Self-Limiting Electrospray Deposition of Nanoparticle Composites via Sub-Percolation Assembly

Electrospray deposition (ESD) uses strong electric fields to produce monodisperse droplets from dispersions and solutions that are driven toward grounded targets. Self-limiting electrospray deposition (SLED) is a regime of ESD that achieves controlled targeted coatings by trapping charge in the growing film and redirecting incoming droplets to uncoated areas of the substrate. However, when conductive nanoparticles are added to self-limiting (SL) sprays, the buildup of charge required to repel incoming material becomes disrupted as particle loading increases. In the literature on polymer composites, the bulk conductivity of a composite has been best understood through percolation theory. The goal of this work was to maximally deliver nanoparticles in the controlled SL regime. Methylcellulose (MC) was chosen as the SL binder due to its ability to form high aspect ratio nanowires, which should increase the interparticle spacing. MC was sprayed with nanoparticles varying in size, aspect ratio, electrical properties, and surface chemistry. The SL behavior was characterized by measuring the growth of films on 2D patterned gratings as well as on flat wafers. The results are interpreted in light of our prior work regarding the dynamics of nanowire formation as well as percolation theory.