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10/31/2000
Bubbles Revisited
A year ago, I wrote a column about bubbles of various sorts. A
few weeks ago, we talked about the role of sound waves in the
evolution of galaxies. Now, let''s see what happens when we
combine bubbles with sound. Ultrasound is sound that is pitched
at such a high frequency that we can''t hear it, although our dog
might. You''ve probably had some medical experience with
ultrasound. In my case, it was used to confirm my gallstone and
a mitral valve prolapse. At Bell Labs, I occasionally used an
ultrasonic bath to clean glassware or other items. When it comes
to chemistry and ultrasound, "bubbles" is the name of the game.
My renewed interest in bubbles was stimulated by an article
titled "Sonochemistry" by Barbara Maynard in the American
Chemical Society publication Chemistry Summer 2000.
Sonochemistry, chemistry driven by ultrasound, depends on the
formation and collapse of bubbles, a process known as
cavitation. Like myself, sonochemistry was born in 1927, when
Alfred Loomis first found that ultrasound could affect chemical
reactions. Nothing much happened in sonochemistry until the
1980s, when inexpensive and reliable means of generating high
intensity ultrasound became available. If we could hear it, some
intense beams of ultrasound would be louder than a jet engine.
You can put a lot of energy into ultrasound!
Sometimes the apparatus used in ultrasound experiments is
borrowed from the medical field. The lithotripter is a device that
generates ultrasound that is used to break up kidney stones.
Lawrence Crum of the University of Washington used a
lithotripter to study single bubbles and "sonoluminescence". If
you missed my earlier column, one of the surprising things in
this field is this phenomenon of sonoluminescence, in which a
collapsing bubble produces a tiny flash of blue light. One theory
for the source of this light is that inside a collapsing bubble is a
very hot soup of electrons and ions. It''s sort of like the proton-
electron soup after the Big Bang that we discussed a few weeks
ago. The blue light is thought to arise in the bubble when the
electrons and ions bump into each other.
In the earlier columns, I mentioned that the actual temperatures
inside collapsing bubbles were thought to be anywhere from a
few thousand to a million degrees Fahrenheit. Measuring the
temperature inside a tiny bubble isn''t easy, as you might guess.
Enter Ken Suslick and his colleagues at the University of Illinois
at Urbana-Champaign. They went at the problem in two
different ways. First, they decided to use a kind of chemical
reaction involving organometallic compounds. Don''t worry
about what the reactions are. All we have to know is that Suslick
and company knew how the rate of these reactions changed as
the temperature increases. (Most reactions speed up with
increasing temperature.) They then studied how fast the
reactions occurred with ultrasound. From this, they could then
deduce the temperature in the bubbles. They got 5200 K (5200
degrees Kelvin or about 9000 degrees Fahrenheit), about the
same as the temperature on the surface of the sun!
They also measured the spectra, the pattern of light, emitted in
the sonoluminescence, a much easier measurement. This pattern
also depends on temperature. Their measurements yielded a
temperature of 5100 K, in super agreement with the 5200 K
obtained by the other method. However, both Suslick and Crum
point out that this is the temperature of the emitting surface in the
bubble. Comparing the bubble to the sun, you may know the
temperature on the surface of the sun but that doesn''t tell you the
temperature inside the sun. So, the temperature deeper inside the
bubble could be much higher. As I mentioned a year ago, some
hope it could be made high enough to induce nuclear fusion.
While I''m skeptical that fusion will be achieved in bubbles, there
are practical applications of sonochemistry in use today. One
example is in the production of microspheres. Microspheres are
little balls, smaller than red blood cells, that can be filled with
air or maybe a drug such as Taxol. The tiny spheres have walls that
are made of proteins formed when the collapsing bubbles form a
"superoxide", a compound of one atom of hydrogen and two
atoms of oxygen. (Compare with water, with two atoms of
hydrogen and one of oxygen.) This superoxide is the key to
forming the protein to form the walls of the microspheres.
Who cares about microspheres? You might if you have a heart
problem and the physician injects the air-filled balls into your
bloodstream. The tiny spheres go to your heart and increase the
contrast in an echocardiogram so the doctor can see the pattern of
blood flow in the heart. The application of microspheres for
drug delivery is in clinical trials now, for example, for the
treatment of breast cancer with Taxol. If you''re worried about
those spheres in your body, they''re biodegradable.
Ultrasound and collapsing bubbles may be of practical use
outside the medical arena. One possible application is in the
field of metallurgy. To appreciate the possibilities, let''s consider
first "splat cooling". This rather unsophisticated term was
applied a number of decades ago to a process in which molten
metal or alloy droplets were essentially shot at a plate made of a
metal such as copper. When the droplet hit the plate it would
splat on the surface just like a drop of rain splats on a
windowpane. However, when the metallic drop splats it
solidifies and, because the copper plate is a good conductor of
heat, cools very rapidly. This rapid cooling doesn''t allow the
atoms in the metal or alloy enough time to arrange themselves in
an orderly crystal structure, as they would do normally, e.g., as
when pouring a molten metal or alloy into a mold. The splat
materials are more like a glass than a crystalline solid.
With this in mind, let''s go back to sonochemistry and make some
iron from a reaction in a liquid solution that typically forms
crystalline iron particles. As Suslick and his colleagues did, let''s
apply high intensity ultrasound to this reaction. The iron atoms
get together at the high temperatures in the collapsing bubbles
and are cooled in the neighborhood of a million degrees a
second! Sure enough, like the splat materials, the iron particles
are not crystalline. So what, you say? Well, these "ultrasonic"
iron particles turn out to be soft, and like ordinary iron, can be
magnetized. This soft iron, though, loses its magnetism almost
immediately when the magnetic field is removed. This property
makes it ideal for use in transformers and in magnetic recording
heads. This ultrasonic iron is also a good catalyst for certain
reactions, one of which is a process for forming a liquid fuel
from coal, a prospect not to be sneezed at in this era of high gas
prices and dependence on foreign oil.
A year ago, I mentioned that in the late 1800s collapsing bubbles
were shown to be responsible for the erosion of ship''s propellers.
When a bubble collapses near a solid surface, a microjet of
liquid forms and shoots through the bubble at a speed of a few
hundred miles an hour. The accumulation of zillions of little jets
like this hitting a propeller can add up over time to a sizeable
amount of damage to the propeller. Now let''s apply ultrasound
to a slurry of metallic powder in a liquid. The shock waves and
microjets from the collapsing bubbles cause the powdered
particles to bang into each other at speeds as high as a thousand
miles an hour. This can cause all kinds of interesting things to
happen. For example, a high-melting metal particle of a metal
like tungsten can melt at the point of contact when another
particle slams into it at such high speeds.
An example of possible practical importance - ordinary nickel is
not a good catalyst, although an expensive, porous form of nickel
is. The reason that ordinary nickel performs poorly is that a
layer, possibly an oxide or some other compound(s), is present
on the surface. If you look at the surface of ordinary nickel
particles with a scanning electron microscope, you''ll see each
particle is covered with a bunch of nodules of some sort. So,
stick these particles in a slurry and pop the slurry into good old
ultrasound. Now look at the particles and they''re relatively as
smooth as the proverbial baby''s bottom! All those particles
hitting each other at high speeds have knocked off the nodules.
Now the nickel particles are a hundred thousand times better as
catalysts, about the same as their expensive porous counterparts.
Another promising application is in the area of cleaning up
polluted water. Michael Hoffmann at the California Institute of
Technology has been working on this approach. When you
apply ultrasound to water, you get not only the high temperatures
but also various highly reactive species such as the superoxide
mentioned above. The high local temperatures can help vaporize
some volatile pollutants, while those reactive species can react
with certain other pollutants to change them into less toxic and/or
more volatile compounds. So, if pesticides or other pollutants
have gotten into your water supply, maybe someday you''ll have
an ultrasonic zapper to clean it up so it''s fit to imbibe. Here''s to
your health! (For more on sonochemistry, Suslick, Crum and
Hoffmann all have informative Web sites.)
Allen F. Bortrum
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