Fluid
A substance that flows and deforms under shear stress.
In physics, a fluid is a liquid, gas, or other material that may continuously move and deform (flow) under an applied shear stress or external force. Fluids have zero shear modulus, meaning they cannot resist any shear force applied to them. The term generally includes both liquid and gas phases, though its definition varies among branches of science.
- field
- Physics, Fluid Mechanics
- known_for
- Continuous deformation under shear stress, inability to support shear stress in static equilibrium, ability to flow and take shape of container
Lore & Background
Fluids display properties such as lack of resistance to permanent deformation, resisting only relative rates of deformation in a dissipative, frictional manner, and the ability to flow. These properties are typically a function of their inability to support a shear stress in static equilibrium. By contrast, solids respond to shear with a spring-like restoring force or require a certain initial stress before deforming. Ideal fluids only respond with restoring forces to normal stresses, called pressure, and can be subjected to both compressive and tensile stress. Both solids and liquids have free surfaces, which cost free energy to form; for liquids this is called surface tension, leading to rounded droplets, while solids tend to form crystals. Gases lack free surfaces and freely diffuse.
Reader's Guide
The study of fluids is fluid mechanics, subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion. Fluids can be classified as Newtonian (stress directly proportional to rate of strain) or non-Newtonian (stress not proportional). They can also be classified by compressibility: compressible fluids change volume under pressure, while incompressible fluids do not. Newtonian and incompressible fluids do not actually exist but are assumed for theoretical settlement; virtual fluids ignoring viscosity and compressibility are called perfect fluids. The behavior of fluids is described by the Navier–Stokes equations, based on conservation of mass, linear momentum, angular momentum, and energy. Pascal's law describes the role of pressure in characterizing a fluid's state. Non-Newtonian fluids like Silly Putty appear solid under sudden force, and very high viscosity substances like pitch appear solid over time.
Did You Know?
- Fluids have zero shear modulus and cannot resist any shear force applied to them.
- Non-Newtonian fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied.
- In medicine, fluid refers to any liquid constituent of the body, whereas 'liquid' is not used in this sense.
- Ideal fluids only respond with restoring forces to normal stresses, called pressure.
Frequently Asked Questions
What is a fluid in physics?
A fluid is any substance—liquid, gas, or otherwise—that continuously deforms and flows when subjected to a shear stress or external force. Unlike a solid, it cannot maintain a fixed shape and instead takes on the form of its container.
Why can't a fluid support shear stress at rest?
Fluids have a zero shear modulus, which means they offer no elastic resistance to shear deformation. Even the tiniest sustained shear force will cause them to keep flowing rather than holding a static shape.
Do both liquids and gases count as fluids?
Yes. In fluid mechanics the term broadly encompasses both the liquid and gas phases, since each can continuously deform under applied shear. Some sub-disciplines refine the definition, but the core idea of flow under stress applies to both.
What is the key difference between a fluid and a solid under applied force?
A solid resists shear by developing an internal restoring stress, while a fluid simply continues to deform as long as the force is present. This is why a fluid flows and a solid merely strains.
What does 'continuous deformation under shear stress' actually mean in practice?
It means that, given any nonzero shear force, a fluid will keep changing shape over time without ever reaching a static equilibrium. The rate of deformation depends on the magnitude of the applied stress and the fluid's viscosity.
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