@article {1568, title = {Tracer diffusion in hard sphere fluids from molecular to hydrodynamic regimes}, journal = {Journal of Chemical Physics}, volume = {125}, number = {20}, year = {2006}, note = {ISI Document Delivery No.: 110UZTimes Cited: 6Cited Reference Count: 43Sokolovskii, R. O. Thachuk, M. Patey, G. N.}, month = {Nov}, pages = {10}, type = {Article}, abstract = {Molecular dynamics is employed to investigate tracer diffusion in hard sphere fluids. Reduced densities (rho(*)=rho sigma(3), sigma is the diameter of bath fluid particles) ranging from 0.02 to 0.52 and tracers ranging in diameter from 0.125 sigma to 16 sigma are considered. Finite-size effects are found to pose a significant problem and can lead to seriously underestimated tracer diffusion constants even in systems that are very large by simulation standards. It is shown that this can be overcome by applying a simple extrapolation formula that is linear in the reciprocal cell length L-1, allowing us to obtain infinite-volume estimates of the diffusion constant for all tracer sizes. For higher densities, the range of tracer diameters considered spans diffusion behavior from molecular to hydrodynamic regimes. In the hydrodynamic limit our extrapolated results are clearly consistent with the theoretically expected slip boundary conditions, whereas the underestimated values obtained without extrapolation could easily be interpreted as approaching the stick limit. It is shown that simply adding the Enskog and hydrodynamic contributions gives a reasonable qualitative description of the diffusion behavior but tends to overestimate the diffusion constant. We propose another expression that fits the simulation results for all densities and tracer diameters. (c) 2006 American Institute of Physics.}, keywords = {BOUNDARY-CONDITIONS, BROWNIAN PARTICLE, COEFFICIENT, DYNAMICS SIMULATION, friction, KINETIC-THEORY, RIGID PARTICLES, SINGLE-PARTICLE MOTION, SIZE DEPENDENCE, TRANSPORT-COEFFICIENTS, VELOCITY-AUTOCORRELATION FUNCTION}, isbn = {0021-9606}, url = {://000242408100031}, author = {Sokolovskii, R. O. and Thachuk, M. and Patey, G. N.} } @article {983, title = {Neutral particle collisional relaxation of suprathermal distributions in aeronomy}, journal = {Planetary and Space Science}, volume = {52}, number = {10}, year = {2004}, note = {ISI Document Delivery No.: 858UKTimes Cited: 3Cited Reference Count: 46}, month = {Aug}, pages = {915-922}, type = {Article}, abstract = {There are numerous situations in laboratory experiments and in atmospheric science that are characterized by distributions of energetic neutral species with extended high-energy tails. This paper provides a detailed analysis of the relaxation of such isotropic nonequilibrium distributions of neutral species. We consider a minor constituent, referred to as the test-particle dilutely dispersed in a second component that acts as a heat bath at equilibrium. A hard sphere cross section is assumed for the collisions of the test particles and the heat bath particles, and collisions between test particles are not included in the analysis. We study the approach to equilibrium with a finite difference method of solution of the Boltzmann equation. The solution of the Boltzmann equation is also presented as an expansion in the eigenfunctions of the Boltzmann collision operator. The main objective of the paper is the calculation of the energy-dependent relaxation times for the distribution function. It is anticipated that these relaxation times will not exhibit a strong energy dependence owing to the energy-independent hard sphere cross section. This relaxation behaviour is important in the characterization of nonthermal populations of energetic atoms in aeronomy. The results are compared with a similar analysis for Coulomb collisions described in the preceeding paper. (C) 2004 Elsevier Ltd. All rights reserved.}, keywords = {BINARY GASES, Boltzmann equation, CHARGE-EXCHANGE, distribution functions, ENERGY-TRANSFER, FAST NITROGEN-ATOMS, FOKKER-PLANCK EQUATION, HYDROGEN, KINETIC-THEORY, MARS, O(P-3) ATOMS, OXYGEN-ATOMS, relaxation times, suprathermal populations}, isbn = {0032-0633}, url = {://000224217300003}, author = {Shizgal, B. D.} } @article {5009, title = {Molecular dynamics study of the collision-induced rotational alignment of N-2(+) drifting in helium}, journal = {Journal of Chemical Physics}, volume = {114}, number = {15}, year = {2001}, note = {ISI Document Delivery No.: 418VGTimes Cited: 9Cited Reference Count: 32}, month = {Apr}, pages = {6662-6671}, type = {Article}, abstract = {The full velocity-angular momentum distribution function for gas-phase N-2(+) drifting in helium is calculated using a molecular dynamics method, and utilized to examine collision-induced rotational alignment in detail. These results are also compared with experimental measurements, most especially those of Anthony [J. Chem. Phys. 112, 10269 (2000)] and those appearing in the preceding article [Anthony , J. Chem. Phys. 114, 6654 (2001)]. Both the calculations and experiments show a number of interesting features including, drift velocities which depend upon rotational state, and quadrupolar alignment parameters which change from negative at high velocities to positive at low velocities. (C) 2001 American Institute of Physics.}, keywords = {EXCITATION, GENERALIZED CROSS-SECTIONS, HE, IONS, KINETIC-THEORY, LIGHTER GASES, STATE, TRANSPORT, TUBE EXPERIMENTS, VELOCITY DISTRIBUTIONS}, isbn = {0021-9606}, url = {://000167914400020}, author = {Baranowski, R. and Wagner, B. and Thachuk, M.} } @article {4468, title = {Molecular-dynamics study of rotational alignment of NO+ drifting in helium-velocity and angular momentum distribution functions}, journal = {Journal of Chemical Physics}, volume = {111}, number = {22}, year = {1999}, note = {ISI Document Delivery No.: 258RKTimes Cited: 8Cited Reference Count: 27}, month = {Dec}, pages = {10061-10068}, type = {Article}, abstract = {Collision-induced rotational alignment of NO+ ions drifting in a helium buffer gas is studied with molecular dynamics using the ab initio potential surface of S. K. Pogrebnya [Int. J. Mass Spectrom. Ion Proc. 149/150, 207 (1995)], obtained via a coupled-cluster singles-doubles approximation. We examine average translational and rotational temperatures, velocity and angular momentum distributions, and the dependence of these quantities on the applied electric field. The distributions show that angular momentum is preferentially aligned perpendicular to the electric field vector. We investigate the mechanism of this alignment through a multipolar moment expansion, and propose and demonstrate the accuracy of a bi-Maxwellian analytic form for describing the angular momentum distribution. (C) 1999 American Institute of Physics. [S0021-9606(99)00546-2].}, keywords = {ATOMIC GASES, HE, IONS, KINETIC-THEORY, MOBILITIES, NA+, TRANSPORT-COEFFICIENTS, TUBE EXPERIMENTS}, isbn = {0021-9606}, url = {://000083852700018}, author = {Baranowski, R. and Thachuk, M.} } @article {3813, title = {Nonequilibrium effects in reactive systems; The effect of reaction products and the validity of the Chapman-Enskog method}, journal = {Physica a-Statistical Mechanics and Its Applications}, volume = {223}, number = {1-2}, year = {1996}, note = {ISI Document Delivery No.: TQ402Times Cited: 30Cited Reference Count: 62}, month = {Jan}, pages = {50-86}, type = {Article}, abstract = {The rates of gas phase reactions can be calculated from the averages of the appropriate reactive cross sections with the velocity distribution functions of the reacting species. The reactive process, especially for reactions with activation energy, removes translationally energetic species and the velocity distribution functions depart from Maxwellian. The rate coefficients can differ from the equilibrium rate calculated with the Maxwell-Boltzmann distribution. The extent of the departure of the distribution function from Maxwellian can be estimated from solutions of the Boltzmann equation with appropriate choices for the elastic and reactive collision cross sections. If there is a good separation in the elastic and reactive collision time scales, a steady solution of the Boltzmann equation can be obtained with a procedure analogous to the Chapman-Enskog method for transport coefficients. In the present paper, the nonequilibrium effects for model reactive systems of the type A + A reversible arrow B + B, with and without the reverse reaction, and the reaction A + C {\textendash}> products are examined with both a Chapman-Enskog method along with an explicitly time-dependent solution for the irreversible reaction A + A {\textendash}> B + B. The main objectives are to study the effect of the inclusion of the products with and without a reverse reaction as well as the range of validity of the Chapman-Enskog method.}, keywords = {ANOMALOUS TRANSPORT, DISTRIBUTIONS, EQUILIBRIUM, HEAT-FLUX, hydrodynamics, KINETIC-THEORY, LASER-INDUCED FLUORESCENCE, PERTURBATION, PLASMA, STRONGLY INHOMOGENEOUS SYSTEMS, VELOCITY}, isbn = {0378-4371}, url = {://A1996TQ40200005}, author = {Shizgal, B. D. and Napier, D. G.} } @article {2908, title = {THE EQUILIBRIUM PAIR DISTRIBUTION FUNCTION OF A GAS - ASPECTS ASSOCIATED WITH THE PRESENCE OF BOUND-STATES}, journal = {Journal of Chemical Physics}, volume = {99}, number = {11}, year = {1993}, note = {ISI Document Delivery No.: MJ902Times Cited: 1Cited Reference Count: 36}, month = {Dec}, pages = {9111-9121}, type = {Article}, abstract = {At thermal equilibrium the momentum distribution of atoms in a gas is usually assumed to be Maxwellian, whether classically or quantally. However, if an atom is bound in a diatomic molecule, the atom{\textquoteright}s momentum distribution is non-Maxwellian. This paper explores the consequent singlet and pair particle distribution functions in a gas having both unbound atoms and bound pairs of atoms. Comment is made on the range of behavior associated with whether the chemical equilibrium constant for diatom formation is small or large. Calculations of distribution functions and their moments for atoms which are members of dimers are presented for some specific model potentials.}, keywords = {2ND, DIMER, formation, INITIAL DENSITY DEPENDENCE, KINETIC-THEORY, PHASE-SPACE SUBDIVISION, POTENTIALS, TEMPERATURE, THERMAL-CONDUCTIVITY, TRANSPORT-PROPERTIES, VIRIAL-COEFFICIENT, VISCOSITY}, isbn = {0021-9606}, url = {://A1993MJ90200074}, author = {Rainwater, J. C. and Snider, R. F.} }