Rotational viscosity of fluids composed of linear molecules: An equilibrium molecular dynamics study

Abstract

Non-Newtonian behavior and molecular structure of Cooee bitumen under shear flow: A non-equilibrium molecular dynamics study J. Chem. Phys. 142, 244501 (2015) In this paper, we investigate the rotational viscosity for a chlorine fluid and for a fluid composed of small linear molecules by using equilibrium molecular dynamics simulations. The rotational viscosity is calculated over a large range of state points. It is found that the rotational viscosity is almost independent of temperature in the range studied here but exhibits a power-law dependency on density. The rotational viscosity also shows a power-law relationship with the molecular length, and the ratio between the shear and rotational viscosities approaches 0.5 for the longest molecule studied here. By changing the number of atoms or united atomic units per molecule and by keeping the molecule length fixed, we show that fluids composed of molecules which have a rodlike shape have a lower rotational viscosity. We argue that this phenomenon is due to the reduction in intermolecular connectivity, which leads to larger fluctuations around the values possessed by the fluid on average. The conclusions here can be extended to fluids composed of uniaxial molecules of arbitrary length.

Generalized extended Navier-Stokes theory: Correlations in molecular fluids with intrinsic angular momentum

“…(36), but the resulting rotational viscosity depends somewhat on the range of s chosen. While the method is satisfactory, it is still desirable to devise a first-principle procedure for evaluating the rotational viscosity without assuming a functional form for η r valid over a range of s. Equation (24) shows that this is possible in the framework of GENS theory. Figure 2 shows the TVACFs and TAVACFs for a range of wavevectors for all three state points.…”

“…η 0 and ζ are evaluated using a Green-Kubo integral, 13 whereas η r is calculated from Eqs. (24) and (36), see text for details. The standard errors associated with η r (third column) in the gas and fluid phases are in the order of 10 −5 and not given in the (29) lies in that the ENS equations are based on a description where the molecular pressure tensor is allowed to have an anti-symmetric part, 10 and where an additional balance equation for the internal molecular degrees of freedom is introduced.…”

“…One important point to note here is that the rotational viscosity does not have a Green-Kubo integral and its evaluation involves an assumption about its Laplace transform. 33 It has been shown 6,14,24,33 that the following simple Lorentzian form is a good approximation where s is the complex Laplace variable, η r (0) = η r is the zero-frequency rotational viscosity, and τ is a characteristic relaxation time. This functional form was first suggested by Evans and Hanley.…”

“…The chlorine molecule is modelled as in Ref. 24, by letting the two chlorine atoms be bonded via a flexible bond. The total potential energy in the system is then…”

“…It has been shown that this spring constant keeps the average bond length close to l B with vibrations that are still slow enough to be accurately integrated forward in time with the time step used, h = 0.001. 24 Initially, 1200 molecules are arranged on a simple lattice with mass density 0.05. The simulation box is then compressed with relatively high compression rate in the x and z directions and small rate in the y direction.…”