Physicists have magnetically levitated various solid objects - including gold, platinum and diamond - in cold oxygen gas for the first time. Laurence Eaves and colleagues at Nottingham University were also able to move the floating objects by adjusting the strength of the applied magnetic field. They believe that the effect could find applications in mineral separation technology (A T Catherall et al. 2003 Nature 422 579).
Magnetic levitation occurs when the force on a diamagnetic object is strong enough to balance the weight of the object itself. If the object is immersed in a paramagnetic fluid, such as gaseous oxygen, the levitation can be enhanced by the effect of buoyancy caused by the “magneto-Archimedes” effect. The levitation force per unit volume of the object depends on the vertical magnetic field gradient in the vessel.
The fact that the density of a gas at fixed pressure is inversely proportional to temperature (Charles’ law), as is the paramagnetic susceptibility (Curie’s law) means that near the boiling point of liquid oxygen, 90 K, the magnetic buoyancy of oxygen gas is ten times stronger than at room temperature.
Liquid oxygen can provide even greater buoyancy – enough to float dense diamagnetic objects at relatively low magnetic field strengths (figure 1). Each object floats at the point at which the local magneto-Archimedes force balances its weight, and the position of the object can be adjusted by changing the magnetic field strength.
The researchers also noticed the formation of a regular pattern of peaks on the surface of the liquid oxygen (figure 2). The peaks are a result of magnetic and surface energy effects and have only ever been observed in synthetic ferrofluids before.
