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. Fluids are substances that have zero shear modulus,
or, in simpler terms, a fluid is a substance which cannot resist any
shear force applied to it.
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 incompressible
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.
2 See also 3 References
Physics Fluids display properties such as:
not resisting 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. In contrast, solids
respond to shear with a spring-like restoring force, which means that
small deformations, whether shear or normal, are reversible.
Solids respond with restoring forces to both shear stresses, and to
normal stresses—both compressive and 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 negative pressure. Both solids and liquids also have
tensile strengths, which when exceeded in solids makes irreversible
deformation and fracture, and in liquids causes the onset of
cavitation. Gases do not have tensile strength, and freely expand in
response to changes in pressure.
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. The ability
of liquids to flow results in very different behaviour in response to
surface tension than in solids, although in equilibrium both will try
to 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 diffuse.
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
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
Bird, Byron; Stewart, Warren; Lightfoot, Edward (2007). Transport Phenomena. New York: Wiley, Second Edition. p. 912. ISBN 0-471-41077-2.
GND: 4017690-3 N