Material functions of liquid n-hexadecane under steady shear via nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects

Abstract

Computer experiments of rheology regarding the effects of temperature (T), pressure (P), and density (ρ) on steady shear flow material functions, which include viscosity () and first and second normal stress coefficients (ψ1 and ψ2) depending on shear rate (γ), have been conducted via nonequilibrium molecular dynamics simulations for liquid n -hexadecane. Straightforwardly, using both characteristic values of a zero-shear-rate viscosity and critical shear rate, - γ flow curves are well normalized to achieve the temperature-, pressure-, and density-invariant master curves, which can be formulary described by the Carreau-Yasuda rheological constitutive equation. Variations in the rate of shear thinning, obviously exhibiting in - γ, ψ1 - γ, and - ψ2 - γ relationships, under different T, P, and ρ values, are concretely revealed through the power-law model's exponent. More importantly, at low shear rates, the fluid explicitly possesses Newtonian fluidic characteristics according to both manifestations; first and second normal stress differences decay to near zero, while nonequilibrium states are close to equilibrium ones. Significantly, the tendency to vary of the degree of shear thinning in rheology is qualitatively contrary to that of shear dilatancy in thermodynamics. In addition, a convergent transition point is evidently observed in the - ψ2 / ψ1 - γ curves undergoing dramatic variations, which should be associated with shear dilatancy, as addressed analytically. © 2009 American Institute of Physics.