Impacting emulsion drop on a steel plate: Influence of the solid substrate

We have investigated the influence of roughness and of various organic coatings on the impact of a single emulsion drop onto steel plates using a high-frequency visualization technique (one picture every 100 mu s). The drop falls by gravity and reaches the solid substrate with a 3.5 m/s velocity. Whatever the investigated system, the drop flattens itself against the substrate under the action of its initial kinetic energy and a peripheral film is formed around it in which viscous dissipation occurs and which is unstable to viscous fingering. Then the drop retracts under the action of capillarity despite a positive spreading coefficient at equilibrium. The role of the viscous dissipation on spreading has been analyzed, and the relevant surface property is shown to be the dynamic surface tension which is usually higher than that at equilibrium. Emulsification plays a major role in the existence and aspect of the splashing phenomenon at the impact and in the shape of the contact line instabilities initiated during the inertial spreading. (C) 1998 Academic Press.