the chain length of the alcohol molecule. Some changes in the values of the dynamic contact angle may be attributed to differences between the orientation of the adsorbed layer of the alcohol solution vapour on a smooth and a rough paraffin wax surface. The results indicate that the classical equilibrium contact angle approach to surface chemistry problems is not valid in the dynamic systems of drop impaction for example, waterproofing and spraying.