Molecular Dynamics Studies Of Fluid Viscosities In Micro/nano-scale Channels

With the development of nano science and technology,the micro and nano fluid system is used more and more widely.When the device size decreases,the flow appears against the law of conventional scale flow phenomenon.The current studies of micro-nano scales fluid flow is not complete.This paper,by using molecular dynamics method,studies the fluid viscosity properties of nanoscale channel,has great significance for the precise design of micro-nano scale devices.First,for nano scale channel flow simulation,the surface has too much atomics,which creates large amount of calculation.This paper puts forward the algorithm of virtual wall,simplifies the simulation of the reaction between the surface atoms and molecules fluid during calculation,improves the efficiency of the simulation.Second,in molecular dynamics,the velocity gradient and the shear stress distribution of the fluid flow is obtained by simulation,then the distribution of the fluid viscosity can be obtained by using Newton's law of viscosity.And then the method was used to study the nano scale gas Couette flow and Poiseuille flow,and analyze the viscosity distribution in the channel.The simulation result shows that the equivalent viscosity in channel can be divided into two areas: central region and near wall region.Central region equivalent viscosity agrees well with experimental value in the references,but equivalent viscosity on near wall region is off the experiment value because of the affect of the wall atomics,the closer distance from the wall,the greater velocity gradient of fluid motion,the smaller equivalent viscosity,wall influence thickness is around 35 nm.When the channel height is small,the whole equivalent viscosity belongs to the near wall region,at this point,the equivalent viscosity values are less than the experimental value.Then the paper compares different gas density,temperature,shear rate,channel height,fluid-structure molecular interaction strength on the distribution of the equivalent viscosity of the fluid.The study founds that in Couette flow,wall influence thickness decreases with the increase of atomic repulsion of fluid molecules;In Poiseuille flow,the wall thickness decreases with the increase of gas density.It is also found that the higher temperature,the greater gas equivalent viscosity,which agrees with the gas viscosity change rule.Finally,the molecular dynamics method was used to study the liquid Couette flow in nano scale channel.The study founds that the greater liquid density,the greater the equivalent viscosity values;When the channel height is less than 20 nm,the smaller the height,the greater the equivalent viscosity values.