Experimental Investigation of Inertial Mixing in Colliding Droplets

Achieving the increasingly fast mixing requirements posed by the chemical, biological, and life science community for confined microchannel droplet flows remains an engineering challenge. The viscous and surface tension forces that often dominate microflows undermine fast, efficient mixing. A novel mixing arrangement based on droplet collisions has been developed that significantly improves mixing rates by utilizing inertia to rapidly rearrange fluid contents. This article experimentally investigates inertial droplet mixing in micro-flows following high-speed droplet pair collisions. The technique utilizes a gaseous flow for liquid droplet generation and transport with collisions occurring in Y-junction microchannel geometries. Mixing rates are quantified using differential fluorescent optical diagnostics, custom image processing algorithms, and statistical analysis. Measured droplet mixing times are compared to the characteristic time scales for mass and viscous diffusion and bulk convective transport. Results show that mixing times are decreased as the droplet pair collision inertia is increased, indicating the potential benefit for inertial collision mixing.