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 = 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.

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A single microfluidic chip consisting of six microfluidic flow-focusing devices operating in parallel was developed to investigate the feasibility of scaling microfluidic droplet generation up to production rates of hundreds of milliliters per hour. The design utilizes a single inlet channel for both the dispersed aqueous phase and the continuous oil phase from which the fluids were distributed to all six flow-focusing devices. The exit tubing for each of the six flow-focusing devices is separate and individually plumbed to each device. Within each flow-focusing device, the droplet size was monodisperse, but some droplet size variations were observed across devices. We show that by modifying the flow resistance in the outlet channel of an individual flow-focusing device it is possible to control both the droplet size and frequency of droplet production. This can be achieved through the use of valves or, as is done in this study, by changing the length of the exit tubing plumbed to the outlet of the each device. Longer exit tubing and larger flow resistance is found to lead to larger droplets and higher production frequencies. The devices can thus be individually tuned to create a monodisperse emulsion or an emulsion with a specific drop size distribution.

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Using a microfluidic flow-focusing device, monodisperse water droplets in oil were generated and their interface populated by either 1 μm or 500 nm amine modified silica particles suspended in the water phase. The deformation and breakup of these Pickering droplets were studied in both pure extensional flow and combined extensional and shear flow at various capillary numbers using a microfluidic hyperbolic contraction. The shear resulted from droplet confinement and increased with droplet size and position along the hyperbolic contraction. Droplet deformation was found to increase with increasing confinement and capillary number. At low confinements and low capillary numbers, the droplet deformation followed the predictions of theory. For fully confined droplets, where the interface was populated by 1 μm silica particles, the droplet deformation increased precipitously and two tails were observed to form at the rear of the droplet. These tails were similar to those seen for surfactant covered droplets. At a critical capillary number, daughter droplets were observed to stream from these tails. Due to the elasticity of the particle-laden interface, these drops did not return to a spherical shape, but were observed to buckle. Although increases in droplet deformation were observed, no tail streaming occurred for the 500 nm silica particle covered droplets over the range of capillary numbers studied.