It seems natural to see the origin of viscosity in terms of the attractive and repulsive forces between molecules. However, gases have substantial viscosity even though their inter-molecular forces are weak suggesting some other mechanism.
Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow. The kinetic theory of gases allows accurate prediction of the behaviour of gaseous viscosity, in particular that, within the regime where the theory is applicable:
- Viscosity is independent of pressure; and
- Viscosty increases with temperature.
In liquids, the additional forces between molecules become important. This leads to an additional contribution to the shear stress though the exact mechanics of this are still controversial. Thus, in liquids:
- Viscosity is independent of pressure (except at very high pressure); and
- Viscosity tends to fall with temperature.