Too Many Particles May Impede Ultrasonic Cleaning

This discussion is a bit anecdotal in nature because my attempts to demonstrate the effects I describe in the following in the laboratory have been less than conclusive.  But, the fact that I have seen them occur on several occasions over a period of more than 50 years gives it a degree of credence.  By this time the reader is probably wondering what mysterious and magical thing can he possibly be talking about?  In fact, it’s all about the possible cause and effect of too much cavitation!  Yup, too much – can you believe it?

It is generally accepted that ultrasonic cleaning requires the formation and catastrophic collapse of cavitation bubbles in implosion.  But we also know that cavitation bubbles probably form preferentially where there are “seeds” which weaken the structure of the liquid.  These cavitation “seeds” may be imperfections in the linking of molecules but, more likely, they are discontinuities in the liquid due to suspended foreign particles.

Note – We know from experience that extremely pure water (usually de-ionized water) is difficult to cavitate.  This is attributed to the lack of “seeds” for the formation and growth of cavitation bubbles.

The premise here is that too many “seeds” could result in the formation of too many cavitation bubbles and that because there are so many bubbles, there may not be enough energy present to support to growth of a significant enough portion of them to the size that results in implosion.  This, then, would dampen or potentially even completely kill the ultrasonic effect.

Allow me to site a couple of real-life examples.

Some years ago, I worked on an application to remove ash from metal parts in a re-work application.  Parts were heated to burn off the defective paint in preparation for re-painting.  After the burning process a very fine powdered ash was left on the parts which we were attempting to remove using ultrasonics.  The whole application seemed totally logical and classic.  The small scale laboratory trials had gone perfectly.  However, during the customer start-up, it was quickly discovered that although a first load of parts cleaned well, cleaning became increasingly ineffective on subsequent loads.  This effect was re-created several times by dumping and refreshing the tank before it was concluded that the excess of small ash particles was apparently resulting in the demise of ultrasonic cavitation and implosion.

In another instance, the parts being cleaned were silicon or quartz wafers that had been polished using a polishing compound.  Again, although the first few parts cleaned well, the process stopped working after several batches had been cleaned.  In this case, even sub-micron filtration was not able to restore cleaning.

In trying to re-create the first of the above in the laboratory I used wood ash (collected from my fireplace) to “saturate” a tank.  This attempt was unsuccessful as the system kept cleaning despite the amount of wood ash introduced.  In an attempt to re-create the second case, I found that the addition of just a small amount of contaminated liquid collected from the trials at the customer was effective in suppressing cavitation while he addition of the polishing compound itself showed little effect.

So, what’s going on?  Truthfully, I don’t know, but I have some (unproven) postulates.

• The particle size must be very small – probably sub-sub-micron.  The filter used in the wafer application was 1 micron so the particles had to be smaller than that.  Although I haven’t had the opportunity to determine exactly how large the wood ash particles were, they were probably well in excess of 1 micron but although untested, filtration to that level was felt impractical.
• There may be something about the “wettability” of the particles.  If the particles are able to be wet by the liquid, they may present less of a barrier as the liquid stays adhered to them.  Shape (round vs. irregular or “sharp”) may play a role as well.
• The phenomenon may be the result of a combination of effects perhaps including the above and more that are, as yet, not identified or understood.

The purpose here is to document this effect for future reference and possible further investigation.

 – FJF –