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

Abstract

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.

Nonlinear Phenomena in Microfluidics

“…Droplets are forced to travel through the narrow gaps demarcated by the barrier and the converging nozzle. Due to the velocity differential between the front and back ends of a droplet as it emerges from the gap, it elongates in the direction perpendicular to the flow, which counteracts the elongation in the streamwise direction that leads to breakup …”

“…Nonlinearities in microfluidic flows can occur in devices with geometric barriers. When placed at the opening of a converging channel, a cylindrical barrier can suppress 82 the breakup of droplets in a dense packing of flowing droplets. 83 Droplets are forced to travel through the narrow gaps demarcated by the barrier and the converging nozzle.…”

“…Due to the velocity differential between the front and back ends of a droplet as it emerges from the gap, it elongates in the direction perpendicular to the flow, which counteracts the elongation in the streamwise direction that leads to breakup. 82 Particles can also be separated by size when flowing through a microfluidic channel containing an array of cylindrical posts. The array consists of rows of cylinders that are evenly spaced by a distance λ.…”

Advanced microfluidic devices for fabricating multi‐structural hydrogel microsphere

“…Alison et al. [ 110 ] reported a simple approach to avoid the simultaneous extrusion of droplets by fixing an obstacle in the outlet channel. When the obstacle was placed, droplets were forced to alternate and exit the outlet one at a time.…”

Hopper flows of deformable particles

“…Microfluidic devices also incorporate flow constrictions to control the pressure and flow rate of complex fluids, such as bubbles and emulsion droplets. [8][9][10][11][12][13] Cell sorting devices [14][15][16] and cell analysis tools [17][18][19] also utilize hopper structures. Despite the fact that hopper and silo flows are ubiquitous in industry, we do not yet have a fundamental understanding of the outflow properties from hoppers and silos.…”