Flow induced coalescence of drops in a viscous fluid

This paper summarizes recent experimental studies of flow-induced coalescence of viscous drops in a viscous fluid in the absence of inertial effects. These studies are based on visual observations of small drops (20-100 mum diameter) that collide in the linear flows generated by a four-roll mill. We consider a pair of polymeric fluids that are Newtonian under the flow conditions relevant to coalescence phenomena, and in addition consider the effect of adding a "copolymer" of the two polymers to the interface, which acts as a surfactant. By direct observation, we can generate quantitative data on the collision trajectories and the conditions for coalescence. These observations have also uncovered several "new" phenomena. Among these are the fact that coalescence frequently occurs during the part of the collision after the drops have already rotated to a configuration where they are being pulled apart by the external flow. This occurs for at least some of the collision trajectories for all fluids where the viscosity ratio of the drop to suspending fluid exceeds 0.1. It is also favored by the addition of surfactant to the drop. We also find that the conditions for coalescence are indicative of a complicated history of film configurations during the "draining" or thinning process, with minimum film thickness (or at least the most unstable configuration) occurring early in the collision, considerably prior to the point where the force along the line of centers changes sign and the drops begin to be pulled apart by the external flow. (C) 2004 American Institute of Physics.