Electrospray Deposition of Colloidal Particles to Controllable Water-Air Interfaces

Electrospray deposition is capable of deploying nano- or micro-scale materials as a thin film. With electrospray, material suspended in a volatile solvent can be deposited onto a wide array of targets, including curved fluidic interfaces. In this work, we introduce a water-air interface with adjustable curvature as the target for electrosprayed microparticles. We developed a simple fluidic device to create spherical cap droplets across a wide range of contact angles with a fixed contact line. The fluid is transported via a 3D printed manifold and is dispensed through a thin sheet of FR4 glass epoxy with circular holes to achieve the spherical cap shape. The contact angle can be reliably modulated from approximately -10° to 60° via a syringe pump. By continuously adding fluid at the appropriate rate, we can also compensate for volume reduction due to evaporation to maintain a constant interfacial shape. With this system, we are able to evaluate the assembly of different colloidal materials at the water-air interface. Previously, we observed the assembly dynamics of polystyrene, silica, and silver-coated silica particles on evaporating droplet interfaces. Now, we apply this controllable fluid interface to further investigate the effect of electrostatic charge on the interfacial assembly over longer time scales. Broadening our knowledge of interfacial self-assembly is essential for developing new thin film and monolayer deposition techniques.