In
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...
, a fluid is a
liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, an ...
,
gas, or other material that continuously
deforms (''flows'') under an applied
shear stress, or external force. They have zero
shear modulus
In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain:
:G \ \stack ...
, or, in simpler terms, are
substances which cannot resist any
shear force applied to them.
Although the term ''fluid'' generally includes both the liquid and gas phases, its definition varies among
branches of science
The branches of science, also referred to as sciences, scientific fields or scientific disciplines, are commonly divided into three major groups:
* Formal sciences: the study of formal systems, such as those under the branches of logic and m ...
. Definitions of ''solid'' vary as well, and depending on field, some substances can be both fluid and solid.
Viscoelastic fluids like
Silly Putty appear to behave similar to a solid when a sudden force is applied. Substances with a very high
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
such as
pitch appear to behave like a solid (see
pitch drop experiment) as well. In
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and ...
, the concept is extended to include fluidic
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic par ...
s other than liquids or gases. A fluid in medicine or biology refers any liquid constituent of the body (
body fluid),
whereas "liquid" is not used in this sense. Sometimes liquids given for
fluid replacement, either by drinking or by injection, are also called fluids (e.g. "drink plenty of fluids"). In
hydraulics
Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid coun ...
,
fluid is a term which refers to liquids with certain properties, and is broader than (hydraulic) oils.
Physics
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 (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 stress in static
equilibrium. In contrast, solids respond to shear either with
a spring-like restoring force, which means that deformations are reversible, or they require a certain initial
stress before they deform (see
plasticity).
Solids respond with restoring forces to both shear stresses and to
normal stresses—both
compressive and
tensile. In contrast, ideal fluids only respond with restoring forces to normal stresses, called
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
: fluids can be subjected to both compressive stress, corresponding to positive pressure, and to tensile stress, corresponding to
negative pressure. Both solids and liquids also have tensile strengths, which when exceeded in solids makes
irreversible deformation and fracture, and in liquids causes the onset of
cavitation
Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
.
Both solids and liquids have free surfaces, which cost some amount of
free energy to form. In the case of solids, the amount of free energy to form a given unit of surface area is called
surface energy, whereas for liquids the same quantity is called
surface tension. The ability of liquids to flow results in different behaviour in response to surface tension than in solids, although in equilibrium both will try to
minimise their surface energy: liquids tend to form rounded
droplets, whereas pure solids tend to form
crystals.
Gases do not have free surfaces, and freely
diffuse.
Modelling
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
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
in characterizing a fluid's state.
The behavior of fluids can be described by the
Navier–Stokes equations
In physics, the Navier–Stokes equations ( ) are partial differential equations which describe the motion of viscous fluid substances, named after French engineer and physicist Claude-Louis Navier and Anglo-Irish physicist and mathematician G ...
—a set of
partial differential equations
In mathematics, a partial differential equation (PDE) is an equation which imposes relations between the various partial derivatives of a multivariable function.
The function is often thought of as an "unknown" to be solved for, similarly to ...
which are based on:
* continuity (
conservation of mass),
* conservation of
linear momentum,
* conservation of
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed sy ...
,
*
conservation of energy.
The study of fluids is
fluid mechanics
Fluid mechanics is the branch of physics concerned with the mechanics of fluids (liquids, gases, and plasmas) and the forces on them.
It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and ...
, which is subdivided into
fluid dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including '' aerodynamics'' (the study of air and other gases in motion) ...
and
fluid statics depending on whether the fluid is in motion.
Classification of fluids
Depending on the relationship between shear stress and the rate of strain and its
derivative
In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
s, 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 follow Newton's law of viscosity and may be called
viscous 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 called
perfect fluids.
See also
*
Matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic par ...
*
Liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, an ...
*
Gas
*
Supercritical fluid
References
*
{{Authority control
Fluid dynamics