Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session G02: Electrokinetic Flows (5:00pm - 5:45pm CST)Interactive On Demand
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G02.00001: Theoretical and experimental analysis of superhydrophilic and superhydrophobic nanostructured surfaces with tunable zeta potentials Aktaruzzaman Al Hossain, Mengying Yang, Antonio Checco, Gregory Doerk, Carlos Colosqui We present a theoretical and experimental analysis of the zeta potential of superhydrophilic and superhydrophobic nanostructured surfaces with different heights (60 to 200 nm) over a range of pH (4 and 8) and concentrations (.3 to 30 mM) of aqueous KCl and NaCl solutions. The experimental determination of the zeta potential via macroscale electrokinetic flow measurements is enabled by a surface fabrication protocol based on block-copolymer self-assembly that is suitable for producing conical nanopillars of precisely controlled height and period over macroscale surface areas. Surfaces fabricated on silicon display superhydrophilicity and superhydrophobicity is attained after coating with octadecyltrichlorosilane (OTS). Negative zeta potentials are reported for all the studied surfaces. While varying the nanostructure height can suppress the zeta potential of hydrophilic surfaces at certain pH and electrolyte concentrations, the hydrophobic OTS coating generally enhances the zeta potential. Our experimental results are accounted for by a site-dissociation model for the surface charge and a hydrodynamic model for the slip length The studied surfaces are relevant for applications in energy conversion/storage and membrane-based separation. [Preview Abstract] |
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G02.00002: Electrokinetic method to study the surface and electronic properties of silicon nanowire diodes Minh Thang Hoang, Amar Mohabir, Amy Brummer, Leonard Feldman, Micheal Filler, Jerry Shan Colloidal dispersions of nanoelectronic devices offer many new opportunities to develop self-propelled nanomachines and advanced nanosystems. A better understanding of their surface and electronics properties, and how they may vary statistically within a sample and as a function of processing, is needed for nanodevice manipulation and assembly. In this study, we demonstrate efficient electrokinetic methods to characterize VLS grown silicon nanowire diodes. In comparison to a homogeneous nanowire, a nanowire diode has a specific rotational direction under an external applied DC electric field, due to its permanent dipole. Contrary to previous work that has assigned this permanent dipole to opposite charges at the junction of n- and p- regions, we show that the dipole is formed due to a non-uniform surface charge distribution. In addition, the nanowire diode, when powered by an external AC electric field, will have a rectified motion that depends on the orientation of the permanent dipole, with a field strength velocity that is dependent on the built-in voltage. These electrokinetic methods, capable of ensemble sample analyses, offer a new approach to efficiently determining the surface and electronic properties of p-n nanowires. The approach represents a crucial step toward the manipulation and separation of complete nanoelectronics devices. [Preview Abstract] |
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G02.00003: Toward predicting the electrokinetic zeta potential in slit micro/nanochannels with nanoscale surface features: Continuum and molecular dynamics analysis. Troy Singletary, Amir Rahmani, Sijia Huang, Gerard Miles, Carlos Colosqui We present a recently developed continuum description of electrokinetic flows for predicting the zeta potential in the case of slit channels with nanoscale surface features of dimensions comparable to the Debye length. The proposed analytical model considers the average effects that such nanoscopic features commonly found on macroscopically smooth surfaces have on the streamwise-averaged fluid velocity and ion density by solving a unidirectional Navier-Stokes and Poisson-Boltzmann equation for such variables. The proposed analytical model is simple and compact and quantitatively accounts for results from molecular dynamics simulations that consider the finite size of ionic species as well as the presence of ion solvation shells and hydration layers on the channel surface. Our theoretical and computational analysis indicates that under conditions for which the Onsager symmetry holds, a simultaneous knowledge of the electroosmotic flow rate and the pressure-driven flow rate or streaming current is instrumental to unambiguously determine the zeta potential and the effective height of the nanoscale surface features. [Preview Abstract] |
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G02.00004: Thick Double Layers: From Energy Storage to~Diffusiophoresis Ankur Gupta, Pawel Zuk, Suin Shim, Howard Stone We~study two distinct out-of-equilibrium processes relating to thick double layers:~(a) charging dynamics of electrical double layers in a nanopore, and (b) diffusiophoretic~mobility of colloidal particles in a~time-dependent concentration field. \newline \newline The~charging dynamics of electrical double layers inside a cylindrical pore has~been widely studied for the thin double layer limit. However, a model for the~thick double layer limit is not readily available.~Here, we demonstrate that the~charging dynamics in the limit of thick double layers can be represented through~an effective circuit model, much like the thin double layer limit. However, the~physical~meaning of the effective circuit elements, i.e., resistors and~capacitors, is different between the two limits.~~~ \newline \newline Diffusiophoresis~refers to the movement of colloidal particles in a concentration gradient of~an electrolyte. It is typically assumed that the diffusiophoretic mobility does~not vary with electrolyte~concentration. Here, we show that the~diffusiophoretic mobility is coupled with electrolyte concentration, especially~in a time-dependent concentration field where the regions of thick double layer~may be~more prevalent. We demonstrate that the diffusiophoretic mobility possesses~a maximum with electrolyte concentration.~ [Preview Abstract] |
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G02.00005: Numerical computation of charge transport in flows of dielectric liquids Mathieu Calero, Holger Grosshans, Miltiadis Papalexandris Flow electrification during transport of dielectric liquids constitutes a major safety hazard. More specifically, at sufficiently high Reynolds numbers and for low-conductivity fluids such as liquid hydrocarbons, the thickness of the hydrodynamic boundary layer can become comparable to that of the electrical double layer. In turn, this leads to increased transport of electric charges away from the wall region and towards the bulk of the flow. Moreover, the transport of charges is further enhanced due to turbulent mixing. However, quantitative information on the underpinning mechanisms of this phenomenon is still lacking. In the first part of this presentation we outline a computational framework that we recently developed for the simulation of wall-bounded flows of dielectric liquids and, in particular, for the numerical study of flow electrification. In the second part, we present results from two cases of laminar flow electrification that we studied numerically in order to assess the efficiency of the proposed numerical procedures. Finally, preliminary results of turbulent flow electrification at weak turbulent intensities are also presented herein. [Preview Abstract] |
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G02.00006: Integral jump conditions for time-dependent Debye layer phenomena Philipp G. Marthaler, Andreas G. Class In lab-on-chip systems, flow propulsion, mixing and control can be performed by induced electrohydrodynamic (EHD) effects, such as AC electro-osmosis (ACEO). Close to charged solid walls or electrodes, Debye layers emerge, inhabiting strong gradients of the relevant physical parameters. In many numerical simulations of EHD flow, the detailed solution of Debye layers is avoided, due to the high computational effort. We present integral jump conditions that can be utilized for cheap simulations of such Debye layer phenomena. Our asymptotic approach is applied to the model by Yariv et al. (2011) who were able to re-establish the Smoluchowski slip condition. In addition to their results, obtained by matched asymptotics, we derive jump conditions of integral form for the set of governing equations. We follow a theoretical procedure published e.g. by Class et al. (2003). Our scaling takes time-dependent effects into account, which are crucial for the understanding and computation of flow excited by time-variant electrodynamics like ACEO. \renewcommand{\section}[2]{} \begin{thebibliography}{} \bibitem{} Yariv et al. (2011) J. Fluid Mech. \textbf{685}. \bibitem{} Class et al. (2003) J. Fluid Mech. \textbf{491}. \end{thebibliography} [Preview Abstract] |
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G02.00007: Approximate Time-Dependent Current-Voltage Relations for Currents Exceeding the Diffusion Limit Yoav Green In the limit of an infinitesimally thin Debye length and sufficiently large voltages, the steady-state current transported through a permselective membrane/nanochannel is predicted to saturate to a limiting value. In practice, the Debye lengths are finite and the current exceeds the predicted diffusion-limited value [1]. This peculiar steady-state response has been investigated for four decades [1,2]. However, the time-dependent response has yet to be resolved. Leveraging the steady-state approach of Yariv [2], I derive three separate expressions for the potential drop for short, intermediate, and long times for currents exceeding the diffusion limit [3]. I will demonstrate that the potential drop correlates to the time-evolution of the non-equilibrium space-charge-layer adjacent to the permselective interface. These approximations are compared to numerical simulations and show remarkable correspondence. [1] Rubinstein and Shtilman, J. Chem. Soc., Faraday Trans. 275, 231 (1979). [2] Yariv, Phys. Rev. E, 80, 051201 (2009). [3] Green, Phys. Rev. E, 101, 043113 (2020). [Preview Abstract] |
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G02.00008: Electrostatic interfacial instability: Weakly nonlinear analysis Ranga Narayanan, Dinesh Bhagavatula We analyse the stability of an interface between a perfectly conducting fluid in contact with a perfect dielectric under the influence of an electric field imposed normal to it. The fluids are confined between two rigid plates, which are maintained at a constant voltage difference, D, counteracting gravity. This work has its implications to patterning of polymer films, where the conducting fluid self assembles into arrays of pillars. These patterns arise due the competition between the applied voltage on the one hand and gravity and surface tension on the other. These competing effects lead to a minimum in a plot of D vs. the wavenumber, k, of the disturbance at the onset of the instability. The voltage difference and surface tension compete at high k, reminiscent of the subcritical nature of the Rayleigh-Taylor instability. However, the presence of a minimum in the D vs. k plot is indicative of a supercritical instability, much like the Benard problem. This suggests that there is a super to subcritical transition, which we investigate using a weakly nonlinear analysis about the neutral state where the applied voltage difference is advanced slightly beyond the critical point. Plots of this transition are presented as a function of the Bond number and explained. [Preview Abstract] |
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G02.00009: A Spontaneous Electrokinetic Magnus Effect James Swan, Zachary Sherman h $-abstract-$\backslash $pardColloids dispersed in electrolytes and exposed to an electric field produce a locally polarized cloud of ions around them. Above a critical electric field strength, an instability occurs causing these ion clouds to break symmetry leading to spontaneous rotation of particles about an axis orthogonal to the applied field, a phenomenon named Quincke rotation. In this Letter, we characterize a new mode of electrokinetic transport. If the colloids have a net charge, Quincke rotation couples with electrophoretic motion and propels particles in a direction orthogonal to both the applied field and the axis of rotation. This motion is a spontaneous, electrokinetic analogue to the well-known Magnus effect. Typically, motion orthogonal to a field requires anisotropy in particle shape, dielectric properties, or boundary geometry. Here, the electrokinetic Magnus (EKM) effect occurs for spheres with isotropic properties in an unbounded environment, with the Quincke rotation instability providing the broken symmetry needed to drive orthogonal motion. We study the EKM effect using explicit ion, Brownian dynamics simulations and develop a simple, continuum, analytic electrokinetic theory, which are in agreement. We also explain how nonlinearities in the theoretical description of the ions affect Quincke rotation and the EKM effect.$\backslash $pard-/abstract-$\backslash $\tex [Preview Abstract] |
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G02.00010: Electrocatalytic Reaction Driven Flow: The role of pH on flow reversal. Abimbola Ashaju, Veerle Otten, Jeffery Wood, Rob Lammertink Bimetallic nanorods move autonomously within an aqueous solution through self-electrophoresis. They can be immobilized in the form of a micropump to generate fluid flow which is driven from the anode to the cathode similar to an electroosmotic flow. However, under low reactive regimes, the flow becomes fully reversed. This work unravels the origin and dynamics of this flow hysteresis through a combined experimental and numerical approach. The key electrocatalytic parameters that contribute to flow reversal, including electrode switch, are analyzed under low reactive regimes induced by pH variations. The proton gradient that initiates electrocatalytic actuation is probed using fluorescence lifetime imaging. The fluid flow and electric field are quantified using the two-particle phoresis correlation. Our numerical simulations elucidate the role of pH variations and additional ionic species (counterions) towards flow reversal. The combination of these techniques highlights the interplay between these electrokinetic phenomena in conjunction with the bielectrode zeta potential towards flow reversal. Our work contributes towards the fundamental understanding of fluid flow powered by an immobilized electrocatalytic micropump that applies to mass transport enhancement in electrochemical systems. [Preview Abstract] |
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G02.00011: On fluid modeling of plasmas and connections to aqueous electrokinetics Arunraj Balaji, Shahab Mirjalili, Ali Mani Though similar to the Poisson-Nernst-Planck model for the transport of charged species in aqueous electrolytes, the fluid model for ion and electron transport in non-thermal plasmas must consider several additional phenomena. Bulk reactions, boundary fluxes, strong species asymmetry, and large/rapidly-oscillating applied voltage all contribute to the complex behavior of such plasma systems. Plasmas provide a novel context in which the effects of these phenomena can be studied, particularly in regimes that exceed the typical conditions observed in aqueous electrolytes. In this work, an existing AC electrokinetics solver is adapted to simulate a 1D non-thermal plasma. Structures and patterns in the distribution of charged species are identified and characterized. Connections to aqueous electrokinetics are explored, particularly with regard to the effects of strong bulk reactions, boundary fluxes, species asymmetry, and large/rapidly-oscillating applied voltage. The findings of this work reveal behavior that might be observed in extreme problems in aqueous electrokinetics and connections between these phenomena and typical observations in the study of non-thermal plasmas. [Preview Abstract] |
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G02.00012: The Influence of Asymmetric Rectified Electric Fields on Particle Aggregation Near Electrodes Timothy Hui, S. M. H. Hashemi Amrei, Gregory H. Miller, William D. Ristenpart Micron-scale particles near an electrode with an applied oscillatory field either aggregate or separate laterally, depending on the identity of the electrolyte. Although electrically induced fluid flows are believed to drive the aggregation, to date predictive models have failed to capture the behavior universally for a wide variety of electrolytes. Here, we assess the role of Asymmetric Rectified Electric Fields (AREFs), which are steady fields induced by an oscillatory potential that occur whenever there is mismatch in the electrolyte mobilities. We demonstrate that including the influence of AREFs on the electroosmotic flow induced on the particle and the electrode surface by the applied oscillatory potential yields a prediction for particle behavior that accords with experimental observations. The results suggest that AREFs play an important role in governing the electrokinetic behavior of systems with applied oscillatory fields. [Preview Abstract] |
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G02.00013: Multimodal Asymmetric Rectified Electric Fields and Long-Range Symmetry Breaking in Electric Field Orientation S. M. H. Hashemi Amrei, Timothy Hui, Gregory H. Miller, William D. Ristenpart Recent numerical, theoretical, and experimental work has established the existence of asymmetric rectified electric fields (AREFs), a type of long-range steady field induced by a sinusoidal potential between parallel electrodes containing an electrolyte with unequal ionic mobilities. Here, we show that certain classes of multimodal applied electric potentials yield AREFs that are spatially asymmetric, causing a net nonzero electric field at the midplane between the electrodes. A profound consequence is that swapping the powered and grounded electrodes alters the long-range direction of the steady field component, even though the applied wave-form has zero time average. We provide experimental observations with micron-scale colloids that corroborate the existence of multimodal AREFs. The findings have implications for a broad variety of systems that involve oscillatory fields, including low-energy water desalination and manipulation of flows and particles in microfluidic systems. [Preview Abstract] |
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G02.00014: Effect of a porous polymer layer on electroconvection Ankush Mukherjee, Gaojin Li, Lynden Archer, Donald Koch Electroconvection enhances the growth of dendrites on the surface of electrodes, reducing the life of a battery cell. Experimental studies have shown that adsorbed polymers on the surface of the electrode increase the stability of lithium deposition and reduce electroconvection. In this presentation, we analyze the linear stability of one-dimensional ion transport near an ion selective surface in the presence of a thin polymer layer modeled as a rigid porous medium. The ultraspherical spectral method is used to fully resolve the thin double layer, space charge layer, Brinkman length scale and the polymer layer. We show that electroconvection can be suppressed by a porous layer of sufficiently small permeability when the layer is thicker than the space charge layer. The attenuation of fluid velocities driven by electrical forces in the space charge layer by drag forces occurs when the polymer layer thickness is larger, and the Brinkman length scale is smaller than the space charge layer thickness. [Preview Abstract] |
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G02.00015: Fast Initiation of Electroconvection on an Ion Exchange Membrane by Coupled Redox Potential Minchan Kim, Rhokyun Kwak, Junbeom Lim In electromembrane processes (e.g. electrodialysis), the use of overlimiting currents has been spotlighted with accelerated ion transport through ion exchange membranes. To trigger this overlimting regime, we generally apply a relatively high voltage (\textgreater 1.5V) to initiate electroconvective vortices (EC) on the membrane, and they facilitate convective ion transport on there. In this scenario, one of the most tantalizing problems in using this overlimiting current is how we can initiate EC in a lower voltage. Here, we suggest the way to accelerate the EC's occurrence by utilizing electrochemical reactions of metals (i.e. Iodine and Zinc) coupling with majority salts (i.e. sodium and chloride ions). During electrodialysis process followed by two chemical reactions, Zn$+$2Cl$\to $ZnCl$_{\mathrm{2}}$ and NaI$_{\mathrm{3}} \quad +$2Na$^{\mathrm{+}}\to $3NaI with redox potential of E$_{\mathrm{0\thinspace }}=$ 1.3V\textit{ vs}~SHE, we can shift the critical voltage of EC initiation forward up to 0V; accordingly, the overlimiting current regime is also started at 0V. As we utilize the overlimiting currents and EC under a lower voltage, power consumption and a required energy of desalination are achieved as 0.82mW and 1.37kwh/m$^{\mathrm{3}}$, significantly reduced from typical ED system (1.47mW and 2.45kwh/m$^{\mathrm{3}})$. Also, even we take into account the cost of chemical fuels to facilitate EC initiation (i.e. Iodine and Zinc), the products (i.e. ZnCl$_{\mathrm{2}}$ and NaI) are more economically valuable than the reactants. [Preview Abstract] |
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G02.00016: Chaotic and periodic dynamics of a hovering Quincke rotor Gerardo Pradillo, Hamid Karani, Matthew Oline, Petia Vlahovska The Quincke effect is an electrohydrodynamic instability which gives rise to a spontaneous rotation of a charge--free dielectric particle in a uniform DC electric field. The equations describing the dynamics of a sphere have been previously mapped onto the Lorenz equations (Lemaire and Lobry (2002)). Experiments have shown the existence of a bifurcation at high electric fields in which chaotic rotation around a fixed axis occurs (Peters et al, Chaos (2005)). In this talk we describe the there-dimensional dynamics of the Quincke rotor using the hovering Quincke state (Pradillo et al, Soft Matter (2019)). We experimentally discover three-dimensional chaotic motion, which allows abrupt changes in axis of rotation, and periodic dynamics characterized by a constant re-orientation of the axis of rotation, coupled with time dependent oscillations. Noisy periodicity has been predicted for the Lorenz system (Sparrow 1982). We develop a fully three-dimensional model to describe the observed dynamics. [Preview Abstract] |
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G02.00017: Transverse migration of polyelectrolytes in microfluidic channels: Concentrating and purifying DNA Anthony Ladd, Benjamin Valley, Jason Butler I will describe recent experiments where DNA is injected into a microfluidic channel and convected through the device by a pressure driven flow, while simultaneously being subjected to an opposing electric field. Epifluorescent and confocal microscopy have previously shown that DNA then migrates to the walls of a microfluidic channel. An interesting consequence is that DNA rapidly accumulates at the junction of two channels of significantly different width, suggesting a possible means to both concentrate and purify DNA. Unlike a number of electrokinetic-based separations, trapping of DNA does not depend on complex flows or fields. Rather, it exploits a little studied aspect of flexible polyelectrolytes; namely that an electric field can generate a long-range flow around an elongated molecule, stretched (for example) by a shear flow. This leads to novel length-dependent motions of the polymer and in particular migration perpendicular to the flow. Because transverse migration is specific to flexible charged molecules and does not depend on the solvent properties, it is specific to nucleic acids among other biological molecules. Recent experiments show a high degree of purification of DNA from solutions containing large amounts of protein, without requiring additional reagents. [Preview Abstract] |
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G02.00018: The effect of multiple nanofluidic channels connected in series on the resulting ion concentration-polarization phenomenon Barak Sabbagh, Sinwook Park, Elad Stolovicki, Gilad Yossifon Ion concentration-polarization (ICP) phenomenon results from an electric current passing through an ionic permselective medium (e.g. nanochannel/membrane). This phenomenon has been intensively studied in relation with microfluidic applications, e.g. on-chip desalination and enhanced biosensing sensitivity. Herein, we extend previous studies of ICP by investigating both the transient and steady-state effect of multiple nanofluidic channels connected in series. A simplified analytical one-dimensional modeling of the system along with experiments demonstrated both the electrical response and ICP layers propagation under conditions of net flow. Moreover, the formation of a preconcentrated plug of molecules in between two adjacent nanochannels, exhibiting a third species effect, has been studied. To realize multiple nanofluidic channels connected in series, a soft elastomeric valves were used, where the cross section dimensions of the channel are changeable from micro- to nano- meter scale by deforming the valve. [Preview Abstract] |
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G02.00019: Ion conduction in nanoscale Janus membranes: Molecular dynamics vs Continuum. Joan M. Montes de Oca, Johnson Dhanasekaran, Juan J. de Pablo An accurate, comprehensive continuum theory of the physical processes that underlie ionic conduction in nanometric size pores is essential for the design and development of new technologies, such as Janus membranes used for power generation. In this work, we compare extensive molecular dynamics (MD) simulations of a complex Janus membrane embedded in an explicit solvent, with classical continuum given by coupling of Poison electrostatics, steady ion transport via Nernst-Planck, and convection described by Navier-Stokes. We find the continuum approach provides good qualitative and quantitative agreement of features such as Ionic distribution and potential with respect to MD simulations. We show the critical role convection plays in achieving this. However, we observe significant discrepancies in the dynamics (Current, ionic velocities, convection flux, etc). After inspection of the molecular mechanisms of ionic conduction, we argue that the significant slip velocity observed in MD simulations, enhanced by the predominantly electroosmotic nature of the flux, plays an important role in the discrepancies observed with the continuum. [Preview Abstract] |
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