Strange corona discharge growths from red hot wire

[Odysseus]

I noticed this odd phenomenon that I've never seen or heard about before while playing with a high voltage Royer oscillator.  By striking an arc between two copper wires, the tips will heat up until melting and turning into a small ball of molten copper oxide.  As this ball cools after the arc is extinguished, small growths of the oxide spontaneously form in the direction of the corona discharge emanating from the wire.

http://youtu.be/xJhEweg3W2w (Don't mind me chatting to my GF on skype in the second half where I adjusted the exposure)

I presume it's some form of thermal sputtering process, but I am not knowledgeable enough to say for sure.  Trippiest thing I've seen in a while, so I'd though I'd share.   I'd be interested in any other references to this phenomenon.

[T3sl4co1l]

Cool.  Never noticed that before.  Possibly to do with sudden freezing, oxides and electric field effects on surface tension and stuff.

Tim

[VK5RC]

To my eye it seems to form just as the tip is cooling, I assume that the tip starts as some irregularity of the conductor and then starts to accumulate more charge which is repelled from the body of the conductor, as the conductor is liquid (or very close) it forms a new shape, the more pointy the shape the more charge concentration, which continues the process until the metal cools and solidifies

[Odysseus]

To my eye it seems to form just as the tip is cooling, I assume that the tip starts as some irregularity of the conductor and then starts to accumulate more charge which is repelled from the body of the conductor, as the conductor is liquid (or very close) it forms a new shape, the more pointy the shape the more charge concentration, which continues the process until the metal cools and solidifies

Ahh, so maybe it's like your hair standing on end when touching a Van de Graaff generator, except in this case it's liquid metal, cooling and hardening as it's repelled.  In that case I would expect the effect to be similar or even more pronounced when using a DC source.  Something to try.

[MacAttak]

To me it looks like the outer surface of the blob cools first into a crust and contracts - quickly increasing pressure on the still-molten inside. The pressure causes it to jet out through a fault in the crust, and as that jet material expands it quickly cools into a needle. Like a miniature volcano. The discharge arcs could be keeping the crust slightly warmer in those spots, providing the weaker fault sites.

For that to work, the molten metal would need to be less dense than when solid, and it would need to be fairly brittle right below the melting point.

[T3sl4co1l]

To me it looks like the outer surface of the blob cools first into a crust and contracts - quickly increasing pressure on the still-molten inside. The pressure causes it to jet out through a fault in the crust, and as that jet material expands it quickly cools into a needle. Like a miniature volcano. The discharge arcs could be keeping the crust slightly warmer in those spots, providing the weaker fault sites.

For that to work, the molten metal would need to be less dense than when solid, and it would need to be fairly brittle right below the melting point.

The factors are present -- oxygen is soluble in molten copper, and precipitates suddenly on freezing (just as freezing a soda is, er...a bad idea).  Normally, this makes a porous or foamy result, but if all the right factors come together, who knows?

Likely relevant:
http://en.wikipedia.org/wiki/Electrospray
Presumably the charge sign does not matter, so it would work for AC as well.

Tim

[Odysseus]

The factors are present -- oxygen is soluble in molten copper, and precipitates suddenly on freezing (just as freezing a soda is, er...a bad idea).  Normally, this makes a porous or foamy result, but if all the right factors come together, who knows?

Likely relevant:
http://en.wikipedia.org/wiki/Electrospray
Presumably the charge sign does not matter, so it would work for AC as well.

Tim

Thank's for the link Tim, I hadn't known about electrospray phenomenon. The electrospray process sounds like the most likely explanation, as the high surface tension and rapid cooling in this case prevents an actual spray from forming.  Seems like I've got some more experimenting to do.

[qwaarjet]

This look like a macro scale version of phenomena that I have seen with regenerable indium cathodes. normally surface tension and electrostatic forces would  would cause  a taylor cone to form and ions would be emitted but in the case the current is so high ( current ? ions emitted)a jet of molten copper is emitted, since this jet cool from the outside in  it will for some what of pipe, like lava tube. once the tube is long enough it will radiat enough energy to cool and solidify the metal and the process stops.

[Psi]

A macro video of it would look cool and might help to see whats actually happening.

[VK5RC]

A great observation Odysseus! I had not heard of electrospray/Taylor cone, thanks T3sl4co1l for the reference. Amazing what you keep on learning.