S. T. Cui

Molecular dynamics simulations of the rheology of normal decane, hexadecane, and tetracosane

Extensive nonequilibrium molecular dynamics simulations have been carried out for liquid decane, hexadecane, and tetracosane at densities corresponding to atmospheric pressure and near ambient temperatures. The strain‐rate‐dependent viscosity has been obtained for strain rates ranging over several orders of magnitude. At high strain rate, the viscosities for all alkanes studied here have similar values and exhibit similar power‐law shear‐thinning behavior with a slope between about −0.40 and −0.33. Accompanying this shear thinning is the onset of orientational order and the alignment of the alkane molecules with the flow direction. The alignment angle tends to 45° at very low strain rate and is significantly smaller at high strain rate. This suggests that the chains substantially align in the flow direction and that the dominant motion at high strain rate is the sliding of the chains parallel to the flow. At low strain rate, the shear viscosity shows a transition to Newtonian behavior. The Newtonian visco...

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

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.

Multiple time step nonequilibrium molecular dynamics simulation of the rheological properties of liquid n‐decane

Nonequilibrium and equilibrium molecular dynamics simulations are reported for a united‐atom model of n‐decane at a state point in the liquid phase. The viscosity calculated by our nonequilibrium molecular dynamics simulations is in good agreement with that obtained from our equilibrium molecular dynamics simulations via the Green–Kubo relation and with that obtained by Mundy et al. [J. Chem. Phys. 102, 3776 (1995)] using the same potential model at the same state conditions. Additionally, the viscosity calculated by nonequilibrium molecular dynamics is in very good agreement with experimental results for n‐decane. The algorithm used for the equilibrium molecular dynamics simulations is an application to alkanes of the multitime step Nose dynamics algorithm developed by Tuckerman and Berne. For the nonequilibrium molecular dynamics simulations, an extension of the multitime step method is derived for the nonequilibrium equations of motion describing planar Couette flow with Nose thermostat. The contributi...

Comparison of the Hydration and Diffusion of Protons in Perfluorosulfonic Acid Membranes with Molecular Dynamics Simulations

Classical molecular dynamics (MD) simulations were performed to determine the hydrated morphology and hydronium ion diffusion coefficients in two different perfluorosulfonic acid (PFSA) membranes as functions of water content. The structural and transport properties of 1143 equivalent weight (EW) Nafion, with its relatively long perfluoroether side chains, are compared to the short-side-chain (SSC) PFSA ionomer at an EW of 977. The separation of the side chains was kept uniform in both ionomers consisting of -(CF 2) 15- units in the backbone, and the degree of hydration was varied from 5 to 20 weight % water. The MD simulations indicated that the distribution of water clusters is more dispersed in the SSC ionomer, which leads to a more connected water-channel network at the low water contents. This suggests that the SSC ionomer may be more inclined to form sample-spanning aqueous domains through which transport of water and protons may occur. The diffusion coefficients for both hydronium ions and water molecules were calculated at hydration levels of 4.4, 6.4, 9.6, and 12.8 H 2O/SO 3H for each ionomer. When compared to experimental proton diffusion coefficients, this suggests that as the water content is increased the contribution of proton hopping to the overall proton diffusion increases.

Molecular simulation of the transition from liquidlike to solidlike behavior in complex fluids confined to nanoscale gaps

We report molecular dynamics simulations at ambient temperature and pressure of dodecane films of thickness between three and eight molecular layers confined between mica surfaces. We use an accurate united-atom model for dodecane and an effective interaction between the dodecane and the confining mica surfaces that is consistent with the surface energy of a mica surface. At ambient normal pressure, the strong surface–fluid interaction leads to increased dodecane density as the wall spacing is narrowed, crossing into a density region corresponding to bulk solid when the confined film becomes narrower than six molecular layers. Correspondingly, we observed a dramatic transition from a liquidlike to an ordered, solidlike structure when the confined dodecane film is reduced from seven to six molecular layers, consistent with experimental observation of many orders of magnitude increase in viscosity at the same film thickness. The solidlike structure is characterized by the layering as well as the in-plane or...

A molecular dynamics study of a short-chain polyethylene melt. I. Steady-state shear

Utilizing a united atom potential model and reversible reference system propagator algorithm (rRESPA) multitimestep dynamics, we have performed equilibrium and nonequilibrium molecular dynamics simulations of a monodisperse C100H202 polyethylene melt at 448 K and 0.75 g/cm 3 . We report a variety of properties calculated at equilibrium including rotational relaxation time and self-diffusion coefficient as well as shear-enhanced diffusion and rheological properties calculated under steady-state shearing conditions. Shear thinning is observed in the viscosity and normal stress coefficients over the range of strain rates studied. A minimum in the hydrostatic pressure is observed at an intermediate strain rate that is associated with a minimum in the intermolecular Lennard‐Jones potential energy as well as transitions in the strain-rate-dependent behavior of several other viscous and structural properties of the system. The shear field also imposes significant alignment of the chains with the flow direction, approaching a limiting angle of approximately 3 at high strain rate. In addition, the self-diffusion coefficients (calculated in terms of the unconvected positions according to the Cummings‐Wang formalism) are markedly enhanced under shear compared to the equilibrium state (up to two orders of magnitude at the highest shear rate studied).

Intermolecular potentials and vapor-liquid phase equilibria of perfluorinated alkanes

Abstract We propose 2 sets of intermolecular potentials for a united atom model of linear perfluorinated alkanes in their fluid states. Configurational-bias Monte Carlo simulations in the Gibbs ensemble have been carried out to determine the vapor–liquid phase equilibria for C n F 2 n +2 with n =5, 6, 7, 8, 10 and 16. Both sets of parameters yield critical temperatures which are in excellent agreement with experiment data. However, the critical densities are slightly too high for one model and slightly too low for the other. The saturated liquid densities agree with experiment to better than 5%. Variations of the critical properties with chain length show the correct scaling for both models.

Molecular dynamics study of the nano-rheology of n-dodecane confined between planar surfaces

Realistic molecular simulations agree with previously published surface force experiments that n-dodecane confined between mica surfaces displays shear-thinning starting at shear rate orders of magnitude less than in the bulk fluid. We probe the origin of this behavior by studying rotational and diffusional relaxations in the simulated fluid and find a freezing-out of the rotational degrees of freedom and a power-law diffusional relaxation, resulting in over seven orders of magnitude increase in the relaxation time.

Rheology of lubricant basestocks: A molecular dynamics study of C30 isomers

We have performed extensive equilibrium and nonequilibrium molecular dynamics (EMD and NEMD) simulations of three isomers of C30H62 at temperatures of 311 and 372 K employing a united atom model. Using the rotational relaxation time calculated from the EMD simulation, the Rouse model predicts a zero-shear viscosity for n-triacontane within 16% of the value determined by NEMD. Compared to experiment, NEMD and the united atom model underpredict the kinematic viscosities of n-triacontane and 9-n-octyldocosane but accurately predict the values for squalane (within 15%). In addition, the predicted values of the kinematic viscosity index for both 9-n-octyldocosane and squalane are in quantitative agreement with experiment and represent the first such predictions by molecular simulation. This same general potential model and computational approach can be used to predict this important lubricant property for potential lubricants prior to their synthesis, offering the possibility of simulation-guided lubricant des...

Configurational bias Gibbs ensemble Monte Carlo simulation of vapor-liquid equilibria of linear and short-branched alkanes

We report Gibbs ensemble Monte Carlo simulations of the linear alkane, n-nonane, and its branched isomer, branched 2,6-dimethylheptane; n-eicosane, and its branched isomer 2,6,11,15-tetramethylhexadecane; and an isomer of triacontane, 2,6,10,15,19,23-hexamethyltetracosane (squalane), using the configurational-bias technique to determine the vapor-liquid phase equilibria of these systems. For linear alkanes, quantitative agreement is obtained between simulation and experiment for both critical temperature and density. For short-branched alkanes for which there are experimental data available, the calculated vapor-liquid phase equilibria compare well with experiments. The critical density for branched alkanes studied here from the simulation is in quantitative agreement with experiment, while the critical temperature from simulation is slightly lower. Examination of the vapor-liquid phase equilibria for the applicability of the law of corresponding states shows very minor differences between a linear alkane and its short-branched isomers. In attempting to evaluate the accuracy of the current intermolecular potential model for the equation of state for the branched alkane molecules, we also performed molecular dynamics simulations. The combination of the Gibbs-ensemble and molecular dynamics results suggest that the potential model developed by Siepmann et al. gives reasonable agreement with experiment.