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

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

Configurational temperature profile in confined fluids. I. Atomic fluid

Two configurational expressions for the temperature are applied to the calculation of temperature profiles within a confined atomic fluid in a narrow slit pore. The configurational temperatures profiles so obtained are compared to the kinetic temperature, calculated from the equipartition principle, in equilibrium (EMD), and nonequilibrium molecular dynamics (NEMD) simulations of planar Poiseuille flow. We show that one of the configurational expressions exhibits a system-size dependence which prevents its application to the determination of high-resolution temperature profiles. The other expression yields good agreement with the kinetic temperature profile in both equilibrium and nonequilibrium systems

Comparison of thermostatting mechanisms in NVT and NPT simulations of decane under shear

Nonequilibrium molecular dynamics (NEMD) simulations play a major role in characterizing the rheological properties of fluids undergoing shear flow. However, all previous studies of flows in molecular fluids either use an “atomic” thermostat which makes incorrect assumptions concerning the streaming velocity of atoms within their constituent molecules, or they employ a center of mass kinetic (COM) thermostat which only controls the temperature of relatively few degrees of freedom (3) in complex high molecular weight compounds. In the present paper we show how recently developed configurational expressions for the thermodynamic temperature can be used to develop thermostatting mechanisms which avoid both of these problems. We propose a thermostat based on a configurational expression for the temperature and apply it to NEMD simulations of decane undergoing Couette flow at constant volume and at constant pressure. The results so obtained are compared with those obtained using a COM kinetic thermostat. At equilibrium the properties of systems thermostatted in the two different ways are of course equivalent. However, we show that the two responses differ far from equilibrium. In particular, we show that the increase in the potential energy of the internal modes with increasing shear is only observed with a Gaussian isokinetic COM thermostat in both NVT and NPT simulations. There is no such increase with the configurational thermostat, which, unlike the Gaussian isokinetic COM thermostat, correctly accounts for the internal degrees of freedom of the molecular fluid