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We present results from nonequilibrium molecular dynamics (NEMD) simulations of simple fluids undergoing planar Poiseuille flow in a slit pore only a few molecular diameters in width. The calculations reported in this publication build on previous results by including the effects of attractive forces and studying the flow at narrower pore widths. Our aims are: (1) to examine the role of attractive forces in determining hydrodynamic properties, (2) to provide clearer evidence for the existence of a non-Markovian generalization of Newtons law, (3) to examine the slip-stick boundary conditions in more detail by using a high spatial resolution of the streaming velocity profiles, (4) to investigate the significance of the recently proposed cross-coupling coefficient on the temperature profiles. The presence of attractive interactions gives rise to interesting packing effects, but otherwise, does not significantly alter the spatial dependence of hydrodynamic quantities. We find the strongest evidence to date that Newton’s Law breaks down for very narrow pores; the shear viscosity exhibits singularities. We suggest a method to test the validity of the non-Markovian generalization of Newton’s Law. No-slip boundary conditions are found to apply, even at these microscopic length scales, provided one takes into account the finite size of the wall atoms. The effects of any strain rate induced coupling to the heat flow are found to be insignificant.

We propose two new thermostats which can be employed in computer simulations to ensure that two different variants of the configurational temperature fluctuate around their equilibrium values. These new thermostats differ from one previously introduced by Delhommelle and Evans [Mol. Phys. 99, 1825 (2001)] in several important ways. First, our thermostats are derived in the same spirit as the Nosé-Hoover thermostat and therefore generate the canonical phase-space distribution. Second, our thermostats involve simpler equations of motion, which do not involve spatial gradients of the configurational temperature. They do not suffer from problems stemming from stiff equations of motion and furthermore, in large temperature perturbation simulations, the measured temperature follows the set-point temperature without any overshoot, and with good damping of oscillations. We show that both of our configurational thermostats are special cases of a more general set of Nosé-Hoover equations proposed by Kusnezov et al. [Ann. Phys. 204, 155 (1990)]. The new thermostats are expected to be highly useful in nonequilibrium simulations, particularly those characterized by spatial inhomogeneities. They should also find applicability in simulations involving large changes in temperature over small time scales, such as temperature quench molecular dynamics and radiation damage modeling.

Doremus' model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The non-linear nature of the equation obtained means that the fitting process is non-trivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials including SiO 2 , GeO 2 , B 2 O 3 , anorthite, diopside, xNa 2 O-(1 -x)SiO 2 , xPbO-(1 -x)SiO 2 , soda-limesilica glasses, salol, and α-phenyl-ο-cresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems, the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anti-correlation between fragility ratio and configuron percolation threshold which determines the glass transition temperature in the analysed materials was found. 66.20.1d, 66.10.Cb, 71.55.Jv. PACS:

In this paper we present results for the wave-vector dependent shear viscosity for a model atomic fluid with short ranged repulsive interactions computed by molecular dynamics simulations. It is shown that the data can be fitted to two different simple functional forms over a large density range, namely, a function composed of two Gaussian terms and a Lorentzian type function with a variable wave-vector exponent. The parameters of both functional forms are found to obey simple density dependencies. While the first functional form has the advantage that the inverse Fourier transform can be found analytically, the Lorentzian type function fits the wave-vector dependence better over the range of wave vectors and densities studied here. The results show that the real space viscosity kernel has a width of 2 to 3 atomic diameters. This means that the generalized hydrodynamic constitutive relation is required if the strain rate varies significantly over this distance, a situation commonly encountered for nanofluidic flows.