On Jun 7, 2007, at 4:57 AM, Michel Jullian wrote:
Hi Horace,
Let's try to agree on simple things, in a simple example. Say at
t=0 the HV is turned on instantly and the +ve anode's tip starts
emitting a dotted line of slow-flying +ve ions
I'm curious - why do you use the notation +ve? Does the "e" represent
someting? I assume here you mean the needle is an anode of potential
+ve, and the plate is ground. Hopefully you are not implying the
filament is comprised of single molecule charged ions? Single
molecule charged ions would distribute themselves across the area of
the plates, following the field lines. The only hope of maintaining
anything like a filament or jet is a high m/q ratio, which charged
drops provide to a degree, and continuous filaments provide as well.
Also, I think Bill said a metal target plate accumulated water, which
would be consistent with either a droplet or water filament model.
Such models are also consistent with the need to keep the environment
wet, humid, or full of CO2. Say, maybe flowing water provides a way
to visualize the filament - a powdered dye on a metal plate. When it
gets wet it changes color. Only good for a one time shot, but it
still would be impressive and maybe could be engineered to leave a
permanent trace for study.
which won't arrive before t=50ms. Say the flow of charge is a
constant 10nA flowing out of the tip. The anode current waveform is
therefore a step function, zero for t<0, 10nA for t>=0. What I am
saying is that the cathode current is exactly the same step
function, as it would be for any electronic component, without a
delay corresponding to the flight time, i.e. cathode current won't
wait for 50ms to turn on. Do you agree with this?
No, not necessarily. Bill said the filaments were fairly neutral,
and a pair could in fact travel parallel and close to each other for
long distances. I therefore don't think the majority of the current
flows until the filament completes contact with the plate, in which
case the filament conducts current through itself. I think the
voltage and thus the repulsion between two *conducting* filaments
declines as the point of observation approaches the ground plate.
I agree that in the *droplet model* the current onset would be
immediate, as the droplets carried off the charge. We clearly are
basing (biasing) our assumptions regarding conductivity depending on
our personal models of the situation. In the filament model there
are two currents involved: (1) the current that leaves some charges
on the filament which is assembled from polar molecules held in place
primarily by their polar (hydrogen bond) attraction with the
alignment coordinated by the tip field E, and (2) the conduction
current that occurs when contact is "made" between the two
electrodes. If we knew the surface area of the filament we could
compute the current required to eject a filament at velocity V and
maintained at needle potential ve.
I think it is an interesting question as to the exact mechanics the
allows the formation of a jet instead of droplets. It may relate to
the voltage, the power supply characteristics, the needle geometry,
and maybe the resistance and surface physics of the needle. It may
be highly related to the manner in which the needle attracts and
feeds water and/or CO2 to the jet. It certainly should be affected
by the surface tension at the needle tip, and thus may be highly
related to the thickness of the water feed to the tip, and to the
chemistry of the water, i.e. its CO2 content. But first there is a
need to prove the continuous filament exists at all and to detect its
presence and characteristics.
Even without an imposed AC signal, the two plate method may be useful
for determining the presence of filaments vs. drops. The experiment
would consist of (1) establishing a current with the needle over one
plate and (2) then moving the needle electrode back and forth between
the centers of the two plates slowly, maintaining the current. Given
two adjacent coplanar target plates, each with its own nano-ammeter,
and slowly moving a source needle from above one to above the other,
we would see the current jump from one plate to the other very fast
if a filament were involved, and more gradually switch between the
two plates if droplet current transmission were involved, especially
at longer needle-plate separations. If lots of filaments from the
source electrode were involved, we should still see the change in
currents occur in jumps, while if droplet conduction is occurring
then the transition of signal strengths would be more continuous.
Another way to diagnose filament vs drop formation at the needle tip
is through current fluctuations. If very pure DC is used, then an
oscilloscope probe wire attached to the needle, but capacitively
isolated from the needle using a small value high voltage capacitor,
should be able to detect the difference between ejection of droplets
vs a continuous jet. Droplets would cause a ripple in current and
voltage, while a jet would form with a smooth waveform.
Regards,
Horace Heffner
