Heat Transport in Fluids and Interfaces via Non-Equilibrium Molecular Dynamics Simulations

摘要

In this thesis non-equilibrium molecular dynamics is used to investigate effects relating to thermal transport in fluids and interfacial systems.udNon-equilibrium molecular dynamics (NEMD) simulations of liquid waterudwere undertaken using the Modified Central Force model (MCFM) ofudwater. Non-equilibrium thermodynamics predicts dipolar alignment as audresponse to an applied temperature gradient. This effect was systematicallyudinvestigated by applying thermal gradients of up to 4 K/ Å to a system ofudMCFM water. This yielded induced electric fields of up to ~ 109 Vm-1.udThe predictions of non-equilibrium thermodynamics were supported by theudsimulations. The mechanism of thermal transport was investigated.udThe effect of electrostatic interactions on the thermal transport propertiesudwas also investigated in this model comparing the Ewald summation andudWolf methods. It was found that whilst the change in equation of stateudusing each method is small, the truncation of the electrostatic interactionsudleads to a lower heat flux density and values for the thermal conductivityudthat are ~ 5 - 10% lower. The relaxation of the system to a steady-stateudtemperature gradient was also investigated and the timescales involved wereudfound to agree with the results using the macroscopic heat equation.udThe hydrogen bonding contribution to the heat flux vector was investigated. This was found to contribute to around 30-40% of the total heat fluxudfor MCFM water. The potential energy contribution was found to becomeudnegative towards lower temperatures. Also investigated was the thermaludconductivity of glassy water with the aim of identifying a difference in theudthermal conductivity from liquid to the glass state. The SPC/E model wasudemployed for this purpose but no significant change was identified.udNEMD simulations were employed to investigate the interfacial thermaludresistance of liquid/vapour and solid/vapour interfaces in a Lennard-Jonesudsystem. For energy fluxes of ≈107 Wm-2 a significant interfacial thermaludresistance was observed, particularly at low temperatures. To investigateudthe microscopic origin of the interfacial thermal resistance, the intrinsicudsampling method was employed in the liquid/vapour interface. The temperatureuddrop was found to occur in front of the interface in a region whereudadsorbed atoms at the surface correspond to a density peak in the vapourudphase.