Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G30: Nanofluids: Theory |
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Chair: Reza Sadr, Texas A&M University at Qatar Room: 33A |
Monday, November 19, 2012 8:00AM - 8:13AM |
G30.00001: Spatial Diffusion of Water in Carbon Nanotubes Amir Barati Farimani, N.R. Aluru Water desalination and transport are the great applications of carbon nanotubes (CNTs). Understanding the dynamics of water molecules in carbon nanotubes can shed light on the physics of transport, diffusion and other thermodynamic properties of water. Among all these properties, diffusion is of great importance as it affects most of other key properties. In this article, the spatial variation of the axial, radial, and tangential diffusion coefficients in carbon nanotubes (CNTs) of various diameters were computed. Based on the spatial variation of the diffusion coefficient, the diffusion mechanisms in different regions of the nanotube are defined. The effect of confinement and CNT wall on the diffusion coefficient is studied and discussed. The dependence of the diffusion coefficient on the carbon water interaction parameter is investigated. The average diffusion coefficient in the nanotube as a function of the nanotube diameter is calculated, and the diffusion mechanisms, including the transition regimes, are identified. It has been shown that the axial diffusion coefficient is enhanced in the adjacent water layer to the wall. The results are explained via hydrogen bond network and water orientations. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G30.00002: Transient response at the microchannel-nanochannel interface: chronopotentiometry, chronoamperometry, and electrochemical impedance Jarrod Schiffbauer, Yoav Green, Sinwook Park, Gilad Yossifon Transient response of the interface between a permselective membrane and electrolyte has been studied both theoretically and experimentally in the context of several well-developed electrochemical measurement paradigms. However, such studies of the microchannel-nanochannel interface are conspicuously lacking. One of the fundamental distinctions between the two types of system is the role of convective transport normal to and through the nanochannel. Here we present several recent experimental and theoretical results concerning the transient response of the microchannel-nanochannel interface to a variety of input signals and discuss the relevance of these results in terms of both fundamentals and applications. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G30.00003: Orientation selection of block copolymer lamellar phases under oscillatory shear Chi-Deuk Yoo, Jorge Vinals A hydrodynamic description of complex fluids that are structured at the nanoscale necessitates the introduction of appropriate order parameters reflecting the broken symmetries of the fluid, and their coupling to velocity fields and transport. We describe the equations of motion for a uniaxial fluid, and use them to study rheology and orientation selection of a block copolymer under shear. Viscoelastic response is also introduced at this scale, which is shown to lead to local, effective viscoelastic contrast that depends on the local orientation of the lamellae. We further explore domain boundary instabilities that arise from viscoleastic contrast, and their relationship to domain orientation selection in large samples under oscillatory shears. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G30.00004: An analytical model of heat transfer in sheared flows of dilute nanofluids Oleksii Rudenko, Victor L'vov, Itamar Procaccia, Federico Toschi We discuss a model for the enhancement of the heat flux by spherical and elongated (spheroidal) nanoparticles in sheared laminar flows of dilute nanofluids in the presence of a constant temperature gradient. Besides the heat flux carried by the nanoparticles, the model accounts for the contribution of their rotation to the heat flux inside and outside the particles. The rotation of the nanoparticles has a twofold effect: it induces a fluid advection around the particle and it strongly influences the statistical distribution of particle orientations. These dynamical effects are responsible for changing the thermal properties of flowing fluids as compared to quiescent fluids. The proposed model demonstrates a potential increase of the heat flux far beyond the Maxwell-Garnett limit for the spherical nanoparticles, while the spheroidal nanoparticles may either enhance or suppress the heat flux comparing to the spherical nanoparticles. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G30.00005: Continuum-based coarse-grained water potentials for structural prediction in confined environments S.Y. Mashayak, N.R. Aluru We develop the single site coarse-grained (CG) potential for structural prediction of the confined water. CG potentials allow computationally efficient simulations, which can access larger time- and length-scales compared to fully atomistic simulations. In the literature, various CG techniques have been used to develop CG potentials for structural prediction of bulk water. Due to the inherent inhomogeneity of the confined water system and water's ability to form directional hydrogen-bonds, development of CG potentials for confined water is a formidable task, and not much progress has been done to solve this problem. Herein, we systematically derive the single site CG potentials for the confined water, and show that these potentials can be used in the multi-scale quasi-continuum framework as well as in the CG molecular dynamics (MD) simulations to predict, in a computationally efficient manner, the atomic-level structure of the water confined in two types channels- a silicon slit channel and a graphite slit channel. Our results show that the center of mass density profiles of the water predicted by the CG model are in good agreement with all-atom MD results across multiple length scales, i. e., from few atomic diameters channel widths to 100s of nm. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G30.00006: On the Effects of Brownian particle Movement on the Overall Fluid Velocity Distribution Way Lee Cheng, Anoop Baby, Reza Sadr The suspension of nano-sized particles in a base liquid, known as nanofluids, are reported to display anomalous, often shown enhancement, thermal properties. This enhancement suggests a potential for industrial applications, in particular, for cooling systems. However, the underlying physics that leads to such enhancements in thermal performance is not fully understood. While nano particles/fluid interaction seems to be the main source of the observed phenomena, proposed theories in this regard are often disputed, and not conclusive, pointing to the need for more research in this field. In this study a simple approach is used to study the flow field in nanofluids due to randomly moving Brownian particles in a stationary fluid using numerical simulations. The unconfined Brownian motion of the particles is implemented via Langevin equation. The induced velocity field in the surrounding fluid is obtained by solving the governing hydrodynamic equations accounting for the motion of the particles and statistics of the flow field is then obtained. Effect of other parameters such as temperature and particle density is also investigated. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G30.00007: Electron Beam Artifacts in Liquid-Cell Electron Microscopy Joseph Grogan, Frances Ross, Haim Bau Liquid-cell electron microscopy has recently emerged as a powerful technique for \textit{in-situ} studies of nanoscale processes in liquids such as nanoparticles' formation, agglomeration, and oriented-assembly and electrochemical plating and imaging of the structure of macromolecules in their native environment. However, many of these phenomena are strongly influenced by the electron beam used for imaging, resulting in artifacts. To utilize the full potential of liquid cell electron microscopy, it is necessary to obtain a good understanding of the interactions of the electron beam with the imaged medium. This study explores the interactions of electrons with water, finding that radiolysis plays a key role in many studies while heating is typically insignificant. We derived a reaction-diffusion model to predict the concentration distribution of radiolysis products H, H$_{2}$, OH, H$_{2}$O$_{2}$, and hydrated electrons, and imaged homogeneous and heterogeneous nucleation of gas bubbles in the liquid cell. The presence of radiolysis products explains many of the observations of crystal formation, growth, and dissolution recently reported in the literature and observed in our laboratory. The study also suggests how to control electron beam effects to suppress or exploit them as desired. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G30.00008: Surface tension relaxation time in liquid-gas and liquid-solid interfaces of simple LJ liquids Alex Lukyanov, Alexei Likhtman We use molecular dynamics (MD) to answer a classical question: how does the surface tension on an interface appear? After defining surface tension from the first principles and conducting several consistency checks, we perform a series of dynamic MD experiments. First, we use a single simple liquid nanodroplet to study liquid-gas (LG) interface dynamics. At time zero, we remove the outer layer of molecules in the droplet, creating a fresh bare interface with the bulk arrangement of molecules. After that the system evolves towards equilibrium, and the expected surface tension is re-established. We found that in the case of LG interfaces, the system relaxation consists of three distinct stages. We have observed this scenario for monatomic Lennard-Jones (LJ) liquids as well as for binary LJ mixtures at different temperatures, monitoring a wide range of physical observables. Second, we use an equilibrated liquid film on a solid substrate. At time zero, we change the strength of the interaction potential between the substrate and liquid molecules and observe how the liquid-solid interface evolves towards equilibrium. We apply results to representative nanoflows over chemically structured substrates and discuss implications to macroscopic modelling of dynamic contact angle. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G30.00009: Mobility of a Semiflexible Chain in a Nanochannel Douglas Tree, Yanwei Wang, Kevin Dorfman The fundamental understanding of the dynamics of biopolymers in nanoscale devices is an important problem with real-world applicability in high-tech genomic mapping devices. Accordingly, we have developed a comprehensive picture of tube-confined polymers that goes beyond the limiting cases given by the decades-old scaling laws of de Gennes and Odijk for polymer mobility in weak and strong confinement. By using a numerical solution of the confined Green's function and by sampling polymer configurations using a Metropolis Monte Carlo algorithm, we are able to estimate the Kirkwood mobility of long, semiflexible polymers (e.g. DNA) in a square nanochannel with full hydrodynamic interactions. We will present results using this approach that show a broad plateau exists in the mobility as a function of the chain extension for moderate confinement, and that the width of the plateau depends on the anisotropy of the monomers (and hence the ionic strength of the buffer). For dilute, high-ionic strength solutions of DNA, our analysis indicates that the classic results of Odijk and de Gennes apply over a distinctly narrow range of extensions, and we predict that the Rouse-like behavior of moderate confinement will be observed for most of the measurable chain extensions seen in experiments. [Preview Abstract] |
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