Harnessing complex fluid interfaces to control colloidal assembly and deposition

Hypothesis: Capillary interactions play an important role in directing colloidal assembly on fluid interfaces. Interface curvature is expected to influence not only individual particle migration on interfaces but also capillary forces between nearby particles. In drying droplets, we hypothesize that the assembly and deposition of particles bound to droplet surface are controlled by the interplay between capillary effects and evaporation-driven flow. Experiments: Using lattice Boltzmann–Brownian dynamics (LB–BD) simulations, we modeled large-scale assembly of nanoparticles on fluid interfaces that have complex geometries and investigate the subsequent deposition upon complete evaporation. A systematic study was performed for geometrically-controlled sessile droplets whose surfaces exhibit varying curvature fields. Findings: The simulations show that the particle dynamics on nonuniformly curved interfaces are anisotropic and governed by particle-pair capillary interactions and curvature-induced capillary migration. A theoretical model was developed to predict the capillarity-induced assembly. Using the curved surface as a template, drying droplets with surface-bound particles deposit distinct patterns as a result of the competition between the capillary effects and evaporation-induced convection. These findings could provide new opportunities in the directed assembly and deposition of colloidal particles with potential applications in fabricating functional materials from nanoscale building blocks.