View source for Fluid

{{short description|Substance which continuously deforms under an applied shear stress, including liquids and gases}}
{{refimprove|date=July 2022}}
{{about|the concept in [[physics]]}}
{{distinguish| liquid}}
{{Continuum mechanics|cTopic=fluid}}

In [[physics]], a '''fluid''' is a [[liquid]], [[gas]], or other material that may continuously [[motion|move]] and [[Deformation (physics)|deform]] ('''''flow''''') under an applied [[shear stress]], or external force.<ref>{{cite web |title=Fluid {{!}} Definition, Models, Newtonian Fluids, Non-Newtonian Fluids, & Facts |url=https://www.britannica.com/science/fluid-physics |website=Encyclopedia Britannica |access-date=2 June 2021 |language=en}}</ref> They have zero [[shear modulus]], or, in simpler terms, are [[Matter|substances]] which cannot resist any [[shear force]] applied to them. 

<!-- The distinction between [[solid]]s and fluid is not entirely obvious. The distinction is made by evaluating the [[viscosity]] of the substance. << This does not seem to be right. Maybe someone can provide some source?-->Although the term ''fluid'' generally includes both the liquid and gas phases, its definition varies among [[branches of science]]. Definitions of ''[[solid]]'' vary as well, and depending on field, some substances can have both fluid and solid properties.<ref>{{Cite journal|title=What's That Stuff? Silly Putty|journal=Chemical & Engineering News|url=https://pubsapp.acs.org/cen/whatstuff/stuff/7848scit3.html|last=Thayer|first=Ann|volume=78|publisher=American Chemical Society|issue=48|year=2000|publication-date=2000-11-27|page=27|doi=10.1021/cen-v078n048.p027|archive-url= https://web.archive.org/web/20210507045350/http://pubsapp.acs.org/cen/whatstuff/stuff/7848scit3.html|archive-date= 2021-05-07|url-status=live}}</ref> Non-Newtonian fluids like [[Silly Putty]] appear to behave similar to a solid when a sudden force is applied.<ref>{{Cite web|url=https://www.science.org/content/article/silly-putty-potholes| title=Silly Putty for Potholes|date=2012-04-11|access-date=2021-06-23|website=Science|last=Kroen|first=Gretchen Cuda}}</ref> Substances with a very high [[viscosity]] such as [[Pitch (resin)|pitch]] appear to behave like a solid (see [[pitch drop experiment]]) as well. In [[particle physics]], the concept is extended to include fluidic [[matter]]s other than liquids or gases.<ref>Example (in the title): {{Cite journal|title=Measuring Hall viscosity of graphene's electron fluid|journal=Science|url=https://www.science.org/doi/10.1126/science.aau0685|last1=Berdyugin|first1=A. I.|date=2019-04-12|volume=364|pages=162–165|last2=Xu|first2=S. G.|issue=6436|doi=10.1126/science.aau0685| others=F. M. D. Pellegrino, R. Krishna Kumar, A. Principi, I. Torre, M. Ben Shalom, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, A. K. Geim, D. A. Bandurin|pmid=30819929|language=en|arxiv=1806.01606|bibcode=2019Sci...364..162B|s2cid=73477792}}</ref> A fluid in medicine or biology refers to any liquid constituent of the body ([[body fluid]]),<ref>{{Cite encyclopedia |title=Fluid (B.1.b.) |date=1933 |encyclopedia=Oxford English Dictionary |publisher=Oxford University Press |location=Oxford |url=https://archive.org/details/in.ernet.dli.2015.271841/page/n361/mode/1up |access-date=2021-06-22 |orig-year=1901 |edition=1978 reprint |volume=IV F–G |page=358 |language=en}}</ref><ref name=Tabers_BodyFluid>{{Cite web|url= https://www.tabers.com/tabersonline/view/Tabers-Dictionary/748149/all/fluid?q=body+fluid#4|title=body fluid|access-date=2021-06-22|website=Taber's online – Taber's medical dictionary|archive-url= https://web.archive.org/web/20210621125044/https://www.tabers.com/tabersonline/view/Tabers-Dictionary/748149/all/fluid?q=body+fluid|archive-date=2021-06-21|url-status=live}}</ref> whereas "liquid" is not used in this sense. Sometimes liquids given for [[fluid replacement]], either by drinking or by injection, are also called fluids<ref>Usage example: {{Cite journal|title="Drink plenty of fluids": a systematic review of evidence for this recommendation in acute respiratory infections|journal=BMJ|last1=Guppy|first1=Michelle P B|date=2004-02-28|volume=328|pages=499–500|last2=Mickan|first2=Sharon M|issue=7438|doi=10.1136/bmj.38028.627593.BE|language=en|pmc=351843|last3=Del Mar|first3=Chris B|pmid=14988184}}</ref> (e.g. "drink plenty of fluids"). In [[hydraulics]], [[Hydraulic fluid|fluid]] is a term which refers to liquids with certain properties, and is broader than (hydraulic) oils.<ref>{{Cite web|url=https://www.nfpa.com/home/About-NFPA/What-is-Fluid-Power.htm|title=What is Fluid Power?|access-date=2021-06-23|website=National Fluid Power Association|archive-url=https://web.archive.org/web/20210623024634/https://www.nfpa.com/home/About-NFPA/What-is-Fluid-Power.htm|archive-date=2021-06-23|url-status=live|language=en|quote=With hydraulics, the fluid is a liquid (usually oil)}}</ref>

==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]].

==Modelling==
{{main|Fluid mechanics}}
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. 

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.

===Classification of fluids===
Depending on the relationship between shear stress and the rate of strain and its [[derivative]]s, fluids can be characterized as one of the following:
*[[Newtonian fluid]]s: where stress is directly proportional to rate of strain
*[[Non-Newtonian fluid]]s: where stress is not proportional to rate of strain, its higher powers and derivatives.

Newtonian fluids follow [[Newton's law of viscosity]] and may be called [[viscous fluid]]s.

Fluids may be classified by their compressibility:
*Compressible fluid: A fluid that causes volume reduction or density change when pressure is applied to the fluid or when the fluid becomes supersonic.
*Incompressible fluid: A fluid that does not vary in volume with changes in pressure or flow velocity (i.e., ρ=constant) such as water or oil.

Newtonian and incompressible fluids do not actually exist, but are assumed to be for theoretical settlement. Virtual fluids that completely ignore the effects of viscosity and compressibility are called [[perfect fluid]]s.

==See also==
*[[Matter]]
*[[Liquid]]
*[[Gas]]
*[[Supercritical fluid]]

==References==
{{Reflist}}
*{{Cite book |title=Transport Phenomena |last=Bird |first=Robert Byron |author2=Stewart, Warren E. |author3=Lightfoot, Edward N.|year=2007 |publisher=Wiley, Revised Second Edition |location=New York |isbn=978-0-471-41077-5 |pages=912 }}

{{Authority control}}
[[Category:Fluid dynamics]]