On the origin and evolution of the depletion zone in coffee stains.

The variation in the concentration of surfactant molecules along the air-water interface of a drying sessile drop containing colloidal particle-surfactant mixtures is known to inhibit the formation of coffee stains. This also leads to the formation of particulate deposits with a region almost deprived of the particles, often called the depletion zone. The molecular size of the surface-active species used in such experiments poses limitations on the direct visualization of the build-up of surfactant molecules at the interface and how it correlates with the nucleation and growth kinetics of the depletion zone. We report a quantitative analysis of the origin and evolution of the depletion zone that forms in a drying sessile drop. By evaporating an aqueous sessile droplet containing a mixture of surface-active poly(N-isopropylacrylamide) (pNIPAM) microgels and polystyrene (PS) colloids, we establish that the concentration fluctuations of the PS particles along the air-water interface trigger a surface tension driven Marangoni flow along the interface. As a result of this, a depletion zone forms within the particulate deposit, which is analogous to the depletion zones observed for dried drops of particle- surfactant mixtures. The critical time at which the depletion zone forms correlates to the time at which surface tension-driven stress along the interface is maximum. Moreover, we show that the increase in the initial concentration of PS particles results in a decrease in the critical time. By using in situ video microscopy, we established that the rate at which the depletion zone grows is non-linear in time. The growth rate of the depletion zones for drops containing different concentrations of PS particles collapses into a master curve upon scaling with concentration and radius of the drop.

Colloidal monolayers with cell-like tessellations via interface assisted evaporative assembly.

HYPOTHESIS: Evaporating sessile drops containing surface active colloids is a promising route to self-assemble two-dimensional nanostructures. The standard protocol is to first self-assemble surface active nanoscale particles at the water-vapour interface and subsequently transfer it on to a solid surface. Colloidal monolayers with very few morphologies have been fabricated, exploiting this bottom-up self-assembly technique. However, the evaporation kinetics under controlled humidity conditions may dramatically alter the microstructure of self-assembled colloidal monolayers at the liquid-vapor interface and that on the solid surfaces, an aspect that has not been fully addressed in the prior studies. EXPERIMENTS: To this end, we present an experimental study of evaporation driven self-assembly of soft poly(N-isopropylacrylamide) (pNIPAM) microgel particles loaded in a sessile drop. The surface-active microgel particles spontaneously populate the water-vapour interface facilitating the suppression of the coffee-ring effect and the formation of monolayer stains. The role of evaporation kinetics under controlled humidity conditions on the colloid's microstructure adsorbed to the solvent-air interface and on the morphology of the colloidal monolayer transferred onto the solid surface are studied in detail. FINDINGS: The formation of particle-free and particle-rich regions at the water-vapor interface is observed for sessile drops evaporated under saturated humidity conditions. We show that the evaporation induced shrinkage of the interface area and the enhancement of the areal density of microgel particles adsorbed onto the interface leads to a restructuring of the particle-laden interface. The rearrangement of microgel particles along the water-vapor interface resembling the de-wetting assisted patterns is transferred to the solid substrate upon complete evaporation of the solvent. The microgel particles in the deposit assemble into domains with enhanced crystalline order. The evolution of Voronoi entropy across the monolayer deposit patterns obtained by the standard and slow evaporation routes are presented.

Loosely packed monolayer coffee stains in dried drops of soft colloids.

We report a complete suppression of the coffee-ring effect resulting in the formation of loosely packed two-dimensional particle monolayers by controlled evaporation of sessile drops containing soft microgel particles. These particulate deposits show gradual order-to-disorder transitions that are not abrupt like deposits of hard colloids. The areal coverage of the monolayer deposits can also be precisely controlled by tuning the particle concentration. The preferential adsorption of soft-microgel particles to the water-vapor interface, and their radial flow along the interface towards the drop edge facilitates the creation of the novel monolayer deposits.