On the validity of Stokesʹ law at the molecular level

In order to investigate the dependence of the viscosity on the mass of the molecules in a liquid, and thus check the validity of Stokesʹ law for molecules, several molecular dynamics simulations of `waterʹ molecules with different mass and different molecular mass distributions were performed. The viscosity is shown to be sensitive to the mass but less sensitive to the mass distribution. The product of diffusion coefficient and viscosity, which according to Stokesʹ law should be independent of the mass, varies. We may therefore conclude that Stokesʹ law is not valid for small molecules.