Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D11: Electrokinetic Flows: General |
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Chair: Alexander Yarin, University of Illinois-Chicago Room: C120-121-122 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D11.00001: Electrostatic atomization: Effect of electrode materials on electrostatic atomizer performance Abhilash Sankaran, Christopher Staszel, Babak Kashir, Anthony Perri, Farzad Mashayek, Alexander Yarin Electrostatic atomization was studied experimentally with a pointed electrode in a converging nozzle. Experiments were carried out on poorly conductive canola oil where it was observed that electrode material may affect charge transfer. This points at the possible faradaic reactions that can occur at the surfaces of the electrodes. The supply voltage is applied to the sharp electrode and the grounded nozzle body constitutes the counter-electrode. The charge transfer is controlled by the electrochemical reactions on both the electrodes. The electrical performance study of the atomizer issuing a charged oil jet was conducted using three different nozzle body materials -- brass, copper and stainless steel. Also, two sharp electrode materials -- brass and stainless steel -- were tested. The experimental results revealed that both the nozzle body material, as well as the sharp electrode material affected the spray and leak currents. Moreover, the effect of the sharp electrode material is quite significant. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D11.00002: Confinement effect on liquid and ion transport in nanochannels coated with environmental-stimuli-responsive polyelectrolyte(PE) brushes Guang Chen, Siddhartha Das We study the confinement effect in the electrokinetic transport in polyelectrolyte(PE)-brush-grafted nanochannels. Starting with thermodynamically self-consistent description, i.e., accounting for the elastic, excluded volume and electrostatic effects of the PE brush and the effects of the induced electric double layer, we first probe the equilibrium brush height. We show that this height is dictated by PE size, grafting density, concentration of electrolyte solution and the extent of confinement. Shrinking-swelling behavior of PE brush with various configurations are compared: 1) short sparse end-charged PE brush swells as the salt concentration increases, while long dense end-charged PE brush shrinks; 2) PE brush with constant volume charge along the backbone always shrinks with the increase of the salt concentration. This shrinking-swelling behavior as well as the monomer distribution of PE interplay with the PE-induced drag force to dictate the overall electroosmotic and ionic current transport in such PE-brush-grafted nanochannels. We exhibit that among other factors, height of the nanochannels can be tuned to regulate this transport. We anticipate that our study will shed new light on structure of nano confined PE brushes with implications in ionic current rectifier design. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D11.00003: Unsteady Capillary Filling By Electrocapillarity In Seok Kang, Jung A Lee Unsteady filling of electrolyte solution inside a nanochannel by the electrocapillarity effect is studied. The filling rate is predicted as a function of the bulk concentration of the electrolyte, the surface potential (or surface charge density), and the cross sectional shape of the channel. Since the driving force of the flow is the electrocapillarity, it is first analyzed by using the solution of the Poisson-Boltzmann equation. From the analysis, it is found that the results for many different cross sectional shapes can be unified with good accuracy if the hydraulic radius is adopted as the characteristic length scale of the problem. Especially in the case of constant surface potential, for both limits of $\kappa h\to 0$and$\kappa h\to \infty $, it can be shown theoretically that the electrocapillarity is independent of the cross sectional shape if the hydraulic radius is the same. In order to analyze the geometric effects more systematically, we consider the regular $N$-polygons with the same hydraulic radius and the rectangles of different aspect ratios. Washburn's approach is then adopted to predict the filling rate of electrolyte solution inside a nanaochannel. It is found that the average filling velocity decreases as $N$ increases in the case of regular $N$-polygons with the same hydraulic radius. This is because of that the regular $N$-polygons of the same hydraulic radius share the same inscribing circle. . [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D11.00004: Sample Preconcentration in Nanochannels with Tunable Surface Charge Alexander Eden, Christopher McCallum, Brian Storey, Carl Meinhart, Sumita Pennathur We present a novel method for field amplified sample stacking (FASS) and focusing in nanochannels by taking advantage of the nonuniform ion distributions produced by thick electric double layers (EDLs) in channels with heterogeneous surface charge. This is accomplished by applying a voltage bias to a gate electrode embedded within the channel wall, tuning the surface charge in a region of the channel and significantly altering the charge density and ionic strength in that region relative to the rest of the channel. The resulting nonuniform electromigration fluxes in the different regions serve to stack charged sample ions at an interface where a step change in zeta potential occurs, providing enhancement ratios superior to those exhibited in traditional microchannel FASS. Numerical simulations are performed to demonstrate the phenomenon, and resulting velocity and salt concentration profiles show good agreement with analytical results. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D11.00005: Membraneless water filtration using CO$_2$ Sangwoo Shin, Orest Shardt, Patrick Warren, Howard Stone Water purification technologies such as ultrafiltration and reverse osmosis utilize porous membranes to remove suspended particles and solutes. These membranes, however, cause many drawbacks such as a high pumping cost and a need for periodic replacement due to fouling. Here we show an alternative membraneless method for separating suspended particles by exposing the colloidal suspension to CO$_2$. Dissolution of CO$_2$ into the suspension creates solute gradients that drive phoretic motion of particles, or so-called diffusiophoresis. Due to the large diffusion potential built up by the dissociation of carbonic acid, colloidal particles move either away from or towards the gas-liquid interface depending on their surface charge. Our findings suggest a means to separate particles without membranes or filters, thus reducing operating and maintenance costs. Using the directed motion of particles induced by exposure to CO$_2$, we demonstrate a scalable, continuous flow, membraneless particle filtration process that exhibits very low pressure drop and is essentially free from fouling. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D11.00006: Diffusiophoresis of a charged drop Fan Yang, Sangwoo Shin, Howard Stone Diffusiophoresis describes the motion of colloids in an electrolytic solution under a concentration gradient. Most of the previous studies in diffusiophoresis have dealt with motion of rigid particles. Here, we study the diffusiophoresis of fluid particles analytically and experimentally. We obtain the analytical solution of the diffusiophoretic velocity of fluid particles by perturbation methods. Using charged oil droplets, we measure the droplet speed under solute concentration gradient and compare it with the analytical solution. Our findings have potential applications for oil recovery and drug delivery. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D11.00007: The Influence of Soft Layer Electrokinetics on Electroporation of Gram-positive Bacteria Naga Neehar Dingari, Jeffrey L. Moran, Paulo A. Garcia, Cullen R. Buie Bacterial electroporation involves subjecting cells to intense (\textasciitilde 10 kV/cm) electric pulses, to open pores on the cell membrane for intracellular delivery of exogenous molecules. Its high efficiency in genetic transformation makes it an attractive tool for synthetic biology. While mammalian cell electroporation has received extensive theoretical and experimental investigation, bacterial electroporation has received markedly less attention. In this work, we develop a theoretical model of electroporation for gram-positive bacteria, taking into account the effect of the bacterial cell envelope on the cell's response to an electroporation pulse. We model the influence of the cell wall charge on the electrokinetic transport (and hence the pore properties) around the bacterial cell envelope using the Poisson-Nernst-Planck equations. Further, we account for the influence of the cell wall's mechanical elasticity on the pore radius evolution during electroporation, which is typically neglected in mammalian cell electroporation. This yields valuable information about favorable conditions for pore formation and will enable designing optimal platforms for bacteria electroporation. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D11.00008: Electro-hydrodynamic force field and flow patterns generated by a DC corona discharge in the air. Nicolas Monrolin, Franck Plouraboue, Olivier Praud Ionic wind refers to the electro-convection of ionised air between high voltage electrodes. Microscopic ion-neutral collisions are responsible for momentum transfer from accelerated ions, subjected to the electric field, to the neutral gas molecules resulting in a macroscopic airflow acceleration. In the past decades it has been investigated for various purposes from food drying through aerodynamic flow control and eventually laptop cooling. One consequence of air acceleration between the electrodes is thrust generation, often referred to as the Biefeld-Brown effect or electro-hydrodynamic thrust. In this experimental study, the ionic wind velocity field is measured with the PIV method. From computing the acceleration of the air we work out the electrostatic force field for various electrodes configurations. This enables an original direct evaluation of the force distribution as well as the influence of electrodes shape and position. Thrust computation based on the flow acceleration are compared with digital scale measurements. Complex flow features are highlighted such as vortex shedding, indicating that aerodynamic effects may play a significant role. Furthermore, the aerodynamic drag force exerted on the electrodes is quantified by choosing an appropriate control volume. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D11.00009: Induced Charge Capacitive Deionization Shimon Rubin, Matthew Suss, Maarten Biesheuvel, Moran Bercovici We demonstrate the phenomenon of induced-charge capacitive deionization (ICCDI) that occurs around a porous and conducting particle immersed in an electrolyte, under the action of an external electrostatic field. The external electric field induces an electric dipole in the porous particle, leading to capacitive charging of its volume by both cations and anions at opposite poles. This regime is characterized both by a large RC charging time and a small electrochemical charge relaxation time, which leads to rapid and significant deionization of ionic species froma volume which is on the scale of the particle. We show by theory and experiment that the transient response around a cylindrical particle results in spatially non-uniform charging and non-steady growth of depletion regions which emerge around the particle's poles. Potentially, ICCDI can be useful in applications where fast concentration changes of ionic species are required over large volumes. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D11.00010: A perturbative thermal analysis for an electro-osmotic flow in a slit microchannel based on a Lubrication theory Ali Ramos, Federico Mendez, Oscar Bautista, José Lizardi In this work, we develop a new thermal analysis for an electro-osmotic flow in a rectangular microchannel. The central idea is very simple: the Debye length that defines the length of the electrical double-layer depends on temperature $T$. Therefore, if exists any reason to include variable temperature effects, the above length should be utilized with caution because it appears in any electro-osmotic mathematical model. For instance, the presence of the Joule effect is a source that can generate important longitudinal temperature gradients along the microchannel and the isothermal hypothesis is no longer valid. In this manner, the Debye length is altered and as a consequence, new longitudinal temperature gradient terms appear into the resulting governing equations. These terms are enough to change the electric potential and the flow field. Taking into account the above comments, in the present study the momentum equations together with the energy, Poisson and Ohmic current conservation equations are solved by using a regular perturbation technique. For this purpose, we introduce a dimensionless parameter $\alpha \ $ that measures the temperature deviations of a reference temperature. [Preview Abstract] |
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