Transient molecular dynamics simulations of viscosity for simple fluids

Area of Science:

• Computational physics and chemistry
• Fluid dynamics
• Materials science

Background:

• Accurate simulation of fluid viscosity is crucial for understanding and predicting fluid behavior.
• Traditional equilibrium molecular dynamics (EMD) can be computationally intensive for viscosity calculations.
• Developing efficient and accurate non-equilibrium methods is an ongoing challenge.

Purpose of the Study:

• To develop and validate a novel transient molecular dynamics (TMD) method for simulating fluid viscosity.
• To compare the performance of TMD with EMD for a Lennard-Jones fluid.
• To investigate the influence of different constitutive models (Newtonian vs. viscoelastic) on simulation accuracy.

Main Methods:

• Developed a transient molecular dynamics (TMD) approach involving the simulation of decaying sinusoidal velocity profiles.
• Obtained viscosity by fitting the analytical solution of the momentum transport boundary-value problem to simulated velocity decay.
• Benchmarked TMD against equilibrium molecular dynamics (EMD) for a Lennard-Jones fluid using Newtonian and linear viscoelastic models.

Main Results:

• The TMD method, when employing a linear viscoelastic model, shows excellent agreement with EMD results across a wide range of fluid densities.
• A Newtonian fluid model introduces significant positive errors at high densities, which are mitigated by the viscoelastic model.
• Viscoelastic behavior was observed in simulations at dimensionless densities above 0.7.

Conclusions:

• The developed transient molecular dynamics (TMD) method provides an accurate and efficient alternative for calculating fluid viscosity.
• A linear viscoelastic model is essential for accurate viscosity simulations at higher densities, capturing observed transient velocity behavior.
• The TMD method, particularly with a viscoelastic approach, offers reliable viscosity predictions comparable to EMD.