Although the termfluid generally includes both the liquid and gas phases, its definition varies amongbranches of science. Definitions ofsolid vary as well, and depending on field, some substances can have both fluid and solid properties.[2] Non-Newtonian fluids likeSilly Putty appear to behave similar to a solid when a sudden force is applied.[3] Substances with a very highviscosity such aspitch appear to behave like a solid (seepitch drop experiment) as well. Inparticle physics, the concept is extended to include fluidicmatters other 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,fluid is a term which refers to liquids with certain properties, and is broader than (hydraulic) oils.[8]
lack of resistance to permanent deformation, resisting onlyrelative 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 ashear stress in staticequilibrium. By contrast, solids respond to shear either witha spring-like restoring force—meaning that deformations are reversible—or they require a certain initialstress before they deform (seeplasticity).
Solids respond with restoring forces to both shear stresses and tonormal stresses, bothcompressive andtensile. 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 deformation and fracture, and in liquids cause the onset ofcavitation.
Both solids and liquids have free surfaces, which cost some amount offree energy to 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.
In a solid, shear stress is a function ofstrain, but in a fluid,shear stress is a function ofstrain rate. A consequence of this behavior isPascal's law which describes the role ofpressure in characterizing a fluid's state.
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.
^"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.Archived from the original on 2021-06-21. Retrieved2021-06-22.
^"What is Fluid Power?".National Fluid Power Association.Archived from 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.
