Re: Squeeky Snow

[William Beaty <billb@ESKIMO.COM>]

On Mon, 18 Dec 2000, Doug Craigen wrote:

> 1) is it the snow that squeeks, or is it the boots rubbing against it?
> 2) what is the mechanism?  At some particular temperature does it lose
> some kind of lubrication that normally makes it slick enough to not
> squeek?  I recall something about a thin water film on the surface of
> ice, which provides the basic slickness.  Does the snow squeek when that
> film reaches some critical thinness?


I encountered some articles about the unsolved mystery of *squeeking
sand*, so I wonder if the effect is similar.  If it is, it might have
very little to do with stick/slip squeeks (like the kind you get when
rubbing a wineglass with a soapy finger.)

Of several "barking sand" theories, my favorite is the "acoustical laser."
A pocket of roiling particles acts like a resonant cavity, but with
a very high density and very low wave-propagation velocity (so a small
cavity can generate a very low note, e.g. the Hot Choclate Effect.)  Since
there are shear forces caused by feet rubbing across the sand, nonlinear
conditions in the sand can lead to amplification of any propagating waves.
(It would be a type of Parametric amplifier, and the waves would feed off
the energy stored in the shearing sand).   A combination of an amplifier
medium with a cavity resonator gives you an acoustic analog of a laser.
The waves might grow until the particle cloud was compressed into a solid
during the high side of the pressure wave.

The family stopped at a beach in Oregon on the way back from vacation last
year, and that beach had the "barking sand" effect.  I collected about a
cubic foot of that sand in a plastic trash bag, which is now sitting in
the basement.  The effect might require a narrow window of humidity
conditions, but I thought, "what the heck", and dragged the heavy ameboid
sand bag back to Seattle anyway.  At the rate I get around to doing
personal experimentation, the darned thing will probably still be sitting
there twenty years from now.



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