Physics of the droplet-to-ion transition in electrosprays of highly conducting liquids

We investigate the physical mechanisms governing the continuous transition from the droplet-dominated to the ion-dominated regime in electrosprays of highly conducting liquids. We characterize electrosprays of four ionic liquids using time-of-flight spectrometry and direct flow rate measurements. In the droplet regime, the jet breakup process exhibits self-similar lognormal mass-to-charge distributions with a constant coefficient of variation. In the mixed and ionic regimes, the average solvation state of the emitted ions decreases with decreasing flow rate, consistent with a shift of the primary ion emission zone toward the cooler cone-jet neck. Modeling ion evaporation from the post-breakup droplet population yields an estimate for the ion solvation energy of $\Delta G_0 \gtrsim 1.9$~eV, a value difficult to reconcile with jet-less ion emission from a Taylor cone tip. Furthermore, we identify two fundamental limits on the performance of highly conducting electrosprays near minimum flow rate: substantial neutral mass losses driven by the evaporation of small droplets, and a dissociation limit imposed by the finite fraction of free ions in the bulk liquid. The dissociation limit yields an analytical expression for the maximum specific impulse of electrospray thrusters, showing excellent agreement with experimental data across multiple propellants and electrospray sources.