Viscosity

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The viscosity (m) of a fluid measures its resistance to flow under an applied shear stress. Representative units for viscosity are kg/(m.sec), g/(cm.sec) (also known as poise designated by P). The centipoise (cP), one hundredth of a poise, is also a convenient unit, since the viscosity of water at room temperature is approximately 1 centipoise.

The kinematic viscosity (n) is the ratio of the viscosity to the density:

n = m/r,

and will be found to be important in cases in which significant viscous and gravitational forces exist.

Viscosity of liquids:

Viscosity of liquids in general, decreases with increasing temperature.

The viscosities (m) of liquids generally vary approximately with absolute temperature T according to:

ln m = a - b ln T

The viscosity (m) of many gases is approximated by the formula:

m = m_o(T/T_o)ⁿ

in which T is the absolute temperature, m_o is the viscosity at an absolute reference temperature T_o, and n is an empirical exponent that best fits the experimental data.

Viscosity of liquids are generally two orders of magnitude greater than gases at atmospheric pressure. Fow example, at 25^oC, m_water = 1 centipoise and m_air = 1 x 10^-2centipoise.