An investigation of droplet mobility and the ultra-mild internal mechanical microenvironment in cylindrical microchannels

Abstract

The mechanical microenvironment inside droplets acts directly on encapsulated cells and reactive substances. We used microparticle image velocimetry to explore the flow characteristics inside droplets moving in cylindrical microchannels. Two kinds of flow behaviors were found inside droplets with increasing capillary number Ca. When Ca < 5.73 × 10−3, the oil phase cannot bypass droplets forward or backward because there is no gutter flow around the droplets, the droplets move in cylindrical microchannels in the form of rigid bodies, and the difference in velocity and gradients inside the droplets is very low. The fluids inside the droplets remain almost stationary with respect to the surrounding oil phase, and the droplets are driven only by compression. When Ca > 1.43 × 10−2, the droplets move faster than the oil phase, which creates a pair of counter-rotating eddies in the front of droplets, and the droplets are driven by both compression and shearing. The critical Ca range for the two flow behaviors is from 5.73 × 10−3 to 1.43 × 10−2 in this study. Comparisons are made between droplet behaviors in rectangular and cylindrical channels; in the latter, the shear and strain rate inside droplets are reduced by factors of 5.02 and 6.86, respectively, and acceleration and viscous dissipation are reduced by even greater factors of 42.53 and 41.56, respectively.