Seriate microfluidic droplet coalescence under optical forces in a channel flow

The microfluidic holding and coalescence behaviors of droplets subjected to an optical trap within a channel flow were numerically studied by using the lattice Boltzmann method and the dynamic ray tracing. A tightly focused Gaussian laser beam positioned laterally with respect to the channel flow direction was used as the optical trap. In such a system, seriate droplet coalescence was observed between an optically trapped droplet and the subsequent droplet. The numerically predicted droplet coalescence behavior agreed well with the experimental results. A sufficiently high laser power was required to capture an incoming droplet and induce coalescence with the subsequent droplet. The effects of various flow and optical parameters on the coalescence behavior were investigated.