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Significance Statement
The polymer lubrications generating significant viscous heating as in the high-speed micro devices are investigated by the Synchronized Molecular-Dynamics simulation [Phys. Rev. X 4, 041011 (2014)]. In the SMD method, the molecular dynamics simulations are performed in small MD cells associated to each fluid element to calculate the local microscopic dynamics involving the viscous heating, but they are synchronized each other at a certain time interval via global heat and momentum transports. This method enables us to investigate the microscopic dynamics such as the conformation of polymer chains in local fluid elements under highly heterogeneous velocity and temperature distributions in the system.
The effects of variation in thermal conductivity of polymeric liquid on the rheological properties and conformational changes are investigated in the present paper. The remarkable result is a transitional behavior of the conformational change of polymer chains due to the competition of shear deformation and viscous heating, both of which are caused by the local shear flow. This also related to the re-entrant transition of stress-optical relation which was first found in our previous study.
Journal Reference
Computers & Fluids, Volume 124, 2 January 2016, Pages 185-189.
Shugo Yasuda1, Ryoichi Yamamoto2
[expand title=”Show Affiliations”]- Graduate School of Simulation Studies, University of Hyogo, Japan
- Department of Chemical Engineering, Kyoto University, Japan
Abstract
The Synchronized Molecular-Dynamics simulation, which was recently proposed by authors (Yasuda and Yamamoto, 2014), is applied to the analysis of polymer lubrication between parallel plates. The changes in rheological properties, conformational change of polymer chains, and temperature rise due to the viscous heating are investigated with varying values of thermal conductivity of the polymeric liquid. It is found that with a small applied shear stress on the plate, the temperature of the polymeric liquid only slightly increases in inverse proportion to the thermal conductivity; the apparent viscosity of the polymeric liquid is little affected by changing the thermal conductivity. However, at a large shear stress the transitional behaviors of the polymeric liquid are observed due to the interplay of the shear deformation and viscous heating by changing the thermal conductivity. This transition is characterized by the Nahme–Griffith number Na, which is defined as the ratio of the viscous heating to the thermal conduction at a characteristic temperature. When the Nahme–Griffith number exceeds unity, the temperature of the polymeric liquid increases rapidly and the apparent viscosity also exponentially decreases as the thermal conductivity decreases. The conformation of polymer chains is stretched and aligned by the shear flow when Na < 1, but the coherent structure becomes disturbed by the thermal motion of molecules when Na > 1.
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