Flow characteristics inside droplets moving in a curved microchannel with rectangular section

Abstract

A micro-particle image velocimetry system is used to study the internal flow field of droplets in a curved channel, with consideration of the effects of capillary number, viscosity ratio, droplet size, channel curvature, and interfacial tension on the flow field. It is found from the experiment that there is a clockwise eddy in the upper part of the droplet, and three more in the lower part, of which the left and right ones are counterclockwise and the middle one is clockwise. By adjusting the above parameters, the flow field structure transitions can be realized. Either the decrease in the viscosity ratio, droplet size, channel curvature, and interfacial tension or the increase in capillary number will all weaken the effect of the lower gap oil film on the movement of the droplets, resulting in disappearance of the lower intermediate eddy. In case the droplet size is further reduced, the two eddies below will gradually approach and merge. After the addition of more surfactants, the eddy at the lower portion of the droplet may not form a complete rotating structure, or be transferred to other planes inside the droplet, and the eddy originally presented in the upper portion of the droplet expands to occupy the entire droplet. The results of this study are useful for screening high-throughput cell manipulation application reaction conditions, further expanding the application range of flow cytometry, and providing experimental support for extraction, synthesis, and heat and mass transfer directly relying on the internal convection of droplets.