The flow topology inside a droplet acts directly on the cells or substances enclosed therein and is, therefore, of great significance in controlling the living environment of cells and the biochemical reaction process. In this paper, the flow characteristics inside droplets moving in rectangular microchannels are studied experimentally by particle image velocimetry for capillary numbers ranging from 10⁻⁵ to 10⁻². In order to decouple the effects of total flow, droplet spacing, viscosity ratio, droplet size, and the depth-to-width ratio of the channel on the flow field, the droplet trains with a designed initial state are first produced by controlling the two-phase flow rate and setting up an auxiliary inlet, which is used to adjust the droplet size and spacing, and then run at a set flow rate. As the total flow increases, the flow topologies inside the plunger droplet gradually change from four eddies to two at relatively high viscosity ratios, whereas the opposite transition direction is observed in the low-viscosity-ratio system. The flow topology inside spherical droplets is unaffected by the total flow or capillary number, invariably producing double vortices. The effect of the channel wall on the droplet boundary decreases as the droplet spacing increases or the droplet size decreases. Assuming the continuity of the fluid mass, the competition between the gutter-flow driving stress and the oil-film resistance determines the boundary velocity of the droplet. The oil-film resistance dominates the motion of the droplet boundary in high-aspect-ratio channels, resulting in the negative rotation of the boundary velocity vectors and six vortices in the interior of the droplet. The results are conducive to the further development of microfluidic flow cytometry, particle concentration control, and droplet micromixers.

中文翻译：

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矩形微通道内液滴内部的流动拓扑及其转换

液滴内部的流动拓扑直接作用于细胞或封闭在其中的物质，因此在控制细胞的生存环境和生化反应过程中具有重要意义。在本文中，通过颗粒图像测速技术，对毛细管数在10 ^-5到10 ^-2范围内的液滴在矩形微通道中的流动特性进行了实验研究。。为了使总流量，液滴间距，粘度比，液滴尺寸和通道的深宽比对流场的影响脱开，首先通过控制两者来产生具有设计初始状态的液滴列相流速并设置辅助入口，该辅助入口用于调节液滴大小和间距，然后以设定的流速运行。随着总流量的增加，柱塞液滴内部的流动拓扑在相对较高的粘度比下从四个涡流逐渐变为两个涡流，而在低粘度比系统中观察到相反的转变方向。球形液滴内部的流动拓扑不受总流量或毛细管数的影响，始终会产生双涡旋。随着液滴间距的增加或液滴尺寸的减小，通道壁对液滴边界的影响减小。假设流体的连续性，则水流驱动应力与油膜阻力之间的竞争决定了液滴的边界速度。油膜阻力主导着高纵横比通道中液滴边界的运动，导致边界速度矢量和液滴内部的六个涡旋负向旋转。结果有利于微流式细胞仪，颗粒浓度控制和液滴微混合器的进一步发展。沟槽流动驱动应力与油膜阻力之间的竞争决定了液滴的边界速度。油膜阻力主导着高纵横比通道中液滴边界的运动，导致边界速度矢量和液滴内部的六个涡旋负向旋转。结果有利于微流式细胞仪，颗粒浓度控制和液滴微混合器的进一步发展。沟槽流动驱动应力与油膜阻力之间的竞争决定了液滴的边界速度。油膜阻力主导着高纵横比通道中液滴边界的运动，导致边界速度矢量和液滴内部的六个涡旋负向旋转。结果有利于微流式细胞仪，颗粒浓度控制和液滴微混合器的进一步发展。