Date: Mon, 22 Nov 1999 15:05:02 -0800 (PST)

Author: William Beaty

Subject: mutually-repelling beads as "fluid"


I saw something cool in the mall yesterday. A "wave drum" musical
instrument in a toy store was doing something I've seen before long ago.
Once upon a time I trapped a bunch of disk magnets between two sheets of
plexiglas so that the self-repelling magnets would organize into a crystal
array. The "wave drum" was doing the same, except it was using

The "drum" consists of a hollow disk-shaped cell with a heavy plastic
membrane on the back and plastic beads within. When tilted, the layer of
beads rolls along and makes the sound of crashing surf. However, the
contact of the individual beads with the plastic membrane was charging
both the surface and the beads. The beads were self-repelling, but also
were clinging to the oppositely-charged membrane. When tilted, packed
array of beads would develop a layer of "atomic fluid". The "fluid" was
made of mutually-repelling beads at the top edge of the packed region, and
I could make this "fluid" slosh around by moving the chamber back and
forth. The beads were also clinging to the plastic membrane, and when I
turned the entire device upside down, they kept rolling across the
"ceiling." When held still, the "fluid" tried to become a hexagonal
array, but the friction was a bit high to make a good one, and the beads
each had a flat spot caused by a drilled hole in each one.

I've seen these "wave drums" before, but usually they're opaque and
contain steel shot. This one was "Learning Smith Wave Drum."

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William J. Beaty SCIENCE HOBBYIST website
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