Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J9: Fluid Structure and Properties |
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Sponsoring Units: DFD Chair: Nikolai Priezjev, Michigan State University Room: Morial Convention Center RO7 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J9.00001: The Interplay of Short- and Long-Ranged Forces in Simulations of Confined Water using Local Molecular Field Theory Jocelyn Rodgers, John Weeks A molecular model of water confined between walls is studied using local molecular field (LMF) theory. LMF theory splits the long-ranged Coulomb $1/r$ potential between charge sites into a short-ranged core potential and a long-ranged, slowly-varying potential ideal for mean-field averaging. The core potential may be treated explicitly by simulations using the minimum image convention with a renormalized external field defined by mean field averaging of the longer-ranged potentials. Here we apply local molecular field theory to molecular dynamics simulations of molecular water confined between walls, with and without an electric field. This is a geometry where short-ranged spherical truncations of Coulomb interactions can fail spectacularly, but in tandem with the effective external field defined by LMF theory such truncations correctly predict structural and electrostatic properties of water. Further the concepts behind LMF theory elucidate the varying contributions of hydrogen-bonding and dipolar interactions in determining the structure of water at surfaces. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J9.00002: Effects of a solute on a simple model solvent Paolo De Gregorio, Jonathan C. Toledo, B. Widom We studied the effect of the addition of a solute on a one-dimensional model solvent (high density, low compressibility, low coefficient of thermal expansion), at infinite dilution. The solute has a solubility which is low and decreases with increasing temperature. The effect of the addition of solutes on the chemical potential of the solvent at constant volume differs from that at constant pressure in a way similar to that of non-polar solutes in water. The solvent-solvent pair distribution function determines fully the modes of decay of the solute-solute counterpart. At the largest distances, the ultimate decay is strictly monotonic (exponential) for both. But while for the solvent-solvent correlations the amplitude associated with that mode is negligible, it is huge for the solute-solute case. Formally, the correlations vanish in identical fashion at infinite distances, but they differ substantially over an extended range of physical interest. The osmotic second virial coefficient is very large and negative, not only as an effect of the proximity `attraction' between the solutes, but also of the very long tail in the correlations. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J9.00003: Ultrafast Phase-Contrast Imaging Study of Finite-time Hydrodynamic Singularities Yujie Wang Most of the nonlinearity induced hydrodynamic singularities are transient and requires high-speed imaging to be studied. There exist some intrinsic problems of visible-light imaging on fluid mechanical research.. The key advantage of x-ray phase-contrast-imaging is that it is interface-based technique and the boundaries are highlighted naturally. It is a highly penetrative technique in which all complex structures along the path will be picked up. Additionally, it is naturally immune of the complexity of multiple scattering and strong optical reflection or refraction. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J9.00004: Liquid State Properties from \emph{ab initio} Density Functional Theory Calculations Nicolas Bock, Travis Peery, Eric Chisolm, Giulia De Lorenzi-Venneri, Duane Wallace, Erik Holmstr\"{o}m, Raquel Lizarraga For the solid state, density functional theory (DFT) has been successfully applied to calculate material properties in a large range of materials. In the liquid state however, thermodynamic properties are calculated by molecular dynamics (MD) simulations in which the forces are calculated with DFT. These simulations are computationally significantly more expensive than comparable solid state calculations. We present a novel approach which does not rely on MD simulations, but instead uses Vibration-Transit (V-T) theory to make predictions of the thermodynamic properties of the liquid phase. This approach is computationally significantly less expensive than an MD simulation. The accuracy of this approach is demonstrated by a comparison to experiment. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J9.00005: Reconstructing the structure and dynamics of density fluctuations in water near a moving proton Robert Coridan, Ghee Hwee Lai, Nathan Schmidt, Peter Abbamonte, Gerard C. L. Wong ~~~~~~The structure and dynamics of water on femtosecond timescales is relevant to many topics in physical chemistry such as ion solvation. We computationally reconstruct the angstrom-scale spatial and femtosecond-scale temporal evolution of density fluctuations in water using high-resolution inelastic x-ray scattering (IXS). The imaginary part of density propagator $\chi $(q,$\omega )$ is directly extracted from the IXS data, and the real part recovered using Kramers-Kronig relations.~ The resultant complex-valued $\chi $(q,$\omega )$ is the Fourier transform of the real-space density-density response function $\chi $(r,t) which measures the dynamical density fluctuations of water due to a point-like instantaneous pulse.~ We use this density propagator from IXS data and linear-response theory to reconstruct the hydration behavior of a proton moving at different speeds through water. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J9.00006: Classical Density Functional Theory of Inhomogeneous Polar Molecular Liquids Johannes Lischner, T.A. Arias We show how free energy functionals for classical assemblies of interacting rigid molecules, composed of an arbitrary number of atoms, can be constructed, such that the entropy of the noninteracting assembly, the thermodynamic properties and the microscopic order of the uniform phase and the dielectric properties in both weak and strong electrostatic fields are reproduced. We use our approach to predict density profiles of liquid hydrogen choride in a parallel plate capacitor with different wall potentials and varying external fields. We show that our theory can easily be coupled to electronic structure calculations within the Joint Density Functional approach and will comment on potential application to water. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J9.00007: Localized Voronoi analysis of quenched liquid configurations Travis Peery, Nicolas Bock, Giulia De Lorenzi-Venneri, Duane Wallace, Erik Holmstrom We developed a set of {\it localized\,} statistical tools to explore and characterize condensed matter particle configurations, particularly amorphous distributions associated with the liquid state. Typically global measures of atomic packing are used to characterize atomic configurations, such as pair distribution functions. For large systems, such calculations can be computationally expensive and tend not to be sensitive to localized symmetries. Our localized tools are based upon the geometric or topological analysis of (static) atomic arrangements using Voronoi polyhedra. As each atom in the configuration has a unique Voronoi polyhedron defined by its near neighbors, our tools can describe the geometry and symmetry of local neighborhoods. We have defined, for example, a local, Shannon-type entropy for the Voronoi coordination number for each atom in a 500-atom, monatomic system. This {\it localized} entropy tool was able to find small (9--40 atom) crystallites or regions of high symmetry in an otherwise random 500-atom configuration quenched from a liquid MD state. These tools will help to define and characterize not only random liquid state configurations and the minimum structures associated with liquid potential energy surfaces, but also the symmetry properties of the quenching process itself. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J9.00008: Phase Separation in the Dipolar Hard-Sphere System Revisited Wonki Roh, Erik Luijten We investigate the liquid-vapor transition in the dipolar hard-sphere system. Since the suggestion of de Gennes and Pincus [Phys.Kondens. Mater.\ \textbf {11}, 189 (1970)] this phase transition has proven both elusive and controversial, with conflicting numerical results regarding its existence and its nature. Employing extensive and efficient grand-canonical Monte Carlo simulations, we revisit this issue. High-precision results on the low-temperature heat capacity are presented along isotherms as well as isochores. In addition, we study the density distribution function and its moments for a wide range of chemical potentials, and identify anomalous finite-size effects that can give rise to incorrect conclusions. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J9.00009: X-ray tracer study of Rheology and Hydrodynamics of Fatty acids Mengning Liang, Ross Harder, Ian Robinson In wormlike micelles, the breaking and reforming of the micelle rods and the shearing of the rods and between the carbon chains themselves result in a complex diffusive behavior with more than one characteristic time constant. This is one of the characteristics of a Maxwellian fluid. We have studied the rotational Brownian motion of an alumina crystal suspended in a fatty acid liquid. Synchrotron generated hard x-rays are used to do single particle tracking of the rotational orientation by tracking the Bragg intensity of alumina crystals in diffraction geometry. This technique allows the tracking of particles to sub-milliradian precision. We have observed multiple time scales of relaxation which is evidence of subdiffusive behavior. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J9.00010: Statics and dynamics of a cylindrical droplet under an external body force James Servantie, Marcus M\"uller We study the rolling and sliding motion of droplets on a corrugated substrate by Molecular Dynamics simulations. Droplets are driven by an external body force (gravity) and we investigate the velocity profile and dissipation mechanisms in the steady state. The cylindrical geometry allows us to consider a large range of droplet sizes. The velocity of small droplets with a large contact angle is dominated by the friction at the substrate and the velocity of the center of mass scales like the square root of the droplet size. For large droplets or small contact angles, however, viscous dissipation of the flow inside the volume of the droplet dictates the center of mass velocity that scales linearly with the size. We derive a simple analytical description predicting the dependence of the center of mass velocity on droplet size and the slip length at the substrate. In the limit of vanishing droplet velocity we quantitatively compare our simulation results to the predictions and good agreement without adjustable parameters is found. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J9.00011: Velocity-dependent friction coefficient at the interface between a polymer melt and a solid substrate Nikolai Priezjev, Anoosheh Niavarani Molecular dynamics simulations are carried out to investigate the dynamic behavior of the slip length in thin polymer films confined between atomically smooth thermal surfaces. For weak wall-fluid interactions, the shear rate dependence of the slip length acquires a distinct local minimum followed by a rapid growth at higher shear rates. With increasing the fluid density, the position of the local minimum is shifted to lower shear rates. We found that the ratio of the shear viscosity to the slip length, which defines the friction coefficient at the liquid/solid interface, undergoes a transition from a nearly constant value to the power law decay as a function of the slip velocity. In a wide range of shear rates and fluid densities, the friction coefficient is determined by the product of the value of surface induced peak in the structure factor and the contact density of the first fluid layer near the solid wall. A relation to recent slip flow experiments is discussed. Reference: A. Niavarani and N.V. Priezjev, Phys. Rev. E (2008) (cond-mat/0711.0178). [Preview Abstract] |
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