Nanoparticle Motion and Deposition Pattern From Evaporating Binary Droplets

The effect of ethanol in the binary solution sessile droplet is investigated on the flow field, nanoparticle motion and nanoparticle deposition pattern. It is found that the droplets with ethanol exhibited three distinct flow regimes through the Particle Image Velocimetry (PIV) analysis on the flow field of droplets suspended with fluorescent microspheres. Regime I features furious flows and vortices which transport particles to the liquid-vapor interface and make them aggregate. In regime II, the aggregates of particles move towards the central area of the droplet dominated by Marangoni flow led by non-uniformity of ethanol along the droplet surface. As the droplet enters regime III, most ethanol has evaporated and it is dominated by the drying of the remaining water. The loading of ethanol in the solution prolongs the relative durations of regimes I and II, resulting in the variety of the final drying pattern of nanoparticles.