Finite amplitude electroconvection induced by strong unipolar injection between two coaxial cylinders

We perform a theoretical and numerical study of the Coulomb-driven electroconvection flow of a dielectric liquid between two coaxial cylinders. The specific case where the inner to outer diameter ratio is 0.5 is analyzed. A strong unipolar injection of ions either from the inner or outer cylinder is considered to introduce free charger carriers into the system. A finite volume method is used to solve all governing equations including Navier-Stokes equations and a simplified set of Maxwell’s equations. The flow is characterized by a subcritical bifurcation in the finite amplitude regime. A linear stability criterion and a nonlinear one that correspond to the onset and stop of the flow motion, respectively, are linked with a hysteresis loop. In addition, we also explore the behavior of the system for higher values of the stability parameter. For inner injection, we observe a transition between the patterns made of 7 and 8 pairs of cells, before an oscillatory regime is attained. Such a transition leads to a second finite amplitude stability criterion. A simple modal analysis reveals that the competition of different modes is at the origin of this behavior. The charge density as well as velocity field distributions are provided to help understanding the bifurcation behavior.Ministerio de ciencia y tecnología FIS2011-25161Junta de Andalucía P10-FQM-5735Junta de Andalucía P09-FQM-458

Self-consistent modeling of laminar electrohydrodynamic plumes from ultrasharp needles in cyclohexane

This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. Physics of Fluids 12, 2809 (2000) when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a highdensity charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the Finite Element Method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage (IV) characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.Ministerio de Economía y Competitividad FIS2014-54539-P

Modelling the air cleaning performance of negative air ionisers in ventilated rooms

Negative air ionisers have seen increasing use as devices for improving indoor air quality, including some success in clinical environments for reducing the transmission of infection. This study uses a ventilation model and a CFD model to examine the physical effects of negative ionisers in indoor environments. The results demonstrate how the negative ion distribution and electric field due to an ioniser are influenced by both the room airflow and the ion generation rate. It is shown that ion concentrations greater than 1010 ions/m3 are necessary for the electrical effects to be significant. The effect on particles is also considered, with the results demonstrating that the ioniser will only increase the deposition of particles when the particle concentration is high enough to contribute to the space charge in the room

Modelling the air cleaning performance of negative air ionisers in ventilated rooms

Negative air ionisers have seen increasing use as devices for improving indoor air quality, including some success in clinical environments for reducing the transmission of infection. This study uses a ventilation model and a CFD model to examine the physical effects of negative ionisers in indoor environments. The results demonstrate how the negative ion distribution and electric field due to an ioniser are influenced by both the room airflow and the ion generation rate. It is shown that ion concentrations greater than 1010 ions/m3 are necessary for the electrical effects to be significant. The effect on particles is also considered, with the results demonstrating that the ioniser will only increase the deposition of particles when the particle concentration is high enough to contribute to the space charge in the room

Induced-Charge Electro-Osmosis

We describe the general phenomenon of `induced-charge electro-osmosis' (ICEO) -- the nonlinear electro-osmotic slip that occurs when an applied field acts on the ionic charge it {\sl induces} around a polarizable surface. Motivated by a simple physical picture, we calculate ICEO flows around conducting cylinders in steady (DC), oscillatory (AC), and suddenly-applied electric fields. This picture, and these systems, represent perhaps the clearest example of nonlinear electrokinetic phenomena. We complement and verify this physically-motivated approach using a matched asymptotic expansion to the electrokinetic equations in the thin double-layer and low potential limits. ICEO slip velocities vary like $u_s \propto E_0^2 L$, where $E_0$ is the field strength and $L$ is a geometric length scale, and are set up on a time scale $\tau_c = \lambda_D L/D$, where $\lambda_D$ is the screening length and $D$ is the ionic diffusion constant. We propose and analyze ICEO microfluidic pumps and mixers that operate without moving parts under low applied potentials. Similar flows around metallic colloids with fixed total charge have been described in the Russian literature (largely unnoticed in the West). ICEO flows around conductors with fixed potential, on the other hand, have no colloidal analog and offer further possibilities for microfluidic applications.Comment: 36 pages, 8 figures, to appear in J. Fluid Mec

Effects of Nanoparticle Geometry and Size Distribution on Diffusion Impedance of Battery Electrodes

The short diffusion lengths in insertion battery nanoparticles render the capacitive behavior of bounded diffusion, which is rarely observable with conventional larger particles, now accessible to impedance measurements. Coupled with improved geometrical characterization, this presents an opportunity to measure solid diffusion more accurately than the traditional approach of fitting Warburg circuit elements, by properly taking into account the particle geometry and size distribution. We revisit bounded diffusion impedance models and incorporate them into an overall impedance model for different electrode configurations. The theoretical models are then applied to experimental data of a silicon nanowire electrode to show the effects of including the actual nanowire geometry and radius distribution in interpreting the impedance data. From these results, we show that it is essential to account for the particle shape and size distribution to correctly interpret impedance data for battery electrodes. Conversely, it is also possible to solve the inverse problem and use the theoretical "impedance image" to infer the nanoparticle shape and/or size distribution, in some cases, more accurately than by direct image analysis. This capability could be useful, for example, in detecting battery degradation in situ by simple electrical measurements, without the need for any imaging.Comment: 30 page