Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C36: Electrokinetic Flows: Induced Charge Flows and Instabilities |
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Chair: Igor Novosselov, University of Washington Room: 618 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C36.00001: Analytical Model for Electrohydrodynamic Thrust Ravi Sankar Vaddi, Igor Novosselov Electrohydrodynamic (EHD) thrusters produce a thrust force by using two or more electrode to ionize the fluid and accelerated the flow in an electric field. Ionized fluids gain momentum due to the momentum transfer between charged species and neutral molecules. This phenomenon is termed as EHD flow. We extend the previously reported analytical model for EHD flow that describes the relationship between the corona voltage, electric field, and ion charge density [1] to the analysis of EHD thrust in 1D planar coordinates. The electric current density derived from the model is in agreement with Mott-Gurney law. This analytical model is compared to the experimental data [2,3] sawtooth anode to rod corona discharge in the surface boundary layer, and wire to mesh corona EHD thruster in the miniature ionocraft [4]. The model sheds new insights into the EHD thrust force and energy conversion efficiencies and can be implemented in the numerical model. [1] Y. Guan et al, Physics of Plasmas 25, 083507 (2018). [2] K. Masuyama et al, Proceedings: Mathematical, Physical and Engineering Sciences 469, 1 (2013). [3] E. Moreau et al, Journal of Physics D: Applied Physics 46, 475204 (2013). [4] E. Dedic et al, arXiv preprint arXiv:1906.10210 (2019). [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C36.00002: Asymmetric Rectified Electric Fields Generate Flows that can Dominate Induced-Charge Electrokinetics S. M. H. Hashemi Amrei, Gregory Miller, William Ristenpart We derive a generalized induced-charge electrokinetic (ICEK) velocity around a conducting object placed in an arbitrary multimodal electric field. The generalized model allows consideration of asymmetric rectified electric fields (AREFs), which have recently been established to occur in liquids where the ions present have unequal mobilities. Including the AREF yields fluid velocities in which both the direction and the magnitude depend sensitively on the applied potential, frequency, ionic type and strength, and even the exact placement of the object between parallel electrodes. The results provide a new explanation for the long-standing question of flow reversals observed in ICEK systems. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C36.00003: Electric-Field Induced Pattern Formation in Layers of DNA Molecules at the Interface between Two Immiscible Liquids Johannes Hartmann, Steffen Hardt, Sicheng Zhao, Aditya Bandopadhyay Electrohydrodynamic/electrokinetic flows play a key role as driving mechanism of pattern formation in colloidal dispersions and at liquid-liquid and liquid-solid interfaces with adsorbed particles. We report an analogous phenomenon of electric field-induced concentration patterns of DNA molecules at the interface of an aqueous two-phase system. The electric field, applied normal to the liquid-liquid interface, drives the molecules to the interface where they get trapped. Hydrodynamic interactions between the molecules arise by electroosmotic flow due to the Debye layer around the polyelectrolytes, leading to pattern formation in the DNA concentration field at the interface. We describe the time evolution of the concentration field by a non-linear integro-differential equation. A linear stability analysis of the equation yields a critical time after which the system destabilizes if exited by a mode of given wavelength. We find that the scaling behavior predicted by theory agrees with experimental results. The presented scheme could be used as an efficient method to pre-concentrate DNA molecules at an interface. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C36.00004: Enhanced Ion Transport by Controlling Electroconvection on Ion Exchange Membranes with Patterned Structures Joonhyeon Kim, Sangha Kim, Rhokyun Kwak Patterned structure in fluid systems has been used to generate vortices, enhancing mass transfer. Here, we investigate new role of this vortex promoter, i.e. controlling an electrically driven hydrodynamic instability ( a.k.a electroconvection (EC) ) on ion exchange membranes. The patterned structures not only generates vortices as a vortex promoter, but also acts as a shelter to keeping EC from being suppressed by shear flow. To verify these effects, we visualized EC over six different patterns under various applied voltages. The strength of EC was then quantified by visualizing velocity and vorticity fields. In current-voltage response, we found that i) conductive ion flux is inversely proportional to the occupied area of the pattern in Ohmic regime, ii) stronger vortex promotion with a larger vortex area induces a shorter or even negligible length of limiting regime, and iii) the area of sheltered EC benefiting from the pattern is directly proportional to the convective ion flux in overlimiting regime. Considering relationship between ion flux and roles of structures synthetically, isosceles triangle pattern shows the highest ion flux through the membrane as it is the best option for a conductor and the second best for the vortex promoter and EC shelter. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C36.00005: Pattern formation of three-dimensional electroconvection on a charge selective surface Soohyeon Kang, Rhokyun Kwak Electroconvection (EC) has been in the spotlight for enhancing ion flux in various electrochemical systems, but its dynamics is yet to be probed in three-dimensions. In this paper, we describe the first laboratory observation of 3-D EC on an exchange membrane and its pattern diversification. Combining experiment and scaling analysis, we successfully categorized three distinguished patterns of 3-D EC according to Reynolds number (Re), electric Rayleigh number (\( \mathrm{Ra_E} \)) and Schmidt number (Sc) as i) polygonal, ii) transverse, or iii) longitudinal rolls. If Re increases or \( \mathrm{Ra_E} \) decreases, pure longitudinal vortices are presented. On the other hand, transverse rolls are formed between longitudinal rolls, and two rolls are transformed as polygonal rolls at higher \( \mathrm{Ra_E} \) or lower Re. In this pattern selection scenario, Sc determines the critical electric Rayleigh number (\( \mathrm{Ra^*_E} \)) for the onset of each transverse or polygonal rolls, resulting \( \mathrm{Ra^*_E \sim Re^2 Sc} \). We also verify that convective ion flux by EC (represented by an electric Nusselt number, \( \mathrm{Nu_E} \)) is fitted to a power law, \( \mathrm{Nu_E \sim Ra_E^{0.48} Sc^{0.39}} \). [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C36.00006: Numerical analysis of 2D and 3D electrohydrodynamic convection instability with crossflow Yifei Guan, James Riley, Igor Novosselov The study focuses on the electro-hydrodynamic (EHD) instability for flow between to parallel electrodes with unipolar charge injection with cross-flow. Lattice Boltzmann Method (LBM) with two-relaxation time (TRT) model is used to study flow pattern [1]. Under strong charge injection and high electrical Rayleigh number, the system exhibits electroconvective vortices. Disturbed by different perturbation patterns, the flow patterns develop according to the most unstable modes. The unstable modes are obtained by dynamic mode decomposition (DMD) on the transient numerical solutions. Once the steady-state solution is obtained, Couette and Poiseuille cross-flow are applied. The flow patterns change according to the strength and direction of the cross-flow. When the cross-flow velocity is greater than a threshold value, in the 2D scenario, the vortices are suppressed [2], and in 3D the instability flow patterns would develop into streamwise rolling patterns. References: [1] Y. Guan and I. Novosselov, Two Relaxation Time Lattice Boltzmann Method Coupled to Fast Fourier Transform Poisson Solver: Application to Electroconvective Flow, Journal of Computational Physics (accepted for publication) (2019). [2] Y. Guan and I. Novosselov, Numerical Analysis of Electroconvection Phenomena in Cross-flow, arXiv preprint arXiv:1812.10899 (2018). [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C36.00007: A Front Tracking Method for the Investigation of the Electrohydrodynamic Instability Between Two Immiscible Fluids Ilke Kaykanat, Metin Muradoglu, Kerem Uguz When an electric field is applied to the flat interface between two immiscible liquids flowing in a microchannel, the interface can be deflected and microdroplets can be obtained. In this study, a front-tracking method is presented for direct numerical simulations of two-phase systems to study the effects of the electric field applied normal to the flat interface between two immiscible, Newtonian fluids. The method is developed using a one-field formulation of the governing equations. The interface is tracked explicitly using a Lagrangian grid and the flow equations are solved on a fixed Eulerian grid. The effects of the applied voltage, the viscosity, and the base-flow strength on the nonlinear evolution of the interface are studied. Furthermore, in this study, the results obtained by the interface tracking method will be compared with the results obtained by both long-wave analysis and the experimental results obtained in our group. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C36.00008: Electroconvection near a metal electrode surface Gaojin Li, Lynden Archer, Donald Koch Electroconvection in electrodeposition leads to fast dendrite growth undermining the efficacy of the process. Above a critical voltage, the electrohydrodynamic instability generates an electro-osmotic slip velocity at the edge of the space charge layer, creating convective flows in the electrolyte which cause an overlimiting current and a nonuniform ion flux. The nonuniform deposition of cations on a metal anode and its coupling with the electroconvection cause fast dendrite growth and lead to premature cell failure. The deposition rate of cation on the metal surface, which is described by the well-known Butler-Volmer equation, determines the ion transport below the limiting current. However, how does it affect the electroconvection in the overlimiting regime is not clear yet. In this work, we use the stability analysis and direct numerical simulation to investigate the linear instability and the dynamics of the electroconvection. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C36.00009: Surface Reaction Driven Flow Abimbola Ashaju, Jeffery Wood, Rob Lammertink Bimetallic nanorods in form of microswimmers within an aqueous solution exhibits self-propulsion that is powered by self-electrophoresis. This bimetallic catalytic system can be immobilized to generate convective flow thereby acting as a micropump. In this work, we focus on experimental and numerical analysis that provides fundamental insight on the key elements including the generated electric field, reaction kinetics and diffusio-electroosmotic phenomena that control the resulting mass transport characteristics in these systems. The catalytic current between the electrodes and the induced potential that governs the reactive fluxes are measured electrochemically, proton concentration gradients originating from the catalytic reaction are imaged and quantified using fluorescence lifetime imaging, while the fluid flow is visualized with 3D particle tracking. Numerical simulations reveals the interplay of electrodes surface reactivity pattern represented by the dimensionless Damköhler number, with the electrokinetic phenomena that controls the release and depletion of protons and consequently the resulting induced fluid flow. This work highlights the ability of surface induced convective fluid flow in electrochemical systems to reduce mass transport limitations. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C36.00010: Experimental and numerical determination of flow mixing enhancement in electrokinetics cross shaped microchannel flows due to spatially periodic wall perturbations Amador Guzman, Mario DiCapua, Daming Chen, Francisco Montero, Maria Canales Liquid flows in microchannels usually occur at very low Reynolds numbers (\textless 1) because inertial forces are strongly dumped by the very small microchannel characteristic length and the dominant viscous forces. When an electrical field is applied to the microchannel ends, a time-dependent stable flow arises at a very low Reynolds number because another non dimensional parameter, the Rayleigh number, takes a dominant role. When the Rayleigh numbers surpasses a threshold value, a unstable behavior sets in convecting the flow instabilities backward and forward and consequently leading to an increase of the flow mixing that is particularly important when liquids with different electrical conductivities need to be mixed, particularly in physiological flows. One way of achieving this type of behavior but to a lower critical Rayleigh number is by adding spatially periodic inhomogeneities to the channel walls. In this research, we investigate the flow mixing enhancement occurring in a cross-shaped microchannels when liquids with different electrical conductivities get in the microchannel at different inlets and flows downstream due to the application of an electrical field and get together in the outlet branch of the microchannel, which contain spatially periodic wall inhomogeneities. Experiments and numerical simulations are carried out to determine and evaluate stable and unstable behaviors and evaluate the flow mixing enhancement. [Preview Abstract] |
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