Mechanisms of ion migration in droplet coalescence and non-coalescence behaviors under direct current electric fields

In electrostatic coalescence, ion migration in controlling the critical transition between coalescence and non-coalescence remains unclear. This study employs many-body dissipative particle dynamics simulations to examine ion migration under varying electric field strengths, ion concentrations, and other factors. During droplets approaching, ions first accumulate at the surfaces and, upon surface charge saturation, migrate to the droplet edge regions. In the liquid bridge evolution stage, low fields produce a neutralization zone where charge density drops sharply, weakening the driving force and suppressing ion transfer. In moderate fields, partial ion escape from this zone slows bridge expansion. At high fields, ions traverse the bridge to the opposite droplet, causing edge electric forces to exceed interfacial tension and preventing coalescence. Moreover, increasing ion concentrations elevates tip charge density and the number of ions transferred through the bridge, thereby lowering the critical field strength and promoting non-coalescence. Ion valence influences the stability of the neutralization zone and the proportion of ions transferred by modulating the microscopic electric force acting on the ions. These findings elucidate ion migration mechanisms governing droplet behavior.