Surface-guided templating of particle assemblies inside drying sessile droplets.

The particles suspended inside evaporating sessile droplets can be assembled into microscopic objects with long-ranged ordered structure. The air-water droplet interface guides the assembly and determines the shape of the resulting micropatches. We report the results of a systematic study of the mechanism of interface-templated assembly on substrates of controlled contact angle. The kinetics of drying were examined by measurements of droplet profiles, and it was found that the rate matched diffusion-limited evaporation well. The shape of the droplets and of the resulting assemblies was correlated to the dynamics of the receding contact line. The effects of major parameters controlling the process, including contact angle, particle concentration, and electrolyte, were investigated in detail. A variety of micropatch shapes were observed and categorized within the parameter space. The in-depth characterization of the process allowed the optimization of the assembly and the formulation of protocols for the deposition of nanostructured patches of different diameter, thickness, and shape.