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  • Open Access

Synchronized Molecular-Dynamics Simulation via Macroscopic Heat and Momentum Transfer: An Application to Polymer Lubrication

Shugo Yasuda1,* and Ryoichi Yamamoto2,†

  • 1Graduate School of Simulation Studies, University of Hyogo, Kobe 650-0047, Japan
  • 2Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan

  • *yasuda@sim.u-hyogo.ac.jp
  • †ryoichi@cheme.kyoto-u.ac.jp
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Phys. Rev. X 4, 041011 – Published 21 October, 2014

DOI: https://doi.org/10.1103/PhysRevX.4.041011

Abstract

A synchronized molecular-dynamics simulation via macroscopic heat and momentum transfer is proposed to model the nonisothermal flow behaviors of complex fluids. In this method, the molecular-dynamics simulations are assigned to small fluid elements to calculate the local stresses and temperatures and are synchronized at certain time intervals to satisfy the macroscopic heat- and momentum-transport equations. This method is applied to the lubrication of a polymeric liquid composed of short chains of ten beads between parallel plates. The rheological properties and conformation of the polymer chains coupled with local viscous heating are investigated with a nondimensional parameter, the Nahme-Griffith number, which is defined as the ratio of the viscous heating to the thermal conduction at the characteristic temperature required to sufficiently change the viscosity. The present simulation demonstrates that strong shear thinning and a transitional behavior of the conformation of the polymer chains are exhibited with a rapid temperature rise when the Nahme-Griffith number exceeds unity. The results also clarify that the reentrant transition of the linear stress-optical relation occurs for large shear stresses due to the coupling of the conformation of polymer chains with heat generation under shear flows.

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Predicting the transport phenomena of complex fluids such as colloids, polymers, and liquid crystals in biological systems using computer simulations is challenging from both a scientific and an engineering point of view. Molecular dynamics simulations are often used to investigate the intrinsic properties of a material based on a tiny sample of the material. However, applying a full molecular dynamics simulation to macroscopic behavior of actual systems is not relevant since significant computational resources are required to solve the dynamics of all of the molecules. We propose a new multiscale simulation scheme, termed synchronized molecular dynamics, which enables us to treat the overall problem.

We assign the molecular dynamics simulations to small fluid elements to calculate the local stresses and temperatures, which are synchronized at a certain time interval to satisfy the macroscopic heat and momentum transfer. We use a nondimensional parameter related to viscous heating and thermal conduction to model how polymeric liquids flow. The synchronized molecular dynamics simulation can drastically reduce the computational requirements compared with the full molecular dynamics simulation, although we find that it is necessary to take into account spatial variations in temperature in real fluids. We apply the method to a polymer lubrication with viscous heating, assuming that the liquid is composed of chains of ten beads linked by a spring potential. After applying different shear stresses, the simulation results also reveal an interesting behavior of the conformation of polymer chains: The reentrant transition occurs in the linear stress-optical relation for large shear stresses due to the coupling of heat and momentum transfer. We highlight several limitations of our simulations, such as time resolution and the size of the molecular dynamics cells, to inform future studies.

We expect that our synchronized molecular dynamics scheme will be useful in various fields of scientific research and engineering developments of new functional materials.

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Phys. Rev. X 4, 041011– Published 21 October, 2014
  • Received 16 January 2014
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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