Coalescence analysis through small-angle light scattering

Flow-induced coalescence in complex flow fields is studied using an in situ, real-time small-angle light scattering technique. Coalescence in a model polyisobutene - polydimethylsiloxane immiscible blend is triggered through step-downs in the flow rate, creating a transition from a breakup-controlled morphology to one controlled by coalescence. Inducing droplet breakup, by increasing the flow rate during the coalescence process, is a reliable probe of the structural changes occurring during the step-down. The coalescence rate, duration of the transient structure, and final droplet size depend strongly on the blend viscosity ratio. Coalescence occurs at extremely dilute droplet concentrations, and an inverse relationship between concentration and time to steady state is observed. Models, developed for simple shear flow, qualitatively describe the coalescence and breakup processes, indicating their potential for predicting these phenomena in processing flows.