1. General

In contrast to solids, liquid or gaseous fluids are easily deformable, without the need to apply great stresses to their contours. The same material can change state from solid to fluid (liquid to gas) by increasing temperature at constant pressure. In the mechanics of deformable media, fluids are described by a behavioral equation that links local stress to the rate (or speed) of deformation, whereas solids are described by a stress-strain relationship.

The behavioral relationship is mechanical. The fluid is also described in terms of thermodynamics through the equation of state, which expresses the relationship between density, pressure and temperature in the case of thermodynamic equilibrium.