Protrusion formation during the collisional process of ethanol and water droplets: Capillary wave propagation on the water droplet

Collision processes of liquid droplets were observed to study the dynamics of molecules in solution with an apparatus which allows us to observe the collision sequence stroboscopically with time resolution of ∼1 μs. The collision sequences of water–water and ethanol–water droplets were observed by production of droplets through piezo-driven nozzles. Formation of characteristic protrusion was traced in the collision of ethanol and water droplets. A model mechanism of protrusion in these systems is proposed based on the observed behavior indicating release deformation of surface tension that propagates as a capillary wave on the droplet surface.