Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R26: Nanofluids IV |
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Chair: Jerry Shan, Rutgers University Room: 329 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R26.00001: Electroosmosis in a nanofilm of chloride-aqueous solution with counter-charged surface patches Harvey Zambrano, Marie Pinti, A.T. Conlisk, Shaurya Prakash We study Electroosmotic flow (EOF) by conducting Non-Equilibrium MD Simulations (NEMDS) of water and chloride on a silica substrate. The system response is studied as axial electric fields (AEF) are imposed and as the surface charge (SC) is modified by implementing counter-charged patches (CP). The density profiles reveal that the CP result in an ionic depletion in bulk solvent and in a higher hydrophilicity than on regular silica. We compute lower velocities for the cases with higher SC on the CP. Our velocities are in agreement to results from previous MD studies. Density and velocity profiles reveal a stationary chloride layer and a stagnant water region on the CP. This stationary layer grows as CP with higher charges are settled and as weaker AEF are imposed. We infer that ionic accumulation on the CP and hydrophilicity are responsible for the EOF velocity changes for systems with different CP and the same AEF imposed. We perform continuum calculations and the EOF velocities agree with the results obtained from NEMDS. We show that by modifying SC on a substrate, systematic changes can be induced in EOF at the nanoscale. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R26.00002: Electrokinetics of Correlated Electrolytes and Ionic Liquids Brian Storey, Martin Bazant Perhaps the most basic assumption of classical electrokinetic theory is the mean-field approximation, where the each ion feels only the electric field produced by the mean charge density (via Poisson's equation) rather than the fluctuating Coulomb forces with individual neighbors. Here, we present a simple continuum model for electrostatic correlations between finite-sized ions, which leads to a 4th order modified Poisson equation, convenient for the analysis of electrokinetic phenomena. When the mean-field approximation breaks down, e.g. due to large ion concentrations, large ion valences, and/or nanoscale confinement, the zeta potential loses its significance, and the model predicts that electro-osmotic flows are typically reduced - or even reversed - by correlation effects, compared to the prediction of the Helmholtz-Smoluchowski formula. This may help to explain the over-prediction of induced-charge electro-osmotic flows by classical models. An interesting limit of the model describes electro-osmosis in solvent-free ionic liquids and molten salts, which may be important in energy storage and electroactuation applications. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R26.00003: Characterization of Electrical Properties of Nanowires by Electro-orientation Cevat Akin, Jerry Shan We investigate the electro-orientation of large-aspect-ratio particles in liquid suspension as a possible technique to determine the particles' electrical conductivity and/or permittivity, which are often poorly known and difficult to measure directly. With the application of a spatially uniform AC electric field, ellipsoidal particles in suspension will rotate into alignment with their longest axis along the field. In the low frequency limit, the alignment rate itself does not distinguish between equally sized particles of different properties. However, it is possible to characterize the particle's electrical properties by measuring the crossover frequency at which its alignment rate transitions from conductivity-dominated to permittivity-dominated behavior. Moreover, the crossover frequency is insensitive to the particle aspect ratio for large aspect ratios, making the electro-orientation technique suitable even for nanowires, whose precise length is difficult to resolve optically. We present experimental results obtained by optical microscopy on the alignment rate of nano- and micro- wires under applied fields of different frequency. Experiments are conducted with particles of different type and size to determine how the electro-orientation crossover frequency varies with particle conductivity and aspect ratio. We compare our experimental results with theoretically obtained values, and assess electro-orientation as a nanowire-characterization technique. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R26.00004: Examining Permittivity Effects in Electric Double Layers using Molecular Dynamics and Atomistic-to-Continuum Modeling Jeremy Templeton, Kranthi Mandadapu, Jonathan Lee, Reese Jones, Jonathan Zimmeran Charged surfaces exposed to an ionic solution attract an electric double layer: stratifications of solvent and solute molecules significantly deviating from their bulk concentrations. The double layer screens the charges in solution from those at the surface, but due to the anisotropy in the near-wall fluid, characterizing the precise mechanics by which this happens has proven difficult. Molecular dynamics (MD) simulations are capable of explicitly resolving the complex layering effects in the vicinity of the charged interface, but much work remains to analyze the results and improve the understanding of these phenomena. In this work, we use atomistic-to-continuum methods to analyze the spatially-varying electrical permittivity in these layers at different potentials and ion concentrations, focusing on salt water solutions. The permittivity is a particularly relevant quantity because it determines the capacitance of the different ionic layers. We will present a mathematical formalism for extracting this quantity from an MD realization and provide examples of how this coarse-grained quantity varies with a simulation. It will then be demonstrated how the resolved permittivity informs the realized voltage drop across the various parts of the double layer. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R26.00005: Atomistic simulations of nanoscale electrokinetic transport Jin Liu, Moran Wang, Shiyi Chen, Mark Robbins An efficient and accurate algorithm for atomistic simulations of nanoscale electrokinetic transport will be described. The long-range interactions between charged molecules are treated using the Particle-Particle Particle-Mesh method and the Poisson equation for the electric potential is solved using an efficient multi-grid method in physical space. Using this method, we investigate two important applications in electrokinetic transport: electroosmotic flow in rough channels and electowetting on dielectric (EWOD). Simulations of electroosmotic and pressure driven flow in exactly the same geometries show that surface roughness has a much more pronounced effect on electroosmotic flow. Analysis of local quantities shows that this is because the driving force in electroosmotic flow is localized near the wall where the charge density is high. In atomistic simulations of EWOD, we find the contact angle follows the continuum theory at low voltages and always saturates at high voltages. Based on our results, a new mechanism for saturation is identified and possible techniques for controlling saturation are proposed. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R26.00006: Molecular Dynamics Models of the Electric Double Layer for Large Zeta Potentials Jonathan W. Lee, Jeremy A. Templeton, Robert H. Nilson, Stewart K. Griffiths, Bryan M. Wong, Andy Kung The Classical Poisson-Boltzmann (PB) theory for the electric double layer (EDL) breaks down at the nanoscale as zeta potential increases. The ability to accurately model the EDL for large potentials is important for engineering high energy storage devices. To better understand behavior at large potentials, various molecular dynamics (MD) models were developed. MD models range from an idealized Lennard-Jones ionic fluid between unstructured walls to a salt water solution between solid substrates. All models feature a bulk fluid region in order to obtain a reference state. Models are compared using charge density profiles, solvent and solute concentration profiles, and zeta potentials as metrics. Local polarization structure can be obtained from the salt water MD models. Despite its inability to capture these effects, the idealized model similarly deviates from PB theory at large potentials. Ion concentration and surface charge density are varied in a parametric study using the idealized model. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R26.00007: Characterization of the Velocity and Power Consumption of Electroosmotic Flow in Nanochannels Using Numerical Simulations Joshua D. Shawala, Francisco J. Diez A numerical simulation of the electroosmotic flow in a nanochannel is performed. The study focuses on characterizing the velocity behavior when changes are applied to the zeta potential in the electrical double layer and the electric field. Similarly, the characteristics of power consumption are indentified by the ratio of convection to conduction current, which changes with flow velocity and ion concentration. In the 2D numerical simulations the electrostatic potential is obtained by the Poisson Boltzmann dilute solution theory which is solved in its nonlinear form and is validated from published work. Aqueous solutions of 1:1 electrolytes at bulk ion concentrations from 1-100 mM are considered. The viscous-driven nature of the flow outside the electric double layer causes an adverse pressure gradient along the center of the channel which is examined in detail. Pressure effects at the inlet and outlet are also considered. The need for higher inlet pressure increases with velocity, which is proportional to the applied electric field. Increasing the bulk ion concentration causes minimal change in average velocity under conditions of zero slip at the channel walls. In the case when a slip length is imposed at the walls, the flow velocity increases significantly with ion concentration. The increase in slip length also increases the ratio of convection to conduction current. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R26.00008: Lattice Boltzmann simulation of electrostatic double layer interaction force for nanoparticles Grace X. Shi, Yan Jin, Volha Lazouskaya, Chao Wang, Lian-Ping Wang Modeling the transport and retention of nanoparticles (NPs) through soil porous media requires an accurate description of the electrostatic interaction force between a nanoparticle and soil grain. In this study, we apply the lattice Boltzmann method to directly solve the nonlinear Poisson Boltzmann (PB) equation for several geometric configurations including plate-plate, NP-plate, and NP-NP interactions, for any surface potentials and interaction distances and for different boundary conditions. Interaction energy and force are then derived from the simulations. For the case of plate-plate interaction, the simulation results are compared to the exact solution of the nonlinear PB equation. It is shown that the linear PB solution is valid when the nondimensional surface potential is less than one, and that the linear PB solution over-predicts the interaction force for intermediate gap distances but under-predicts the force for small gap distances. For NP-plate and NP-NP interactions, an axisymmetric lattice Boltzmann formulation is developed to solve the governing equations. The results will be compared to the classic approximate expressions of interaction force to evaluate their validity and to study the effect of nanoparticle size. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R26.00009: The effects of external electric field on the structure of water inside and outside single-walled carbon nanotubes Zhen Xu, Guo-Hui Hu, Zhe-Wei Zhou In the present work, all-atom molecular dynamics (MD) simulations are utilized to examine the structure of water inside and outside the armchair SWCNT in the presence of external electric field parallel or perpendicular to the tube axis. Extensive MD simulations have been performed in wide ranges of $E$ (0-3V/nm) at room conditions (300 K and 1 bar). The dependence of liquid density profile, orientation of dipole moment and hydrogen bonds profiles are discussed on the electric fields. With the parallel electric field, the structure of water outside the SWCNT changes slightly while inside the SWCNT the water structure is found to be more ordered. With the perpendicular electric field, the structure of water both inside and outside the SWCNT has changed dramatically. When the strength of field is above 1V/nm, the chains or the layers structures inside the SWCNT are broken and even the isolated water molecule is found. Outside the SWCNT, liquid density profiles, orientation of dipole moment and hydrogen bonds profiles are found to be the non-axisymmetric. This work may be helpful in understanding the physics of the confined water and in the design of future nanofluidic devices. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R26.00010: Modeling of Ion Transport in Nanochannels Justyna Czerwinska Transport of ions in fluidic environment is a basis for many biological processes. Microchannel ions flow is characterized by formation of electric double layer (EDL) near the solid wall. In nanochannel, however, the formation of EDL can be prevented by the constrained geometry leading to the creation of a single ion layer and resulting in the selective movement of ions. The confinement effects can be controlled by the wall charges providing a controlability to the nanoscale diffusion (pumping effect). This study will present model and molecular dynamics simulations of three-dimensional nanochannel flow of charged fluid. The various concentration of ions in solution was studied as well as the influence of the external force. [Preview Abstract] |
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