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 It has been recognized that traditional hydrodynamics theory like Navier-Stokes (N-S) is no longer applicable in systems with lower dimension.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.