Fluids display properties such as: • lack of resistance to permanent deformation, resisting only
relative rates of deformation in a dissipative, frictional manner, and • the ability to flow (also described as the ability to take on the shape of the container). These properties are typically a function of their inability to support a
shear stress in static
equilibrium. By contrast, solids respond to shear either with
a spring-like restoring force—meaning that deformations are reversible—or they require a certain initial
stress before they deform (see
plasticity). Solids respond with restoring forces to both shear stresses and to
normal stresses, both
compressive and
tensile. By contrast, ideal fluids only respond with restoring forces to normal stresses, called
pressure: fluids can be subjected both to compressive stress—corresponding to positive pressure—and to tensile stress, corresponding to
negative pressure. Solids and liquids both have tensile strengths, which when exceeded in solids creates
irreversible deformation and fracture, and in liquids cause the onset of
cavitation. Both solids and liquids have free surfaces, which cost some amount of
free energy to form. In the case of solids, the amount of free energy to form a given unit of surface area is called
surface energy, whereas for liquids the same quantity is called
surface tension. In response to surface tension, the ability of liquids to flow results in behaviour differing from that of solids, though at equilibrium both tend to
minimise their surface energy: liquids tend to form rounded
droplets, whereas pure solids tend to form
crystals.
Gases, lacking free surfaces, freely
diffuse. ==Modelling==