Translation-orientation coupling and Cox-Merz rule of liquid hexane

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Fluid Dynamics

Background:

• The Cox-Merz rule empirically relates frequency-dependent linear viscoelasticity to shear-rate-dependent nonlinear viscosity in polymers and other complex fluids.
• Understanding the molecular origins of this rule is crucial for predicting fluid behavior under various flow conditions, particularly for chain-like molecules.

Purpose of the Study:

• To elucidate the molecular-level origins of the Cox-Merz rule in liquids composed of chain-like molecules, using liquid hexane as a model system.
• To investigate the relationship between viscoelastic relaxation, shear-induced molecular ordering, and the validity of the Cox-Merz rule.

Main Methods:

• Employed both equilibrium and non-equilibrium molecular dynamics (MD) simulations to model liquid hexane.
• Analyzed frequency-dependent complex shear viscosity and shear-rate-dependent nonlinear viscosity.
• Investigated the slowest viscoelastic relaxation mode and shear-induced orientational order.

Main Results:

• Confirmed that the frequency-dependent complex shear viscosity and shear-rate-dependent nonlinear viscosity of liquid hexane adhere to the Cox-Merz rule.
• Identified a translation-orientation coupling mechanism as the primary contributor to the slowest viscoelastic relaxation mode.
• Observed saturation of shear-induced orientational order at the onset of shear thinning in non-equilibrium MD simulations.

Conclusions:

• The Cox-Merz rule in chain-like liquids is fundamentally linked to the translation-orientation coupling mechanism.
• This coupling governs both the slowest viscoelastic relaxation and the shear-induced orientational ordering, explaining the observed adherence to the Cox-Merz rule.