Physics: Bubbling Hot
By Henry Gee
October 21, 1999
In terms of physics, sound is the consequence of waves of compression and tension
passing through a medium. If a really energetic sound is blasted through a liquid, a
wave of tension can literally pull the liquid apart, forcing bubbles into existence.
These bubbles contain gases that would otherwise be dissolved in the liquid -- the
bubbles feel the squeeze when the tension is succeeded by a wave of compression,
and their contents heat up dramatically. Theory predicts that temperatures in
bubbles generated by this process, known as 'acoustic cavitation' may reach
thousands of degrees Kelvin, comparable with the surface of the Sun.
Perhaps understandably, acoustic cavitation presents a few experimental
challenges: good measurements inside a hot, cavitating bubble are scarce, and
nobody has studied how temperature is influenced by external parameters such as
the composition of the solution used in the experiments. Enter Kenneth S. Suslick
and colleagues from the University of Illinois and Urbana-Champaign, Illinois,
who have come up with an ingenious method to take the temperatures of cavitating
bubbles. They present their findings in a report in Nature.1
Given that inserting a thermometer into a cavitating bubble is impossible, Suslick
and colleagues get the solution to report its own temperature. Chemical compounds
containing the so-called 'transition' metals -- such as iron, and less familiar
elements such as molybdenum and chromium -- are inherently colourful by virtue
of peculiarities in their electronic structure. Some of these compounds emit light of
a given colour the intensity of which is a direct measure of temperature. Suslick
and colleagues show that if solutions of these metals are subject to acoustic
cavitation, the intense light emitted by compound trapped inside cavitating bubbles
will provide a direct reading of temperature inside the bubbles. Using this
technique, the researchers establish beyond doubt that cavitating bubbles in a
solution of the compound iron hexacarbonyl may reach 5,100 kelvin.
Now that the researchers have a precise probe of temperature, they can get to work
on understanding the factors that influence temperature. The chemical composition
of the solution is an important determinant: temperatures in solutions of
molybdenum hexacarbonyl, for example, do not get as high as in an iron
hexacarbonyl solution. But what makes the bubbles so hot to begin with? The
researchers think that there is more to it than simple compression. One reason is
that the heated bubbles become tiny chemical reactors, the consequences of which
will naturally change the environment inside the bubble.
1 McNamara III, W.B., Didenko, Y.T. & Suslick, K.S. Sonoluminescence
temperatures during multi-bubble cavitation. Nature 401, 772 (1999).
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