The effect of compatibilizer on the coalescence of two drops in flow

This paper reports results from an experimental study of the effects of copolymer/compatibilizer on the coalescence of two equal size drops in the flow field produced by a four-roll mill. The data encompass two different fluid systems, both with PDMS as the suspending fluid and PBd as the drops, and an acid-base complex of PDMS–NH 3 + − OOC–PBd adsorbed at the interface that we shall refer to as a copolymer. The two systems differ in the ratio of viscosities (λ) of the drop to the suspending fluid, one having λ=0.19 and the other λ=1.3. For the lower viscosity ratio system, as the amount of adsorbed copolymer is increased, the drainage time for coalescence in a head-on collision is increased monotonically and the critical capillary number for coalescence in a glancing collision is also reduced monotonically in a manner that appears qualitatively consistent with a slowing of the film drainage process due to Marangoni stresses. Detailed trajectory measurements for drops with copolymer show agreement with predicted theoretical results for spherical drops without copolymer, but with an increased viscosity ratio. With copolymer present, we also find that coalescence occurs for the largest capillary numbers only after the drops begin to be pulled apart by the external flow. For the higher viscosity ratio system, the effect of increasing the copolymer concentration is nonmonotonic. For very small concentrations, there is a major decrease in the critical capillary number for coalescence and a corresponding increase in the drainage time prior to coalescence, but as the copolymer concentration is further increased, the film drainage time decreases and the critical capillary number increases to a value that is intermediate between the clean interface result, and the result for the smallest copolymer concentration. This is shown to be due to a dependence of the critical coalescence angle on copolymer concentration that was not present in the lower viscosity ratio system. We conclude by speculating about mechanisms, in addition to the Marangoni effect, that might “explain” these observations.