Showing papers by "Peter T. Cummings published in 1997"

Quantitative comparison and optimization of methods for evaluating the chemical potential by molecular simulation

Abstract: The precision of several methods for computing the chemical potential by molecular simulation is investigated. The study does not apply molecular simulation to the analysis but instead works with models of the simulation process. These models enable the variance of the chemical potential to be computed accurately and very quickly and thereby permits the methods (freeenergy perturbation, expanded ensembles, thermodynamic integration, and histogram-distribution methods) to be optimized and compared over a range of densities. The study focuses exclusively on the hard-sphere model. This model is simple and well characterized; yet it exhibits the essential features that make the chemical potential calculation difficult; arguments are presented to support the broader applicability of the study. The severe asymmetry of particle insertion against particle deletion is highlighted, and it is shown that any staged free-energy perturbation method with a 'deletion' component is highly prone to systematic error. More g...

Comparison of shear flow of hexadecane in a confined geometry and in bulk

Abstract: We examine the shear flow of hexadecane confined between plates with separation of 1–10 nm using molecular dynamics simulations. We also performed non-equilibrium molecular dynamics (NEMD) simulations of bulk hexadecane to compare with the simulations in the confined geometry. The stiffness of hexadecane and its high melting temperature result in a tendency to crystallize at room temperature or large load. We find that when confined between hydrocarbon walls, shearing hexadecane exhibits a velocity profile with substantial slip at the wall and essentially constant velocity over most of the interior space between the walls. As the strength of the wall-fluid interaction increases the amount of slip decreases, but slip always occurs at the boundary for the range of parameters studied. The results are compared with recent surface force apparatus experiments on hexadecane and with similar simulations of model bead-spring fluids.

Shear behavior of squalane and tetracosane under extreme confinement. I. Model, simulation method, and interfacial slip

Abstract: In this three part study, nonequilibrium molecular dynamics simulation of the rheology of confined films is used to explore the microscopic properties and response of model lubricants under shear. The rheological behavior of two alkanes that differ in molecular structural complexity is examined: tetracosane (C24H50), which is a linear alkane, and squalane (C30H62), which has six symmetrically placed methyl branches along a 24 carbon backbone. The model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. Shear flow is generated by moving the walls at constant velocity, and various properties are calculated after attainment of steady state. Heat generated by viscous dissipation is removed by thermostatting the first two atoms of the tethered molecules at 300 K, which allows a temperature profile to develop across the width of the lubricant layer. This paper details the molecular model and simulation method, and examines interfacial slip at the interface between the tethered chains and the fluid alkane. The effects of various parameters on the slip behavior are presented. Two subsequent papers respectively address the structural features of these liquid alkanes under shear flow and compare the viscosities from independent calculations of the bulk and confined fluids.

Molecular Simulation Study of Tetraalkylammonium Halides. 1. Solvation Structure and Hydrogen Bonding in Aqueous Solutions

Abstract: The hydration structure in aqueous solutions of three tetraalkylammonium halides (TAAX), tetramethylammonium chloride (TMACl), tetrapropylammonium bromide (TPABr), and tetrabutylammonium bromide (TBABr) at concentrations of 2.2, 1.4, and 1.0 m, respectively, is studied via molecular dynamics (MD) simulation at ambient conditions. The results are compared directly with neutron diffraction with isotopic substitution (NDIS) experimental results for the same systems. In agreement with the NDIS results, we find evidence of a small enhancement of water structure around the apolar solutes, based on water H−H radial distribution functions. Decomposition of the water−water O−H radial distribution function into hydrogen-bonded and non-hydrogen-bonded components, using the geometric criterion for hydrogen bonding of Mezei and Beveridge (1981), allowed us to calculate the hydrogen-bonded (HB) contribution to the total O−H radial distribution function, both in the bulk solution and in the first solvation shell of the ...

Shear behavior of squalane and tetracosane under extreme confinement. II. Confined film structure

Abstract: This paper focuses on the structural characteristics of confined squalane and tetracosane under shear flow conditions. Nonequilibrium molecular dynamics simulation is used to explore the rheology of these model lubricants. A preceding paper describes the molecular model and the simulation method, and examines interfacial slip. The lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. In this paper we examine the density profiles and chain conformations of the alkanes under shear flow conditions. Our results indicate a profound influence of the walls on the fluid structure. In particular, when the wall spacing is close to an integral multiple of the molecular diameter, tetracosane shows the formation of distinct layers with the molecules being in a fully extended state. This behavior is not observed for squalane. Under shear flow conditions the molecules tend to orient parallel to the walls, as would be expected, with a greater degree of orien...

Shear behavior of squalane and tetracosane under extreme confinement. III. Effect of confinement on viscosity

Abstract: This study uses nonequilibrium molecular dynamics simulation to explore the rheology of confined liquid alkanes. Two alkanes that differ in molecular structural complexity are examined: tetracosane (C24H50), which is a linear alkane, and squalane (C30H62), which has six symmetrically placed methyl branches along a 24 carbon backbone. These model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. This paper, the third of a three part series, compares the viscosities of the confined fluids to those of the bulk fluids. The alkanes are described by a well-documented potential model that has been shown to reproduce bulk experimental viscosity and phase equilibria measurements. Details of the simulation method, and structural information can be found in the preceding two papers of this series. The measured strain rates in these simulations range between 108 and 1011 s−1, which is typical of a number of practical applications. The confined fluids...

Lubricant Characterization by Molecular Simulation

Abstract: Lubrication is a phenomenon of immense practical importance and fundamental scientific interest, and the automobile engines of the future are envisioned by the Partnership for a New Generation Vehicle will require the development of improved lubricants that perform well at higher operating temperatures and higher engine speeds. The rheological properties of liquid alkanes of intermediate molecular sizes (C20H42-C40H82) are among the most important properties in lubricant performance. Though realistic study of these systems by molecular simulation has previously been limited by both high computational costs and the lack of potential models accurate over a wide range of physical conditions, the advent of massively parallel supercomputers has now made such studies possible. As an illustration of the ability of molecular simulation as a useful tool for lubricant development, we present the first molecular-simulation-based calculation of the kinematic viscosity index of an alkane liquid, viz. 2,6,10,15, 19,23-hexamethytetracosane, commonly called squalane. In the mass range of interest, squalane is one of the few commercially available isoparaffins and has been the subject of previous studies by molecular simulation (Mondello and Grest 1995; Mundy et al., 1997). Though numerous properties of a lubricant are important to end-use applications, the viscosity is considered most significant. Mundy et al. (1996) investigated the variation of decane’s viscosity with pressure, calculating its pressureviscosity coefficient by equilibrium molecular dynamics simulations. The kinematic viscosity index (VI) is another widelyused industrial characterization of automotive lubricants. It was proposed by Dean and Davis (1929) as an indication of an oil’s viscosity-temperature characteristics in terms of its Saybolt viscosities at 311 K (100°F) and 372 K (210°F). Two series of reference lubricating-oil fractions (H and L) were used for comparison. Series H exhibited little change of viscosity with temperature while the viscosities of series L oils exhibited large variation with temperature. Series H and L represented, respectively, the best and worst oils available in 1929. Series H oils were assigned a VI of 100, series L a

Electrofreezing of water in molecular dynamics simulation accelerated by oscillatory shear

Abstract: We present a study of the effect of oscillatory shear on the crystallization of supercooled water in nonequilibrium molecular dynamics simulations of simple water models. [The TIP4P model has been used throughout; calculations using the extended simple point charge (SPC/E) model were checked for consistency and gave qualitatively the same results.] The application of a planar Couette flow field alone did not result in crystallization but in combination with a static electric field different isomorphs of ice emerged depending on the state of the system. At high pressures ($3\ensuremath{-}5$ kbar) the oscillatory shear substantially speeded up the formation of the conjectured ice polymorph (ice XII). At ambient pressures lower density systems could be forced to order in situations where hitherto the application of the electric field alone has not resulted in ordering. These results suggest that this method will be very useful in exploration of the fluid-solid boundaries of the phase diagram of models for water.

Computer Simulations of the Static Scattering from Model Polymer Blends

Abstract: To understand the unusual composition and wave vector, q, dependent Flory interaction parameters, χ, obtained from small angle neutron scattering (SANS), we have calculated the three partial structure factors for symmetric binary polymer blends with relatively strong interactions between dissimilar monomers using molecular dynamics simulations. In agreement with past work we find that the radii of gyration of the chains are altered on blending. The following new results emerged from our simulations. The single-chain form factors in the blends follow Gaussian statistics at all distances larger than a monomer diameter, confirming the Flory−de Gennes conjecture that correlations in condensed polymer phases are screened over short length scales. The Rg's deduced from these form factors are in agreement with simulation-determined values and illustrate conclusively that the single chain term in the incompressible random-phase approximation (i-RPA) is altered on blending. The three partial structure factors obta...

Directional dependence of the random kinetic energy in planar couette flow

Abstract: Non-equilibrium steady state systems do not obey the equipartition theorem in that the distribution of the random kinetic energy among the three directions is not independent of the mechanism which removes the dissipative heat. This becomes important beyond the linear regime because system properties will be functions of the thermostatting mechanism as well. For homogeneous non-equilibrium molecular dynamics simulations, in most of the cases, the simplest synthetic thermostat is used in which, analogously to equilibrium algorithms, the friction coefficient is identical in the x, y and z directions. The simplicity is attractive, but computer simulation results are presented that demonstrate that this may not be the most sensible choice physically.

Nanorheology of Liquid Alkanes

Abstract: We report molecular dynamics simulations of liquid alkanes, squalane and tetracosane, confined between moving walls to which butane chains are tethered, effectively screening the details of the wall. As in an experiment, heat is removed by thermostatting the tethered molecules. Results obtained at high strain rates, typical of practical applications, suggest little or no difference between the bulk rheology and confined flow, and the occurrence of a high degree of slip at the wall-fluid interface at the conditions studied. At relatively low velocities and high densities, tetracosane shows the formation of fully-extended chains at certain wall spacings.