Axial driving characteristics of water in rotating black phosphorus nanotubes

Abstract

Since the advent of two-dimensional materials, the micro/nano technology has been greatly developed, and the design of micro/nano fluid devices has become an important research area. As a new two-dimensional material, the black phosphorus (BP) has attracted wide attention because of its excellent properties such as anisotropy, and it has been applied to many areas. In this paper, the axial motion properties of water molecules in the rotating black phosphorus nanotube (BPNT) are studied by the molecular dynamics method. The results show that water molecules in the rotating chiral BPNT can move along the axis, and the moving direction of water molecules is determined by the rotating direction of the nanotube. The velocity of water molecules and the resultant force of water molecules received from the nanotube in the axial direction increase with the angular velocity increasing. The friction coefficient and slip characteristics of the water-BP interface are calculated by using the Couette flow model, and it is clarified that the natural anisotropic microstructure on the surface of BP is the essential reason for the axial motion of water molecules in the rotating BPNT. Besides, we construct a model of filling water molecules between two BPNTs. It is found that the axial movement of water molecules between two nanotubes will be enhanced when the internal and external tube rotate simultaneously. The radius of the nanotubes will also affect the directional motion of the water molecules. Specifically, at the same angular velocity of BPNTs, with the increase of the radius, the axial motion velocity of water molecules in the BPNT will decrease, while the force received from the BPNT will increase. The axial motion of water molecules in the double-walled BPNT is little different from that in the single-walled BPNT, which proves that the number of layers has no significant influence on the driving effect of water molecules. The influence of temperature on the motion properties of water molecules depends on the coupling effect of pressure and temperature in the tube on the convection-solid interface friction coefficient. When the temperature is lower than the normal temperature, the axial velocity of water molecules and the force exerted by the BPNT will increase with the increase of temperature, and when the temperature reaches the normal temperature, it will become stable. The results will provide a theoretical basis for the study of the flow characteristics of the fluid in BPNTs and the application of the fluid drive devices based on BPNTs.