Electrohydrodynamics of an ionic liquid meniscus during evaporation of ions in a regime of high electric field

Abstract

Numerical investigations are presented for an ionic liquid meniscus undergoing evaporation of ions in a regime of high electric field. A detailed model is developed to simulate the behavior of a stationary meniscus attached to a liquid feed system. The latter serves as a proxy for commonly utilized electrospray emitters such as needles and capillary tubes. Two solution families are identified for prototype liquid analogous to the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4). The first belongs to a regime of low electric field in which the meniscus is blunt and does not emit charge. The second belongs to a regime of high electric field in which a conelike meniscus produces charge from a sharp tip. Electrohydrodynamic features of the meniscus in this regime are presented. These reveal that the meniscus is Stokesian and hydrostatic and governed by conduction. The applied electric field influences both the shape of the meniscus and the current that it produces while the impedance of the feed system-which must be above a threshold value in order to ensure that the current, and therefore the flow, remains below a maximum value-influences the meniscus current but not the macroscopic shape. In general, this shape deviates from Taylor's idealized cone.

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

“…The results presented in this section follow the same characteristic non-dimensional numbers based on the properties of standard ionic liquids as defined in Coffman et al (2019). These properties are similar to those of EMI − BF 4 , which is a widely used ionic liquid in the literature of pure-ion evaporation (Romero-Sanz et al 2003;Legge & Lozano 2011).…”

“…Menisci in this region resemble those explored by Higuera (2008). As discussed by Coffman et al (2019), the non-dimensional critical electric field Ê * = R1/2 is of the order of those found near the apex of the hyperboloidal shapes described by Basaran & Scriven (1990). The pressure drop created by the evaporation process compensates for the electric stress before the Rayleigh instability is triggered.…”

“…where Λ = k T 0 /κ 0 is the non-dimensional sensitivity of the electric conductivity to changes in temperature. While this non-dimensionalization has been mostly used in the numerical procedure to keep consistency with existing literature (Coffman, Martínez-Sánchez & Lozano 2019), it has been noted that dimensionless magnitudes referencing the emission region (E 0 /E * , I/I * , r 0 /r * ) provide very useful physical interpretations. Non-dimensionalization referencing the emission region can be easily obtained by postprocessing solutions without modifying any physical result.…”

“…These static menisci have a characteristic emission region of non-dimensional size r * /r 0 = R−1 , where the non-dimensional electric fields are of the order of the critical field Ê * ∼ R1/2 . The surface charge on these menisci is not relaxed and the temperature is around 3 %-5 % higher than in the bulk ionic liquid due to heating by ohmic dissipation (Coffman et al 2019). Figure 6 includes the flow structure of a prototypical equilibrium interface in subregion II.a.…”

“…Experimental challenges have hindered a clear understanding of ILIS, specially the role of key operating parameters such as the external electric field (Krpoun & Shea 2008;Pérez-Martínez & Lozano 2015), liquid temperature (Lozano & Martínez-Sánchez 2005), and other physical and geometrical tip characteristics relevant to passive-type sources, such as the size of the inlet pores (Courtney & Shea 2015), electrode shape or hydraulic impedance of the feeding material (Castro & Fernández De La Mora 2009) and material dielectric properties (Coffman et al 2013). Among these challenges are the current lack of non-destructive techniques to resolve the small scales of ILIS menisci (∼1-5 μm) to interrogate the system in-situ, e.g.…”