Inner circulation flow characteristics of coalescence droplets in microchannel

Controllable coalescence of multiple droplets is an important part of droplet manipulation, which is used to add samples to existing droplets, complete multi-step biological and chemical reactions inside droplet. It is of great importance to study the internal circulation characteristics for enhancing heat and mass transfer inside droplets. In this paper, microchannel experiments and volume of fluid method are conducted, and the evolution process of droplet coalescence is analyzed. The velocity distribution inside droplet is used as the main index to study the internal circulation characteristics under different surface velocities conditions. A characterization method of internal circulation strength based on central radial velocity and axial velocity inside the droplet is proposed, which can characterize the internal circulation region inside droplet caused by shearing action of microchannel wall of continuous phase. Results show that with the decrease of droplet length, the velocity distribution inside the droplet changes from being dominated by shearing action of microchannel wall to shearing action of continuous phase. In the process of droplet coalescence, there is a convective trend in the contact area between two droplets, and the maximum radial and axial velocities reach 3.5 times and 7.5 times of the droplet surface velocity respectively. This work can provide a reference for improving droplet control accurancy and mixing efficiency.