Liquid chlorine in shear and elongational flows: A nonequilibrium molecular dynamics study

A diatomic fluid (chlorine) is studied by the nonequilibrium molecular dynamics (NEMD) technique in various isochoric homogeneous flows at a unique state point. Rheological and orientational properties are investigated in planar Couette flow and also, for the first time for a molecular system, in various elongational flows. Moreover, the phenomenological coefficients (zero strain rate viscosity η0, Maxwell constant for molecular orientations) describing the linear regime in strain rate are probed through appropriate time correlation functions measured along an equilibrium trajectory. Large statistics are required given the presence in all relevant time autocorrelation functions of an exponential tail with a characteristic time close to the single molecule reorientation time at equilibrium τr (≈103 integration steps). At low strain rates, rheological and reorientational properties of molecular fluids are difficult to measure with accuracy by NEMD, not only because of the usual signal-to-noise ratio problems, but also as a result of the relatively slow reorientational dynamics which follows the imposition of an homogeneous flow. All flows lead to a somewhat similar shear thinning behavior for the generalized viscosity in terms of the second scalar invariant of the strain rate tensor, a quantity which provides a measure, valid for all flows, of the deformation rate based on the heat dissipation. The effective shear viscosity almost decreases by a factor of 2 with respect to η0 at a reduced shear rate of τrγ≈ 1. Orientational coupling with flow is studied in detail both in linear (Newtonian) and nonlinear regimes. A strong analogy is observed be-tween nonlinear behavior of stress and orientation tensors under flow.