Levitation is the process by which an object is suspended by a
physical forceIn physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
against gravity, in a stable position without solid physical contact. A number of different techniques have been developed to levitate matter, including the
aerodynamicAerodynamic levitation is the use of gas pressure to levitate materials so that they are no longer in physical contact with any container. In scientific experiments this removes contamination and nucleation issues associated with physical contact with a container.-Overview:The term aerodynamic...
,
magneticMagnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields...
,
acousticAcoustic levitation is a method for suspending matter in a medium by using acoustic radiation pressure from intense sound waves in the medium. Acoustic levitation is possible because of the non-linear effects of intense sound waves....
, electromagnetic,
electrostaticElectrostatic levitation is the process of using an electric field to levitate a charged object and counteract the effects of gravity. It was used, for instance, in Robert Millikan's oil drop experiment and is used to suspend the gyroscopes in Gravity Probe B during launch.Due to Earnshaw's...
, gas film, and
opticalOptical levitation is a method developed by Arthur Ashkin whereby a material is levitated against the downward force of gravity by an upward force stemming from photon momentum transfer...
levitation methods.
Physics
For levitation on
EarthEarth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, first, a force is required directed vertically upwards and equal to the gravitational force, second, for any small displacement of the levitating object, a returning force should appear to stabilize it. The stable levitation can be naturally achieved by, for example,
magneticMagnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
or aerodynamic forces.
Levitation techniques are useful tools in physics research. For example, levitation methods are useful for high-temperature melt property studies because they eliminate the problem of reaction with containers and allow deep undercooling of melts. The containerless conditions may be obtained by opposing gravity with a levitation force, or by allowing an entire experiment to freefall.
Magnetic levitation
Although any electromagnetic force could be used to counteract gravity, magnetic levitation is the most common. Diamagnetic materials are commonly used for demonstration purposes. In this case the returning force appears from the interaction with the screening currents. For example, a
superconductingSuperconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
sample, which can be considered either as a perfect diamagnet or an
ideally hard superconductorAn ideally hard superconductor is a type II superconductor material with an infinite pinning force . In the external magnetic field it behaves like an ideal diamagnet if the field is switched on when the material is in the superconducting state, so-called "zero field cooled" regime...
, easily levitates in an ambient external magnetic field. In very strong magnetic field, by means of diamagnetic levitation even small live animals have been levitated.
It is possible to levitate pyrolytic graphite by placing thin squares of it above four cube magnets with the north poles forming one diagonal and south poles forming the other diagonal.
Magnetic levitation is in development for use for transportation systems. For example the Maglev transportation includes trains that are levitated by a very large number of magnets and, due to the lack of friction on guide rails, they are potentially faster, quieter and smoother than wheeled mass transit systems.
Electromagnetic
Electromagnetic levitation (EML), patented by Muck in 1923 , enables the levitation of an object using
electromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
and is a commonly used containerless technique in experimental condensed matter physics. A typical EML coil has reversed winding of upper and lower sections energized by an
Radio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) power supply. The RF field induces eddy currents in the sample which interact with the inhomogeneous magnetic field of the levitation coil, inducing a force which by
Lenz's lawLenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...
acts on the sample in the direction of low magnetic field strength. This can be adjusted in order to compensate against the force of gravity and hence levitate the sample. The eddy currents can lead to inductive heating of the sample which can allow melting of the sample and hence study of the liquid state.
Other levitation methods
A number of different techniques, that don't involve magnetism, have been developed and are commonly used to produce the stable levitation of matter. These methods are discussed below.
Electrostatic
In electrostatic levitation an
electric fieldIn physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
is used to counteract gravitational force.
Aerodynamic
In aerodynamic levitation, the levitation is achieved by floating the object on a stream of gas, either produced by the object or acting on the object. For example, a ping pong ball can be levitated with the stream of air from a vacuum cleaner set on 'blow'.
With enough thrust very large objects can be levitated using this method. Helicopters and hovercraft, for example, use powerful downthrusts to achieve stable levitation.
Acoustic
Acoustic levitation uses sound waves to provide a levitating force.
Gas film
This technique enables the levitation of an object against gravitational force by floating on a thin
gasGas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
film formed by gas flow through a porous membrane. A common example of this is
air hockeyAir hockey is a game for two competing players trying to score points in the opposing player's goal.-Equipment:Air hockey requires an air-hockey table, two player-held mallets, and a puck....
, where the puck is lifted by a thin layer of air.
Optical
Optical levitation is a technique in which a material is levitated against the downward force of gravity by an upward force stemming from
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
momentumIn classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
transfer.
Casimir Force
Scientists have discovered a way of levitating ultra small objects by manipulating the so-called Casimir force, which normally causes objects to stick together due to forces predicted by
quantum field theoryQuantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...
. This is, however, only possible for micro-objects.
Buoyant levitation
Gases at high pressure can have a density exceeding that of some solids. Thus they can be used to levitate solid objects through
buoyancyIn physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
. Noble gases are preferred for their non-reactivity.
XenonXenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
is the densest noble gas, at 5.894g/L. Xenon has been used to levitate polyethylene, at a pressure of 154atm.
Animal levitation
Scientists have levitated frogs, grasshoppers, and mice by means of powerful electromagnets utilizing superconductors, producing diamagnetic repulsion of body water. The mice acted confused at first, but adapted to the levitation after approximately four hours, suffering no immediate ill effects.
Maglev trains
Magnetic levitation is used to suspend trains without touching the track. This permits very high speeds, and greatly reduces the maintenance requirements for tracks and vehicles, as little wear then occurs.