Fluid

Continuum mechanics
Laws Conservation of mass Conservation of momentum Conservation of energy Entropy inequality
Solid mechanics Solids Stress Deformation Compatibility Finite strain Infinitesimal strain Elasticity linear Plasticity Bending Hooke's law Material failure theory Fracture mechanics Contact mechanics Frictionless Frictional
Fluid mechanics Fluids Fluid statics Fluid dynamics Navier–Stokes equations Bernoulli's principle Poiseuille equation Buoyancy Viscosity Newtonian Non-Newtonian Archimedes' principle Pascal's law Pressure Liquids Surface tension Capillary action Gases Atmosphere Boyle's law Charles's law Gay-Lussac's law Combined gas law Plasma
Rheology Viscoelasticity Smart fluids Magnetorheological Electrorheological Ferrofluids Rheometry Rheometer
Scientists Bernoulli Boyle Cauchy Charles Euler Gay-Lussac Hooke Pascal Newton Navier Stokes
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In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.

Although the term "fluid" includes both the liquid and gas phases, in common usage, "fluid" is often used as a synonym for "liquid", with no implication that gas could also be present. For example, "brake fluid" is hydraulic oil and will not perform its required function if there is gas in it. This colloquial 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. The distinction between solids and fluid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered to behave like a solid or a fluid, depending on the time period over which it is observed. It is best described as a viscoelastic fluid. There are many examples of substances proving difficult to classify. A particularly interesting one is pitch, as demonstrated in the pitch drop experiment currently running at the University of Queensland.

Physics

Fluids display properties such as:

• not resisting deformation, or resisting it only lightly (viscosity), and
• the ability to flow (also described as the ability to take on the shape of the container).This also means that all fluids have the property of fluidity.

These properties are typically a function of their inability to support a shear stress in static equilibrium.

Solids can be subjected to shear stresses, and to normal stresses—both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

Modelling

In a solid, shear stress is a function of strain, but in a fluid, 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 (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

References

• Bird, Byron; Stewart, Warren & Lightfoot, Edward (2007). Transport Phenomena. New York: Wiley, Second Edition. p. 912. ISBN 0-471-41077-2.  Cite uses deprecated parameters (help)