Molecular Dynamics Modeling of the Forces on [EMIM][BF₄] Ion Emissions and Emission Rates Due to Applied Electric Fields

Ionic liquids are stable propellants that are candidates for use with electrospray propulsion systems. Ionic liquids such as [EMIM][BF₄] can be ionized by electric fields to produce emissions through various mechanisms including fragmentation. Processes like fragmentation can lead to reduced efficiency or erosion and need to be modeled at high-fidelity to develop a relationship between operating parameters such as the strength of the applied electric field and the resulting plume composition. By introducing a nano pore that allows us to maintain a curved liquid-vacuum interface, we are able to replicate the meniscus that forms at the tip of an electrospray emitter where ion emissions are produced when the applied electric field is able to overcome the surface tension of the liquid. Using ensembles of molecular dynamics simulations, it is possible to quantify the emissions produced by applying uniform electric fields of 1, 2, 4, or 6 V/nm for over 2 ns. These simulations reveal that the number of emissions and cumulative charge increase with the applied electric field. As the magnitude of the electric field increases from values of 1 or 2 V/nm to 4 or 6 V/nm, the predominant emission mechanism changes from heavier species such as dimers to lighter monomers. This atomistic model shows that as the electric field increases, it becomes easier to overcome the surface tension forces on the oscillating chains of ions that emerge at the liquid-vacuum interface to produce ion emissions. Lighter monomers can be cleaved from these chains more easily with stronger applied electric fields leading to fewer dimer and trimer emissions overall. The emissions counts and cumulative charge profiles computed from these simulations reveal that by controlling the applied electric field, it is possible to bias the ion emissions toward fewer dimer or trimer emissions. This is crucial to preventing the emission of heavier ion species that may fragment to create charged products that move at reduced velocities or neutral [EMIM][BF₄] that cannot be accelerated further by the electric field.