Electrokinetic and Electroconvective Effects in Ternary Electrolyte Near Ion-Selective Microsphere.

The paper presents theoretical and experimental investigations of the behavior of an electrolyte solution with three types of ions near an ion-selective microparticle with electrokinetically and pressure-driven flow. A special experimental cell has been developed for the investigations. An anion-selective spherical particle composed of ion-exchange resin is fixed in the center of the cell.

Electric-permittivity-based instability of two dielectric miscible liquids under DC field.

This paper considers the mixing of two dielectric miscible viscous liquids with different electric permittivities bounded by solid walls in an external electric field normal to the interface of the liquids. The mutual diffusion of these two liquids leads to the formation of an unsteady self-similar 1D diffusion layer. This layer is found to be unstable to the perturbations of the interface.

Novel electroosmotic micromixer configuration based on ion-selective microsphere.

In this paper, a micromixer of a new configuration is presented, consisting of a spherical chamber in the center of which an ion-selective microsphere is placed. Stratified liquid is introduced through the chamber via inlet and outlet holes under an external pressure gradient and an external electric field is directed in such a way that the resulting electroosmotic flow is directed against the pressure-driven flow, resulting in mixing. The investigation is carried out by direct numerical simulation on a super-computer.

Transitions and Instabilities in Imperfect Ion-Selective Membranes.

Numerical investigation of the underlimiting, limiting, and overlimiting current modes and their transitions in imperfect ion-selective membranes with fluid flow through permitted through the membrane is presented. The system is treated as a three layer composite system of electrolyte-porous membrane-electrolyte where the Nernst-Planck-Poisson-Stokes system of equations is used in the electrolyte, and the Darcy-Brinkman approach is employed in the nanoporous membrane. In order to resolve thin Debye and Darcy layers, quasi-spectral methods are applied using Chebyshev polynomials for their accumulation of zeros and, hence, best resolution in the layers.

Tonks-Frenkel instability in electrolyte under high-frequency AC electric fields.

The instability of an electrolyte surface to a high-frequency, 10 to 200kHz, electric field, normal to the interface is investigated theoretically. From a practical viewpoint, such a high frequency leads to the absence of undesired electrochemical reactions and provides an additional control parameter. The theory of unsteady electric double layer by Barrero and Ramos is exploited.

Overlimiting current due to electro-diffusive amplification of the second Wien effect at a cation-anion bipolar membrane junction.

Numerical simulations are presented for the transient and steady-state response of a model electrodiffusive cell with a bipolar ion-selective membrane under electric current. The model uses a continuum Poisson-Nernst-Planck theory including source terms to account for the catalytic second Wien effect between ionogenic groups in the membranes and resolves the Debye layers at interfaces. The resulting electric field at the membrane junction is increased by as much as four orders of magnitude in comparison to the field external to the membrane.

Stability of two layers dielectric-electrolyte microflow subjected to an alternating external electric field.

The stability of the electroosmotic flow of the two-phase system electrolyte-dielectric with a free interface in the microchannel under an external electric field is examined theoretically. The mathematical model includes the Nernst-Plank equations for the ion concentrations. The linear stability of the 1D nonstationary solution with respect to the small, periodic perturbations along the channel, is studied.

Stability of the electroosmotic flow of a two-layer electrolyte-dielectric system with external pressure gradient.

The stability of the electroosmotic flow of electrolyte-dielectric viscous liquids under the influence of the DC and AC electric fields along with the external pressure gradient is studied theoretically. Liquids are bounded by two infinite parallel plates. The lower wall bordering the electrolyte is assumed to be a charged surface, and the upper wall is electrically isolated.

Modes of electrokinetic instability for imperfect electric membranes.

The direct transition to overlimiting current bypassing the stage of limiting currents is considered for imperfect membranes. Instability of the quiescent steady-state one-dimensional solution, which is the result of a balance of diffusion and electromigration, is investigated on the basis of the full Nernst-Planck-Poisson-Stokes system and a simplified quasielectroneutral system. A three-layer geometry, electrolyte-porous membrane-electrolyte, is considered.

On two-liquid AC electroosmotic system for thin films.

Lab-on-chip devices employ EOF for transportation and mixing of liquids. However, when a steady (DC) electric field is applied to the liquids, there are undesirable effects such as degradation of sample, electrolysis, bubble formation, etc. due to large magnitude of electric potential required to generate the flow.

Thermoelectroconvection near charge-selective surfaces.

A new kind of instability caused by Joule heating near charge-selective surfaces (permselective membranes, electrodes, or systems of micro- and nanochannels) is investigated theoretically using a model based on the Rubinstein-Zaltzman approach. A simple relation is derived for the marginal stability curves: Joule heating can either destabilize or stabilize the steady state, depending on the location of the space charge region relative to the gravity vector. For the destabilizing case, the short-wave Rubinstein-Zaltzman instability is replaced by a long-wave thermal instability.

Three-dimensional coherent structures of electrokinetic instability.

A direct numerical simulation of the three-dimensional elektrokinetic instability near a charge-selective surface (electric membrane, electrode, or system of micro- or nanochannels) has been carried out and analyzed. A special finite-difference method has been used for the space discretization along with a semi-implicit 31/3-step Runge-Kutta scheme for the integration in time. The calculations employ parallel computing.

Electrokinetic instability near charge-selective hydrophobic surfaces.

The influence of the texture of a hydrophobic surface on the electro-osmotic slip of the second kind and the electrokinetic instability near charge selective surfaces (permselective membranes, electrodes, or systems of microchannels and nanochannels) is investigated theoretically using a simple model based on the Rubinstein-Zaltzman approach. A simple formula is derived to evaluate the decrease in the instability threshold due to hydrophobicity. The study is complemented by numerical investigations both of linear and nonlinear instabilities near a hydrophobic membrane surface.

Competition between Dukhin's and Rubinstein's electrokinetic modes.

The combined effect of two modes of electroconvection, i.e., (a) the electro-osmotic flow of the second kind induced by a curved membrane surface and (b) electrokinetic instability, is studied numerically.

Electrokinetic instability in microchannels.

The effect of geometric confinement on electroconvective instability due to nonequilibrium electro-osmotic slip at the interface of an electrolytic fluid and charge-selective solid is studied. It is shown that the topology of the marginal stability curves and the behavior of the critical parameters depend strongly on both channel geometry and dimensionless Debye length at low voltages for sufficiently deep channels, corresponding to the Rubinstein-Zaltzman instability mechanism, but that stability is governed almost entirely by channel depth for narrow channels at higher voltages. For shallow channels, it is shown that above a transition threshold, determined by both channel depth and Debye length, the low-voltage instability is completely suppressed.

Linear and nonlinear evolution and diffusion layer selection in electrokinetic instability.

In the present work, four nontrivial stages of electrokinetic instability are identified by direct numerical simulation (DNS) of the full Nernst-Planck-Poisson-Stokes system: (i) a stage of the influence of the initial conditions (milliseconds); (ii) one-dimensional (1D) self-similar evolution (milliseconds-seconds); (iii) a primary instability of the self-similar solution (seconds); (iv) a nonlinear stage with secondary instabilities. The self-similar character of evolution at moderately large times is confirmed. Rubinstein and Zaltzman instability and noise-driven nonlinear evolution toward overlimiting regimes in ion-exchange membranes are numerically simulated and compared with theoretical and experimental predictions.

Taylor cones in a leaky dielectric liquid under an ac electric field.

Conical points of a leaky dielectric drop surrounded by a dielectric gas in an external ac electric field are investigated. A novel class of steady conical tips depending on the permittivity ratio and applied signal frequency is presented. It is found that conical solutions with very small angles are possible (angles much smaller than the classical Taylor cone angle 49.

Three-dimensional localized coherent structures of surface turbulence: Model validation with experiments and further computations.

We validate experimentally the Kapitsa-Shkadov model utilized in the theoretical studies by Demekhin [Phys. Fluids 19, 114103 (2007)10.1063/1.

The geometry and motion of nematode sperm cells.

The nematode sperm cell crawls by recycling major sperm protein (MSP) from dimers into subfilaments, filaments, and filament complexes, as a result of thermal writhing in the presence of hydrophobic patches. Polymerization near leading edges of the cell intercolates MSP dimers onto the tips of growing filament complexes, forcing them against the cell boundary, and extending the cytoskeleton in the direction of motion. Strong adhesive forces attach the cell to the substrate in the forward part of the lamellipod, while depolymerization in the rearward part of the cell breaks down the cytoskeleton, contracting the lamellipod and pulling the cell body forward.

Self-organization of two-dimensional waves in an active dispersive-dissipative nonlinear medium.

We consider the pattern-formation dynamics of a two-dimensional (2D) nonlinear evolution equation that includes the effects of instability, dissipation, and dispersion. We construct 2D stationary solitary pulse solutions of this equation, and we develop a coherent structures theory that describes the weak interaction of these pulses. We show that in the strongly dispersive case, 2D pulses organize themselves into V shapes.

Nonlinear electrokinetics and "superfast" electrophoresis.

Nonlinear and nonequilibrium electrophoresis of spherical particles of radius a is shown to be possible when the solid surface allows field or current penetration. At low particle Peclet numbers, transient capacitative charging occurs until the surface polarization completely screens the external field. For a DC applied field [see text], the resulting electrokinetic velocity reaches Dukhin's maximum value of [formula: see text], where [see text] and mu are the liquid permittivity and viscosity.

Circular Hydraulic Jumps Triggered by Boundary Layer Separation.

When a high-flow-rate circular jet impinges vertically on a horizontal plane, it flows out radially and then undergoes a distinctive hydraulic jump on the plane because of boundary layer separation induced by hydrostatic back pressure. The jump radius is shown to be 0.37 a Re(1/3) Lambda(-1/8), where Lambda=(ga(3)/nu(2)) Re(-7/3) is a modified Froude number, Re=(Q/anu) is the jet Reynolds number, a is the jet radius, and Q the liquid flow rate, which is favorably compared to experimental data in the limit of small Lambda.