LEVITATION

Hello everybody. In this post I'm going to talk about levitation. Levitation is the process by which and object is suspended by a physical force against gravity, in a stable position without solid physical contact. There are a number of different methods that are available in order to levitate a matter, and these methods are:

              1)  Aerodynamic levitation,

        2) Magnetic levitation,

              3) Acoustic levitation,

              4) Quantum levitation,

              5)  Electromagnetic levitation

              6) Electrostatic levitation and

              7)  Optical levitation 

Aerodynamic levitation – is the use of gas pressure to levitate materials so that they are no longer in physical contact with any container. The term aerodynamic levitation could be applied to many objects that use gas pressure to counter the force of gravity, and allow stable levitation. Helicopters and air hockey pucks are two good examples of objects that are aerodynamically levitated. However, more recently this term has also been associated with a scientific technique which uses a cone-shaped nozzle allowing stable levitation of 1-3mm diameter spherical samples without the need for active control mechanisms. 

Magnetic levitation - maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. In this case the magnetic pressure is used to counteract the effects of gravitational and any other acceleration. There are two stabilities of magnetically levitated objects and these stabilities are:

-          Static stability means that any small displacement of levitated object from a stable equilibrium position will cause a net force to push it back to the equilibrium point.

-          Dynamic stability occurs when the levitation system is able to damp out any vibration-like motion that may occur.

For successful levitation and control of all 6 axes (3 spatial and 3 rotational) a combination of permanent magnets and electromagnets or diamagnets or superconductors as well as attractive and repulsive fields can be used. From Earnshaw's theorem at least one stable axis must be present for the system to levitate successfully, but the other axes can be stabilized using ferromagnetism.The primary ones used in maglev trains are servo-stabilized electromagnetic suspension (EMS), electrodynamic suspension (EDS).

Acoustic 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 nonlinear effects of intense sound waves. The objects can be levitated with sound which is heard by the human ear or the sound that levitates object is far from human hearing range.

The first theoretical study was done by L.V.King in 1934 in a paper called “On the Acoustic Radiation Pressure on Spheres”. Unfortunately it was only theoretical because it wasn’t possible to develop the machine that would levitate objects. In 70s and 80s the acoustic levitation was a field of research for NASA and MIT engineers. There is couple of studies available on the internet. In 2013 Swiss scientists made controllable acoustic levitator. So far the levitators could levitate the objects but these hovering objects where motionless. Now they can controllably move these hovering objects. 

 

Acoustic levitation is usually used for containerless processing which has become more important of late due to the small size and resistance of microchips and other such things in industry. Containerless processing may also be used for applications requiring very-high-purity materials or chemical reactions too rigorous to happen in a container. This method is harder to control than other methods of containerless processing such as electromagnetic levitation but has the advantage of being able to levitate nonconducting materials.

Here are couple of videos:

There are many ways of creating this effect, from creating a wave underneath the object and reflecting it back to its source, to using an acrylic glass tank to create a large acoustic field.

Quantum levitation as it is called is a process where scientists use the properties of quantum physics to levitate an object (specifically, a superconductor) over a magnetic source (specifically a quantum levitation track designed for this purpose).

The Science of Quantum Levitation-The reason this works is something called the Meissner effect and magnetic flux pinning. The Meissner effect dictates that a superconductor in a magnetic field will always expel the magnetic field inside of it, and thus bend the magnetic field around it. The problem is a matter of equilibrium. If you just placed a superconductor on top of a magnet, then the superconductor would just float off the magnet, sort of like trying to balance two south magnetic poles of bar magnets against each other.

    Superconductivity and magnetic field do not like each other. When possible, the superconductor will expel all the magnetic field from inside. This is the Meissner effect. In our case, since the superconductor is extremely thin, the magnetic field DOES penetrate. However, it does that in discrete quantities (this is quantum physics after all! ) called flux tubes. (Note: This is demonstrated in the graphic associated with this article.)Inside each magnetic flux tube superconductivity is locally destroyed. The superconductor will try to keep the magnetic tubes pinned in weak areas (e.g. grain boundaries). Any spatial movement of the superconductor will cause the flux tubes to move. In order to prevent that the superconductor remains "trapped" in midair. Let's think about what a superconductor really is: it's a material in which electrons are able to flow very easily. Electrons flow through superconductors with no resistance, so that when magnetic fields get close to a superconducting material, the superconductor forms small currents on its surface, cancelling out the incoming magnetic field. The result is that the magnetic field intensity inside the surface of the superconductor is precisely zero. If you mapped the net magnetic field lines (again, see the graphic) it would show that they're bending around the object. When a superconductor is placed on a magnetic track, the effect is that the superconductor remains above the track, essentially being pushed away by the strong magnetic field right at the track's surface. There is a limit to how far above the track it can be pushed, of course, since the power of the magnetic repulsion has to counteract the force of gravity.

A disk of a type-I superconductor will demonstrate the Meissner effect in its most extreme version, which is called "perfect diamagnetism," and will not contain any magnetic fields inside the material. It'll levitate, as it tries to avoid any contact with the magnetic field. The problem with this is that the levitation isn't stable. The levitating object won't normally stay in place. (This same process has been able to levitate superconductors within a concave, bowl-shaped lead magnet, in which the magnetism is pushing equally on all sides.)One of the key elements of the quantum locking process is the existence of these flux tubes, called a "vortex". If a superconductor is very thin, or if the superconductor is a type-II superconductor, is costs the superconductor less energy to allow some of the magnetic field to penetrate the superconductor. That's why the flux vortices form, in regions where the magnetic field is able to, in effect, "slip through" the superconductor. In the case described by the Tel Aviv team above, they were able to grow a special thin ceramic film over the surface of a wafer. When cooled, this ceramic material is a type-II superconductor. Because it's so thin, the diamagnetism exhibited isn't perfect ... allowing for the creation of these flux vortices passing through the material.

Flux vortices can also form in type-II superconductors, even if the superconductor material isn't quite so thin. The type-II superconductor can be designed to enhance this effect, called "enhanced flux pinning."

Other Types of Quantum Levitation

The process of quantum levitation described above is based on magnetic repulsion, but there are other methods of quantum levitation that have been proposed, including some based on the Casimir effect. Again, this involves some curious manipulation of the electromagnetic properties of the material, so it remains to be seen how practical it is.

The Future of Quantum Levitation

Unfortunately, the current intensity of this effect is such that we won't have flying cars for quite some time. Also, it only works over a strong magnetic field, meaning that we'd need to build new magnetic track roads. However, there are already magnetic levitation trains in Asia which use this process, in addition to the more traditional electromagnetic levitation (maglev) trains. Another useful application is the creation of truly frictionless bearings. The bearing would be able to rotate, but it would be suspended without direct physical contact with the surrounding housing, so that there wouldn't be any friction. There will certainly be some industrial applications for this, and I'll keep my eyes open for when they hit the news.

Electrostatic 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 theorem no static arrangement of classical electrostatic fields can be used to stably levitate a point charge. There is an equilibrium point where the two fields cancel, but it is an unstable equilibrium. By using feedback techniques it is possible to adjust the charges to achieve a quasi-static levitation.

Optical levitation is a method whereby a material is levitates against downward force of gravity by an upward force stemming from photon momentum transfer. Typically photon radiation pressure of a vertical upwardly directed and focused laser beam of enough intensity counters the downward force of gravity to allow for a stable optical trap capable of holding small particles in suspension.