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.

Shell Crystallization in Granular Hopper Flow

An interesting phenomenon that a layer of crystallized shell formed at the container wall during hopper flow is observed experimentally and is investigated in DEM simulation. Different from shear or vibration driven granular crystallization, our simulation shows during the hopper flow the shell layer is formed spontaneously from the stagnant zone at the base and grows at a constant rate to the top with no external drive. The growth rate of the shell is found linearly proportional to the rate of the hopper flow. This shell is static and served as a new wall, which changes the flow profiles and its stress properties, and in turn guarantees a constant flow rate.

Effect of Particle Shape on Repose Angle Based on Hopper Flow Test and Discrete Element Method

The repose angle of granular material is an essential parameter to understand the microbehavior of the granular material and, then, to relate it with the macrobehavior. In this study, a self-design large-scale hopper flow test apparatus has been developed to measure the repose angle of the ballast using a fixed funnel method. Then, the numerical simulation using the realistic clump is compared with the experimental test to prove its validity. Meanwhile, the idealized clumps with custom shape parameters, including roughness of particle and ground, angularity, aspect ratio, and sphericity, were chosen to analyze the influences of particle shape on the repose angle. The results show that the angle of repose generally tends to increase with the increase of the friction coefficient of particles and the roughness of the ground. With the increase of the angularity from 0 to 4, the pile height and the repose angle increase. Meanwhile, the extended area decreases accordingly. For cuboid particles, with aspect ratio increasing from 1.0 to 1.67, the angle of repose increases firstly and then maintains a constant between aspect ratio 1.25–1.67. For ellipsoid particles, the angle of repose decreases, then reaches a minimum at aspect ratio around 1.3, and finally increases.