Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EP: Nanofluids III |
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Chair: Xiangchun Xuan, Clemson University Room: Long Beach Convention Center 203A |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EP.00001: Atomistic-continuum hybrid simulations for nano-scale flows Pandurang Kulkarni, Gaurav Tomar, Chia-Chun Fu, M. Scott Shell, L. Gary Leal It is known that the continuum assumption breaks down when the length scale of a flow approaches few nanometers. Examples include dynamics of thin films and interfaces, slippage in nanochannels and complex biological flows. In this work we develop a hybrid multiscale model, which combines atomistic description in a spatially localized region with continuum description in larger part of the flow domain. The atomistic region is simulated using standard molecular dynamics (MD) with particles interacting via Lennard-Jones pair potential. The continuum part of the problem is solved using the boundary-integral method. The spatio-temporal coupling between the two descriptions is achieved through constrained dynamics in the overlap region. The proposed model is validated by simulating shear flows in channels. A quantitative agreement is found between the computed flow fields and the analytical solutions. The boundary-integral based continuum solver offers improved efficiency and stability over conventional CFD methods. The potential applications of the method in emerging nano-fluidics are discussed. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EP.00002: Molecular dynamics simulation of non-Newtonian phenomena and shear-induced structural changes in atomic fluids Xin Yong, Lucy Zhang The rheology and microstructure of dense simple fluids have been mysterious subjects for decades due to the lack of direct experimental investigation. In this study, we present a planar Couette flow using molecular dynamics simulations to investigate the properties such as the velocity profile, density distribution, temperature profile and fluid structure within a wide range of wall velocities or shear rates. Here, we examine both boundary-driven shear (with walls) and homogeneous shear (without walls). In the boundary-driven shear, we model a fluid slab confined between two smooth and rigid solid walls. The upper wall is assigned a velocity to induce a planar Couette flow. In the homogeneous shear, only fluid atoms are modeled. The shear flow is generated from a canonical ensemble with a superimposed linear velocity profile associated with the Lees-Edwards periodic boundary conditions. Solid-like fluid layers were observed in the boundary-driven shear. In the homogeneous shear, the string phase is formed at high shear rates, which results in dramatic shear thinning. At lower shear rates, crystallization of fluid atoms induced by a large-scale secondary flow may appear. The physical features of the fluid structures and the corresponding viscosities are compared in the two models. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EP.00003: A Long-Range Electric Field Solver for Molecular Dynamics of Fluid-Solid Interfaces Based on Atomistic-to-Continuum Modeling Jeremy Templeton, Reese Jones, Jonathan Zimmerman, Bryan Wong, Jonathan Lee Understanding charge transport processes at a molecular level using computational techniques is currently hindered by a lack of appropriate models for incorporating anistropic electric fields, as occur at charged fluid/solid interfaces, in molecular dynamics (MD) simulations. In this work, we develop a model for including electric fields in MD using an atomistic-to-continuum framework. Our model represents the electric potential on a finite element mesh satisfying a Poisson equation with source terms determined by the distribution of the atomic charges. The method is verified using simulations where analytical solutions are known or comparisons can be made to existing techniques. A Calculation of a salt water solution in a silicon nanochannel is performed to demonstrate the method in a target scientific application. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EP.00004: A New Approach in Processing Atomistic Simulations Leopold Grinberg, George Karniadakis Computing an ensemble average in unsteady flow simulations performed with the Molecular Dynamics or coarse-grained versions, e.g. the Dissipative Particle Dynamics method, typically requires phase averaging of numerical solution over large number of time periods and realizations. For faster and more accurate processing we propose a new approach based on the window-Proper Orthogonal Decomposition (WPOD) methodology developed by Grinberg et. al. (ABME, vol. 37, 2009). WPOD helps to extract components of the velocity fields with high correlation time, i.e., ensemble average, in a hierarchical manner. The method is very robust and easy to implement. It leads to at least ten-fold computational savings and is appropriate for steady and unsteady non-periodic in time flows. We will review the new technique and present results of 3D numerical simulations of unsteady flows in microvessels. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EP.00005: Multiscale analysis of structure of confined simple fluids Tarun Sanghi, Narayana Aluru We discuss our recently proposed multiscale approach, an empirical potential based quasi-continuum theory (EQT), to predict the equilibrium structure of confined fluids across multiple length scales. In EQT, Nernst-Planck's equation is used to obtain self-consistent concentration and potential profiles of the confined fluid. The robustness, accuracy and computational efficiency of the framework are demonstrated by obtaining concentration and potential profiles of several simple Lennard-Jones type fluids (non-polar, spherical molecules such as Methane, Oxygen, Argon) confined in slit like geometries and comparing the results with molecular dynamics (MD) simulations. The extension of the framework for confined polyatomic fluids (linear rigid chain like molecules such as Ethane and Carbon-dioxide) is also discussed. [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EP.00006: Origin of line tension for a Lennard-Jones nanodroplet Joost H. Weijs, Antonin Marchand, Jacco H. Snoeijer, Bruno Andreotti, Detlef Lohse The existence and origin of line tension has remained controversial in literature. To address this issue we compute the shape of nanodrops using molecular dynamics and compare them using density functional theory in the approximation of the sharp kink interface. We show that the deviation from Young's law is very small and would correspond to a typical line tension length scale (defined as line tension divided by surface tension) similar to the molecular size. It turns out that, for Lennard-Jones droplets, line tension is always negative and most pronounced at small contact angles. We propose an alternative interpretation based on the geometry of the interface at the molecular scale. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EP.00007: Contact line dynamics of sessile nanofluid droplets under inert and saturated atmospheres Stuart Jack, Khellil Sefiane, Prashant Valluri, Omar Matar We present experimental results concerning contact line dynamics of sessile ethanol droplets laden with TiO2 nanoparticles under unsaturated and saturated environments. The measuring apparatus comprises of a special motorised stage designed to allow for a range of forced speeds to study the dynamic effects. An isolated metallic chamber wherein the droplet was deposited allowed for maintenance of saturated or unsaturated conditions. Results show dependence of the driving force and the Capillary number (based on contact line velocity) - but, not in accordance with the traditional Cox-Voinov and hydrodynamic theories. Our analysis shows that the deviations from these standard theories could be due to local increase of viscosity at the contact line. Results also show that evaporation has little effect on the contact line behaviour of TiO2-Ethanol nanofluids. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EP.00008: Optimization of Nanoparticle Separation through Solid State Nanopore Prashanta Dutta, Talukder Jubery, Anmiv Prabhu, MinJun Kim Recently there has been a growing interest on solid state nanopores to separate biological molecules such as proteins, DNA, and RNA. However, efficient separation of biomolecules through nanopores is a challenging task as a number of factors such as size and charge density of particle, size and charge density of membrane pore, and the concentration of bulk electrolyte influence the translocation behavior of nanoparticles through pores. To address this issue, a mathematical model is developed based on mass, momentum, and charge conservation equations to study the behavior of particles through pores. The surface charge density of the membrane pore was identified as the most critical parameter that determines the selectivity of the membrane and the throughput of the separation process. Based on this model, a single 150 nm pore was fabricated in a 50 nm thick free standing silicon nitride substrate by focused ion beam milling. This pore was subsequently chemically modified with (3-Aminopropyl) triethoxysilane to change its surface charge density. This chemically modified nanofluidic architecture was then used to separate 22 nm and 58 nm polystyrene nanoparticles. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EP.00009: Separation of nanoparticles by flow past a patterned substrate Rui Zhang, Joel Koplik We use molecular dynamics simulations to investigate trajectory deflection and particle trapping in flows of nanoparticle suspensions along patterned surfaces. Rigid atomistic particles are suspended in a viscous liquid driven by a pressure gradient through a channel, one side of which has a pattern of alternating stripes which attract or repel the particles. The full wall interaction is obtained by summing over semi-infinite slabs of material with alternating van der Walls interactions, and has a non-trivial three-dimensional spatial variation. This wall interaction can either trap particles on the attractive stripes or deflect the trajectories of mobile particles away from the direction of mean flow. We determine how the motion of particles of different sizes is affected by the wall interactions, and in particular show that trajectory deflection is size dependent and that such flows may be used as a ``vector chromotography'' separation technique. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EP.00010: Spontaneous Imbibition Dynamics of an n-Alkane in Nanopores: Evidence of Meniscus Freezing and Monolayer Sticking Patrick Huber, Simon Gruener Capillary filling dynamics of liquid n-tetracosane (n-C$_{\rm 24}$H$_{\rm 50}$) in a network of cylindrical pores with 7 and 10~nm mean diameter in monolithic silica glass (Vycor) exhibit an abrupt temperature-slope change at $T_{\rm s}=54\,^{\circ}$C, $\sim 4\,^{\circ}$C above bulk and $\sim 16\,^{\circ}$C, $8\,^{\circ}$C, resp., above pore freezing. It can be traced to a sudden inversion of the surface tension's $T$-slope, and thus to a decrease in surface entropy at the advancing pore menisci, characteristic of the formation of a single solid monolayer of rectified molecules, known as surface freezing from macroscopic, quiescent tetracosane melts. The imbibition speeds, that are the squared prefactors of the observed square-root-of-time Lucas-Washburn invasion kinetics, indicate a conserved bulk fluidity and capillarity of the nanopore-confined liquid, if we assume a flat lying, sticky hydrocarbon backbone monolayer at the silica walls.\\ (1) Simon Gruener and Patrick Huber, \textit{Physical Review Letters} \textbf{103}, 174501 (2009). [Preview Abstract] |
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