Bubbles and cavitation

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. 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".
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.
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.
Michael Koblic, Campbell River, BC
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Not anomalous, unless you were electrolyzing, say, water.... heh, where's the O2????? Oh, OH, Cold Fusion!!!!
Yet the bubbles seem

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? :)

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.

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.
--
EA


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> Michael Koblic,
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On Tue, 10 May 2011 07:31:13 -0400, "Existential Angst"

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...
Michael Koblic, Campbell River, BC
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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


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On Wed, 11 May 2011 22:26:38 -0400, "Existential Angst"
[...]

My understanding of cavitation is that it is simply formation of gas pockets in a liquid and their collapse. The gas can be anything, air and water vapor being the most common.
I have counted several different ways these pockets (or bubbles) can form not including introducing the gas in the way I do. The damage is done by the bubbles collapsing suddenly, not by their collision. It is particularly pronounced at a fluid/solid/vapor interface where the bubbles collapse in a way that produces fluid microjets directed towards the solid surface - a perfect fit for my case. That there is a significant amount of energy generated by this happening I have demonstrated adequately.
Not too sure what you mean by "electrochemical short circuit".
All in all, I tend to lean towards the Occam's razor on this one.
Michael Koblic, Campbell River, BC
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Pretty sure "pure cavitation" needs no gas dissolution, and what is percieved by the eye as being a "bubble" in liquid actually comprises a vacuous space...though, probably upon collapse, gas BEGINS to fills the space first, with the relative propagation velocity of gas v/s the surrounding liquid being dependant on too many factors to get into much detail here.
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