Viscosity

The property of fluids and gases to resist flow, caused by intermolecular forces. In a "streamline flow" the fluid can be thought of as containing parallel layers which move at different rates, a simple shearing motion. The resistance caused by the viscous forces then gives a tangential force opposing differences in speed between the layers. Newton's law of viscosity states that this force per unit area is proportional to the velocity gradient. Fluids that show this behaviour are called Newtonian fluids.

References

[a1]	G.K. Batchelor, "An introduction to fluid dynamics" , Cambridge Univ. Press (1974)