On Sun, 24 Jun 2007, Michel Jullian wrote: > I think you have misunderstood what I suggested, which was not a linear > chain of charged droplets, but one of mere air ions (ultra low current > point to plane corona discharge), only too far apart (due to the low > current) to provoke any significant widening of the beam by sideways > mutual repulsion.
Oops, I was making assumptions: because charged droplets evaporate very rapidly, leaving air ions behind, therefore droplets are a mechanism for emitting low density ions using fairly weak e-fields without corona breakdown of the air. I strongly suspect that full-blown corona discharge from a sharp point will always emit huge numbers of ions in a fan-shaped stream, since it needs a fairly strong e-field to turn on and maintain a plasma. The strong e-field then drags large numbers of ions from the plasma, and they create turbulence or at least self-repel. This is easily demonstrated experimentally when bringing a sharp needle up to the fog layer: there is sometimes no initial small hole from an "air thread," no ions at all, but if the needle approaches the fog layer within a few inches, suddenly a blast of ion wind turns on and blows a 10cm hole in the fog layer. Perhaps there is another explanation, but this looks to me like an example of sudden corona ignition and high-rate ion production. At low fields where "air threads" are seen, I suspect that charged micro- droplets are emitted and then evaporate, leaving ions. Or perhaps if surface details are sharp enough, electrons can be emitted directly through field-emission, and attach to air molecules. > Due to the low field ~1kV/cm, the ions would drift > relatively slowly at ~20 m/s. So the threads would really be narrow ion > streams, this is what you had suggested initially BTW, and which I had > dismissed a bit too hastily. > > The problem I see now with the electrospray explanation is the same I > had seen (mistakenly I now realize in your case of ultra low currents) > with the ion stream explanation, namely lateral expansion. > As the name > implies, charged water droplets will explode by self-repulsion and > evaporation into a spray of smaller droplets, which will explode again > etc... Nope. Well, yes they can explode at certain high values of e-field and low surface tension. Called "Coulomb explosions." But at lower values, the surface charge on each droplet is too low to tear apart the droplet, so droplets are emitted in a very long coherent stream. See the animation on that same website, http://www.newobjective.com/electrospray/spray_anim.html and note the 1KV reference to "cone/droplet oscillation." For electrospray mass spectrometers, they want a diffuse spray where some droplets are drawn into the capillary mouth. At too low a voltage, a narrow chain of droplets is emitted, and this "thin ray" will almost certainly miss the capillary. "Air threads" would involve this cone-droplet oscillation and not the spray of droplet explosions. > (see http://www.newobjective.com/electrospray/ ), so this would > produce a relatively wide beam. Higher viscosity and non-evaporating oil > droplets may behave differently, but I don't think electrospraying water > or an aqueous solution can produce a narrow beam or thread as you > observe. Beware, for "newobjective.com" is an instrument supplier, so they take the viewpoint of advertisers, where narrow droplet streams are a bad thing to be trivialized, and coulombic explosions of the droplets are the selling point. Set up the experiment. Wave your dry hands close over the fog-plate and watch what happens. Now wet your fingers and try again. (((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website billb at amasci com http://amasci.com EE/programmer/sci-exhibits amateur science, hobby projects, sci fair Seattle, WA 425-222-5066 unusual phenomena, tesla coils, weird sci
