Quantitative measurements of nanoparticle layer thicknesses near the contact line region after droplet drying-out

Abstract

Confocal fringe patterns of evaporating sessile drops have provided initial evidence of the presence of a sub-micron thin liquid film emanating from an evaporating sessile droplet. The droplets studied were seeded with 50 nm aluminum oxide particles. Natural evaporation occurs on the substrates that are varied in wettability with static apparent contact angles as θ = 20o, 33o, 85o and 105o for a DI-water. Subsequent SEM, EDX and AFM characterization demonstrates the existence of aluminum oxide nanoparticle deposition in the submicron region. The profile of stuck nanoparticles shows nanometer-scaled height that is comparative in thickness to a transition film. Moreover, the radial extent of the deposition is consistent around the periphery of the original droplet and is found to be dependent upon the surface wettability. This study shows experimentally that the relative length of nanoparticle deposition increases with the surface wettability.