Coalescence of Disc-Shaped Falling Droplets Inside Quiescent Liquid Media

The coalescence of two disc-shaped droplets falling under the effect of gravity inside a quiescent liquid medium is studied using particle image velocimetry (PIV). A smaller droplet, followed by a larger droplet, falls from a rest position inside a confined channel. The larger droplet with the higher terminal velocity collides with the smaller droplet and merges to form a single droplet that continues to fall. Here, three stages of coalescence (film drainage between droplets, film rupture, and growth of connection) of the droplets are shown qualitatively. The glycerol-water mixture is used as a droplet and silicon oil as a continuous surrounding liquid. The droplet is seeded with a 40 μm PMMA particle, which is neutrally buoyant and faithfully follows the flow. Images are recorded and processed at different time intervals to capture the droplet coalescence. It is shown that larger droplet approaches smaller droplet due to higher terminal velocity resulting in movement or leakage of surrounding liquid away from the droplets. Distance between droplets keeps reducing to zero where film rupture occurs. Reshaping of a larger diameter disc-shaped droplet is observed at the end, which keeps falling with a new higher terminal velocity.