What does it mean by momentum transfer is not a vector (3 components) but rather a tensor (9 components)?
Newton says that viscosity is due to molecular diffusion between layers in the fluid. A molecule leaves one layer and transfers it's momentum to the adjoining layer. That transfer creates an acceleration and that acceleration creates shear forces which is related to the viscosity. Stresses in the most basic forms, i.e. no simplifying assumptions, are tensors. There are nine components 3 normal stresses and 6 shear stresses (note that for equillibrium, the 6 shear stresses are 2 groups of 3 that are equal, [tex]\tau_{xy} = \tau_{yx}[/tex]). You might like to read up on tensors by taking a look at this thread that Astronuc created in the tutorial section: https://www.physicsforums.com/showthread.php?t=101414