Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session H14: Electrokinetics: Particles, Semipermeable Membranes, Charged Surfaces |
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Chair: Bruno Figliuzzi, Ecole des Mines Room: 202 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H14.00001: Dielectric decrement effects in electrokinetics Bruno Figliuzzi, Wai Hong Ronald Chan, Cullen Buie, Jeffrey Moran Understanding the nonlinear phenomena that occur in the electric double layer (EDL) that forms at charged surfaces is a key issue in electrokinetics. In recent studies, Nakayama and Andelman [J. Chem. Physics 2015] Hatlo et al. [EPL 2012], and Zhao and Zhai [JFM 2013] demonstrated that dielectric decrement significantly influences the ionic concentration in the electric double layer (EDL) at high zeta potential, leading to the formation of a condensed layer near the particle's surface. In this presentation, we apply the dielectric decrement model to study two archetypal problems in electrokinetics, namely the electrophoresis of particles with fixed surface charges and the electrophoresis of ideally polarizable particles. Our aim is to rely on numerical simulations to incorporate nonlinear effects including crowding effects due to the finite size of ions, dielectric decrement in the EDL, surface conduction, concentration polarization and advection in the bulk solution. In parallel, we derive a simplified composite layer model that enables us to obtain analytical estimates of the physical quantities involved in the physical description of the problem. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H14.00002: The influence of soft layer electrokinetics on bacterial electroporation Jeffrey Moran, Naga Neehar Dingari, Cullen Buie Electroporation of mammalian cells has received a significant amount of theoretical attention over the last decade because of its ability to deliver biologically active molecules into cells using short and strong electric field pulses. However, application of the same theory to bacterial electroporation presents significant challenges because of the presence of charged soft layers around bacteria. The soft layer charge distribution has been found to significantly influence bacterial electrophoretic mobility and polarizability because it alters the electric potential spatial distribution around the cell envelope. In addition, the RC charging time scale of both the soft layer and electric double layer is of the order of microseconds, which is also of similar order of magnitude as the pore creation time scale. Therefore in this study, we investigate the influence of soft layer electrokinetics on the spatial pore distribution and the temporal pore radius evolution during bacteria electroporation, which are quantitative measures of a bacterium's amenability to electroporation. The study will have significant impact on designing and optimizing bacteria electroporation platforms for gene and drug delivery applications. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H14.00003: Frequency dispersion of electrokinetically activated Janus particles Alicia Boymelgreen, Tov Balli, Gilad Yossifon, Touvia Miloh We examine the influence of the applied frequency of the electric field on the induced-charge electroosmotic flow around a metallo-dielectric Janus particle. Previously, we have used three dimensional-two component micro-particle-image-velocimetry (3D-2C $\mu$ PIV) around a \textbf{stagnant} particle, to illustrate the presence of a number of competing effects including dielectrophoresis and electrohydrodynamic flow which distort both the strength and shape of the frequency dispersion predicted for pure induced-charge effects. Here, we extend this work by examining the frequency dispersion of \textbf{mobile} Janus particles of different sizes ($3-15\mu m$ in diameter) at different electrolyte concentrations. In all cases, towards the DC limit, and in the frequency domain where previously EHD flow was shown to dominate, the velocity of a mobile particle decays to zero. At the same time significant variations in the frequency dispersion, including its shape and the value for maximum velocity are recorded as a function of both electrolyte concentration and particle size. This work is of both fundamental and practical importance and may be used to further refine non-linear electrokinetic theory and optimize the application of Janus particles as carriers in lab-on-a-chip analysis systems. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H14.00004: Effects of buoyant forces on chaotic electroconvection Elif Karatay, Matthias Wessling, Ali Mani The transport of ionic species is enhanced by induced electroconvection that arise due to electrokinetic instabilities stemming from coupling of hydrodynamics with ion transport and electrostatic forces. Recent research have shown the contribution of chaotic multi-scale structures beyond a threshold value of applied electric potential. However the buoyant forces have been neglected in the existing studies of chaotic electrokinetic flows where the density gradients of salt depletion can become gravitationally stable or unstable depending on the geometric orientation of electrokinetic systems. In this study we thoroughly examine the interplay of gravitational convection and chaotic induced electroconvection in both gravitationally stable and unstable configurations via direct numerical simulations of a model system consisting of a salt solution confined in between two cation selective membranes. Our results reveal that buoyant forces are not negligible when the Rayleigh number of the system exceeds a critical value $Ra_{cr} \sim 1000$. When the density gradient of salt depletion is gravitationally stable, the growth of the electrokinetic flow structures are saturated by buoyant forces. Whereas gravitationally unstable density gradient leads to buoyant flow structures. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H14.00005: Investigation of Current Hotspots on an Ion-Selective Membrane Subject to Chaotic Electroconvection Clara Druzgalski, Ali Mani We have performed a 3D direct numerical simulation (DNS) of chaotic ion transport associated with electroconvective instability near an ion-selective membrane. Data from the 3D DNS demonstrate that the chaotic fluid motion substantially influences the transport of ions and causes instantaneous hotspots of high current density on the surface. We present a comprehensive statistical analysis of surface current density, including probability density functions (PDFs) and joint-PDFs with other interfacial measures involving flow, conductivity and electric fields. These results provide new insights into the mechanism and characterization of current hotspots. Our results are relevant to industrial applications involving ion-selective interfaces such as electrodialysis for water purification, and emerging microfluidic devices that use ion-selective components for separation processes. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H14.00006: Enhancement of Overlimiting Current through an Ion-Selective Membrane via Surface Conductivity Patterning Scott Davidson, Matthias Wessling, Ali Mani Electroconvection's ability to enhance transport through ion-selective surfaces provides promising opportunities for improving many diffusion-limited electrochemical and microfluidic systems. We have investigated two sources of electroconvection at ion-selective surfaces, electrokinetic instability and surface-patterning with impermeable stripes and their interactions using direct numerical simulation of the governing equations. We show that despite the reduced surface area available for transport, patterned surfaces can lead to an up to 80{\%} increase in current density relative to homogeneous surfaces. At applied voltages below the nominal threshold of instability, patterning enhances transport by inducing flow, while at higher voltages they do so by regularizing the chaotic electroconvective flows. Additionally, we show the formation of novel electroconvective patterns with multiple coexisting length scales due to electrokinetic instability at the homogeneous surface. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H14.00007: Asymmetry induced electric current rectification in permselective systems Yoav Green, Yaron Edri, Gilad Yossifon Permselective systems are inherently asymmetric as they have preference to the transport of one charge carrier over the other. In this work we derive a solution for the concentration distribution, electric potential and current-voltage response for a four-layered system comprised of two microchambers connected by two permselective regions of varying properties. We show that any additional asymmetry in the system, in terms of the geometry, bulk concentration, or surface charge property of the permselective regions, results in rectification of the current. Our work is divided in two parts, when both permselective regions have the same sign surface charge sign and the case of opposite signs. For the same sign case we are able to show that the system behaves as Dialytic battery. For the case of opposite signs, our system exhibits the bipolar behavior of a diode where the magnitude of the rectification can be of order \textit{O(100-1000)}. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H14.00008: Direct Observation of Three-dimensional Electroconvective Vortices on a Charge Selective Surface Rhokyun Kwak, Jongyoon Han, Taikjin Lee, Ho-Young Kwak We present a visualization of three-dimensional electroconvective vortices (EC) by ion concentration polarization (ICP) on a cation selective membrane. The vortices are initiated between two transparent Nafion membranes under no-shear/shear conditions with various applied voltages and flow velocities. Fluorescent imaging and spatial Fourier transform allow us to capture vortex structures. In this 3-D system, EC shows three distinguished structures: i) polygonal shapes with no-shear and ii) transverse and/or iii) longitudinal vortex rolls with shear flow, which is reminiscent of 3-D Rayleigh-Benard instability. Under shear flow, as flow velocity (Reynolds number: Re) increases or voltage (electric Rayleigh number: Ra) decreases, pure longitudinal vortices are presented; in the inverse case, transverse vortices are also formed. It is noteworthy that if we confine EC in quasi 2-D system with high Ra (\textgreater 10,000), we obtain pure transverse vortices (Kwak et al., PRL, 110, 114501 (2013)); high Ra induces chaotic EC in this 3-D system, instead of 2-D stable transverse vortices. To the best of our knowledge, this is the first direct observation of 3-D EC, which will occur in realistic electrochemical devices, e.g. electrodialysis. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H14.00009: Non-isothermal electro-osmotic flow in a microchannel with charge-modulated surfaces Oscar Bautista, Salvador Sanchez, Federico Mendez In this work, we present an theoretical analysis of a nonisothermal electro-osmotic flow of a Newtonian fluid over charge-modulated surfaces in a microchannel. Here, the heating in the microchannel is due to the Joule effect caused by the imposition of an external electric field. The study is conducted through the use of perturbation techniques and is validated by means of numerical simulations. We consider that both, viscosity and electrical conductivity of the fluid are temperature-dependent; therefore, in order to determine the heat transfer process and the corresponding effects on the flow field, the governing equations of continuity, momentum, energy and electric potential have to be solved in a coupled manner. The principal obtained results evidence that the flow patterns are perturbed in a noticeable manner in comparison with the isothernal case. Our results may be used for increasing microfluidics mixing by conjugating thermal effects with the use of charge-modulated surfaces. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H14.00010: Uncertainty Quantification of Nonlinear Electrokinetic Response in a Microchannel-Membrane Junction Shima Alizadeh, Gianluca Iaccarino, Ali Mani We have conducted uncertainty quantification (UQ) for electrokinetic transport of ionic species through a hybrid microfluidic system using different probabilistic techniques. The system of interest is an H-configuration consisting of two parallel microchannels that are connected via a nafion junction. This system is commonly used for ion preconcentration and stacking by utilizing a nonlinear response at the channel-nafion junction that leads to deionization shocks. In this work, the nafion medium is modeled as many parallel nano-pores where, the nano-pore diameter, nafion porosity, and surface charge density are independent random variables. We evaluated the resulting uncertainty on the ion concentration fields as well as the deionization shock location. The UQ methods predicted consistent statistics for the outputs and the results revealed that the shock location is weakly sensitive to the nano-pore surface charge and primarily driven by nano-pore diameters. The present study can inform the design of electrokinetic networks with increased robustness to natural manufacturing variability. Applications include water desalination and lab-on-a-chip systems. [Preview Abstract] |
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