Flow topology and its transformation inside droplets traveling in rectangularmicrochannels

• Published in: Physics of fluids (1994) Vol. 32; no. 5
• Main Authors: Li Mengqi, Liu Zhaomiao, Pang, Yan, Chengjin, Yan, Wang, Ju, Zhao Siyu, Zhou, Qiang
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 01.05.2020
• Subjects: Capillary flow; Droplets; Flow characteristics; Flow cytometry; Flow velocity; Fluid dynamics; Fluid flow; High aspect ratio; Microchannels; Microfluidics; Motional resistance; Particle image velocimetry; Topology; Two phase flow; Viscosity; Viscosity ratio; Vortices
• ISSN: 1070-6631; 1089-7666
• DOI: 10.1063/5.0004549

The flow topology inside a droplet acts directly on the cells or substances enclosedtherein and is, therefore, of great significance in controlling the living environment ofcells and the biochemical reaction process. In this paper, the flow characteristics insidedroplets moving in rectangular microchannels are studied experimentally by particle imagevelocimetry for capillary numbers ranging from 10−5 to 10−2. Inorder to decouple the effects of total flow, droplet spacing, viscosity ratio, dropletsize, and the depth-to-width ratio of the channel on the flow field, the droplet trainswith a designed initial state are first produced by controlling the two-phase flow rateand 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 insidethe plunger droplet gradually change from four eddies to two at relatively high viscosityratios, whereas the opposite transition direction is observed in the low-viscosity-ratiosystem. The flow topology inside spherical droplets is unaffected by the total flow orcapillary number, invariably producing double vortices. The effect of the channel wall onthe droplet boundary decreases as the droplet spacing increases or the droplet sizedecreases. Assuming the continuity of the fluid mass, the competition between thegutter-flow driving stress and the oil-film resistance determines the boundary velocity ofthe droplet. The oil-film resistance dominates the motion of the droplet boundary inhigh-aspect-ratio channels, resulting in the negative rotation of the boundary velocityvectors and six vortices in the interior of the droplet. The results are conducive to thefurther development of microfluidic flow cytometry, particle concentration control, anddroplet micromixers.