Molecular dynamics simulations of EMIM − BF4 ionic liquid for electrospray thrusters
This research discusses the use of the molecular dynamics method to simulate the ionic liquid EMIM − BF4 in conditions associated with electrospray thrusters. In particular, two situations are investigated: the comparison of an all atom and a coarse grained model of EMIM − BF4 under the influence of an external electric field, and the process of fragmentation that occurs in the electrospray’s plume for dimer and trimer ionic species. In order to compare the all atom and the coarse grained model, bulk simulations were performed to replicate the properties and characteristics of the ionic liquid and validate the potentials selected. Then, the bulk systems were subject to the application of an external electric field, to generate ionic emissions. The results show that the coarse grained and the all atom model behave differently with regards to the emissions results, in particular different structural formations are observed between both models. This ultimately has an impact in the physics that is represented, and the differences observed are connected to the lower friction and faster dynamics associated with the coarse grained model. The formation of an electric double layer in the bulk of the ionic liquid is also investigated and discussed. For the investigation regarding the fragmentation of ionic species, the fragmentation rates of negative and positive dimers and trimers are obtained for different conditions of temperature and electric field. These results enable the development of a fragmentation model for higher length scale simulations of electrosprays, such as particle-in-cell. Furthermore, the trajectories of fragmented dimers are analyzed in detail, and the energetic mechanism associated with the fragmentation process is found to be a reduction in the electrostatic interactions between the ions of the system. For the trimer species, the different fragmentation pathways are discussed and the impact of different simulation conditions on the dominant pathway of fragmentation is analyzed for this species.
molecular dynamics, electrospray, fragmentation, ionic liquids
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign