Assembly of Electrically Charged Particles on Asymmetric Droplet Interfaces

Microparticles on the interface of a sessile droplet interact via electrostatic and capillary forces that are governed by particle size, surface charge, and contact line roughness. We created various asymmetric liquid droplets using surface energy patterning and delivered microspheres 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 could significantly influence the colloidal assembly, depending on the particle type. Electrically insulative 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 between interfacial particles and the photoresist used to create the surface energy pattern, both of which retained electric charge from the electrospray. Our experimental results are supplemented by a numerical solution to the electric field near the droplet interface, to corroborate the substrate repulsion and depletion region formation. Finally, to understand the magnitude of particle-substrate interactions, we explored particles with different electrical properties. Understanding the kinetics of electrosprayed particles at the droplet surface is essential for building predictable and structured monolayers via interfacial self-assembly.