Merging of oil droplets at an air-liquid interface under the influence of surface excited waves

The coalescence or merging of two droplets consists of several complex hydrodynamics and interfacial phenomena. When two droplets move nearby, two physical effects can be analyzed, i.e., they either coalesce to form a new drop or move away from each other, indicating non-coalescence. The dynamics of droplet coalescence are independent of their initial size; droplets of varying sizes still exhibit the coalescence phenomenon. The majority of research has focused on the coalescence of droplets of the same liquid. Even though the droplets are composed of distinct liquids, they still exhibit the merging phenomenon; however, the liquids of the droplets must be miscible with one another. In this presented work, the coalescence dynamics of multiple castor oil droplets over the non-viscous liquid (water) surface are experimentally investigated under the influence of low-amplitude surface excited waves. It is found that, irrespective of the number of parent droplets over the water surface, merging always occurs in the presence of excited waves if they approach each other. As the number of droplets increases, the time for complete coalescence to form a new droplet also increases. Furthermore, it is also observed that the coalescence phenomena are independent of the droplet’s initial position. However, the deposition point of the droplets significantly affects their propulsion velocities. The droplets deposited near the center of the container have a higher propulsion velocity than those injected near the walls. The investigation is also carried out to determine the kinematics of two non-identical droplets over the free surface of water before merging. The study reveals that a smaller droplet propagates faster than a larger droplet, and the velocities of both droplets decrease with time.