The emission properties, structure and stability of ionic liquid menisci undergoing electrically-assisted ion evaporation

TL;DR

This study uses electrohydrodynamic simulations to analyze the stability, emission properties, and structure of ionic liquid menisci under electric stress, revealing universal stability limits and effects of temperature and impedance.

Contribution

It introduces a comprehensive model for ionic liquid menisci under electric fields, identifying stability conditions and the universal electric pressure limit for ion evaporation.

Findings

Stable emission occurs within specific impedance and field ranges.

A universal electric pressure limit for stability is identified.

Temperature effects are minimal at constant impedance due to charge relaxation.

Abstract

The properties and structure of electrically-stressed ionic liquid menisci experiencing ion evaporation are simulated using an electrohydrodynamic model with field-enhanced thermionic emission in steady state for an axially-symmetric geometry. Solutions are explored as a function of the external background field, meniscus dimension, hydraulic impedance and liquid temperature. Statically stable solutions for emitting menisci are found to be constrained to a set of conditions: a minimum hydraulic impedance, a maximum current output, and a narrow range of background fields that maximizes at menisci sizes of 0.5-3 microns in radius. Static stability is lost when the electric field adjacent to the electrode that holds the meniscus corresponds to an electric pressure that exceeds twice the surface tension stress of a sphere of the same size as the meniscus. Preliminary investigations suggest this limit to be universal, therefore independent of most ionic liquid properties, reservoir pressure, hydraulic impedance or temperature and could explain the experimentally observed bifurcation of a steady ion source into two or more emission sites. Ohmic heating near the emission region increases the liquid temperature, which is found to be important to accurately describe stability boundaries. Temperature increase does not affect the current output when the hydraulic impedance is constant. This phenomenon is thought to be due to an improved interface charge relaxation enhanced by the higher electrical conductivity. Dissipated Ohmic energy is mostly conducted to the electrode wall. The higher thermal diffusivity of the wall versus the liquid, allows the ion source to run in steady state without heating.