The Science of Magnetic Levitation
Magnetic levitation occurs when the force on a diamagnetic object, due to an inhomogeneous magnetic field, is strong enough to balance the body’s weight
resulting from the effect of the Earth’s gravity1. The effect is not noticeable, however, in a homogeneous magnetic field; the atoms also need to be within a magnetic field gradient. If the product of the field strength and the field gradient is large enough, then the force exerted on the atoms is sufficient to counteract the effects of gravity.
When a team of scientists at Nottingham approached Oxford Instruments, almost ten years ago, to supply a superconducting magnet for their research, they requested several features to enable effective levitation experiments.
The magnet had to be capable of producing a large field gradient, running in persistent mode and was to be used by a large, multidisciplinary group of scientists.
The equipment, therefore, preferably needed to be easy to use and relatively low maintenance.
To meet these needs, Oxford Instruments produced a system that uses Minimum Condensed Volume (MCV™) technology. Unlike conventional superconducting magnets, this only uses a few litres of liquid helium and does not loose any cryogen to evaporation. This keeps maintenance to a minimum,
removes the need for helium refills and keeps the system compact. Additionally, the range of experiments that can be carried out is increased by the fact that the 50 mm diameter bore access is at room temperature and can therefore be used with living systems. Despite its small size, the magnet is capable of field strengths of up to 17 Tesla at the centre of the bore (the earth’s magnetic field is typically 0.07 mT) and, vitally for levitation, can produce a (field gradient x field) product of up to1,470 T2/m. Although this is sufficient to levitate water/oil-based substances and plastics, the Nottingham team wanted to levitate denser materials and tried augmenting the effect.
Enhancing levitation with oxygen
One means of achieving this enhancement, oxygen gas, had already been demonstrated by Ikezoe et al 2. Oxygen molecules (O2) are paramagnetic, and so are attracted to magnetic fields. The resultant enhanced buoyancy, when a diamagnetic object is placed in oxygen gas, is named the magneto-Archimedes effect. In an ingenious development of this experiment, reported in Nature, the Nottingham magnetic levitation team exploited Charles’ law and Curie’s law in order to levitate a much wider range of materials3. A combination of these two effects gives x10 enhancement of the magneto-Archimedes effect near the boiling point of liquid oxygen. As the cold oxygen gas is at ordinary atmospheric pressure, it is very easy and quick to manipulate the samples and to put them into the levitation system. The Nottingham group was able to levitate natural diamonds (which have a density of 3.51 g ml-1) in cold oxygen gas. Liquid oxygen is even more paramagnetic, and therefore provides still greater buoyancy. This enhancement allowed the researchers to float silicon, a £1 coin, a piece of lead and even gold and platinum in liquid oxygen3. These studies also revealed that whilst inside the magnet, the liquid oxygen adopts surface corrugations, similar to the strange instabilities displayed by ferrofluids in a high magnetic field4. This may provide a useful system for theoretical models of crystal formation and dynamics.
Click here to find more about the research on magnetic levitation at the University of Nottingham.
Click on the video to view Episode 2 of the Children's BBC TV programme Little Howard's BIG Question, featuring the flying "Super Carrot", filmed at Nottingham univeristy and featuring the Oxford Instruments levitation magnet.
1. Geim, A. K., Simon, M. D., Boamfa, M. I.,
Heflinger, L. O. Nature 400, 323 - 324 (1999).
2. Ikezoe, Y., Hirota, N., Nakagawa, J.,
Kitazawa, K. Nature 393, 749 - 750 (1998).
3. Catherall, A., Eaves, L., King, P.J., Booth,
S.R. Nature, 422, 579 (2003).
4. Cowley, M.D., Rosenweig, R.E. J Fluid Mech,
30, 671-688 (1967).