Colloidal Assembly of Electrosprayed Particles at a Static Water-Air Interface*

In electrospray deposition, materials dispersed in a solvent are delivered to a substrate, driven by an electric potential. The high voltage source imparts a net electric charge on suspended colloids, which dissipates over long time scales for insulative materials. Particles delivered to a water-air interface via electrospray repel each other due to the charge, and self-assemble depending on the properties of the substrate. Here, we show that the assembly of electrosprayed colloidal materials at a static interface is governed by two parameters: the dissipation of electric charge over time, and the conductivity of the fluid phase. To observe the assembly over an arbitrary length of time, we use a 3D printed fluidic device with integrated water channels to continuously compensate for evaporation. The conductivity of the water was modulated by adding salt to achieve concentrations of ~0, 1, and 5 mM NaCl. We found that the lowest conductivity case exhibited a "frozen" assembly, where relative motion between particles was very limited. However, for the 5 mM salt experiment particles reorganized into a near-hexagonal structure to minimize their interaction energy. We attribute these findings to the capacity of the water-air interface to maintain a static charge for the low conductivity case, which screens electrostatic repulsion between the particles. This result highlights the significance of surface charge in the electrospray deposition process.