Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M3: Electrokinetics: Instability and Chaos |
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Chair: Ali Mani, Stanford University Room: 3004 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M3.00001: Electroconvective Instability in Flow-shear-induced Transport Barrier: Threshold for Stable Vortices and Chaotic Eddies Rhokyun Kwak, Van Sang Pham, Jongyoon Han Suppression of turbulence and transport by shear flow is a common process in plasma fluid dynamics, while it has been rarely observed in nonionized fluids. Here, we visualize this effect in microfluidic nonionized system with electroconvective instability (EC) initiated by ion concentration polarization on ion selective membrane. The membranes act as the source of both instability and flow shear (wall shear of Hagen-Poiseuille (HP) flow) simultaneously, fitting the requisite for this shear suppression effect; turbulence in the domain of flow shear. To the best of our knowledge, this is the first characterization of flow-shear-induced transport barrier in microfluidics, captured by scaling analysis, experiment, and numerical modeling. Selected by balancing flow shear and velocity fluctuation, which generated by HP flow and vortical EC, the threshold for shear suppression scales by EC thickness $d_{\mathrm{ec}}$/$w$ \textless 0.618. Stable unidirectional EC occurs under the threshold, while chaotic EC occurs over the threshold by overcoming flow shear. It also has significant implications on the energy saving of electrochemical systems ($e.g.$ electrodialysis) to prevent chaotic turbulences and corresponding energy dissipations. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M3.00002: Chaotic electroconvection near ion-selective membranes: investigation of transport dynamics from a 3D DNS Clara Druzgalski, Ali Mani We have investigated the transport dynamics of an electrokinetic instability that occurs when ions are driven from bulk fluids to ion-selective membranes due to externally applied electric fields. This phenomenon is relevant to a wide range of electrochemical applications including electrodialysis for fresh water production. Using data from our 3D DNS, we show how electroconvective instability, arising from concentration polarization, results in a chaotic flow that significantly alters the net ion transport rate across the membrane surface. The 3D DNS results, which fully resolve the spatiotemporal scales including the electric double layers, enable visualization of instantaneous snapshots of current density directly on the membrane surface, as well as analysis of transport statistics such as concentration variance and fluctuating advective fluxes. Furthermore, we present a full spectral analysis revealing broadband spectra in both concentration and flow fields and deduce the key parameter controlling the range of contributing scales. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M3.00003: Electrokinetic Instability, Geometric Confinement, and Overlimiting Conductance Jarrod Schiffbauer, Mathias Baekbo Anderson, Ali Mani, Gilad Yossifon For systems containing ion-selective membranes or nanochannels, concentration polarization (CP) under DC voltage beyond the classical Levich limit leads to the loss of local electroneutrality over micron or larger scales at the salt-depleted interface. This manifests itself in the appearance of an extended space charge (ESC) region, which is rendered unstable above a critical voltage drop. The instability drives the the formation of a fast-flowing vortex system with complex, often chaotic, dynamics. In unconfined systems, i.e. large electrolytic cells, this contributes strongly to the overlimiting conductance (OLC) of the system. However, both the role of the instability in OLC as well as its origin and onset become more complicated in highly confined systems such as microchannel devices. The problem of instability under geometric confinement has been studied both analytically and numerically using two different approaches. We compare the two approaches, and discuss relevant experimental evidence. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M3.00004: Suitability of commercial software for direct numerical simulations of chaotic electrokinetic transport Elif Karatay, Ali Mani Many microfluidic and electrochemical applications involve chaotic transport phenomena that arise due to instabilities stemming from coupling of hydrodynamics with ion transport and electrostatic forces. Recent investigations have revealed contribution of a wide range of spatio-temporal scales in such chaotic systems similar to those observed in turbulent flows. Given that these scales can span several orders of magnitude, significant numerical resolution is needed for accurate prediction of these phenomena. The objective of this work is to assess efficiency of commercial software for prediction of such phenomena. To this end we have considered Comsol Multiphysics as a general-purpose commercial CFD/transport solver, and have compared its performance against a custom-made DNS code tailored to the specific physics of chaotic electrokinetic phenomena [1]. We present comparison for small systems, which can be simulated on a single core, and show detailed statistics including velocity and concentration spectra over a wide range of frequencies. Our results indicate that while accuracy can be guaranteed with proper mesh resolution, commercial solvers are generally at least an order of magnitude slower than custom-made DNS codes. \\[4pt] [1] Druzgalski, Andersen, and Mani, Phys. Fluids 25, 1 [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M3.00005: Analysis of chaotic electroconvection near electrodes Scott Davidson, Ali Mani Electroconvective instability has recently been shown computationally to occur near electrode surfaces in induced-charge electro-osmosis (ICEO) in addition to its well-known occurrence near ion-selective membranes under large applied fields. This instability occurs due to the interaction of the extended space charge region of nonequilibrium electrical double layers with the applied field. The presence of the instability causes chaotic flow leading to order one changes to mean flow rates in ICEO as well as leading to flow between parallel electrodes where the fluid would otherwise remain stationary. We present direct numerical simulations (DNS) of the coupled Poisson-Nernst-Planck and Navier-Stokes equations analyzing both flow and transport effects in various regimes of the governing nondimensional parameters. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M3.00006: Scaling law of velocity and conductivity in EK turbulence Wei Zhao, Fang Yang, Guiren Wang In microfluidics, when electrokinetic (EK) flow is applied with sufficiently high electric Rayleigh number (\textit{Ra}$_{e})$, turbulence can be achieved, and there can even be an universal equilibrium range of conductivity field. In this flow, a new scaling law region of velocity and conductivity structures where the energy cascade is dominated by electric body force (EBF) can be found. This is similar to the Bolgiano-Obukhov scaling law (BO59) in Rayleigh-B\'enard (RB) convection. By both directly analyzing Navier-Stokes (N-S) equation and dimensional analysis, the scaling exponent of the second order moment of velocity structure function is 2/5, while that of conductivity structures is 4/5. Compared to the buoyancy in RB convection which decreases with decreasing length scale, EBF actually increases with decreasing spatial scales. This leads to two different microscales depending on the strength of EBF. The scaling law of velocity fluctuation is verified experimentally in a micro-EK turbulent flow. Although due to the restriction of geometry of our microchannel, the bandwidth of the EBF dominant subrange is narrow. By adjusting \textit{Ra}$_{e}$ and other parameters, a wider EBF dominant subrange is predicable. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M3.00007: Electrokinetic flow characteristics of two fluids with different electrical conductivities in cross-shaped microchannels by the lattice-Boltzmann Method Amador Guzman, Alvaro Socias, Diego Oyarzun Electrokinetic inestabilities (EKI) in microchannels flow are important to determine and characterize when either suppressing or enhancing flow features for injection and separation or mixing of multiple species are desired features. Convective and absolute electrokinetic instabilities (EKI) can be triggered or suppressed by active means such as externally applied AC or DC on the channel inlet, outlet and walls, and passively by building geometrical patterns on the wall channels such as grooves or waves. EKI are caused when a strong conductivity gradient between two fluids with different conductivities under an externally applied electric field becomes unstable. We model and simulate electrokinetic flow in a cross-shaped microchannel of two fluids with different electrical conductivity under an applied electrical field among the microchannel wells. We use the lattice-Boltzmann method (LBM) for solving the discretized Boltzmann Transport Equations (BTE) describing the coupled processes of hydrodynamics, electrodynamic and concentration of species of three fluids having different electrical conductivities under an external voltage in a cross-shaped microchannel with grooves in the outlet channel. Our numerical simulations predict well the conductivity gradient across the interface among the fluids and the unstable behavior of this interface when the local Rayleigh electrical number achieved, setting up EKIs. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M3.00008: Electrohydrodynamic Instability of a Capacitive Elastic Incompressible Membrane Yuan-Nan Young, Michael Miksis The electrohydrodynamic instability of a leaky capacitive membrane in a direct current (DC) electric field, both perpendicular and parallel to the membrane in a micro-fluidic channel, is investigated theoretically. Under a parallel electric field, the membrane can be driven unstable with a vanishing membrane conductance. On the other hand a non-conducting capacitive membrane is always stable under a perpendicular electric field, and membrane conductance is essential for membrane instability due to a perpendicular electric field. The effects of membrane conductance, bending modulus, and charge relaxation time on the membrane instability are elucidated for several combinations of conductivity ratio and permittivity ratio in the bulk fluids. The tangential electric field acts similarly to the membrane tension in terms of its damping effects at small length scales (high wave number), while either bending or membrane tension is needed to damp out the small-scale perturbations under a perpendicular electric field. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M3.00009: Two layer flow of thin leaky dielectric films between electrodes Elizaveta Dubrovina, Richard Craster, Demetrios Papageorgiou The flow of two viscous conducting fluids between two electrodes is investigated. The fluids are assumed to be leaky dielectrics and two nonlinear coupled evolution equations are derived for the moving interface and the interfacial charge. These are solved numerically for three different cases in which the magnitude of the ratios of electric conductivities and permittivities is varied. A linear stability analysis indicates that electrical forces destabilize the system. These predictions are confirmed by numerical results which show that increasing the ratios of conductivities and permittivities leads to traveling waves that grow in amplitude. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M3.00010: Electro-hydrodynamic Stability of Electrified Jet - Dharmansh, Paresh Chokshi The axisymmetric stability of the straight jet in electrospinning process is examined for both Newtonian and polymeric fluids using leaky dielectric model. Contrary to previous studies which consider cylindrical jet as the base-state, in the present study the thinning jet profile obtained as steady-state solution of the 1D model is considered as the base-state. The linear stability of the thinning jet is analyzed for axisymmetric disturbances, which are believed to be responsible for the bead formation. The growth rate eigen-specturm is constructed using Chebyshev collocation method. Two different types of axisymmetric instability modes are observed, the Rayleigh mode and the conducting mode. Competition between these two modes is revealed for the thinning jet. The most unstable growth rate for thinning jet is found to be significantly different from that for the uniform jet. The role of various material and process parameters is also investigated. For the viscoelastic fluids, the thinning jet with non-uniform extension rate captures the role of nonlinear rheology of fluid in the stability behavior. The viscoelastic jet profile obtained from steady-state 1D model is analyzed for stability. The role of fluid elasticity on various instability modes is studied. Interestingly, the strain hardening behavior in polymer solution tends to suppress the instability producing smooth fibers. Also, increasing the polymer concentration exhibits stabilizing effect on the axisymmetric instability modes. [Preview Abstract] |
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