A microfluidic method to study demulsification kinetics

Abstract

We present the results of experiments studying droplet coalescence in a dense layer of emulsion droplets using microfluidic circuits. The microfluidic structure allows direct observation of collisions and coalescence events between oil droplets dispersed in water. The coalescence rate of a flowing hexadecane-in-water emulsion was measured as a function of the droplet velocity and droplet concentration from image sequences measured with a high-speed camera. A trajectory analysis of colliding droplet pairs allows evaluation of the film drainage profile and coalescence time t_c. The coalescence times obtained for thousands of droplet pairs enable us to calculate coalescence time distributions for each set of experimental parameters, which are the mean droplet approach velocity 〈v₀〉, the mean dispersed phase fraction 〈ϕ〉 and the mean hydraulic diameter of a droplet pair 〈d_p〉. The expected value E_tc of the coalescence time distributions scales as E_tc ∝ 〈v₀〉^{−0.105±0.043}〈d_p〉^0.562±0.287, but is independent of ϕ. We discuss the potential of the procedure for the prediction of emulsion stability in industrial applications.