Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session AN: Nano Fluids I |
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Chair: Heng Ban, Arizona State University Room: Salt Palace Convention Center 251 B |
Sunday, November 18, 2007 8:30AM - 8:43AM |
AN.00001: On break-down of continuum theories for electroosmotic flow in nanoscale channels Moran Wang, Shiyi Chen Most previous researches on electro-osmotic flows in nanochannels found that the continuum theories, such as PB theory for electric potential distribution and NS equation for fluid flow, broke down at the nanoscale. There has not been a perfect explanation for it yet though several factors were ever suspected, such as the transport properties of water near the charged surface and the immobilization of couterions absorbed on the channel wall. In this work, we present our NEMD simulations of electroosmotic flows in nanochannels by changing the molecular parameters freely, and compare with corresponding continuum theory. Our results show that the break-down of continuum theories for nanoscale electroosmosis is mainly caused by the differences of the non-Coulombic interaction between the solvent molecules and the ions. The effect of Coulombic force on the break-down is negligible. When the difference of interactive potential between solvent molecules and ions is artificially eliminated, the MD results agree well with the continuum predictions. [Preview Abstract] |
Sunday, November 18, 2007 8:43AM - 8:56AM |
AN.00002: The Role of Hydrodynamics in the Polarization of the Double Layer Surrounding a Nanosize, Cylindrical Particle Hui Zhao, Haim Bau The polarization of, the forces acting on, and the flow field around a charged, cylindrical, dielectric particle submerged in an electrolyte solution and subjected to an alternating electric field are calculated as functions of electrolyte concentration and electric field frequency by solving the linearized Poisson-Nernst-Planck (PNP) equations. Consistent with dielectric spectroscopy measurements and with previous theoretical works, we find that at relatively low frequencies, the polarization coefficient is nearly frequency-independent; but once the Strouhl number $St_1 =\frac{a^2}{D^{\ast} \omega^{\ast} }$ increases above 1, the polarization coefficient increases as the frequency increases, attains a maximum, and then decreases. When $St_2 =\frac{1}{1+Du} \frac{\lambda^{{\ast} 2}} {D^{\ast} \omega^{\ast}}>1$, the polarization coefficient decreases rapidly as the frequency is further increased. In the above, $a$ is the particle's radius; $\lambda^{\ast}$ is the Debye screening length; D$^{\ast}$ is the diffusion coefficient of the electrolyte; $\omega^{\ast}$ is the electric field frequency, and \textit{Du} is the Dukhin number. The major contributor to the force acting on the particle is the drag induced by the electroosmotic flow around the particle. The work helps clarify the effect of double layer polarization on dielectrophoresis. [Preview Abstract] |
Sunday, November 18, 2007 8:56AM - 9:09AM |
AN.00003: Generalized Electrokinetic Transport of Ions in Nanochannels Fabio Baldessari, Juan G. Santiago We present a generalized model for calculating transport of dilute analytes in long, thin nanochannels with overlapped electric double layers, and in the presence of an axial electric field. Differently than published models, we adopt equilibrium between the ionic solutions in the wells and inside the nanochannel to self-consistently predict background electrolyte ion densities and the electric potential field. Furthermore, our model includes the (strong) dependence of ion mobility on local ionic strength of the electrolyte. We present predictions solving simple one-dimensional integrals. We validate our predictions by comparing simulations with measurements of effective mobility of two charged fluorescent analytes in fused silica nanochannels (Bodipy with valence -1, and Fluorescein with valence -2). We present results of separation performance, and we compare electrokinetically-driven field flow fractionation to other established separation methods of the same family [Preview Abstract] |
Sunday, November 18, 2007 9:09AM - 9:22AM |
AN.00004: Accurate treatment of external electric field in molecular dynamics simulation of nanofluidics Anjan Raghunathan, Narayana Aluru We propose a self-consistent molecular dynamics (SCMD) formulation for electric-field-mediated transport of water and ions through a nanochannel connected to reservoirs. The SCMD formulation is compared with a uniform field MD approach, where the applied electric field is assumed to be uniform, for 2~nm and 3.5~nm wide nanochannels immersed in a 0.5M KCl solution. The reservoir ionic concentrations are maintained using the dual-control-volume grand canonical molecular dynamics technique. Simulation results with varying channel height indicate that the SCMD approach calculates the electrostatic potential in the simulation domain more accurately compared to the uniform field approach, with the deviation in the results increasing with the channel height. The translocation times and ionic fluxes predicted by uniform field MD can be substantially different from those predicted by the SCMD approach. Our results also indicate that during a 2 ns simulation time K$^{+}$ ions can permeate through a 1 nm channel when the applied electric field is computed self-consistently, while the permeation is not observed when the electric field is assumed to be uniform. [Preview Abstract] |
Sunday, November 18, 2007 9:22AM - 9:35AM |
AN.00005: The Effect of Electrical Double Layer On Nano-Flows Heng Ban, Bochuan Lin, Barton Smith The effect of electrical double layer (EDL) on micro-flows has been studied widely. When the channel width or tube diameter are much greater than the thickness of EDL, the electrical conductivity of the fluid can be assumed constant and the bulk electrical conductivity is often used in calculations. For channel or tube size similar to the EDL thickness, for instance, a fraction of one micron, the effect of overlapping EDL on fluid electrical conductivity needs to be included in the analysis. ~This paper presents an asymptotic analysis of the effect of overlapping EDL on the pressure-driven channel flow. The governing equations for the flow, the Poisson equation for the electric potential, and the charge continuity equation for the net charge were solved analytically. The effect of overlapping EDL on the electrical conductivity and velocity distribution in the micro-channel and the pressure drop were determined. The results showed that, the average electrical conductivity of electrolyte inside the channel increased significantly. As a consequence, the pressure drop for the pressure-driven flow was smaller than that without considering the influence of EDL on conductivity. [Preview Abstract] |
Sunday, November 18, 2007 9:35AM - 9:48AM |
AN.00006: A mechanistic modeling of heat conduction in nanoparticulates suspensions (nanofluids) Rui Qiao, Ping He Nanofluids are a new class of heat transfer fluids consisting of nanoparticles dispersed in a base fluid and often show superior thermal conductivity. The mechanisms of heat conduction in nanofluids are unclear at present, and many mechanisms, e.g., Brownian motion of nanoparticles and particle clustering, have been proposed to play important role in determining the overall heat conduction. Here we examine the role of nanoparticle Brownian motion by using numerical modeling. We propose a mesoscopic particle method for modeling of heat conduction in nanofluids. We show that the proposed method can accurately reproduce 1) the Brownian dynamics of nanoparticles and 2) the heat conduction in strongly heterogeneous media. We then apply the method to investigate the role of nanoparticle Brownian motion in determining the heat conduction in nanofluids by comparing the thermal conductivity for ``frozen'' nanofluids and dynamic nanofluids. Implications of the simulation results for engineering design of nanofluids will be discussed. [Preview Abstract] |
Sunday, November 18, 2007 9:48AM - 10:01AM |
AN.00007: ABSTRACT WITHDRAWN |
Sunday, November 18, 2007 10:01AM - 10:14AM |
AN.00008: Thermal Conductivity Enhancement of Nanofluids in Conjunction with Electrical Double Layer (EDL) Jung-Yeul Jung, Jung Yul Yoo A novel expression for the thermal conductivity of nanofluids is proposed on the bases of both electrical double layer (EDL) and kinetic theory, which is applied to Al$_{2}$O$_{3}$ nanofluids satisfactorily with respect to temperature, volume fraction and particle size. In the case of dilute nanofluids, the effects of the Brownian motion and interparticle interaction due to EDL on enhancing the thermal conductivity of nanofluids are quite comparable, while the effect of interparticle interaction due to EDL is more prominent in the case of dense nanofluids. The model presented in this paper shows that interparticle interaction due to EDL is the most responsible for the enhancement of thermal conductivity of nanofluids. [Preview Abstract] |
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