Simulation of Electrohydrodynamic Distortion Relevant to Liquid Metal Ion Sources

Citation

Im, Cheolmin (2025) Simulation of Electrohydrodynamic Distortion Relevant to Liquid Metal Ion Sources. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qs8q-wj55. https://resolver.caltech.edu/CaltechTHESIS:06022025-014234039

Abstract

The free surface of a liquid metal film in vacuum subject to an imbalance of destabilizing Maxwell (electric) and stabilizing capillary forces can undergo rapidly accelerating electrohydrodynamic (EHD) distortion which culminates in formation of protrusions from whose tips highly energetic ion beams are emitted. Such a phenomenon has been successfully leveraged into liquid metal ion sources (LMIS) which are fundamental to the operation of focused ion beam systems used for micro- and nanofabrication and even microarray devices actuating space micropropulsion. In this thesis, we have conducted a series of computational simulations using the arbitrary Lagrangian Eulerian finite element method designed to track the pre-emission EHD liquid distortion for realistic LMIS geometries characterizing a slender microemitter with a sharp (highly curved) tip positioned beneath an apertured extractor in vacuum.

The simulations unveil various stable and unstable configurations whose protrusions can occur along the liquid surface. The particular configurations are found to correlate with the Reynolds number and electric Weber number based on a handful of initial values, namely the applied electric potential, emitter apex curvature radius and initial liquid thickness. For the parameter space explored, spectral analysis of the unstable configuration yields a dominant wavenumber in close agreement to that predicted from linear stability analysis of the flat liquid layer. This can be traced to the fact that a key aspect ratio indicates that the dynamics are in the flat liquid limit. Examination of the late stage dynamical behavior of the evolving protrusions reveals self-similar growth for all configurations examined. Values of the exponents extracted from the simulations are found to cluster neatly when plotted against Reynolds number and Weber number.

Thesis Files