The Regulation Of Steady Configuration And Flow Behavior Of Gas-liquid Two-phase Fluid In The Micro-and Nano-scale Channels

The liquid-filled micro-and nano-scale channels are widely found in nature,such as pores in soil and rocks,plant veins,and blood vessels in the human body.Moreover,due to the excellent transport properties of the fluid in the micro-and nano-scale channels,artificially prepared micro-and nano-scale channels have been widely used in mechanical manufacturing,electronic technology,biological engineering and other fields.The micro-and nano-scale channels transporting fluids have become an important support for the development of miniaturization and precision of modern industry.Since the fluid in the channels does not exist alone and usually contains other substances such as gas,understanding the transport of gasliquid two-phase fluid in the micro-and nano-scale channels is critical for the design,optimization and fabrication of these devices.In the micro scale,the fluid exhibits a completely different behavior from that in the macro scale.This dissertation investigates the basic properties and regulation relationships of pure fluid in the micro-and nano-scale channels,and further explores the flow behavior and response laws of gas-liquid two-phase fluid in the microand nano-scale channels.In terms of single-phase fluid,molecular dynamics method is used to simulate the static and dynamic wetting phenomena of water on smooth walls and the Poiseuille flow in the different types of micro/nano-scale channels.The variation laws of fluid viscosity,slip length and other properties with solid-liquid interaction and other factors are revealed.The simulation results show that the density and stress distribution of the fluid exhibit an obvious oscillation region at the solid-liquid interface.According to this fluctuation,the thickness of the boundary layer of the solid-liquid interface is determined and the viscosity of the boundary layer of the fluid is calculated.It is found that the viscosity of the boundary layer is much greater than that of the fluid and increases with the increase of the strength of solid-liquid interaction.Based on the static contact angle of droplets,the expression of fluid interface viscosity is proposed.For the channels with the same structure,strong wettability of the wall results in smaller slip lengths.The controlling relationship between wettability of the wall and Navier slip length is further established.As for gas-liquid two-phase fluids,based on molecular dynamics simulation,the Poiseuille flow of a two-phase fluid composed of water and argon in the micro-and nano-scale channels is studied.The basic properties of the fluid such as density,velocity and slip length are calculated,and their response laws to the three-phase interaction are revealed.In addition,we summarize the variation laws of the size and shape of nanobubbles at the boundary.The results show that the peak density of the fluid adjacent the wall containing bubbles increases with the increase of the gas-liquid/solid-liquid interaction,while the fluid velocity and slip length show the opposite variation laws.The Navier slip length of fluid with bubbles is higher than that of single-phase fluid,indicating that bubbles at the boundary can increase the slip length of fluid and improve the transmission efficiency of fluid in the channels.The size and spreading diameter of nanobubbles on the wall surface decrease with the increase of gas-liquid/solidliquid interaction,and the hydrophobic surface is more favorable for the stable existence of nanobubbles than the hydrophilic surface.To summarize,based on the molecular dynamics method,the basic transport properties and flow behavior of single-phase fluid and gas-liquid two-phase fluid in the micro-and nanoscale channels are investigated in this dissertation,and the variation laws of fluid viscosity and slip length with solid-liquid-gas interaction are revealed.It is clarified that the existence of interfacial bubbles can improve the transport efficiency.The research results improve the understanding of the basic properties and transport properties of multiphase fluids at the microand nano-scale,and provide scientific basis and data support for engineering applications based on drag reduction by gas to improve the transmission efficiency of the micro-and nano-scale channels.