Homogeneous Gelation Leads to Nanowire Forests in the Transition Between Electrospray and Electrospinning
The morphology of coatings created by electrostatic deposition can be generally divided into three categories: wire mats (electrospinning), particles (electrostatic spray, electrospray deposition(ESD)), and films (all low-viscosity applications). There should exist nanowire forests as a mixture of wire and particulate deposition. Such a morphology has yet to be observed experimentally, which we propose is the result of spatially-varying viscosity in sprayed droplets. We utilized electrostatic dissipative particle dynamics (DPD) and ESD to explore the spray of methylcellulose (MC) in water:ethanol mixtures. MC possesses a lower critical solution temperature (LCST) in water and water:ethanol blends. DPD simulations reveal that the barrier to forming nanowire forests is the directional nature of evaporation, but they should form were evaporation homogeneous. In ESD conducted above the LCST, MC and water phase separate concurrently with the rapid evaporation of ethanol, forming a homogeneous gel phase. This gel can undergo the elongation of electrospinning on a drop-by-drop basis to create forests of individual nanowires. Our study indicates that this homogenous evolution of viscosity is necessary for nanowire forest formation and that the specific viscosity (along with droplet size) further controls the morphology of the forests.