Coalescence in surfactant-stabilized emulsions subjected to shear flow

Experiments were carried out to study the coalescence dynamics of a neutral ly buoyant liquid-liquid emulsion subjected to a simple shear flow in a Cou ette device. The effect of shear rate and dispersed phase holdup were studi ed. The surfactant stabilized emulsions were prepared in a stirred tank at high shear rates which were applied for long times so as to obtain reproduc ible equilibrium drop size distributions. Low shear rates were used in the Couette device to prevent drop breakup. Sufficiently large drops and densit y matching ensured that coalescence due to Brownian motion and creaming was negligible and thus the change in drop size distribution with time was ent irely due to shear-induced coalescence. The evolving volume density distrib utions were measured using optical microscopy and image analysis. A populat ion balance model based on Smoluchowski's result for the rate of shear-indu ced coalescence and an empirical form for the coalescence efficieney was us ed to describe the system. Shearing results in stabilization of the emulsio n, and the coalescence rate reduces with increasing shear rates. The stabil izing effect due to shearing is greater for higher holdups. The drop size d istribution for high values of the holdup becomes bimodal at long times. Th e coalescence efficiency, back-calculated from experimental data, is indepe ndent of drop size for low holdups but is size dependent for higher holdups . This work demonstrates a simple experimental method for evaluating the co alescence efficiency for emulsions with high holdups where theoretical mode ls are not available.