A New Twist on an Aging Parlor Trick

Magnetism, Snippets

magnet stack2

From copper wire to living frogs, over the past twenty years many common objects have been levitated by a magnetic field.* Disks of pyrolytic graphite (PG), a hybrid material of diamond and graphite, can notoriously be levitated above neodymium magnets, the same type of strong permanent magnet you might find in your computer’s hard drive.

A carbon disk floating above an array of magnets makes for many a nifty and educational online video, but it has been difficult to make these tiny hover-boards do physical work. Chemists Masayuki Kobayashi and Professor Jiro Abe of Aoyama Gakuin University in Japan may have found a solution.

In a communication published on the web this week by the Journal of the American Chemical Society, the authors report the use of light to induce motion in a PG disk suspended over neodymium magnets. The light from a laser heats the disk, and in doing so lessens its repulsion of the permanent magnets. The hotter the disk, the lower it hovers.

When the scientists shine a 300 mW laser pointer at the edge of a PG disk hovering over an array of magnets, the disk will move whenever the laser is moved. This happens because the heat created by the laser causes the disk to tilt, and this change in balance propels it forward.

The laser light can also be used to rotate the PG disk when it is is balanced over a stack of larger, disk shaped magnets. The magnetic field created by a permanent magnet is highest at its edge. Because of this, when the light moves the disk to the boundary of the magnet stack, it will be strongly repeled, and this interplay of forces will cause the disk to spin.

Most intriguingly, when the carbon-disk/disk-magnet combination is placed in the sun, the carbon disk will rotate rapidly—up to 200 rotations-per-minute—and will only stop when a shadow is cast over the ensemble. The scientists believe that this is a good way to generate rotational energy from solar energy, a possible new paradigm in solar energy conversion.

M. Kobayashi and J. Abe, “Optical Motion Control of Maglev Graphite”, J. Am. Chem. Soc., doi: 10.1021/ja310365k

For some fun videos of levitation in action see:
Supporting Information
Floating Graphite
Maglev Toy
Levitating Frog

*The north poles of two permanent magnets repel one another. Much in the same way, in the presence of a magnetic field certain materials including water, gold, glass and plastic will create a new field in the opposite direction that repels the first—a property of the material known as diamagnetism. Graphite (fools gold, pencil lead) and diamond are both diamagnetic materials made of all carbon.

More questions? Leave a comment!

2 thoughts on “A New Twist on an Aging Parlor Trick

    1. Very neat. Thanks, Tim!

      The Crookes does seem to be some sort of photothermal effect, as well (http://www.youtube.com/watch?v=E3oLcut2Hs0), though its rotation seems to have little to do with the material properties of the vanes, and more with how they interact with what little gas is in the bulb. I was hoping to find an example of a motor driven by one, but so far no luck.

      The Nature Nano paper (http://www.nature.com/nnano/journal/v5/n8/full/nnano.2010.128.html) is super cool.

      For those who are interested, the authors of this paper created a very small motor made of gold (100 nanometers in diameter) that will rotate when a beam of light is shined on it. It works very differently from the one in the paper above. Instead of heating the gold, the light interacts with the electrons on the surface of the motor, the vibrations and oscillations of which are collectively known as surface plasmons. Because the shape is not symmetrical, the plasmons will move (like waves) in a certain direction, driving the motor. What is extra cool is that the way these waves move–how much and in what direction–depends on the energy of the light used, and so the motor will rotate in different directions and at different speeds depending on wavelength of light. What is extra special cool is that the scientists could then use this motor to actually rotate a disk 4000 times the size of the motor.

      There is a lot more to plasmons than I have room to talk on here. They are a quantum mechanical, many body phenomenon, and deserve proper space, so I will definitely cover them and the cool things that they can be used for at some point in the future.

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