Coalescence kinetics of oil-in-water emulsions studied with microfluidics

We report the results of experiments on the coalescence dynamics in flowing oil-in-water emulsions using an integrated microfluidic device. The microfluidic circuit permits direct observation of shear-induced collisions and coalescence events between emulsion droplets. Three mineral oils with a range of viscosities 8–70 mPa s and five silicone oils with a range of viscosities 6–100 mPa s were chosen as dispersed phase. Pure water was used as continuous phase. Trajectory analysis of colliding droplet pairs allows evaluation of the film drainage profile and coalescence time. From the coalescence times obtained for ten thousands of droplet pairs we calculate coalescence time distributions for the different emulsions. For all systems, the coalescence time decreases with increasing capillary number and increases with increasing dispersed phase viscosity. Scaling relations for the coalescence time are derived and compared with theoretical predictions. The potential of the procedure as a diagnostic tool for the prediction of emulsion stability in oil/water separation is discussed, as well as alternative applications of the microfluidic circuits.