Onset Voltage for Electrospray from Nanopores

The electrospray of fluids from nanopores is of great significance to electric propulsion technology, while the underlying mechanism of nanoscale fluid jet still remains unclear. We investigated the physical process of the ejection of sodium chloride solution under applied electric field through molecular dynamics simulations. It was found that, when the electric driven force surpassed the resistance caused by the surface tension, the fluid breakup would occur and subsequently the droplet would be ejected out of the nanopore. A systematic investigation of the dependence of the onset electric field on factors, including pore size, system temperature, pore length, and surface wettability, was performed. It was discovered that the onset electric field decreased when reducing the pore size or enlarging the pore length, which amplified the maximum electric field strength inside the pore. The onset electric field decreased as the system temperature was raised, due to the reduction in the surface tension of the liquid. When the surface hydrophobicity was increased, the onset electric field was also raised, because a higher infiltration pressure was required to fill the nanopore for a hydrophobic nanopore. The surface charge of the nanopore would induce the formation of the electric double layer, which can promote or suppress the electrospray.