Investigation Of Properties Of Interfacial Slip And Its Influence On Flow Field Using Molecular Dynamics Simulations

Microfluidic systems are highly efficient and economic,thus pose a greatly promising applications in defense,medicine,pharmaceuticals,and environmental monitoring,etc.,such as,micro sensors,DNA analysis and sequencing,drug delivery and chemical analysis.In microfludics,the reduced chrematistic length of flow induces large ratio of surface to volume leading to that the interfacial slip effect significantly influences the flow properties.To accurately describe the liquid flow in microfluidic device and reduce the flow resistance,the slip properties and its mechanisms are necessary to improve the efficiency and accuracy of microfluidic devices.Therefore,this thesis investigated the properties of the interfacial slip and the involved flow characters using molecular dynamics simulation.The obtained results and innovations of the thesis includes:(1)Investigated the liquid structure and adhesion strength on solid surface.Firstly,from the static point of view,the three-dimensional solid-like structure of liquid at the solid-liquid interface is obtained,and the detailed features of the solid-like structure is given.The obtained structural features are consistent with the experimental results,indicating that the Lennard-Jones potential energy function can accurately characterize the interaction between solid and liquid.Then from the dynamic point of view,the quantitative relationship between the adhesion of the solid surface and the solid wettability is given,and this relationship is consistent with the experimental results of the predecessors.From the static and dynamic perspectives,the solid-liquid interaction potential energy function selected in this paper is verified.Also,the integral equation of motion equation,temperature control method,time step selection and macro-statistical calculation are accurate and reliable.(2)Investigated the influence of large range of solid-liquid interaction on the slip at low shear rate.Two kinds of slip regimes,strong and weak solid-liquid interaction,were found with the change of solid-liquid interaction strength.In the strong liquid interaction regime,the slip increases with the decrease of the solid-liquid interaction strength;while in the weak solid-liquid interaction regime,the slip decreases with the decrease of the solid-liquid interaction strength.When the slip length reaches the maximum value,the corresponding critical solid-liquid interaction strength increases with the increase of temperature,but does not change with the change of the driving force.By examining the atomic hopping energy barrier based on the weighted average of the density distribution in the first liquid layer,the slip model of the predecessors is extended,and the mechanisms of the two slip regimes are well explained.(3)Investigated the influence of strong shear and viscosity heating on the slip behaviors at solid-liquid interface.The average temperature of the liquid at strong and weak solid-liquid interaction strength increases significantly with the increase of the shear rate.And as the shear rate increases,the slip length changes from small(large)to a fixed value at the strong(weak)solid-liquid interaction strength.By examining the influence of the liquid temperature and shear rate on the main peak,S(G₁)/S(0),of the static structural factor in the first liquid layer and the average distance,D_WF,between solids and the first liquid layer,the slip behaviors are analyzed in details,and the influence mechanism of viscous heating effect on the slip is given.(4)Investigated the slip behaviors on gas-liquid interface of nanostructured solid surface.It is proved from the atomic scale that the shear stress of the liquid near the gas-liquid interface decreases gradually with the normal distance,and the shear stress is much lower than that on the solid-liquid interface.Correspondingly,the normal stress of the liquid gradually increases with the horizontal position at the gas-liquid interface.The balance between shear and normal stress maintains the steady flow of the system.It is found that the slip properties of the gas-liquid interface on the nanostructured surface are the composite slip boundary conditions,that is,the finite slip condition at the boundary of the gas-liquid interface,and the nearly infinite slip condition at the center of the gas-liquid interface.This composite slip boundary condition successfully explains the contradictory results of predecessors on the surface slip flow of regular structures at different scales.(5)Investigated the ship boundary condition of polymer fluid on the solid surface using coarse-grained model and a surface friction control term.The analytical expressions of the position and the slip length are theoretically derived,and the slip length and hydrodynamic position are related to the intrinsic properties of the polymer fluid,such as polymer size,degree of polymerization,and viscosity.Both numerical simulation and theoretical prediction show that the hydrodynamic position is independent from the strength of the surface friction term and the degree of coarse-grained.The slip length is in a power-dependent relationship with the surface friction term strength,?_B??_WT^-1/3.This power relationship can be used to calculate the strength of the surface friction term so that the slip boundary conditions are independent of the degree of coarse grained of the polymer.