wrote in message
...
On Tue, 10 May 2011 07:31:13 -0400, "Existential Angst"
wrote:

wrote in message
. ..
Here is a bit of physics that I am struggling with:

I am using an electro-etching process which produces hydrogen gas. It
is produced on the cathode and comes out of the electrolyte as copious
bubbles. There is *no* gas produced on the anode.

Not anomalous, unless you were electrolyzing, say, water.... heh, where's
the O2????? Oh, OH, Cold Fusion!!!!

Yet the bubbles seem
to punch holes in the resist on the anode. I have done this enough
times to convince myself that these bubbles are the cause of
"foul-biting".

Most certainly has to be the complementary electrolytic reaction at the
anode, likely resulting in a more soluble compound at the anode, like
Pb --
PbSO4. The lead would proly be observed as pitting, or your
"foul-biting" -- a Bri'ish term?


I understand about cavitation in general but always thought about it
as a process associated with changes of pressure within the liquid.
Bubbles develop in the low pressure areas of flow e.g. on propellers
and foils that collapse with considerable forces being generated in a
form of small water jets. Similar situation exists when the liquid is
boiling. I am wondering if introduction of gas into the liquid by
electrolysis makes the gas bubbles behave in the same way.

Don't know about small water jets, but indeed you are dealing with
Newtonian/pressure issues in a propeller, forcing dissolved gas out of
solution, as the solubility of a gas in solution is a function of
pressure,
in this case local pressures induced by rapid shear. And ergo some
mechanical wear'n'tear.

Boiling also affects the solubility of a gas in solution, but via
temperature. The solubility of gases in solution is opposite to that of
solutes, like salt, with temp., so gas is expelled in boiling, while more
salt would dissolve.
You will observe this, btw, even without boiling, ie, just by heating, as
bubbles collect on the sides of the beaker, etc.

By definition (afaik -- wiki????) cavitation refers specifically to
pressure-induced bubbles.

The gas created in electrolysis is totally different from either of the
above. Electrolysis is a true chemical process, restructuring covalent
bonding (forming brand new orbitals) while the above is just nipping at
things like vanderwaals, London, hydrogen "bonding", etc, which are like
the
quantum mechanical equivalent of "molecular gravity", for want of a better
analogy.
Breaking these types of bonds will change "state", and mebbe reaction
rates,
but not real chemistry. Same ole orbitals, just bent a little bit.


In practical terms I suspect the solution will be repositioning the
electrodes (I have the anode suspended above the cathode which means
that the bubbles pass over the surface of the anode before being
expelled from the electrolyte) but I would like to know if cavitation
is a reasonable explanation in this case.

Proly not, which you will find after repositioning, and the anode still
pits.
It could be that the hydrogen is then *reacting* with the anode, but I
would
doubt this also, from very defs of electrolysis.

Now, in point of fact, there are newtonian forces as the bubbles "hit" the
cathode, but goodgawd, that would have to be one fragile anode for this to
amount to anything.

I'd be mega-innerested in the outcome. You could post this in sci.chem,
sci.physics, and be amazed at the wild high-tech confabulations, coming
from
the likes of Uncle Al et al.

I run the process with a new stainless steel cathode on top today. The
only other difference is that I baked the resist after the patterns
were engraved and had to solve the electrical connection to the anode
which is now completely immersed in the electrolyte.

The result was a dramatic improvement. Hardly any foul-biting at all.
Crisp, clean and deep pattern etch.

The hydrogen bubbles escaped through the holes in the cathode and
burst generating little geysers of almost 2in in height. If the bubble
burst under the cathode it did so with a loud pop.

Thus I am pretty much convinced that cavitation related to the
hydrogen bubbles is very real and unexpectedly powerful phenomenon.

I have a few issues to solve related to the electrical supply to the
anode but these are minor.

This was something of a revelation...

You may have solved the problem, but it is still not clear what the problem
was.
Cavitation was certainly not the problem, cuz, well, this wadn't cavitation.
It is highly unlikely that it was a mechanical collision effect, of the
formed bubbles.
I think it was some unanticipated chemical reaction, that is avoided by
shifting geometries.
The colliding hydrogen might be forming something of an electrochemical
short circuit, as well.

Anyway, souinds like a fascinating project.
--
EA



Michael Koblic,
Campbell River, BC