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 (rho) on steady shear flow material functions, which include viscosity (eta) and first and second normal stress coefficients (psi(1) and psi(2)) depending on shear rate (gamma), 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, eta-gamma 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 eta-gamma, psi(1)-gamma, and -psi(2)-gamma relationships, under different T, P, and rho 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 -psi(2)/psi(1)-gamma curves undergoing dramatic variations, which should be associated with shear dilatancy, as addressed analytically.

Molecular structural property and potential energy dependence on nonequilibrium-thermodynamic state point of liquid n-hexadecane under shear

“…Two necessary boundary conditions were set in the constant-pressure NEMD simulation box: the periodic boundary condition was adopted in all directions, while the Lees-Edwards boundary condition was used in the xy plane. Using the same molecular model and state point, our data 16 fall reasonably within the predictive calculations of Berker et al 6 and Chynoweth et al 8 In this article, we extended previous studies, [16][17][18] which have imitated the computer rheological experiment of steady state shear flow to obtain the possible microscopic picture of molecular chains undergoing structural deformation. Note that both the melting and boiling points 45 of n-hexadecane are 289-291 and 558 K, respectively.…”

“…Our previous article [16][17][18] provided details about the molecular model and simulation method. The constantpressure (isobaric-isothermal or NPT) NEMD simulations 2, 43 used a set of realistic Ryckaert-Bellemans molecular potentials 44 improved by Chynoweth and Michopoulos (CM).…”

“…Drawing on our previous constant-pressure NEMD simulations, [16][17][18] an objective is to propose a plausible microscopic understanding of the observed macroscopic shear flow phenomena. This study was motivated by the need to present a complete discussion on changes in structural properties with respect to shear rate, temperature, and pressure.…”

Linear viscoelasticity and thermorheological simplicity of n-hexadecane fluids under oscillatory shear via non-equilibrium molecular dynamics simulations

“…Recently, we reported 27,49,50 a whole series of NEMD papers in detail, regarding non-thermodynamic and rheological behaviors of n-hexadecane fluids under steady-state shear flow, which include shear dilatancy, shear thinning, and normal stress effect. The original motivation of the present study is, therefore, geared to ''oscillatory shear'' extended from steady-state shear based on previous studies.…”

“…The original motivation of the present study is, therefore, geared to ''oscillatory shear'' extended from steady-state shear based on previous studies. 27,49,50 The major objective is to show linear viscoelasticity and thermorheological simplicity. Also, a phase shift of oscillatory sheared fluids is presented while a non-linear viscoelastic behavior of strain thinning, 51 which indicates the modulus decreased upon increasing strain amplitudes, is revealed.…”