Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session LK: Nano-Fluids I |
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Chair: Jerry Shan, Rutgers University Room: 102B |
Monday, November 24, 2008 3:35PM - 3:48PM |
LK.00001: Thermal Resistance at the Liquid-Solid Interface Ali Beskok, Bo Hung Kim, Tahir Cagin Heat conduction between parallel plates separated by a thin layer of liquid Argon is investigated using three-dimensional MD simulations employing 6-12 Lennard-Jones potential interactions. Channel walls are maintained at specific temperatures using a recently developed interactive thermal wall model. Heat flux and temperature distribution in nano-channels are calculated for channel heights varying from 12.96 nm to 3.24 nm. Fourier law of heat conduction is verified for the smallest channel, while the thermal conductivity obtained from Fourier law is verified using the predictions of the Green-Kubo theory. Temperature jumps at the liquid/solid interface, corresponding to the well known Kapitza resistance, are observed. Using systematic studies thermal resistance length at the interface is characterized as a function of the surface wettability, thermal oscillation frequency, wall temperature, thermal gradient and channel height. An empirical model for the thermal resistance length, which could be used as the jump-coefficient of a Navier-type boundary condition, is developed. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LK.00002: Slip flow: How slip occurs on a two-dimensional surface Taeil Yi, Q. Jane Wang, Seth Lichter The relative interfacial velocity between a liquid and solid is called slip. Though there are physical measurements and numerical computations of the amount of slip, the mechanics of slip remain unclear. For 2D flows (i.e. over a 1D surface) slip occurs by the propagation of defects along the surface.\footnote{A. Martini, A. Roxin, R. Q. Snurr, Q. Wang {\&} S. Lichter. \textit{J. Fluid Mech.} \textbf{600}: 257-269 (2008).} Here, we show how slip occurs in 3D flow using molecular dynamics (MD) simulation. We study slip in a long Couette channel of fixed height with molecules of size $\sigma $. We carry out a sequence of MD simulations beginning with a 2D computational geometry, i.e. width = $\sigma $, and incrementally increase the width of the computational domain. By examining the dynamics at the liquid/solid interface, we can follow the propagation of interfacial defects, as they evolve from their well-understood 1D form into fully 2D slip. Does slip on 2D surfaces also occur through the propagation of defects as it does over a 1D surface? We hope to answer that question in this presentation. The results of this work have application to optimizing surfaces for slip and to flows in small geometries, such as carbon nanotubes and in tribological flows. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LK.00003: A mechanism based on fluid compressibility to explain molecular scale slip behavior Neelesh A. Patankar, Hua-Yi Hsu We reproduce molecular scale slip behavior by solving continuum equations for the shear flow of a \textit{compressible} fluid between two walls \textit{in the presence of wall potentials}. A constant slip length is obtained at low shear rates which transitions, at a critical shear rate, to a different constant slip length at high shear rates. This is consistent with molecular dynamic (MD) results. The critical shear rate for transition can be estimated based on scaling arguments. The numerical results motivate a theoretical solution, based on continuum equations, for the slip lengths at high and low shear rates. Prior MD results can be consolidated in the context of this model, and it also elucidates the mechanism of slip. It is seen that at low shear rates the fluid experiences an additional body force due to the wall potential (e.g. Lennard-Jones potential). This slows down the fluid in the first molecular layer. This body force is non-zero only if the fluid is compressible. At high shear rates this body force is negligible and the slip length is determined by a friction coefficient between the wall and fluid molecules. [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LK.00004: Identification of concentration polarization regimes in microchannel-nanochannel interfaces using method of characteristics Ali Mani, Tom Zangle, Juan Santiago We developed a simple transport model to study concentration polarization (CP) regimes in microchannel-nanochannel interfaces. The models include advection due to electroosmosis, pressure-driven flow, electro migration, and diffusion. The electric double layer effects are assumed to be confined to near wall regions. This model is used to study CP in a series microchannel-nanochannel-microchannel geometry and found to provide significant insight into dynamics of CP. Consistent with experimental observations, two different CP regimes are identified: In one regime CP enrichment and depletion zones remain local to the channel interfaces (CP without propagation). In another regime, CP zones show long range propagation in the form of concentration shocks (CP with propagation). Solutions based on the method of characteristics are shown to uniquely determine which CP regimes will be selected by the system. We find propagation of CP is determined by two major system parameters: a nanochannel Dukhin number, and the ratio of the co-ion mobility to electroosmotic mobility. We found that after CP propagates, the system cannot be affected by perturbations to the reservoir. Extension of this model to more complex geometries will be discussed. [Preview Abstract] |
Monday, November 24, 2008 4:27PM - 4:40PM |
LK.00005: Improved potential of mean force for Brownian dynamics simulation of nanoparticle aggregation Sergiy Markutsya, Shankar Subramaniam, Rodney Fox Aggregation of nanoparticles in liquid suspension is a phenomenon that affects the design and scale-up of process equipment for nanoparticle synthesis. The radial distribution function of nanoparticle pairs is an essential statistic characterizing multiparticle dynamics that must be captured by Brownian dynamics (BD) simulations in order to accurately simulate the structure of nanoparticle aggregates. Molecular dynamics (MD) simulations of a model aggregating system are used as a benchmark to evaluate the simplest specification of the potential of mean force (PMF) in BD, which yields good agreement in the diffusion-limited regime but performs poorly in the reaction-limited regime. An improved potential of mean force that accounts for the relative acceleration between nanoparticles due to the presence of solvent molecules is developed. Implementation of this improved PMF in BD yields promising preliminary results for the structure of nanoparticle aggregates when compared with benchmark MD simulations. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LK.00006: Diffusivity effects on charged species separation in nanochannels David Boy, Frederic Gibou, Igor Mezic, Sumita Pennathur Recent work has investigated the dispersion and separation of charged molecules in nanochannels. One conclusion has been that the combination of transverse velocity and electric field gradients can provide a mechanism for separation of different-valence ionic species. Building on this, we present a continuum transport model for finite-sized particles in a nanofluidic system and analyze the model both theoretically, with Taylor-Aris dispersion theory, and computationally, with direct numerical simulation. We assume a finite-sized electric double layer, a dilute solution, and an axially applied DC electric field. We show that, under these conditions, finite-sized particles may exhibit qualitatively different separation behavior than predicted by small ion theory. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LK.00007: Spreading of a Nanodroplet on a Solid Surface Physically-Patterned with an Array of Pillars Hongfei Wu, Kristen Fichthorn, Ali Borhan Molecular dynamics simulations are used to study the spreading dynamics of a Lennard-Jones liquid droplet on a heterogeneous solid surface. The solid is physically patterned by placing a set of pillars of square or circular cross section on an otherwise flat surface. The liquid-solid interaction is modeled by a modified Steele's potential derived for arrays of pillars characterized by a desired roughness and solid fraction. Using the modified Steele's potential, liquid-solid interactions can be computed more quickly, and larger droplet sizes can be studied. Simulations indicate that the spreading of nanodroplets depends not only on the surface energy of the solid, but also on the heterogeneity of the solid surface. For a constant solid surface energy, the topographic structure of the solid surface can have a significant effect on its hydrophobic characteristics. By varying the pillar height, spacing, and arrangement on the solid surface, a transition between the Wenzel and Cassie modes of wetting is observed in the simulations. These observations are explored in terms of the interplay between the bulk liquid chemical potential and the liquid-solid interfacial tension, as well as the topology of the liquid-solid potential-energy surface induced by surface heterogeneity. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LK.00008: Particle dynamics and rheology of SWNT suspensions under shear and electric fields Chen Lin, Peter Huang, Jerry Shan The net orientation angle of single-wall nanotubes (SWNTs) in liquid suspension under combined shear flow and electric fields is investigated experimentally with an optical polarization-modulation technique. The macroscopic viscosity of the suspension under the shear and electric fields is also measured simultaneously to the optical measurement. Theoretical predictions of the time scales of two particle-dynamics processes, the orientation of particles and the formation of microstructure in the suspension, are compared with experimental data. The relation between the particle dynamics and the macroscopic rheology of the dilute SWNT suspension is discussed. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LK.00009: High-speed Tracking of Quantum Dots in Microflows using Evanescent Wave Illumination Jeffrey Guasto, Kenneth Breuer Total internal reflection velocimetry (TIRV) is applied to measure the dynamics of colloidal quantum dot (QD) tracer particles within 200 nm of a microchannel wall at shear rates in excess of 20,000 s$^{-1}$. QDs are quickly developing into viable tracer particles for measuring microscale fluid dynamics. However, the low emission intensities of QDs usually require long exposure and inter-frame times, which limit velocity resolution and compromise accuracy (due to their fast diffusion as a consequence of a small, 17 nm hydrodynamic diameter). In this study, a two-stage, high-speed image intensifier and camera were integrated into an evanescent wave microscopy imaging system to provide the necessary high temporal resolution to image the fast diffusion dynamics of QD's in real time (up to 10,000 fps), which allowed individual particles to be tracked continuously for extended periods. In addition to examining the trajectories of individual particles, ensemble-averaged tracking measurements reveal near-wall velocity distributions in high-speed microchannel flows (Re$\sim $10), where velocities on the order of 5 mm/s are measured within 200 nm of the microchannel wall. This data provides a robust confirmation of recent results demonstrating diffusion-induced bias error for near-wall velocimetry. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LK.00010: A Universal AC Cone Angle due to Net Entrainment of Anionic Species Nishant Chetwani, Siddharth Maheshwari, Hsueh-Chia Chang The slender conical meniscus that is obtained by the application of high frequency AC field is quite distinct from DC Taylor cone. The AC cone shows continuous longitudinal growth and has a much smaller half cone angle of $\sim 11^{\circ}$. Mass spectrometry on the microjet from the AC cone shows that dissociation reaction occurs at the tip but only the low-mobility anionic species are entrained to produce a charged cone. These free negative charges relax to the interface to produce a non-uniform surface charge density that scales with respect to the azimuthal radius as $\rho ^{-\textstyle{1 \over 2}}$ to balance the singular normal capillary pressure. Repulsion of this entrained surface charge and the Maxwell pressure they induce are estimated with an elliptic integral and a variational formulation produces a normal stress balance with capillary pressure that is only satisfied at a universal angle of $12.6^{\circ}$ for the liquids with high dielectric constant, in good agreement with the measured values for the organic solvents used in experiments. [Preview Abstract] |
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