The effect of evaporation kinetics on nanoparticle structuring within contact line deposits of volatile drops

This paper deals with the evaporation behaviour of nano-suspensions. An investigation of the relationship between reduced environmental pressure and the ring-stain formation mechanism is presented. Monodisperse, nanosuspension sessile drops formed a variety of patterns, at the macro-scale, consisting of sets of rings, concentric rings and irregularly distributed stains. "Stick-slip" motion of the triple line (TL), constantly pinned TL and very rapid evolution of the evaporation were the reasons for each of the observed patterns respectively. At the particle level (nano-scale), the promotion of close-packed, hexagonal particle structures was observed as a result of the enhanced evaporation rate and hence increased particle velocity. Beyond an optimal/critical pressure, a combination of further enhanced particle velocity with very limited space (for particles to find their most thermodynamically favourable positions) led to the formation of a disordered region at the exterior region and, as expected, to more defects. Lastly, inside the rings, particles formed random groups of particle aggregates, resembling branches, following TL motion during the "slip" phase. On the other hand, particle structures resembling ripples formed on top of a particle monolayer with increasing surface coverage as pressure was reduced. This was a direct result of fluid flow being too weak to reach the TL but strong enough to carry particles over smaller distances during the last stages of the evaporation process. (C) 2012 Elsevier B.V. All rights reserved.