Re: FUN: high-speed electrostatic air-threads

[William Beaty <billb@eskimo.com>]

On Tue, 9 Jun 1998, William Beaty wrote:

> Updates:
>   Electrostatic Air-threads
>   http://www.eskimo.com/~billb/weird/unusual/airthred.html


I have a question with no obvious answer, and I thought phys-l might have
some suggestions

When "electric wind" is emitted by a needle, the repulsion of the alike
charged ions causes the stream of "wind" to fan out from the needle and
then to be partially guided by ambient e-fields (and also by intertia
effects, etc.)  A large, fuzzy "sausage" of charged air extends from the
needle tip.

Sometimes this does not occur.  Instead the stream of "wind" acts like a
filament with diameter less than a mm, and velocity of a few meters per
second.  Why?  I don't understand this at all.  I would expect that the
ions would repel each other, rather than forming an incredibly narrow,
high-speed jet.

To observe these narrow ion-jets, I put about 10KV on a pan of water, drop
in a few chips of dry ice to form a layer of mist, then wave my hands a
few inches over the pan.  Numerous narrow furrows are carved into the
mist.  Each furrow is a narrow stream of air which originates on dust
motes and hairs on the surface of my hand.  If I find that I have a
dust-mote on a fingertip, then I can write my name in the mist from 10cm
away.  A torn edge from a piece of paper will generate a great many of
these "air-threads", all in parallel as they travel to the charged pan. 
By wiggling my hand and observing the delay time before the mist-holes
respond, I estimate the velocity of the air to be 10KPH.  I've made these
air-threads up to 60cm in length by using a steel-wool whisker or a single
carbon fiber, but typical lengths are more like 10cm.

How is such a narrow stream of charged wind able to exist?  Why don't the
parallel streams seem to repel each other (as physical alike-charged
threads would?)  And shouldn't a high-velocity narrow jet instantly break
into turbulence?  Is something weird going on here, or is this normal
electrostatics?


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William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  www.eskimo.com/~billb
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