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Subject: Ultrasound


From: Stefan Michalski <stefan_michalski>
Date: Friday, May 22, 1998
N. Caldararo asks why we did not cite his article on the dangers of
ultrasonics on paper artifacts ("Tests on the effects of the use of
ultrasound in the humidification of paper," Book and Paper Group
Annual, volume 11, (Washington DC: Book and Paper Group,  AIC, 1992)
pp1-20. The answer is that we did not consider it relevant to our
technique, the consolidation of powdery paints by ultrasonic mist.

The long answer begins with clarifying the nature of ultrasonic
effects. All quotes in this  paragraph come from a very recent and
readable review by K.S. Suslick, "Sonochemistry", Kirk-Othmer
Encyclopedia of Chemical Technology, 4th ed., Supplement Volume,
1998: "The origin of sonochemistry and sonoluminescence is acoustic
cavitation: the formation, growth, and implosive collapse of bubbles
in liquids irradiated with high intensity sound....In clouds of
cavitating bubbles, these hot-spots have equivalent temperatures of
roughly 5000 K, pressures of about 1000 atmospheres, and heating and
cooling rates above 10^10 K/s." p. 516  " When liquids that contain
solids are irradiated with ultrasound, related phenomena can occur.
When cavitation occurs near an extended solid surface, cavity
collapse is nonspherical and drives high speed jets of liquid into
the surface. These jets and associated shockwaves can cause
substantial surface damage and expose fresh, highly heated
surfaces." p. 517 and "The chemical effects of ultrasound do not
arise from a direct interaction with molecular species... no direct
coupling of the acoustic field with chemical species on a molecular
level can account for sonochemistry.." p, 517.

Thus, without a liquid carrying the ultrasonic  wave, there is no
means of bubble formation, no impressive cavitation phenomena.  All
soundwaves, and particularly high frequencies like ultrasonics, do
not cross an air/liquid or air/solid interface well at all, due to
severe impedance mismatch. They are reflected. What little energy
does cross such interfaces diminishes rapidly in intensity  with
distance, (square law for the small sources we are concerned with).
Otherwise, ultrasonic humidifiers would drive dogs insane for miles
(kilometres). Our apparatus generates mist in a bottle of liquid. A
slow stream of incoming air pushes the mist/air mixture down an exit
tube to the artifact. The source bottle is typically a meter or more
from the artifact. It is not at all analogous to an ultrasonic bath,
or the deceptively "dry" Cavitron dental tool for example, which is
very close to what it cleans, and which must be connected by a
liquid droplet between the probe and the surface being cleaned.

In Caldararo's compilation of articles on observed physical or
chemical effects, every example deals in ultrasonic exposure
mediated by a continuous liquid between source and target.
Unfortunately, this leaves one with the only evidence provided to
suggest damaging ultrasonics leaping across a considerable air gap
as Caldararo's own study of damage to typing ink on paper. Clearly,
the purpose of the paper and the modest experiments was to provide a
warning about possible side-effects, to encourage more thorough
study, which we all agree is "a good thing", and completely
reasonable in the immersion bath use of ultrasonics. However. I have
never thought an open dist-list is appropriate for "refereeing" a
paper after the fact, but the author has publicly asked why I
ignored his article (Which I did on purpose, not by ignorance.  It
is my job to read thoroughly.)  To give a detailed critique of the
data in this venue would be inappropriate, and would place me well
into the evil empire of arrogant scientists.  I will simply say that
I am not convinced by the published evidence. There are more
plausible explanations for the images in the micro-photographs other
than Caldararo's speculation that "while the energy delivered by
humidification cannot be great , there may be some affinity between
the matrix components (of ink on paper) and that energy."

Stefan Michalski

                  Conservation DistList Instance 11:95
                   Distributed: Friday, May 22, 1998
                       Message Id: cdl-11-95-001
Received on Friday, 22 May, 1998

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