Multiscale modeling of electrospray ion emission

Abstract

A multi-scale approach to electrospray ion source modeling has been developed. The evolution of a single-emitter electrospray plume in a pure ionic regime is simulated with a combination of electrohydrodynamic fluids and n-body particle modeling. Simulations are performed for the ionic liquid, EMI-BF4, firing in a positive pure-ion mode. The metastable nature of ion clusters is captured using an ion fragmentation model informed by molecular dynamics simulations and experimental data. Results are generated for three operating points (120, 324, and 440 nA) and are used to predict performance relevant properties, such as the divergence angle and the extractor surface impingement rate. Comparisons to experimental data recorded at similar operating points are provided.

Comparison of computational algorithms for simulating an electrospray plume with a n-body approach

“…Previous iterations of the n-body simulation framework [ 5 , 7 ] include three types of ion species (monomers, dimers, and trimers) as well as a probabilistic fragmentation model. Moreover, the injection was randomized and hence probabilistic interactions and grid interactions were tracked.…”

“…Moreover, the injection was randomized and hence probabilistic interactions and grid interactions were tracked. In order to produce consistent results and precisely compare the accuracy between the different algorithms, the code was simplified with the following assumptions: The injection procedure is pre-randomized for a subset of particles Only monomers are considered Fragmentation is neglected Particle interactions with the extractor grid are not tracked The geometry however is the same as the one used in [ 7 ] and shown in Fig. 1 .…”

“…The needle is modeled as a hyperboloid with linear eccentricity a , asymptote value and tip curvature . More details can be found in [ 7 ].…”

“…A total current of nA is emitted with a maximum time step of ps which is well in the region of convergence for the time step as shown in [ 7 ]; all time steps below s are found to resolve the trajectory to within 0.1% accuracy. The applied potential is V and EMI-BF4 is used as the ionic liquid.…”

“…These electric fields typically span over multiple scales, from the V/m in the emission region to almost vanishing fields in the field-free region, and are a direct byproduct of the geometry of the acceleration region, including the features of the meniscus shape itself [ 6 ] and the curvature of the tip electrodes. The study of these processes affecting ion plume trajectories has motivated the development of multi-scale simulation frameworks at different levels of accuracy: from kinetic approaches [ 7 ] to particle-in-cell (PIC) models [ 8 , 9 ]. The former model improves the accuracy of the electric field estimation by computing the exact Coulomb force between the particles at the expense of an intense computational scalability 1 , where N is the number of particles being simulated.…”