Fluid
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Inphysics,afluidis aliquid,gas,or other material that may continuouslymoveanddeform(flow) under an appliedshear stress,or external force.[1]They have zeroshear modulus,or, in simpler terms, aresubstanceswhich cannot resist anyshear forceapplied to them.
Although the termfluidgenerally includes both the liquid and gas phases, its definition varies amongbranches of science.Definitions ofsolidvary as well, and depending on field, some substances can have both fluid and solid properties.[2]Non-Newtonian fluids likeSilly Puttyappear to behave similar to a solid when a sudden force is applied.[3]Substances with a very highviscositysuch aspitchappear to behave like a solid (seepitch drop experiment) as well. Inparticle physics,the concept is extended to include fluidicmattersother than liquids or gases.[4]A fluid in medicine or biology refers to any liquid constituent of the body (body fluid),[5][6]whereas "liquid" is not used in this sense. Sometimes liquids given forfluid replacement,either by drinking or by injection, are also called fluids[7](e.g. "drink plenty of fluids" ). Inhydraulics,fluidis a term which refers to liquids with certain properties, and is broader than (hydraulic) oils.[8]
Physics[edit]
Fluids display properties such as:
- lack of resistance to permanent deformation, resisting onlyrelative rates of deformationin 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 ashear stressin staticequilibrium.By contrast, solids respond to shear either witha spring-like restoring force—meaning that deformations are reversible—or they require a certain initialstressbefore they deform (seeplasticity).
Solids respond with restoring forces to both shear stresses and tonormal stresses,bothcompressiveandtensile.By contrast, ideal fluids only respond with restoring forces to normal stresses, calledpressure:fluids can be subjected both to compressive stress—corresponding to positive pressure—and to tensile stress, corresponding tonegative pressure.Solids and liquids both have tensile strengths, which when exceeded in solids createsirreversible deformationand fracture, and in liquids cause the onset ofcavitation.
Both solids and liquids have free surfaces, which cost some amount offree energyto form. In the case of solids, the amount of free energy to form a given unit of surface area is calledsurface energy,whereas for liquids the same quantity is calledsurface 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 tominimise their surface energy:liquids tend to form roundeddroplets,whereas pure solids tend to formcrystals.Gases,lacking free surfaces, freelydiffuse.
Modelling[edit]
In a solid, shear stress is a function ofstrain,but in a fluid,shear stressis a function ofstrain rate.A consequence of this behavior isPascal's lawwhich describes the role ofpressurein characterizing a fluid's state.
The behavior of fluids can be described by theNavier–Stokes equations—a set ofpartial differential equationswhich are based on:
- continuity (conservation of mass),
- conservation oflinear momentum,
- conservation ofangular momentum,
- conservation of energy.
The study of fluids isfluid mechanics,which is subdivided intofluid dynamicsandfluid staticsdepending on whether the fluid is in motion.
Classification of fluids[edit]
Depending on the relationship between shear stress and the rate of strain and itsderivatives,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.
Newtonian fluids followNewton's law of viscosityand may be calledviscous fluids.
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 calledperfect fluids.
See also[edit]
References[edit]
- ^"Fluid | Definition, Models, Newtonian Fluids, Non-Newtonian Fluids, & Facts".Encyclopedia Britannica.Retrieved2 June2021.
- ^Thayer, Ann (2000)."What's That Stuff? Silly Putty".Chemical & Engineering News.78(48). American Chemical Society (published 2000-11-27): 27.doi:10.1021/cen-v078n048.p027.Archivedfrom the original on 2021-05-07.
- ^Kroen, Gretchen Cuda (2012-04-11)."Silly Putty for Potholes".Science.Retrieved2021-06-23.
- ^Example (in the title):Berdyugin, A. I.; Xu, S. G. (2019-04-12)."Measuring Hall viscosity of graphene's electron fluid".Science.364(6436). 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: 162–165.arXiv:1806.01606.Bibcode:2019Sci...364..162B.doi:10.1126/science.aau0685.PMID30819929.S2CID73477792.
- ^"Fluid (B.1.b.)".Oxford English Dictionary.Vol. IV F–G (1978 reprint ed.). Oxford: Oxford University Press. 1933 [1901]. p. 358.Retrieved2021-06-22.
- ^"body fluid".Taber's online – Taber's medical dictionary.Archivedfrom the original on 2021-06-21.Retrieved2021-06-22.
- ^Usage example:Guppy, Michelle P B; Mickan, Sharon M; Del Mar, Chris B (2004-02-28).""Drink plenty of fluids": a systematic review of evidence for this recommendation in acute respiratory infections ".BMJ.328(7438): 499–500.doi:10.1136/bmj.38028.627593.BE.PMC351843.PMID14988184.
- ^"What is Fluid Power?".National Fluid Power Association.Archivedfrom the original on 2021-06-23.Retrieved2021-06-23.
With hydraulics, the fluid is a liquid (usually oil)
- Bird, Robert Byron; Stewart, Warren E.; Lightfoot, Edward N. (2007).Transport Phenomena.New York: Wiley, Revised Second Edition. p. 912.ISBN978-0-471-41077-5.
