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

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 ${\rm \mu}{\rm m}$ 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.

Multiscale modeling of electrospray ion emission

“…Such modified equilibrium shapes (at least those conserving their axial symmetry) exist under a limited range of conditions, namely, a narrow range of external fields, a minimum hydraulic impedance, and particularly small meniscus sizes on the order of 3-5 μm. 7 This micrometer-scale order of magnitude lies close to the diffraction limit for traditional optical microscopy, which has precluded direct observation of these ionic liquid menisci. Attempts for a direct observation of these menisci have been performed with electron microscopy with limited success since electrons were shown to produce abnormal solidification of the ionic liquid during the observation process.…”

“…This model resolves the 2D axially symmetric equilibrium shapes of a meniscus in steady pure-ion evaporation, as a function of the geometry of the emitter and a set of physical parameters of the ionic liquid (density, dielectric constant, surface tension coefficient, conductivity). The model is based on the work by Coffman et al 22 and Gallud and Lozano 7 and adapted to handle curved electrode geometries. This adaptation is only geometrical and maintains the physics described in the cited models.…”

“…The inlet of the channel Γ I is situated at a distance equal to the channel radius r c from the rim of the fluid channel. Similar to Gallud and Lozano 7 and Coffman et al, 22 the channel is considered to be very long compared to the meniscus size, and only a small portion of length equal to r c is treated computationally. The axes are non-dimensionalized by the channel length r c .…”

“…The breakup of quasineutrality in the ionic liquid is due to the presence of conductivity gradients originated by the heating of the meniscus. 7 The ion space charge is neglected in the vacuum for ionic liquids (ρ sc % 0 on Ω v ) since zeroth order of magnitude analysis shows that it does not modify the electric field that the meniscus observes to a sufficient extent. 22 In the bulk and meniscus interface, convective charge transport can be neglected as well, and an Ohmic model for the conductivity is considered: j ¼ κ(T)E. A potential drop V is applied between the tip Γ T and the extractor Γ E ,…”

“…Parameters, such as viscosity or conductivity, depend strongly on the temperature of the liquid. These changes affect the structure of the equilibrium shape in the meniscus 7 and, consequently, the initial conditions of the ions. These effects are captured in the energy conservation equation, which provides temperature distributions in Ω l ,…”