Viscosity arises because of frictional forces within a liquid as it moves or changes shape. When a body moves through a fluid – a liquid or a gas – it experiences a force. This force arises from the fact that the moving body must move the particles of the fluid out of the way in order to move – it must give them momentum and overcome the frictional forces which arise because of the viscosity of the liquid. According to Newton's Third Law of Motion – To every action there is an equal and opposite reaction – the force the body exerts on the fluid is equal to the force the fluid exerts on the body.
The molecules or atoms of a fluid exert forces on one another. In general, the molecules or atoms of a liquid exert greater forces on one another than the molecules or atoms of a gas, and the molecules or atoms of a liquid are closer together. This means that the frictional forces in a liquid are generally greater than the frictional forces in a gas, and the coefficient of viscosity is greater for a liquid than a gas.
In general the coefficient of viscosity decreases with increasing temperature, as the liquid expands and then becomes a gas. Some coefficients of viscosity are shown below.
Viscosity (Pa s)
Hydrogen at 0 degrees Celsius
Blood at 37 degrees Celsius