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Re: mist maker



At 09:00 AM 5/17/00 -0400, Wolfgang Rueckner wrote:
Using 150 V/m for the E-field at the surface of the Earth and Gauss' law, I
calculate an electron density of about 10^-9 per sq m on the surface of the
Earth. That doesn't seem like much of a source for an ion generator. Did
I miss something fundamental here?

Your skepticism is well placed; you didn't miss anything. The post to
which you refer (the following) is mistaken.

At 03:03 PM 5/16/00 -0400, Chuck Britton wrote in part:

Since the earth is surrounded by a downward pointing E field of about
150 V/m, we can use Gauss' Law to calculate an average density of
'un-neutralized' electrons that are scattered across the surface of
the earth and any conducting surface which is 'grounded'. i.e. the
surface of the water in the your 'mister' has excess electrons on it.

That implies that if the mister is used indoors it won't work, because the
terrestrial E-field is shielded.

The mister does not depend on "un-neutralized electrons". It is
ultrasonic. Almost certainly piezoelectric.

==============

So why don't we do some physics?

If temporarily we assume small amplitudes (the linear regime), the
dispersion relation for short-wavelength waves on the surface of water is
w^2 = (sigma/rho) k^3
http://asp.esam.nwu.edu/kath/d30notes/node5.html

where in SI units (not the units used in that reference) we have
sigma = 0.074 kg/sec^2 (surface tension)
rho = 1000 kg/m^3 (density)

and if the back of my envelope is to be trusted that means at 1 MHz the
wavelength is something like 8 microns.

It's then easy to believe that for modest power-density, these waves could
be driven out of the linear regime (amplitude greater than wavelength)
whereupon you get micron-sized "whitecaps" and tons of mist.

At 03:49 PM 5/16/00 -0500, Doug Craigen in part:
As for how the beam generates the mist - all my reading said
"cavitation". In other words, I don't know.

I share Doug's skepticism about "cavitation" and other processes in the
bulk liquid. You can't have _mist_ below the surface. Mist is a mixture
of air and water. How would you get the required air into the bulk? If
you made mist in the bulk, how would it get to the surface?