In this work, the use of molecular dynamics as a predictive tool for modeling the atomistic behavior of electrospray propulsion is discussed. 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF 4 ) and ethylammonium nitrate (EAN) were considered as two limits of ionic liquid (IL) propellants that tend to operate in an ion versus a droplet mode. The emission modes were found to depend on the electro-chemical properties of the IL propellant. The aprotic EMIM-BF 4 -based electrospray emitted primarily monomers and trimers as the dominant species and only small quantities of droplets. In contrast, trimers were the dominant emitted species in the protic EAN emissions with a significantly large contribution from droplets towards the total emission current, suggesting that EMIM-BF 4 -based colloid thrusters operate in ion mode and EAN-based devices operate in the droplet mode. Furthermore, the formation of the Taylor cone was found to depend on the mass flow rate and the external electric field strength. This paper provides a framework that can be extended for use to simulate any other ILs or their combinations.Aerospace 2018, 5, 1 2 of 18 mode [10]. On the other hand, ethylammonium nitrate (EAN) is classified as a protic IL, which are known to be environmentally more benign than other organic solvents [11] and easier to synthesize compared to aprotic ILs [12]. EAN has been reported to emit in the droplet mode, producing a jet of nanodroplets [13]. The electro-chemical properties of the ILs used as propellants and their mass flow rates determine the mode of operation of the colloid thruster. Extensive research has been performed to determine the emission modes of the colloid thrusters. Since future propellants may use mixtures of protic and aprotic types of ILs, molecular dynamics (MD) electrospray simulations can be used to analyze the differences in the emission behavior specific to these two types of ILs.A large number of ILs can be used as propellants for electrosprays [14,15], and the availability of a large number of inter-atomic potentials also makes it possible to model virtually any of the ILs. However, MD simulations scale on the order of O(N 2 ), where, N is the number of atoms, making MD very expensive to model large simulations (>10,000 atoms) typically used to study electrosprays. In this work, effective field coarse graining method [16,17] was used to derive coarse-grained potential of EAN, and its efficacy for the electrospray processes was verified by successfully performing MD simulations of Taylor cone formation and subsequent emission of ion-species. Previous works [16][17][18] have derived coarse-grained (CG) potentials to verify bulk IL physical properties or have used approximations to modify the CG potential to perform electrospray simulations [19,20].Prior to designing colloid thrusters, it is necessary to have a simulation-based ability to predict thrust, required voltages and the mode of operation. MD is a particle-based method that allows one to understand the nature of electrosprays at ...