Droplet migration in emulsion systems measured using MR methods

The migration of emulsion droplets under shear flow remains a largely unexplored area of study, despite the existence of an extensive literature on the analogous problem of solid particle migration. A novel methodology is presented to track the shear-induced migration of emulsion droplets based on magnetic resonance imaging (MRI). The work is in three parts: first, single droplets of one Newtonian fluid are suspended in a second Newtonian fluid (water in silicone oil (PDMS)) and are tracked as they migrate within a Couette cell; second, the migration of emulsion droplets in Poiseuille flow is considered; third, water-in-silicone oil emulsions are sheared in a Couette cell. The effect of (a) rotational speed of the Couette, (b) the continuous phase viscosity, and (c) the droplet phase concentration are considered. The equilibrium extent of migration and rate of migration increase with rotational speed for two different emulsion systems and increased continuous phase viscosity, leads to a greater equilibrium extent of migration. The relationship between the droplet phase concentration and migration is however complex. These results for semi-concentrated emulsion systems and wide-gap Couette cells are not well described by existing models of emulsion droplet migration. 3.2 v% of a 1% Tween 60 aqueous solution emulsified in 12,500 cSt silicone oil, rotating in a Rheo-NMR Couette cell. Signal is received only from the aqueous phase. (a) shows the initial distribution of water droplets. (b) shows the distribution of water droplets after 30 min rotation at 0.31 rev / s. (c) shows the coalescence effect of rotating at 0.62 rev / s for 5 min.