A fluid is defined as a substance that continually deforms (flows) under an applied
shear stress. All liquids and all gases are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.The term "fluid" is often used as being synonymous with "liquid". This can be erroneous and sometimes clearly inappropriate—such as when referring to a liquid which does not or should not involve the gaseous state. For example, "brake fluid" is hydraulic oil which will not perform its required function if gas is present. The medical profession relies on the term "fluids" in dietary references ("take plenty of fluids") where the presence of gases is irrelevant or even possibly dangerous.
Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between solids and
fluidis not entirely obvious. The distinction is made by evaluating the viscosityof the substance. Silly Puttycan be considered to behave like a solid or a fluid, depending on the time period over which it is observed. However Silly Puttyis correctly termed a viscoelasticfluid.
Fluids display such properties as:
* not resisting deformation, or resisting it only lightly (
* 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 stressin 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 viscosityand so are capable of being subjected to low levels of shear stress.
In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of rate of strain. A consequence of this behavior is
Pascal's lawwhich describes the role of pressurein 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:
Newtonian fluids : where stress is directly proportional to rate of strain, and
Non-Newtonian fluids : where stress is 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 equationswhich are based on:
* continuity (conservation of mass),
* conservation of
* conservation of
conservation of energy.
The study of fluids is
fluid mechanics, which is subdivided into fluid dynamicsand fluid staticsdepending on whether the fluid is in motion.
* [http://hyperphysics.phy-astr.gsu.edu/hbase/fluid.html Fluid mechanics Concept Map from Georgia State University]
Wikimedia Foundation. 2010.