DROPLET BREAK-UP IN SIMPLE SHEAR-FLOW IN THE PRESENCE OF EMULSIFIERS

The break-up of droplets in an inhomogeneous flow is a key process in emulsion manufacturing and manipulation. The break-up of droplets in a steady simple shear flow in the absence of emulsifiers has been studi ed extensively, and is by now well understood. In the presence of emul sifiers the interfacial tension is lowered, which promotes emulsificat ion. In addition, however, the interface may acquire viscoelastic prop erties. These are known to be important for the prevention of coalesce nce, but their influence on droplet break-up has not been thoroughly e stablished. In this paper we report a fairly simple phenomenological a pproach, which links the droplet break-up dynamics to the interfacial viscoelasticity, identifying the latter with the elasticity modulus of a deforming planar interface. Experiments were performed in a Couette device, in which break-up conditions for water droplets in oil were d etermined as a function of emulsifier concentration in the droplet pha se. Below the critical micelle concentration (CMC) break-up requires a higher stress than above or far below the CMC. Over a viscosity ratio range of three decades the results have been found to agree with the model, which only uses independently known quantities derived from equ ilibrium interfacial tension properties.