Flow behavior of water in nano confinement is essential to various application fields, including water purification, desalination, energy conversion, DNA sequencing,etc. It has been recognized that traditional hydrodynamics theory like Navier-Stokes (N-S) is no longer applicable in systems with lower dimension. To assess the limits of N-S equation, the molecular dynamics simulation is used to study water flow behavior in nano-slit. The nano-slit is formed by two parallel graphene sheets separated by a certain distance. Flow rate profiles of water in nano-slit with different distance between two graphene sheets show that when the distance between two graphene sheets is less than 3 nm, N-S equation cannot describe the flow behavior correctly. This means, N-S equation is applicable for channels with size larger than 3 nm, which is about ten times the diameter of water molecule. For pores in which N-S equation is applicable, effective viscosity and slip length were obtained by fitting the flow rate profiles with N-S equation. The influences of pore size, driving force, and wall hydrophobicity on the flow behavior were also investigated with emphasis on the effective viscosity and slip length. With the increases of slit pore size or driving force, water average velocity increases, accompanied by an increase in effective viscosity and a decrease in slip length. The increase of the wall hydrophilicity of the nano-slit results in a decrease of slip length while imposes no obvious effect on the effective viscosity.%采用非平衡分子动力学模拟(non-equilibrium molecular dynamics simulation)方法研究了不同间距纳米狭缝之间水的流动行为.研究了纳米狭缝间距、壁面性质和外部压力对水流动速度径向分布、有效黏度、壁面速度和滑移长度的影响,讨论了Navier-Stoke(N-S)方程的适用性.研究结果表明,N-S方程仅适用于3 nm以上的孔道;狭缝尺寸的增加和施加压力的增加均会使得管内流速增加,而造成表观黏度降低以及滑移长度增加.壁面亲水性的增加仅使得滑移长度降低,表观黏度并没有发生较大变化.
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