Regarding ions in air, water droplets in fog, and ferrofluids near
magnets...
Could there be a confusion here between density and pressure? Water
droplets in air can be treated as 'heavy air' with a higher density, but
the pressure is not increased. I know next to nothing about ferrofluids,
but are you sure that near magnetic poles they have a higher pressure? I
wouldn't be surprised if the magnet results in a higher density of
colloidal particles, but I would be very surprised if the static pressure
was actually higher.
In any case, the reason water droplets make 'dense air' is that each
droplet is quite heavy compared to a air molecule. Ions, on the other
hand, have (basically) the same mass as the air molecules, so I wouldn't
expect a density increase either. I think your dyed-water analogy is best:
a charged fluid with the same density and pressure.
How about this for an explanation: as you walk into the ionized region, you
charge up from contact with the air. Then there is an electrostatic
repulsion between your body and the air.
As far as whether charged air would build up, couldn't we figure that from
the mean free flight time for an air molecule? With a guess at the
electric field, you could figure the acceleration during free flight and
from that get how fast the molecule is pushed along through the air. Hmmm,
where is that thermo textbook....
--
--James McLean
jmclean@chem.ucsd.edu
post doc
UCSD