Inferring electrospray emission characteristics from molecular dynamics and simulated retarding potential analysis

Abstract

In this work, we present coordinated molecular dynamics, ion cluster acceleration, and retarding potential analysis simulations to determine cluster fragmentation behavior in a realistic emitter geometry for electrosprays operating in the pure ionic regime. Molecular dynamics simulations are used to determine the fragmentation rates of ionic liquid clusters as a function of internal energy, electric field strength, and cluster size. A simplified model of electrospray cluster acceleration is developed from previous electrohydrodynamic emission models and used to simulate retarding potential analysis curves. Fragmentation rates and beam composition are inferred for experimental data based on the molecular dynamics and cluster acceleration simulations. We find that for these experimental data, temperatures of EMI-BF4 dimers likely range between 590 and 687 K while trimer temperatures are larger between 989 and 1092 K. The percentage of monomers, dimers, and trimers in the beam is approximately 45%, 30%–43%, and 13%–25%, respectively. Both ionic liquid cluster temperatures and beam composition agree with previous analysis of this experimental work, supporting the use of coordinated molecular dynamics and retarding potential analysis as a method of inferring electrospray beam parameters. Insights gained from this simulation process are discussed in the context of currently unexplained electrospray emitter behavior and experimental results including the presence of tetramers and trimers in the beam and fragmentation rates in high electric field regions.