Over the years, there have been several anecdotal references to otherwise unexplained changes in the properties of surfaces exposed to ultrasonic energy in a liquid. In some cases, it would make sense that the change was due to increased cleanliness. In others, however, the benefit of cleanliness alone would seem questionable. One incident in particular sticks in my mind.
Many years ago, a printing company reported that ink rollers (not analox rolls but just smooth rolls) carried ink much more efficiently and uniformly after they were ultrasonically “cleaned.” In a series of (not so) controlled experiments, it was shown that the use of a chemical cleaner played virtually no measurable role in whatever was happening. Rolls exposed to ultrasonics using tap water showed the same change in ink carrying properties as those cleaned ultrasonically using a variety of chemicals. It was finally deduced that ultrasonic exposure had “roughened” the surface of the steel rolls just enough to allow the ink to adhere more effectively. At that time (30 or so years ago) we did not have the means to measure what exactly had changed when the rolls were exposed to ultrasonics. No difference could be seen using a microscope – the result was strictly empirical but significant.
Contemplating the above over the years, I have come to the conclusion that there are two possible explanations for the change seen in the ultrasonically cleaned ink rolls – –
- The implosions of cavitation bubbles may have created small “craters” in the surface which improved the “tooth” of the surface.
- The surface had been “machined” sufficiently to remove a thin skin of oxide or other barrier which otherwise prevented adhesion of ink.
Either of the above is logically possible based on our knowledge of ultrasonic cavitation and implosion. But, the important thing at that time was not what happened but the fact that something happened.
Since that time there have been instances in which, although not quite as dramatic, I have seen evidence of changes in surface characteristics that are not consistent with cleaning alone. But the purpose here is not to teach but to learn.
Ultrasonics is still a growing technology. The more I know (after nearly 50 years) the more I find that I don’t know. I see this potential use of ultrasonics under controlled conditions and with known parameters a rich opportunity to enhance coating technology and to improve bonding technology. On a somewhat larger scale, we already commonly use sand blasting, as an example, as a way to increase bond strength when two surfaces are to be held together using an adhesive. Maybe ultrasonic “machining” would provide a means to improve bond strength on a much smaller scale – maybe in optical coatings, semiconductor and other applications. In fact, this effect may already be benefiting in instances where the operative term should not be cleaning but, rather, “machining.”
With the capability we now have to utilize a variety of ultrasonic (and megasonic) frequencies, power levels and waveforms, I believe it would be worthwhile to research this, as yet, poorly understood potential use for ultrasonics.
– FJF –