The effect of confinement-induced shear on drop deformation and breakup in microfluidic extensional flows

Droplets of de-ionized water and four aqueous surfactant solutions were generated in oil using a microfluidic flow-focusing device. The morphological developments of the drops in extensional flow and confinement-induced shear flow at various extension rates were studied using a hyperbolic contraction. This novel approach to droplet deformation within a microfluidic device allowed the probing of droplets within a nearly uniform extensional flow. The focus of this work was to study the effect of confinement-induced shear on droplet deformation and breakup in extensional flows. Droplet deformation was found to increase with both increasing capillary number and increasing confinement, for a fixed viscosity ratio of λ=0.1, with the effect of the shear induced by confinement being quite dramatic. The addition of surfactant to the droplets resulted in the production of tails, which streamed from the rear of the droplets and produced daughter droplets much smaller than the parent droplet. In the partially confined limit, where the flow was purely extensional, a single tail was formed at the center of the droplets trailing edge. With enhanced confinement, shear effects from the wall became important, the droplets were observed to take on a bullet-like shape, and two tails formed at the trailing edge of the droplet. The critical value of the capillary number and confinement needed for the formation of tails varied with the surfactant used.