Until recently, I was only able to theorize about the mechanism involved in the perforation of aluminum foil by ultrasonic cavitation. I no longer have to theorize!
My theory had always been that perforation of aluminum foil was due to the repeated flexing of aluminum foil as cavitation implosions occurring on opposite sides of the foil bent the foil first one way and then the other until the aluminum foil fractured. Well, I was at least partially right. What I hadn’t anticipated was that some perforations are, apparently, caused by a single cavitation implosion event. Our lab here recently added a Keyence microscope which has opened the door to a greater understanding of this mechanism. Let’s start by looking at a picture of some foil that has been perforated by cavitation implosions.
The silvery area is the aluminum foil and the grey area are perforations of the foil. It is interesting to note that the repeated small perforations are nearly all virtually the same size, about 100 microns. This foil was exposed to 40kHz ultrasonics. Hole size may be a function of frequency but that is yet to be explored. When a number of these perforations occur together, a “lace” effect is created which eventually falls apart due to additional cavitation implosion events. It is also interesting that once the first perforation occurs, more seem to follow in the same area. This may be due to the fact that the residual aluminum particles created by the first perforation act as nucleation sites for the formation of additional cavitation bubbles in the same area. That will also be a subject for further study. It is obvious, however, that once several individual perforations occur they grow into larger and larger holes as cavitation implosions occur preferentially in the compromised area.
In the picture above, it may appear that all of the perforations are occurring in the same direction – that the foil is either being pushed in or out of the plane of the picture. This is not the case. The perforations originate from both sides. The picture below shows an edge view of foil which obviously has been deformed as a result of cavitation implosions on both sides.
Not all cavitation implosion events create perforations. The picture below shows some dimples that were caused by implosions that were not intense enough to cause total perforation. This may be due to the fact that they occurred too far away from the foil surface to deliver enough energy to perforate the foil. The red color indicates protrusion from the plane of the foil while the blue color indicates a depression.
Finally, using the capability of the Keyence scope, we can look at another piece of perforated foil in a moving three dimensional view as you will see in this video
I believe that the above pretty much confirm my theory that cavitation bubbles grow and implode on both sides of the foil. The perforations, however, seem to be due to the composite effect of many implosions. These photographs do not appear to confirm my original theory of repeated bending of the foil to fracture as I had expected. Upcoming blogs will move us forward in our understanding of this phenomenon. Stick around!