Department of Mechanical Engineering, Stanford University, Stanford, CA 94305.
Department of Mechanical Engineering, Stanford University, Stanford, CA 94305
Proc Natl Acad Sci U S A. 2021 May 11;118(19). doi: 10.1073/pnas.2017822118.
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.
当颗粒物质、胶体悬浮液，甚至动物和人群从狭窄的出口排出时，如果多个粒子或实体同时试图排出，就会自发形成堵塞，从而阻塞出口并最终停止流动。出人意料的是，人们发现出口上游的障碍物会抑制堵塞的形成。对于乳液滴等软粒子，由于其可变形性和消失的颗粒间摩擦力，在快速流动极限下不会观察到堵塞现象。相反，当多个液滴同时试图排出时，它们会相互挤压并发生破裂。类似于障碍物如何减少刚性粒子系统中的堵塞，我们假设并证明，障碍物可以通过防止多个液滴同时排出，从而抑制由密集乳液滴组成的微流控晶体二维料斗流中的破裂。通过不同障碍物尺寸和位置的通道中发生破裂的液滴的分数的图谱，描绘了有效抑制破裂所需的几何约束条件。当障碍物被最佳放置时，它会引起液滴的意外排序，使它们交替并一次一个地从出口排出。与没有障碍物时相比，液滴破裂被大幅抑制了近三个数量级。这一结果可以提供一种简单的、被动的策略来防止液滴破裂，并可以用于提高液滴微流控生化分析的稳健性和完整性，以及基于挤压的乳液或泡沫材料的三维打印。
