Editing Fluid (section)

==Physics== 
Fluids display properties such as:
* lack of resistance to permanent deformation, resisting only [[viscosity|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 [[Mechanical equilibrium|equilibrium]]. By contrast, solids respond to shear either with [[Elasticity (physics)|a spring-like restoring force]]—meaning that deformations are reversible—or they require a certain initial [[stress (mechanics)|stress]] before they deform (see [[Plasticity (physics)|plasticity]]).

Solids respond with restoring forces to both shear stresses and to [[normal stress]]es, both [[Compressive stress|compressive]] and [[Tensile stress|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 [[Pressure#Negative pressure|negative pressure]]. Solids and liquids both have tensile strengths, which when exceeded in solids creates [[Plasticity (physics)|irreversible deformation]] and fracture, and in liquids cause the onset of [[cavitation]]. <!--[[Gas]]es do not have tensile strength, and freely expand in response to changes in pressure. << If liquids have a tensile strength then gases should certainly also have a tensile strength. -->

Both solids and liquids have free surfaces, which cost some amount of [[Thermodynamic free energy|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 [[Wulff construction|minimise their surface energy]]: liquids tend to form rounded [[droplets]], whereas pure solids tend to form [[crystals]]. [[Gas]]es, lacking free surfaces, freely [[Diffusion#Elementary theory of diffusion coefficient in gases|diffuse]].