Coalescence of trapped droplets using a microfluidic device

The phenomenon of coalescence of droplets is crucial to destabilization of emulsion systems. For emulsions stabilized by surfactants, the adherence of a droplet to a solid boundary can greatly affect the time scales of coalescence. A microfluidic device with a T-junction and a hydrodynamic trap is employed to characterize the coalescence of trapped droplets. Micrometer sized water droplets are first formed in a T-junction by the shearing of a surfactant-laden hydrocarbon continuous phase. The size of the droplet can be controlled by altering the ratio of the flow rates of both phases. A passive hydrodynamic trap is utilized to capture the newly generated droplet. The continuous generation of droplets allows for coalescence to occur at the trap. The series of coalescence is captured with a high-speed camera and the film drainage time is measured against the varying concentration of surfactant, degree of confinement, and incoming droplet velocity. Coalescence is quantified by the dimensionless drainage time plotted against the capillary number, and consideration is given to the appropriate scaling for the different experimental regimes.