Anisotropic Flow Behaviors And Mechanism Of Fluids On The Surface Of Black Phosphorus

With the rapid development of low-dimensional materials,the opportunity to promote the development of micro/nano fluid devices,a new low-dimensional material black phosphorus(BP)has attracted wide attention due to its excellent properties,and has been applied in many areas.In this paper,the flow characteristics of the fluid in the nanochannels of black phosphorus were studied by using the molecular dynamics method,the mechanism of black phosphorus and solid-liquid interface anisotropy were explored.The results will provide a theoretical basis for the study of the flow characteristics of the fluid in the black phosphorus nanochannels and the design of micro/nano fluid devices based on black phosphorus materials.The anisotropic friction and slip characteristics of black phosphorus-water interface were studied based on Couette shear flow model,the mechanism of the anisotropic flow at the solid-liquid interface is clarified,and the influence of shear velocity,anisotropy,nanochannels width and the number of black phosphorous layers on the flow characteristics of the water molecule Couette flow in the black phosphorous nanochannels is revealed.The results show that the boundary slip velocity and shear stress increase linearly with the increase of shear strain rate,and the viscosity coefficient of water molecule is basically a certain value,which is not affected by the shear strain rate.The anisotropy caused by the natural fold structure on the surface of black phosphorus can affect the flow characteristics of water molecules at the black phosphorus-water interaction interface.With increasing the intersection angle between water flow direction and black phosphorus ripple direction,the boundary slip decreases and the interfacial friction increases continuously,the viscosity coefficient of water molecules is not affected by anisotropy.Through the calculation of the number density distribution,micro configuration diagram of border water molecules and the potential energy cloud reveals the black phosphorus-water interaction interface and influence mechanism;Finally,the influence of the width of the nanochannels and the number of black phosphorus layers was investigated.In the case of the same shear strain rate,it was found that the slip velocity of water molecules in the nanochannels decreased with the increase of the width of the nanochannels.The velocity distribution of water molecules and the number density distribution along the channel width in the bilayer model are basically consistent with that in the monolayer model,except that the increase of the interface interaction energy between the bilayer of black phosphorus and water molecules leads to slight decrease of the boundary slip velocity and increase of the interface friction coefficient,and does not affect the anisotropy of black phosphorus.The influence of anisotropy on the slip and viscosity characteristics of water molecules on black phosphorus surface were studied based on the Poiseuille pressuredriven flow model,the effect of anisotropy on the solid-liquid interface were investigated,and the influence of the driving force,anisotropy,width of the nanochannels and the number of black phosphorus layers on the flow characteristics of the water molecule in the nanochannels were revealed based on Poiseuille model.The results show that the boundary slip velocity increases with the increase of the driving force.The anisotropy will also affect the flow characteristics of water molecules in the nanochannel under the pressure driven Poiseuille flow.Specifically,the boundary slip velocity decreases with the increase of the chirality angle,and the viscosity coefficient of water molecules is still not affected by the anisotropy.Finally,under the condition that the acceleration value remains unchanged,the influences of the width of nanochannels and the number of black phosphorus layers on the boundary slip velocity and viscosity coefficient of water molecules were studied,found that the slip velocity of water molecules in the nanochannels decreased with the increase of the width of the nanochannels;the velocity distribution of water molecules and the number density distribution along the channel width in the bilayer model are still very little different from that in the monolayer model,and the interface interaction energy between the bilayer of black phosphorus and water molecules also shows an increasing trend compared with that in the monolayer black phosphorus,the law of anisotropy remains unchanged.