Colloidal Assembly on Non-Axisymmetric Droplet Interfaces via Electrospray

Microparticles on the interface of a sessile droplet interact via electrostatic and capillary forces governed by particle size, surface charges, and contact line roughness. We created non-spherical droplets using surface energy patterning and delivered microparticles to the interface with electrospray atomization. Using water as the target droplet, we observed the particle assembly over time. We found that the underlying surface energy pattern significantly influenced the colloidal assembly, and drove particles toward the center of the droplet. The particles arranged as a large, single cluster with local hexagonal ordering, but left a clear region free of particles near the contact line. This depletion region is attributed to electrostatic repulsion from the photoresist used to create the surface energy pattern, which retained electric charge from the electrospray. To understand the effect of electrostatic interactions, we explored target droplets with dissimilar dielectric properties. Using patterned substrates and electrospray for particle deposition, we can harness the assembly of colloids at a fluid interface to build repeatable monolayer patterns.