Measurement of shear-induced dispersion in a dilute emulsion

The time-dependent drop distribution of a dilute, polydisperse emulsion is measured in a simple shear flow. The suspending fluid is much more viscous than the dispersed phase (1:1000). Drops are found to drift away from eithe r bounding wall and accumulate near the center of the gap, due to the aniso tropy of droplet-plane interactions. An expression for this drift velocity has been derived for single drops by Chan and Leal [J. Fluid Mech. 92, 131 (1979)] and was in agreement with isolated drop migration observed in our w ork. Eventually the inward drift is balanced by a shear-induced gradient di ffusivity, and a steady-state concentration distribution is reached. When t he drops are sufficiently far from either wall a self-similar, parabolic co ncentration profile is predicted at all times. Droplet diffusivities were d etermined for capillary numbers Ca=(gamma) over dot (a) over bar<<mu>/sigma between 0.17 and 0.92, where (gamma) over dot is the shear rate, (a) over bar is the mean drop radius, mu is the viscosity of the suspending fluid, a nd sigma the interfacial tension. The values obtained are an order of magni tude lower than theoretical predictions of Loewenberg made in the limit of small deformation. (C) 2001 American Institute of Physics.