Coalescence in a surfactant-less emulsion under simple shear flow

Abstract

Shear-induced coalescence is investigated in a neutrally buoyant, surfactant-less oil-in-water emulsion containing an electrolyte. Emulsion drops are large enough for Brownian motion-induced coalescence to be negligible. The emulsion is subjected to a simple shear flow in a tangential Couette apparatus with a rotating inner cylinder, and the evolution of the drop size distribution with time is obtained using optical microscopy and image analysis. The effect of varying the shear rate and the dispersed phase holdup is studied. The evolving drop size distributions become bimodal with increasing coalescence and are not found to be self-similar. A population balance model with an empirical coalescence efficiency (ratio of the actual coalescence rate to the Smoluchowski rate) function containing four fitting parameters describes the time evolution of the experimental drop size distribution reasonably well. The coalescence rate, inferred qualitatively from the measured drop size distributions at different times as well as obtained from the fitted coalescence efficiency, is found to increase with increasing shear rate and dispersed-phase holdup. The coalescence efficiency also increases with holdup, although it decreases with increasing shear rates. The fitted coalescence efficiency results indicate the existence of a critical drop diameter of 12 μm at which the coalescence efficiency is maximum. The computed magnitude of the coalescence efficiency is of the order of 10^-2 for all cases and is about 100-fold larger than that obtained earlier for surfactant-stabilized emulsions. The results indicate that coalescence in surfactant-less emulsions is qualitatively different from that in emulsions containing surfactant. The experimental findings are discussed in the light of recent theories and experimental work in this field.