Evolution of internal flows in mechanically oscillating sessile droplets undergoing evaporation

Abstract

Sessile water droplets on hydrophobic substrates have been subjected to mechanical oscillations to excite mean streaming flows in the liquid phase. The driving frequencies are below 1kHz. The flow features have been imaged at a frame rate much lower than the driving frequencies. Counter-rotating vortices are observed along the oscillating liquid-vapor interface (droplet free surface) while an upward drift originating from the substrate exists in the interior. The mean flow features arise out of steady streaming from the substrate and the oscillating liquid-vapor interface. The upward drift is segregated along the droplet height by seemingly horizontal planes. These planes are a characteristic of the time periodic velocity rather than the mean flow. Furthermore when the oscillating droplet is allowed to evaporate under stationary ambient conditions and constant driving frequency, these flow features evolve in a spatio-temporal fashion. In one of our previous studies, we have shown that the oscillation mode of the droplet changes when allowed to evaporate. Mode transition therefore also leads to evolution of the mean streaming flow. The aim is to provide a physical understanding of the evolution of the time averaged flow. This work is motivated by studies related to manipulation of nano-particle deposition patterns in colloidal droplets using controlled oscillations.