Force autocorrelation function in linear response theory and the origin of friction

Vanishing of the equilibrium Green-Kubo fluctuation expression for the friction coefficient of a massive particle moving in a finite-volume liquid is usually interpreted as an unphysical consequence of the finite volume. Here I show that it is a physical consequence of the finite mass of the rest of the system, which allows it to be dragged by the moving particle. As a consequence, it is sufficient to have two infinite masses in the liquid for the friction coefficient to be finite. In addition, I give the physical interpretation of different friction coefficients for two infinite-mass particles moving in the liquid.Comment: 24 pages text and figure

Shear stress relaxation in liquids

We show that at high densities, as the system size decreases, liquid becomes able to permanently sustain increasing internal shear stress after a constant deformation, although the other characteristic liquid properties, such as the pair distribution function and diffusion coefficient do not change under strain. The system size necessary for observation of this effect increases with the decrease in temperature, and it is stronger in pair potentials with steeper repulsive part. We relate this result to the size of the "cooperatively rearranging regions" of the Adam-Gibbs theory of glass transition

Influence of strain on transport in dense Lennard-Jones systems

We study the shear stress relaxation and temperature dependence of the diffusion coefficient, viscosity, and thermal conductivity along a high-density Lennard-Jones isochore of the reduced density of 1.0, as it crosses the freezing and melting lines, in equilibrium and under constant strain

Approach to the nonequilibrium time-periodic state in a steady shear flow model

The standard nonequilibrium molecular dynamics algorithm for steady shear flow (SLLOD ) employs Lees–Edwards periodic boundary conditions. It is not widely known that these boundary conditions make the system non–autonomous. The "steady state" shear stress is in fact time periodic. The standard sponse theory derivations for steady shear do not take proper account of these non–autonomous terms. In this paper we correct this deficiency. We show that these non–autonomous terms invalidate the Green-Kubo relation for the finite frequency shear viscosity of fluids under Lees-Edwards periodic boundary conditions

Hydrogen bonding in ethanol under shear

We study the dependence of viscosity of ethanol on shear rate using constant volume and constant pressure nonequilibrium molecular dynamics simulations, with the emphasis of the interrelationship between breaking, stability, and alignment of hydrogen bonds and shear thinning at high shear rates. We find that although the majority of hydrogen bond breakings occur at low shear rates, we do not observe shear thinning until there is some shear-induced alignment of the hydrogen bonds with the direction of shear

Shear viscosity of molten sodium chloride

The shear viscosity of molten sodium chloride is determined under a wide range of strain rates using nonequilibrium molecular dynamics (NEMD) simulations in the canonical (N,V,T) ensemble. Questions have been recently raised on the use of kinetic temperature thermostats, based on the equipartition principle, in simulations of nonequilibrium fluids and using a configurational temperature thermostat has been suggested to be more realistic. To further ascertain the results obtained in this work, we study molten NaCl with both kinetic and configurational temperature thermostats. Since configurational thermostats have been so far restricted to simple fluids or alkanes, we first apply configurational expressions for the temperature to molten NaCl, test the values so obtained in equilibrium molecular dynamics simulation for various system sizes and state points and finally use them to thermostat molten NaCl under shear. NEMD results obtained for both thermostats show that except for the so-called normal stress coefficients, molten salt under shear exhibits mostly the same features as a simple fluid under shear, i.e., features in agreement with the mode-coupling theory. The choice of the thermostatting method is found to have little influence on the results for the range of shear rates investigated

Conductivity of molten sodium chloride and its supercritical vapor in strong dc electric fields

We investigate the influence of thermostatting methods on the electrical conductivity and structure of molten and supercritical sodium chloride obtained in nonequilibrium molecular dynamics simulations in strong constant (dc)electric fields. The strong dependence of the results on the type of thermostat employed in simulations becomes apparent only at extremely high fields (>0.5×109 V/m). For this range of fields, quantitative differences of unexpected size can be seen in the melt. In the supercritical fluid, different thermostats predict qualitatively very different behavior and structure. While the kinetic-type thermostats predict increased association of ions in the field, configurational thermostat predicts enhanced dissociation

Conductivity of molten sodium chloride in an alternating electric field

We study the properties of molten sodium chloride in alternating electric fields of two amplitudes and for a large range of frequencies using nonequilibrium molecular dynamics simulations, and compare the responses with two different methods of temperature control to the predictions of linear response theory. We find that the considerable nonlinearity in the resulting current density observed at low frequencies can be explained by the characteristics of the nonlinear response to constant fields. We also comment on the differences in the dissipation mechanisms and the entropy change with two thermostats

Homogeneous shear flow of a hard-sphere fluid: Analytic solutions

Recently, a solution for collision-free trajectories in an N particle thermostatted hard-sphere system undergoing homogeneous shear (the so-called "Sllod" equations of motion) led to a kinetic theory of dilute hard-sphere gases under shear. However, a solution for collisions, necessary for a complete theory at higher densities, has been missing. We present an analytic solution to this problem, which provides surprising insights into the mechanical aspects of thermostatting a system in an external field. The equivalence of constant temperature and constant energy ensembles in the thermodynamic limit in equilibrium, the conditions for the nature of heat exchange with the environment (entropy creation and reduction) in the system, and the condition for appearance of the artificial string phase follow from our solution