Viscosity coefficients can be defined in two ways:

* Dynamic viscosity, also absolute viscosity, the more usual one (typical units Pa·s, Poise, P);

* Kinematic viscosity is the dynamic viscosity divided by the density (typical units cm2/s, Stokes, St).

Viscosity is a tensorial quantity that can be decomposed in different ways into two independent components. The most usual decomposition yields the following viscosity coefficients:

* Shear viscosity, the most important one, often referred to as simply viscosity, describing the reaction to applied shear stress; simply put, it is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this is what is referred to as a velocity gradient).

* Volume viscosity (also called bulk viscosity or second viscosity) becomes important only for such effects where fluid compressibility is essential. Examples would include shock waves and sound propagation. It appears in the Stokes’ law (sound attenuation) that describes propagation of sound in Newtonian liquid.

Alternatively,

* Extensional viscosity, a linear combination of shear and bulk viscosity, describes the reaction to elongation, widely used for characterizing polymers. For example, at room temperature, water has a dynamic shear viscosity of about 1.0×10−3 Pa·s and motor oil of about 250×10−3 Pa·s.

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