Study On The Liquid Wall Slippage And Threshold Pressure Gradient By Molecular Dynamics Simulation

Micro mechanical system takes Micro-Electron Mechanical System (MEMS) as the research object, has received more and more attentions since the1980s. The researches on the character of micro scale flow have gained more and more attentions and are widely used in industries, electronic information technology and bioengineering. In micro scale flow, some factors which can be neglected in macro size, such as size effect, surface force and so on, must be took into consideration. For the complexity of micro scale flow, researchers have some trouble on figuring out its flow mechanism and can not quantificationally measure the accurate value of physical quantity in micro scale flow. The Molecular Dynamics Simulation (MDS) can make up the shortage of theory and traditional experiment in studying the micro flow, and is an effective method to understanding the micro flow.Based on fluid mechanics and MDS, we studied the wall slippage, drag reduction and the generation mechanism of Threshold Pressure Gradient (TPG) in micro flow by focusing on Poiseuille flow between two parallel planes.Began with the background and developments of micro flow, this paper listed some influencing factor to micro flow and development in drag reduction by molecular dynamics simulation. Some basic theories and the latest research progress about TPG were introduced. This paper gave a comprehensive introduction to Molecular Dynamics Simulation, including some basic principles, simulation process, implementation method and future development trend.Then we introduced the current studies of wall slippage, set up the model, and studied the influence of wettability on fluid velocity and boundary slip velocity. The results show that when liquid flow through the strong hydrophobic channel, there exists obvious slip velocity on the boundary; while the liquid flow through the strong hydrophilic channel, the velocity of liquid near the boundary is nearly keep still. The situations of slip length with different driving force on the strong hydrophilic and hydrophobic channel were discussed. No matter the channel surfaces are hydrophobic or hydrophilic, the absolute slip length decreases with the augment of external force and keeps constant zero when the force increases to a certain extent. Besides, the combined influences of the driving force and channel size on the slip velocity and the slip length in different wettability were simulated, effects of the driving force on flow rates and drag reduction were gotten. The results show that either the channel surfaces were hydrophobic or hydrophilic, the slip velocity increases with the augment of external force. The drag reduction significantly increases with the decreasing of channel size, which shows the surface effect in micro scale.At last, from the aspect of wettability of solid wall, we analyzed the influence of boundary stick layers in the hydrophobic boundary. The existence of TPG was proved according the force on the molecular and the generation mechanism of TPG was caused by the solid-liquid attraction which will provide the theoretical foundation for further investigation of micro flow.