Strategic placement of an obstacle suppresses droplet break up in the hopper flow of a microfluidic soft crystal

When granular materials, colloidal suspensions, and even animals and crowds exit through a narrow outlet, clogs can form spontaneously when multiple particles or entities attempt to exit simultaneously, thereby obstructing the outlet and ultimately halting the flow. Counterintuitively, the presence of an obstacle upstream of the outlet has been found to suppress clog formation. For soft particles such as emulsion drops, clogging has not been observed in the fast flow limit due to their deformability and vanishing interparticle friction. Instead, they pinch off each other and undergo break up when multiple drops attempt to exit simultaneously. Similar to how an obstacle reduces clogging in a rigid particle system, we hypothesize and demonstrate that an obstacle could suppress break up in the two-dimensional hopper flow of a microfluidic crystal consisting of dense emulsion drops by preventing the simultaneous exit of multiple drops. A regime map plotting the fraction of drops that undergo break up in a channel with different obstacle sizes and locations delineates the geometrical constraints necessary for effective break up suppression. When optimally placed, the obstacle induced an unexpected ordering of the drops, causing them to alternate and exit the outlet one at a time. Droplet break up is suppressed drastically by almost three orders of magnitude compared to when the obstacle is absent. This result can provide a simple, passive strategy to prevent droplet break up and can find use in improving the robustness and integrity of droplet microfluidic biochemical assays as well as in extrusion-based three-dimensional printing of emulsion or foam-based materials.

Spray-assisted formation of micrometer-sized emulsions

Emulsion drops with defined sizes are frequently used to conduct chemical reactions on picoliter scales or as templates to form microparticles. Despite tremendous progress that has been achieved in the production of emulsions, the controlled formation of drops with sizes of a few µm at high throughputs remains challenging. Drops of this size, however, are in high demand for example for many pharmaceutical, food, and material science applications. Here we introduce a scalable method to produce water-in-oil emulsion drops possessing controlled diameters of just a few microns: We fabricate calibrated aerosol drops and transfer them into an oil bath to form stable emulsions at rates up to 480 µl min-1 of the dispersed phase. We demonstrate that the emulsification is thermodynamically driven such that design principles to successfully form emulsions can easily be deduced. We employ these emulsion drops as templates to form well-defined µm-sized hydrogel spheres and capsules.