The viscosity of liquids increases as temperature decreases, whereas the viscosity of gases increases as temperature increases. Viscosity is a fluid’s resistance to internal motion or flow and results from intermolecular friction.
More precisely, absolute or dynamic viscosity is the ratio of a fluid’s internal shear stress to its velocity gradient, and kinematic viscosity is the ratio of dynamic viscosity to density.
A fluid that has no resistance to internal shear stress is called an ideal fluid. Such behavior has only been observed in materials called superfluids at extremely low temperatures approaching absolute zero. At the other end of the spectrum, many solids exhibit flow over a long time scale with sufficient pressures. In fact, much of the Earth’s crust is subject to such solid flow, and this solid flow can be described as an internal viscosity.
Viscosity plays an important role in geology, biology, medicine, engineering and physics. The viscosity of liquids is encountered in everyday life, such as the viscosity of a cooking oil decreasing and thinning as the oil is heated in a pan. Motor oils are selected for automobiles based on their viscosity-to-temperature relationship. Blood pressure is affected by the viscosity of blood platelets, and there are many medications which “thin” the blood to reduce the risk of heart attack and stroke.