Wall friction should be decoupled from fluid viscosity for the prediction of nanoscale flow

The accurate determination of fluid viscosity based on the microscopic information of molecules is very crucial for the prediction of nanoscale flow. Despite the challenge of this problem, researchers have done a lot of meaningful work and developed several distinctive methods. However, one of the common approaches to calculate the fluid viscosity is using the Green-Kubo formula by considering all the fluid molecules in nanospace, inevitably causing the involvement of the frictional interaction between fluid and the wall into the fluid viscosity. This practice is certainly not appropriate because viscosity is essentially related only to the interactions among fluid molecules. Here, we clarify that the wall friction should be decoupled from fluid viscosity by distinguishing the frictional region and the viscous region for the accurate prediction of nanoscale flow. By comparing the fluid viscosities calculated from the Green-Kubo formula in the whole region and viscous region and the viscosity obtained from the velocity profile through the Hagen-Poiseuille equation, it is found that only the calculated viscosity in the viscous region agrees well with the viscosity from the velocity profile. To demonstrate the applicability of this clarification, the Lennard-Jones fluid and water confined between Lennard-Jones, graphene, and silica walls, even with different fluid-wall interactions, are extensively tested. This work clearly defines the viscosity of fluids at nanoscales from the inherent nature of physics, aiming at the accurate prediction of nanoscale flow from the classical continuum hydrodynamic theory.