Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D07: Electrokinetics: General |
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Chair: William Ristenpart, University of California, Davis Room: Georgia World Congress Center B212 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D07.00001: Scale dependence of flow structures in electroconvection Karen May Wang, Ali Mani Electroconvection (EC) is a hydrodynamic instability that occurs when dissolved ions are driven from a bulk fluid towards an ion-selective surface under a sufficiently large applied voltage. This phenomenon can impact mass transfer and ion residence time in a wide range of electrochemical applications. Simulating EC in regimes relevant to industrial applications is challenging; its wide range of spatiotemporal scales requires costly computations with extremely high resolutions. This study considers chaotic EC in a symmetric binary electrolyte in a canonical geometry. Using direct numerical simulations (DNS), we show that the range of length scales spanned by the EC induced eddies is a strong function of ε, which is the ratio of the nominal Debye length to the macroscopic geometry length. The ratio of large eddy to small eddy size is shown to grow as ε-0.8. Data normalization based on this power law resulted in collapse of statistics in an intermediate asymptotic layer where smallest eddies reside. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D07.00002: Assessment of a reduced-order model for electroconvective flows and applications in optimization Arunraj Balaji, Ali Mani Electroconvection (EC) is a hydrodynamic instability that arises in a wide range of electrochemical applications involving voltage-driven transport of ions through an ion-selective surface. In most applications, such as desalination, EC leads to enhanced transport rate and faster product throughput. Simulation of EC requires direct numerical solution (DNS) of the coupled Poisson-Nernst-Planck and Navier-Stokes equations, which can be extremely expensive since most practical regimes demand high spatial and temporal resolutions. We present the assessment of a reduced-order model for EC flows in which the velocity field is constrained to have a prescribed, smooth, periodic profile, and the charge transport is modeled via quasi-electroneutral approximation. Instead of as a solution to the Navier-Stokes equation, the velocity magnitude is determined by a balance between input mechanical power from the local electrostatic forces and viscous dissipation. We present results from this model and discuss its application in estimating bounds of opportunity for optimization of EC flows. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D07.00003: Electrohydrodynamic Flow induced by the Onsager Effect Jung Min Oh We analyze the electrohydrodynamic flow around a cylindrical/spherical particle in a leaky dielectric medium induced by the Onsager effect, which describes the electric-field-dependent ion dissociation rate of weak electrolytes. From the Onsager effect, the conductivity is shown to increase linearly with respect to the electric field strength and the Coulombic force by the Maxwell-Wagner polarization can be induced in non-uniform electric field. In this paper, the time-averaged Stokes flow around a cylindrical/spherical particle by the Onsager effect is solved analytically using the perturbation expansion to the 1st order accuracy. The flow field solutions are expressed with a finite set of eigenfunctions in cylindrical and spherical coordinates. The flow velocity is proportional to the cubic of the electric field strength. It is also shown that the flow patterns can be classified by a simple straining, intermediate and eight-folded octagonal flow according to the Clausius-Mossoti factor, which is a function of complex conductivities of liquid and particle. The results are compared with full numerical simulations and experiments to discuss the detailed mechanism of flow structures. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D07.00004: Capillary Driven Electrokinetic Power Generator Sankha Shuvra Das, Aditya Bandopadhyay, Partha Saha, Suman Chakraborty Recently, paper-based platform has been used for electrokinetic energy conversion by exploiting the surface energy of the device. In such a device, liquid migrates towards downstream direction through wicking action and the net streaming potential is obtained from coupled capillary and evaporation effect. However, the fabrication technique reported to manufacture such devices essentially scales up the cost. Hence, in this work, we have attempted to fabricate an ultra low-cost paper device and exploit the same for electrokinetic energy generation. We develop an ultra-low cost electrokinetic power generator on a paper platform, by employing simple pencil-sketched electrodes and a frugal fabrication technology. The paper platform ( ~10µm pore size) can induce open circuit potential and short circuit current in the range of ~30-40mV and ~5nA respectively, by utilizing the capillary-driven spontaneous flow of an ionic solution through the internal pores. Such devices turn out to be ideal in empowering portable sensors for biomedical applications in resource-limited settings. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D07.00005: Electrokinetic flows and energy conversion in nanofluidic channels: Assessing continuum predictions via molecular dynamics Carlos E Colosqui, Amir M. Rahmani, Antonio Checco The flow of electrolyte solutions in nanoscale channels can convert mechanical forces into electrical power. High efficiencies (> 50%) are predicted by the classical Poisson-Boltzmann and Navier-Stokes equations. Employing molecular dynamics (MD) we assess the accuracy of such continuum-level descriptions for slit channels with small nanoscale heights (h ∼ 10 nm) where there is significant overlap of the electric double layers at the wall-fluid interface. We study nanochannels with atomically smooth surfaces and periodic geometric features. The hydrodynamic slip length and ion solvation energy are varied over a moderate range, which is found to significantly affect the nanochannel hydrodynamic and electrical resistance, and thus the conversion efficiency. Despite steric effects, nanoscale hydration layers, and localized charge heterogeneities, the employed continuum model predicts with reasonable accuracy electrokinetic coupling coefficients and conversion efficiencies from MD simulations. Our results support the application of conventional continuum descriptions with minor modifications to design nanofluidic devices for electrokinetic energy conversion with high efficiencies. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D07.00006: A numerical study on the effect of the electrode voltage on the near-electrode layers in leaky dielectric liquids Babak Kashir, Anthony Perri, Alexander Yarin, Farzad Mashayek The effect of the electrode voltage on the near-electrode layers in leaky dielectric liquids is investigated numerically employing a code developed in OpenFOAM. The Frumkin-Butler-Volmer kinetics responsible for the ion transfer at the electrode surface is accounted for using the experimental data for the faradaic reactions. A novel approach is proposed to calculate the compact layer thickness ramifying from the seminal Stern model. Moreover, a numerical algorithm is suggested to calculate the compact layer thickness for high values of potential applied at the electrodes. The impact of the electric potential on the thickness of the compact and polarized layers and the ionic transport are also investigated in the context of a two-dimensional microchannel flow. The polarized layer is defined as the distance from the surface of the compact layer where the absolute net charge falls to 1% of the maximum absolute value occurring near the compact layer. The electro-hydrodynamic flow in the channel is subjected to two opposite electrodes with opposite polarities resembling situations occurring in the electrostatic atomizers. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D07.00007: Effects of asymmetric ion sizes and valences on the structure of the electric double layer near electrode Yun Sung Park, Eunseo Kim, In Seok Kang Understanding the electric double layer (EDL) near electrode is crucial for utilizing electrical devices including supercapacitors and batteries. The EDL structure is expected to depend on the sizes and valences of accompanying ions. The sizes of ions determine the maximum packing density, while the valences of ions determine the strength of electrical interactions. In this work, we study the EDL structure of ions of which sizes and valences are asymmetric using the continuum approach. Firstly, we propose the asymmetric ion distribution as a function of electric potential. The EDL structure and the stress exerted on electrodes can be obtained from its solution. Secondly, the equation of asymmetric ion dynamics is solved for confined slit, with certain potential difference imposed between two ends. The charging rate of EDL depends on the size and valence of the counter-ions. The asymmetric potential drop across the EDL is explained along with its ion density distribution. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D07.00008: Computation of the interaction of electrically charged particles Claus Bissinger, Holger Grosshans In industrial applications, the dynamics of particles is frequently controlled through electrical charges, e.g. in electrostatic precipitators or during powder coating. However, the electrification of particulates can also cause the formation of deposits, hazardous sparks and dust explosions. The objective of our work is to propose a new computational model which captures accurately the dynamics of particles during their approximation. This model focuses especially on the precise prediction of the contribution of the electrostatic and collisional forces whose time-scales are typically much smaller than the numerical time-step used to compute forces other than Coulomb force. To this end, binary particle interaction is calculated through the implementation of a sub-grid model and local refinement of the time-step. In our talk we present results concerning binary and multiple particle interaction. Further, we elaborate on the conditions, in terms of particle charges and sizes, for which our model is superior to previously employed approaches. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D07.00009: Streaming current for interfaces covered by clustered particle monolayers Jerzy Blawzdziewicz, Zbigniew Adamczyk, Maria Ekiel-Jezewska Measurements of streaming current (i.e., the current resulting from charge convection in the electric double layer) are widely used to investigate properties of micro- or nano-particle monolayers adsorbed on a planar surface. Our previous study showed that, at a given area fraction, the streaming current is practically the same for the equilibrium and random-sequential-adsorption particle distributions and is only slightly different for square and hexagonal periodic lattices. Numerical results presented here demonstrate that the microstructure dependence of the streaming current is very strong for clustered particle distributions, especially for closely packed compact clusters separated by large particle-free areas. This dependence results from enhanced charge convection in the particle-free regions and reduced charge convection within the clusters, where there is strong hydrodynamic screening. The limiting behavior for large compact clusters is described using interpolation formulas combining streaming current contributions from the clusters, particle-free areas, and border regions. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D07.00010: Strong deformation of the thick electric double layer around a charged particle during sedimentation or electrophoresis Aditya S Khair The deformation of the electric double layer around a charged colloidal particle during sedimentation or electrophoresis in a binary, symmetric electrolyte is studied. The surface potential of the particle is assumed to be small compared to the thermal voltage scale. Additionally, the Debye length is assumed to be large compared to the particle size. These assumptions enable a linearization of the electrokinetic equations. The particle appears as a point charge in this thick-double-layer limit; the distribution of charge in the diffuse cloud surrounding it is determined by a balance of advection due to the particle motion; Brownian diffusion of ions; and electrostatic screening of the particle by the cloud. The ability of advection to deform the charge cloud from its equilibrium state is parameterized by a Peclet number, Pe. For weak advection (Pe « 1) the cloud is only slightly deformed. In contrast, the cloud can be completely stripped from the particle at Pe » 1: consequently, electrokinetic effects on the particle motion vanish in this regime. Therefore, in sedimentation the drag limits to Stokes' law for an uncharged particle as Pe -> ∞. Likewise, the particle velocity for electrophoresis approaches Huckel's result. |
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