Breakup of bubbles and drops in steadily sheared foams and concentrated emulsions

Abstract

This experimental study is focused on the process of bubble breakup in steadily sheared foams, at constant shear rate or constant shear stress. Two different types of surfactants were used and glycerol was added to the aqueous phase, to check how the bubble breakup depends on the surface modulus and on bulk viscosity of the foaming solutions. The experiments show that bubble breakup in foams occurs above a well defined critical dimensionless stress, tau[over]CR identical with(tauCRR/sigma) approximately 0.40, which is independent of surfactant used, solution viscosity, and bubble volume fraction (varied between 92 and 98%). Here tauCR is the dimensional shear stress, above which a bubble with radius R and surface tension sigma would break in sheared foam. The value of the critical stress experimentally found by us tau[over]CR approximately 0.40, is about two orders of magnitude lower than the critical stress for breakup of single bubbles in sheared Newtonian liquids, tau[over]CR approximately 25. This large difference in the critical stress is explained by the strong interaction between neighboring bubbles in densely populated foams, which facilitates bubble subdivision into smaller bubbles. A strong effect of bubble polydispersity on the kinetics of bubble breakup (at similar mean bubble size) was observed and explained. Experiments were also performed with hexadecane-in-water emulsions of drop volume fraction 83%<or=Phi<or=95% to study drop breakup in concentrated emulsions. Qualitatively similar behavior was observed to that of foams, with the critical dimensionless stress for drop breakup being lower, tau[over]CR approximately 0.15, and practically independent of the drop volume fraction and viscosity ratio (varied between 0.01 and 1). This critical stress is by several times lower than the critical stress for breakage of single drops in sheared Newtonian fluids at comparable viscosity ratio, which evidences for facilitated drop subdivision in concentrated emulsions. To explain the measured low values of the critical stress, a different type of capillary instability of the breaking bubbles and drops in concentrated foams and emulsions is proposed and discussed.