Large scale simulations of colloid thrusters: Coarse graining strategies

Work is reported here on developing coarse-graining strategies for carrying out large-scale molecular dynamics simulations of colloid thruster emitters. Colloidal thruster technology is based on the electrostatic acceleration of small droplets and/or ions that are generated by feeding a conducting fluid through a small capillary and applying a large acceleration potential between the capillary and an extraction electrode. A wide range of length and time scales are encountered and modeling the entire flow field using continuum methods is a formidable task that has not yet been achieved, in particular at the tip of the Taylor cone. The advantage of molecular dynamics simulations is that all the pertinent physics can be incorporated in reasonably straightforward fashion. However, modern propellants are most frequently ionic liquids where one or both of the ions are quite large. Thus, it behooves the modeler to attempt to reduce the number of particles in the simulation to gain computational speed. Coarse-graining involves representing a large molecular ion with a smaller number of atomic cluster sites, where the potential energy parameters for each site have been optimized to reasonably reproduce the total potential energy field of the original molecular ion. Measures of success include parameters such as the drift velocities of individual ions, electrical conductivity, surface tension and wettability.