Coalescence time of water-in-oil emulsions under shear

Here we use microfluidic techniques to study the coalescence dynamics of water-in-oil emulsions in a simple shear flow. Single water-in-oil emulsion droplets are produced in a glass-capillary-based emulsion generator under Rayleigh-Plateau instability. This droplet is guided into a collision channel, inside which it shears past a stationary water droplet held by surface tension at a nozzle protruding through the wall. Two high-speed cameras with perpendicular views track the droplet interactions. Over 2,000 trajectories were recorded to investigate the interaction time needed for coalescence of the two drops. We study the dependence of this coalescence time tc on droplet approach velocity V, effective diameter DH, offset Δx and continuous-phase viscosity. We find that tc∼V-1, indicating strong correlation between coalescence time and V in our 3-D system, in contrast to results reported from two-dimensional micro-fluidic collision channels. Finally, the experimental results reveal scaling law for dimensionless time needed for coalescence as tc∼DHΔx-0.5V-1, when the channel confinement effects are minimal.