Electric field control of monomer and dimer emissions in the EMIM-BF4 ionic liquid

Abstract

Achieving fine control over the thrust and specific impulse from electrospray thrusters requires fundamental understanding of the microscale mechanisms governing the types of ion emissions at the emitter tips. Using all-atom molecular dynamics (MD) simulations, we model the pressurized flow of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) ionic liquid out of a nanopore to form a convex meniscus, representative of the liquid surface at the emitter-tip, which in turn is subjected to a uniform electric field. Results show that most emitted species are dimers at low electric fields (∼1 V/nm), transitioning to combined monomer and dimer emissions at intermediate electric fields (∼2-4 V/nm), with predominantly monomer emissions at high electric fields (∼6 V/nm). This dimer-to-monomer transition with electric field strength is correlated with the corresponding increase in per-atom stress fluctuations along single interconnected molecular chains of alternating cations and anions at the tip of the Taylor cone, allowing for scission of shorter chains to emit monomers at higher electric fields and scission of longer chains to emit dimers and trimers at lower electric fields. Our coarse-grain MD simulations reveal qualitatively similar emission mechanisms to the all-atom MD simulations, with comparable instantaneous currents during steady-state ion emissions (∼1-8 nA) under electric field strengths of relevance to experiments.