Brownian dynamics modeling of the assembly of electrosprayed particles on sessile droplet surfaces

Colloidal assembly at fluid interfaces is important in numerous chemical and biological processes. It also enables novel manufacturing of thin film materials. Electrospray is a nonintrusive method for delivering colloidal particles to a fluid interface such as the sessile droplet surface. A unique characteristic of electrospray targeting is imparting significant charge on both sprayed particles and receiving substrate. In this case, electrostatic interactions between particles and between particles and the substrate influence the dynamics and assembly structures at the interface. In this study, we investigated the effects of these electrostatic interactions on particle assembly on triangular and dumbbell-shaped sessile droplets using computational modeling. We combined ANSYS electrostatic simulation and a new Brownian dynamics algorithm to simulate charged particles moving on curved surfaces. Consistent with previous experimental observations, our simulations demonstrated the formation of a depletion region near the droplet contact line driven by the long-range electrostatic repulsion between particles and substrate. We quantitively characterized the effects of particle surface charge and particle concentration on the assembly structure and the depletion region development. The findings provide insights into the competition of various electrostatic interactions in the interfacial assembly of electrosprayed particles.