Molecular Dynamics Study Of Thermophysical Properties Of Nanoscale Thin Liquid Films

Heat transfer enhancement at the nanoscale becomes significantly important with the rapid development of nanotechnology and microelectromechanical technology.Nanoscale thin liquid films play different roles in heat transfer enhancement in a variety of engineering applications and high-tech science and technology depending on their existing forms.However,the mechanism of heat transfer enhancement for nanoscale thin liquid films is still unclear.The studies on thin liquid films are insufficient and it is difficult to obtain conclusive results from experimental observations at the nanoscale.As the computation technology improves,molecular dynamics simulation has gradually become an important scientific research method to provide convenience to studying the heat transfer and thermophysical properties of nanoscale thin liquid film.Therefore,this thesis focuses on the thermophysical properties of nanoscale thin liquid films with different existing forms.The three-dimensional system models of nanoscale freestanding,sessile,and confined thin liquid films are established in this work.Molecular dynamics simulations are performed using LAMMPS(Large-scale Atomic/Molecular Massively Parallel Simulator)software.The specific heat capacity,thermal conductivity,and dynamic viscosity of the thin liquid films are simulated.The fitting formula of the thermophysical properties of the thin liquid films with the film thickness is obtained through the calculation and analysis of the three forms of thin liquid films.The characteristics of anisotropy at the nanoscale are obtained.The size,temperature,interface,and energy effects on the thermophysical properties of the nanoscale thin liquid films are explored.Based on the statistical mechanics in molecular dynamics,the microscopic analysis of the liquid molecules in the thin liquid films is achieved.It is confirmed that the molecular bonds and angles have neglegible effect on the thermophysical properties of the nanoscale thin liquid film,while the vibrational state,distribution structure,and diffusivity of the molecules are the keys to affecting the thermophysical properties and hence the heat transfer process.The effect of nanoparticles on the thermophysical properties of thin liquid films is further studied by simulating the thin nanofluid fims.Using the interface energy analysis method,the influence of the two types of solid/liquid interfaces on the thermophysical properties of the thin nanofluid films is investigated.The asymmetry of the liquid molecular density distribution caused by the simultaneous existence of solid/liquid interface and gas/liquid interface in sessile thin liquid film aggravates the Brownian motion of nanoparticles,resulting in special effects on the thermophysical properties.The vibrational density of states and mean square displacement analysis of the liquid molecules around the nanoparticle and the plate also confirm the effect of the asymmetry in the sessile film on the Brownian motion of the nanoparticles.The influence of interfacial wettability on the thermophysical properties of thin liquid films is further analyzed by changing the potential energy function parameters between solid and liquid molecules.Wettability changing results in variation of thermal conductivity of nanoscale confined thin liquid films in both parallel and perpendicular directions.Furthermore,the wettability is altered by making atomic defects on the metal solid surface,in order to be more realistic and closer to the actual experiment.Modification of a solid surface at the atomic level can reasonably bridge the effect of wettability and the physical phenomenon,so that experiments and simulations can be compared and verified each other.The comprehensive analysis of heat transfer in thin liquid films can then be achived.The effects of the existence form,nanoparticles,and interfacial wettability of nanoscale thin liquid films on their thermophysical properties are studied in this work.The mechanisms of the thermophysical properties variation of the nanoscale thin liquid film are revealed.The relationship between different thermophysical properties and liquid film thickness is given.This present simulations not only systematically improve the understanding of the thermophysical properties and mechanisms of different forms of thin liquid films,but also provide a solid theoretical basis for analyzing the heat and mass transfer characteristics of thin liquid films by bridging experiments and atomic simulations for future studies.