Wall migration and shear-induced diffusion of fluid droplets in emulsions

The spatial distribution of drops in multiphase Stokes flow is derived theoretically as a function of two dimensionless parameters, accounting for wall migration, buoyancy, and shear-induced diffusion. The wall migration effect, which drives drops away from the walls and toward the center of the gap, is often significant even when droplets are 100 times smaller than the gap. By comparison with the experimental drop concentration profile, the shear-induced down-gradient diffusivity is measured and found to be approximately four to five times larger than the prediction for drop self-diffusivity. These are the first such measurements of the diffusivity of drops with clean interfaces and contrast markedly with previous measurements on surfactant-laden drops. Nonuniform concentration along the vorticity axis is also investigated briefly. (C) 2003 American Institute of Physics.