The Ephemeral Jelly

Abstract

Though this jet-like internal flow pattern inside of a water droplet resembles a mushroom, it swims like a jellyfish. This microscopic phenomenon was observed when two micro-droplets coalesce. Internal droplet flow has remained a challenge to visualize for over a century due to the small length and time scales associated with the flows. In order to study droplet coalescence dynamics, we developed a droplet dispensing and visualization system. To track the internal flow front, and ethanol (20 wt%) water mixture was used as the working fluid. Due to the unequal evaporation rates between water and ethanol, a density gradient is developed at the liquid-gas interface of the droplets, resulting in a lower refractive index at the interface compared to that in the bulk liquid. Jet-like internal flow and vortex rings create a jellyfish-like structure inside the merging droplet. The chronophotograph demonstrates the previously unidentified jet-like flow that occurs during the coalescence of droplets having different sizes.