Long-Term Evolution of Interfacial Particles via Electrospray Deposition

Electrospray is a versatile process for delivering a wide range of materials to a substrate. Here, we suspend fluorescent polystyrene particles with a mean diameter of 2 µm in a volatile solvent. Driven by a high electric potential, a stable Taylor cone-jet is formed at the electrospray emitter, atomizing the suspension and imparting a positive electrostatic charge to the particles as the solvent fully evaporates. To probe the assembly of these highly charged particles at a liquid-vapor interface, a 3D printed manifold was used to create an array of 4 target droplets. By monitoring the water-air interface shape, we have a closed-loop feedback control scheme to compensate for evaporation of the water. The deposition of the PS particles was observed in-situ, allowing for coarse control over the interfacial particle density. After the spray was stopped, we recorded the particle assembly for up to 24 hours. When the water subphase has sufficient electrical conductivity (~ 5 mM NaCl), we see regions of hexagonal ordering, indicative of strong electrostatic repulsion. However, when high-purity deionized water was used, the assembly becomes frozen and particles appear immobile, exhibiting minimal Brownian motion. The significance of electrostatic charge at the interface here is clear, as the salt water was able to dissipate charge delivered via electrospray. Conversely, the DI water surface maintains a net positive charge and screens electrostatic repulsion.