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

Abstract

A small amplitude oscillatory shear flows with the classic characteristic of a phase shift when using non-equilibrium molecular dynamics simulations for n-hexadecane fluids. In a suitable range of strain amplitude, the fluid possesses significant linear viscoelastic behavior. Non-linear viscoelastic behavior of strain thinning, which means the dynamic modulus monotonously decreased with increasing strain amplitudes, was found at extreme strain amplitudes. Under isobaric conditions, different temperatures strongly affected the range of linear viscoelasticity and the slope of strain thinning. The fluid's phase states, containing solid-, liquid-, and gel-like states, can be distinguished through a criterion of the viscoelastic spectrum. As a result, a particular condition for the viscoelastic behavior of n-hexadecane molecules approaching that of the Rouse chain was obtained. Besides, more importantly, evidence of thermorheologically simple materials was presented in which the relaxation modulus obeys the time-temperature superposition principle. Therefore, using shift factors from the time-temperature superposition principle, the estimated Arrhenius flow activation energy was in good agreement with related experimental values. Furthermore, one relaxation modulus master curve well exhibited both transition and terminal zones. Especially regarding non-equilibrium thermodynamic states, variations in the density, with respect to frequencies, were revealed.

Theory of harmonic dissipation in disordered solids

“…With this technique, the loss (or internal friction) angle φ is measured between an imposed sinusoidal strain and the resulting internal stress, yielding the energy dissipation Q −1 = tan φ. This approach is widely used experimentally in the Hz to kHz regime to study glasses [57,58], liquids and soft matter systems [59,60], but has also been used numerically at higher frequencies in MD simulations [61][62][63]. Here we show that in the high-frequency regime of harmonic dissipation, the quality factor Q −1 can be expressed analytically, allowing to analyze in details the features that control dissipation in a glass, both below and above the IR limit.…”

Master curves and radial distribution functions for shear dilatancy of liquid n-hexadecane via nonequilibrium molecular dynamics simulations

“…The functions and parameters of those potentials for the CM model are the same as the ones we apply to model n-hexadecane molecule. 16 This model has been adopted previously in the steady state shear flow, 16,36,[43][44][45] oscillatory shear flow, 46 and contraction flow 47 portions of molecular dynamics simulations.…”

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

“…The CM model is adopted in the steady state shear flow, 17,[54][55][56]59 oscillatory shear flow, 36 and nanocontraction flow 60 portions of MD simulations. For completeness, we introduce all potential functions for the CM model below and their parameters are listed in Table I.…”

“…Apart from a few noteworthy reports for oscillatory shear flows, both important features of fluids-linear viscoelastic and thermorheological simplistic-have been recently presented using NEMD simulations. 36 The steady shear material functions [37][38][39] involve viscosity and first and second normal stress coefficients. In rheology, nonlinear manifestations, such as shear thinning, shear thickening, and normal stress differences, have been commonly observed for polymeric fluids.…”