Microfluidic Valves for Selective on-Chip Droplet Splitting at Multiple Sites.

We describe a microfluidic system for control of droplet division at two locations using a T-junction and expansion channel which are placed one after another. Droplets generated at a standard T-junction are introduced into the droplet division section of the microchannel. In the first set of experiments, the droplet division section consists of two consecutive identical T-junctions branching from the main channel. With this geometry, we were able to produce daughter droplets only at the first junction while there was no droplet division at the second junction. Resistive network analysis is used to redesign the microchannel geometry with an expansion channel in place of the second junction, to have the same quantity of flow entering in both the junctions. We observed five different regimes of droplet breakup, namely, (1) no droplet breakup in both junctions, (2) droplet breakup in the first junction, (3) droplet breakup in both junctions with higher daughter droplet volume in the first junction, (4) daughter droplet volume higher in the second junction, and (5) intermittent droplet breakup in both the junctions. Under specific flow conditions, droplet interaction with both the junctions is similar. We then showed design requirements for location of microvalves, simulated by deformation of the main channel wall and by experiments to break the droplet.