Re: Chladni figures...

[William Beaty <billb@eskimo.com>]

On Mon, 2 Sep 1996, Joseph Bellina wrote:

> As I recall, it was Maxwell who figured out why the difference between 
> salt/sand and lycopdium (thanks) powder....Tom Greenslade, are you 
> lurking?  The lighter lycopdium powder goes to the antinodes because of 
> the flow of air over the vibrating plate...but I don't recall the details

Here's something related to the Chladni demo but much less explainable.
Lay a 12" speaker on its back, stretch rubber or mylar across the face,
and place a tall ring of plastic or cardboard on the sheet as a perimeter
wall.  Drive the speaker with an oscillator and power amp, then add
*significant* quantities of fine powder.  Baking soda or talc works, also
try fine sand or salt.

The patterns which form resemble the Chladni plates not at all.  Instead a
mathematician's nightmare of dynamical self-organization forms arise. 
Powder masses interfere with movements of the membrane and movements of
powder, while the membrane movements shuttles the powder around.  The
result is nasty, brain-hurting 2D instabilities.  There are self-building
lenticular piles having convection flows, central vertical powder *jets*,
and sometimes two jets form and rotate, spewing powder which then is
sucked back to the pile.  Moving piles appear and begin rotating around a
common center.  Glactic spiral arms arise and vanish.  Multiple
interlocking fingerlike piles form and compete with each other for powder
mass, so they move and squirm like magnetic bubble domains. Crystalline
arrays of powder piles appear.  At some settings, moving density waves or
sand dunes appear and slowly drift along.  At others, rapid rivers of
powder appear and create closed circular flows.  When the amplitude is
high, thick layers of bouncing grains appear and create ghostly waves and
structures which hover above the membrane.  Often many of these forms
coexist at once and interact. 

Lava Lamps have nothing on this, I've accidentally had hours slip away
while staring at vibrating powder and turning a knob.  :)  Two knobs
actually.  Since the patterns derive from multiple nonlinearities, the
amplitude affects the structure of the patterns even more than the
frequency knob.

......................uuuu / oo \ uuuu........,.............................
William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
EE/Programmer/Science exhibit designer        http://www.eskimo.com/~billb/
Seattle, WA 98117  billb@eskimo.com           SCIENCE HOBBYIST web page