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In
physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Spacetime, spa ...
, a fluid is a substance that continually deforms (flows) under an applied
shear stress Shear stress, often denoted by ( Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel (geometry), parallel to the material cross section. Norma ...

shear stress
, or external force. Fluids are a phase of
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
and include
liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Phase (matter), phase of matter and incl ...
s, gases and plasmas. They are substances with zero shear modulus, or, in simpler terms, substances which cannot resist any shear force applied to them. Although the term "fluid" includes both the liquid and gas phases, in common usage, "fluid" is often used synonymously with "liquid". This usage of the term is also common in medicine and in nutrition ("take plenty of fluids"). Liquids form a free surface (that is, a surface not created by the container) while gases do not. Viscoelastic fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied. Also substances with a very high viscosity such as Pitch (resin), pitch appear to behave like a solid (see pitch drop experiment).


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 Shear stress, often denoted by ( Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel (geometry), parallel to the material cross section. Norma ...

shear stress
in static Mechanical equilibrium, equilibrium. In contrast, solids respond to shear either with Elasticity (physics), a spring-like restoring force, which means 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 stresses—both Compressive stress, compressive and Tensile stress, tensile. In contrast, ideal fluids only respond with restoring forces to normal stresses, called pressure: fluids can be subjected to both compressive stress, corresponding to positive pressure, and to tensile stress, corresponding to Pressure#Negative pressure, negative pressure. Both solids and liquids also have tensile strengths, which when exceeded in solids makes Plasticity (physics), irreversible deformation and fracture, and in liquids causes the onset of cavitation. 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. The ability of liquids to flow results in different behaviour in response to surface tension than in solids, although in equilibrium both will try to Wulff construction, minimise their surface energy: liquids tend to form rounded droplets, whereas pure solids tend to form crystals. Gases do not have free surfaces, and freely Diffusion#Elementary theory of diffusion coefficient in gases, diffuse.


Modelling

In a solid, shear stress is a function of Strain (materials science), strain, but in a fluid, stress (physics), shear stress is a function of strain rate. A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state. Depending on the relationship between shear stress, and the rate of strain and its derivatives, fluids can be characterized as one of the following: *Newtonian fluids: where stress is directly proportional to rate of strain *Non-Newtonian fluids: where stress is not proportional to rate of strain, its higher powers and derivatives. The behavior of fluids can be described by the Navier–Stokes equations—a set of partial differential equations which are based on: * continuity (continuity equation#Fluid dynamics, conservation of mass), * conservation of linear momentum, * conservation of angular momentum, * conservation of energy. The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.


See also

*Matter *Liquid *Gas *Supercritical fluid


References

* {{Authority control Fluid dynamics