Twinkling colored lights of ? (magnetic viewing film)

Charles Krebs · Post by **Charles Krebs** » Thu Dec 29, 2011 4:51 pm

OK, last guess for me (I think )

electromagnetic metamaterial

as in the electromagnetic bandgap materials discussed here:
http://cearl.ee.psu.edu/Projects/Assets ... S-2003.pdf

rjlittlefield · Post by **rjlittlefield** » Thu Dec 29, 2011 5:16 pm

Charles Krebs wrote:electromagnetic metamaterial

I had to look up that one! No, I'm pretty sure this stuff doesn't qualify there.

Pau wrote:A magnetic fluid?

Close -- Pau gets the kudos, and there wasn't a prize anyway.

I believe Pau is thinking of ferrofluids. The metal particles in ferrofluids are very small, "diameter usually 10 nanometers or less" according to http://en.wikipedia.org/wiki/Ferrofluid. Ferrofluids appear featureless to light, except for the large spiky structures that form on their surface when exposed to a strong magnetic field. They are usually found in bulk or in thin layers between mating surfaces.

But the material shown here is magnetic viewing film. In viewing film, the metal particles are much larger and are shaped as flattened flakes so that they reflect light strongly in some orientations but not others. They are encapsulated in small bubbles of oil between two layers of impermeable plastic. The result is a fairly stiff sheet of plastic, roughly 0.15 mm thick in the material shown here.

When exposed to a strong magnetic field, the flakes orient themselves along magnetic flux lines. If the flux lines and thus the flakes are perpendicular to the plastic sheet, then the sheet appears dark. If they are parallel, the sheet appears bright.

What I did to photograph the twinkling lights was to set up some strongly colored lighting and slide a magnet around near the objective. Here's the setup:

Revisiting the animation:

Now we can understand what we're seeing. The yellow color is low power incandescent light transmitted up through the condenser. Red spots are flakes oriented to reflect red-filtered light coming from one side; bluish spots are flakes oriented to reflect blue-filtered light coming from the other side; and green is the color of the oil. The three large fixed blotches of light blue are clumps of metal flakes that got themselves wedged into fixed positions within their droplets and did not shift position as I moved the magnets.

It seems that this material is not widely known, despite being widely available and not very expensive. You can get a small piece of it at Amazon for $2.75. Just search for magnetic viewing film. Googling for the same phrase will get you lots more information, including a Wikipedia article and other informative pages such as http://www.wondermagnet.com/viewingfilm.html.

Magnetic balls are of course very popular toys. What the magnetic viewing film reveals is that they are also much more complicated than one might guess. For example, the blotch patterns that I showed earlier are "simply" the magnetic field patterns that appear on three faces of the standard 6x6x6 cube:

The cube appears very symmetric -- but not the magnetic fields!

I became aware of this material some months ago, when I was buying some magnets for Christmas presents. A magnet vendor's website suggested I might be interested in this magnetic viewing film, too. Well, yes, as a matter of fact I was.

And now you know about it too!

Perhaps with luck and a dedicated shopping spree, we can get even Amazon to suggest viewing film whenever somebody buys a set of magnets. They don't now, despite that I checked all 17 pages of "Customers Who Bought This Item Also Bought".

Even more oddly, the Amazon page for magnetic viewing film does not suggest magnets! I have no idea what's going on there. Something odd about the customer base, I suppose.

--Rik

Pau · Post by **Pau** » Fri Dec 30, 2011 2:21 am

Rik, you're right, I was refering to ferrofluid, and it doesn't match well your images.
I never heard about this magnetic viewing film, a very interesting material.
Unfourtunately my children are too old, if not I would be now ordering some sheets

rjlittlefield · Post by **rjlittlefield** » Fri Dec 30, 2011 2:45 am

Well, I can attest that these toys do not require children to play with them.

In fact I long ago adopted the policy of buying for others only toys that I would like to play with myself.

*

--Rik

* The little helicopters made a fine present --- especially since the recipient was coming to my place for Christmas!

ChrisR · Post by **ChrisR** » Fri Dec 30, 2011 10:31 am

* yeah they're fun. Have you checked out the ones with cameras built in!!