Homogeneous gelation leads to nanowire forests in the transition between electrospray and electrospinning

Abstract

The morphology of coatings created by electrostatic deposition can be generally divided into three categories: wire mats, particles, and films. At intermediate regimes, there should exist nanowire forests as a mixture of wire and particulate deposition, but these have yet to be observed as a dominant morphology. We utilized electrostatic dissipative particle dynamics simulations to reveal that the barrier to forming nanowire forests is the directional nature of evaporation, implying that they should form if evaporation is homogeneous. Experimentally, we utilized electrospray deposition (ESD) to explore the spray of a fibril gel former, methylcellulose (MC) in water:ethanol mixtures. MC possesses a lower critical solution temperature (LCST) in water and water:ethanol blends. Above the LCST, MC and water phase separate concurrently with the rapid evaporation of ethanol, forming a shear-thickening, homogeneous gel phase. In the ESD process, the strongly entangled polymer solution resulting from gelation electrospins on a drop-by-drop basis to create forests of individual nanowires. To verify this mechanism, we employed different viscosities of MC by ESD through changes in spray temperatures, molecular weight, loading concentration, flow rates, spray distances, and additive content. The results reveal that lowering the viscosity of the gel increases the aspect ratio of formed nanowires. Similarly, the incorporation of additives that lead to too high or too low a viscosity prevents the formation of nanowires in ESD. Our study indicates that the homogenous evolution of viscosity is necessary for nanowire forest formation and that the specific viscosity further controls the morphology of the forests.