Synchronized Molecular-Dynamics Simulation of the Thermal Lubrication of an Entangled Polymeric Liquid.

Shugo Yasuda

doi:10.3390/polym11010131

Abstract

The thermal lubrication of an entangled polymeric liquid in wall-driven shear flows between parallel plates is investigated by using a multiscale hybrid method, coupling molecular dynamics and hydrodynamics (i.e., the synchronized molecular dynamics method). The temperature of the polymeric liquid rapidly increases due to viscous heating once the drive force exceeds a certain threshold value, and the rheological properties drastically change at around the critical drive force. In the weak viscous-heating regime, the conformation of polymer chains is dominated by the flow field so that the polymers are more elongated as the drive force increases. However, in the large viscous-heating regime, the conformation dynamics is dominated by the thermal agitation of polymer chains so that the conformation of polymers recovers more uniform and random structures as the drive force increases, even though the local shear flows are further enhanced. Remarkably, this counter-intuitive transitional behavior gives an interesting re-entrant transition in the stress–optical relation, where the linear stress–optical relation approximately holds even though each of the macroscopic quantities behaves nonlinearly. Furthermore, the shear thickening behavior is also observed in the large viscous-heating regime—this was not observed in a series of previous studies on an unentangled polymer fluid. This qualitative difference of the thermo-rheological property between the entangled and unentangled polymer fluids gives completely different velocity profiles in the thermal lubrication system.

Highlights

Polymeric fluids exhibit complicated flow behaviors because the microscopic dynamics of polymer chains are highly correlated with global hydrodynamic transport [1]
We investigate the thermal lubrication of the polymeric liquid composed of entangled chains by using the synchronized molecular dynamics (SMD) simulation [22]
In the SMD method, the gap between the upper and lower walls is uniformly divided into M mesh intervals with ∆y = H/M, and small sub cells are assigned to each mesh interval, where the molecular dynamics of local fluid elements are calculated. (See Figure 1) In each sub-MD cell, the nonequilibrium molecular dynamics (NEMD)

Summary

Introduction

Polymeric fluids exhibit complicated flow behaviors because the microscopic dynamics of polymer chains are highly correlated with global hydrodynamic transport [1]. In high-speed devices, the microscopic dynamics are significantly affected by viscous heating because the polymeric fluid has a large Prandtl number, so the microscopic dynamics and the hydrodynamic heat and momentum transports are mutually correlated. Predicting the microscopic dynamics and the macroscopic thermal flows in high-speed devices is challenging from both scientific and engineering points of view. Computer simulations are expected to be useful to solve these mutually correlated multiscale systems. Multiscale simulations to tackle the flow behaviors of complex fluids have been developed by various researchers. The pioneering research was carried out by Laso and Öttinger [2,3,4]

Methods

Results

Conclusion