Coalescence dynamics of surfactant-stabilized emulsions studied with microfluidics

We report the results of a study on emulsion stability in a microfluidic channel flow using an integrated microfluidic device. The microfluidic circuit enables production of a monodisperse oil-in-water emulsion and monitoring of emulsion stability upon shear-induced collisions. Sodium-n-dodecyl sulfate was used as emulsifier, and hexadecane as dispersed phase. We measure the mean drop size at the end of the collision channel as a function of the surfactant and sodium chloride bulk concentration, and as a function of the total flow rate. We find that emulsions are stable against coalescence for SDS bulk concentrations down to 10(-6) M within the residence time of the droplets in the channel in the absence of added NaCl. The stability of the emulsion at these low SDS bulk concentrations is interpreted in terms of a reduced mobility of the droplet interfaces, which slows down drainage of the film of the continuous phase between the droplets. Emulsions stabilized by SDS with added NaCl in the continuous phase display a transition from a stable to unstable regime when increasing the NaCl bulk concentration from 0.1 M to 0.3 M, which is in agreement with predictions using simple DLVO force calculations for colloidal stability. We also estimate the characteristic coalescence time between droplets using a simple coalescence theory and compare the results with values obtained by us previously from trajectory analysis of colliding droplet pairs.