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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 phase 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

Main article: Fluid mechanics

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:

• 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:

The study of fluids is fluid mechanics, which is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion.

## References

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

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Fluid — Please support Wikipedia.
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 ThinkProgress At Least 10 Percent Of Fracking Fluid Is Toxic ThinkProgress Thu, 21 Aug 2014 09:16:29 -0700 At least 10 percent of the contents of fracking fluid injected into the earth is toxic. For another third we have no idea. And that's only from the list of chemicals the fracking industry provided voluntarily. That's according to an analysis by William ... Philly.com Oronde Gibson, 43, doorman and manager at Club Fluid Philly.com Fri, 22 Aug 2014 00:06:01 -0700 Oronde was the manager and guardian of the door at the former Club Fluid on South 4th Street, a nightclub popular with fans of hip-hop, soul and rap, delivered by exuberant DJs, who made the records shriek like bouncing banshees. The crowd loved it. 9NEWS.com CSU research could help track fracking fluid 9NEWS.com Fri, 22 Aug 2014 09:41:15 -0700 CSU research could help track fracking fluid. Research now underway at Colorado State University could eventually help track where chemicals used in hydraulic fracturing wind up once they're injected into the ground. Loading… Post to Facebook. Dakota Fluid Power purchases Rapids Hydraulic and Machine Herald Review Fri, 22 Aug 2014 06:41:15 -0700 With this acquisition, Rapids Hydraulic and Machine will now operate under the name of Dakota Fluid Power but will continue its long tradition of providing outstanding customer service and high-quality products and services to meet the industry and ... Brake Fluid Safety Test Strip Helps Safeguard Stopping Power Midland Daily News Fri, 22 Aug 2014 03:52:30 -0700 Brake fluid is what transfers pressure from your brake pedal to your brakes and safety depends on the proper and timely replacement of brake fluid. Contaminants in your brake fluid compromise the effectiveness of your brake system and decrease stopping ... The Progressive Pulse With second hearing on drilling rules set for Friday, new worries about ... The Progressive Pulse Fri, 22 Aug 2014 06:48:15 -0700 Hydraulic fracturing, or fracking, is the practice of injecting fluid at high pressure into the earth, which breaks up oil- and gas-filled rock formations that is then extracted to the surface. The contents and makeup of that fluid have been a subject ... Victory Blue and CSX Transflo begin Diesel Exhaust Fluid (DEF) terminal ... DiscoverDEF.com Fri, 22 Aug 2014 05:45:35 -0700 Victory Blue has announced a collaboration with bulk commodity transloading company CSX Transflo. Victory Blue is already active in producing and distributing Diesel Exhaust Fluid (DEF) and the new partnership will allow it to extend its coverage to ... What Is Fluid Dynamics? LiveScience.com Tue, 19 Aug 2014 18:30:00 -0700 Fluid dynamics is "the branch of applied science that is concerned with the movement of liquids and gases," according to the American Heritage Dictionary. Fluid dynamics is one of two branches of fluid mechanics, which is the study of fluids and how ...
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