Flow topology and its transformation inside droplets traveling in rectangular microchannels

Abstract

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.

Electrochemical Detection in Nanoliter Droplets

“…We believe this is intrinsic to the system and not random noise. Two possible explanations were considered where (i) this could be due to an increase in mass transfer due to internal recirculation towards the end of the droplets [28][29][30][31] or (ii) due to an increased wetting area of the electrodes toward the end of the droplet as it has been shown in literature that the oil film is the thinnest towards the end of the droplets. 12,13 This increase in current toward the end of the droplet led to a large variation which was between 12 and 22%.…”