Unprecedented actions were taken by the editorial board of
Science, a famous scientific journal of the US, for an article
published in the March issue of the journal in 2002. The editor
had to write an explanation for the publication of the article
which claimed that nuclear reactions were produced by sound
wave in room temperature with very a simple setup. The publication
of this finding was controversial, partly because it is difficult
to imagine that nuclear reactions can be produced in such
a simple experiment. In fact, the simple experimental setup
was described in details by a popular science magazine, Scientific
American (SC), a few years ago. Even high school students
should have the skills required to conduct the experiment.
A company offered a kit for the needed parts of the experiments
for less than US$100. This cost seems to be incredibly low
for a nuclear reaction experiment!
Of course, the SC article was not intended to
teach high school students to produce nuclear reactions in
school or at home. The article only gave instructions on how
to produce light from sound. However, the scientists from
a national laboratory claimed that they found neutrons from
a similar experiment which indicated that nuclear reaction
had taken place in the experiment. Figure 1 is the schematic
diagram of the experiment described in the SC article.
(Figure 1. The setup of sonoluminescence)
The main apparatus of the experiment is the spherical glass
flask filled with water in the diagram. The main point of
the experiment is to suspend an air bubble in the middle of
the flask by using sound waves. The mechanical vibration of
the sound wave is generated by the two piezo-electric
transducers glued to the flask as shown. The
piezo-electric transducer can produce mechanical motions when
an electric energy is applied to them; similar to the loud-speakers
used in stereo-systems in our homes. The signal generator
and the power amplifier shown in the Figure are used to provide
the signal and energy needed by the transducers. From the
schematic diagram, the design of the experiment is similar
to the cleaning chamber of an ultrasonic cleaner which we
usually see in shops selling eye-glasses. The difference is
that the experiment discussed here is specially designed not
only to suspend a single air bubble in the middle of the flask
but also to provide the needed conditions to make the bubble
glow. If the experiment is performed in a dark room, the suspended
bubble will look similar to the dim stars in a night sky;
emitting blue light, visible to the naked eyes. First-time
viewers of this light-emitting bubble would always complain
at first that nothing can be seen but then be amazed by the
brightness of the little "star" and its charm.
(Figure 2. An experimental setup of sonoluminescence:
the dot in the middle of the flask is the sonoluminesence
glow. This picture is obtained with permission of Wan Kwok
Tung Chris, Dr H F Chau and Dr F C C Ling of HKU Department
of Physics.)
Ship builders were among the first to realize that bubbles
can produce huge amount of energy and damage. They found that
many small cavities will appear on the surface of the propeller
after a ship had been traveling at high speeds. These cavities
seemed to be blasted off the surface of the propeller by some
sharp objects in the water. They were puzzled by these unusual
damages of the propeller because the origin of these sharp
objects was unknown until they discovered that a huge amount
of bubbles could be produced on the propeller when it was
rotating at high speeds. These bubbles were not in the water
initially. They are created by the fast rotating propeller
which acts like a "knife" to cut open the water
to release the dissolved gaseous in it. However, these bubbles
are highly unstable and will disappear when they contract
and dissolve back into the water after the "knife"
is gone. If some bubbles are attached to the surface of the
propeller, a huge amount of energy will be released and concentrated
to a small point and therefore producing the observed cavities
when the bubbles collapse. Bubbles are common in ultrasonic
cleaner. The cleaning properties of these machines are probably
related to the collapses of these bubbles; similar to the
damage of the propeller.
- Ultrasonic Cleaner
(Ultrasonic Cleaner)
Ultrasound is used most frequently in Chemistry
among other fundamental studies. Extreme conditions such as
high temperature or pressure are needed for some important
chemical reactions to proceed. Sometimes, these extreme conditions
are inconvenience and ultrasound is found, by accident, to
be a good alternative to replace these conditions. Some reactions
requiring high reaction temperature can be finished even in
room temperature when carried out under the irradiation of
ultrasound. Bubbles can always be seen in chemical reactions
under ultrasound together with short flashes of light. In
the beginning, chemists are not too interested because these
randomly created bubbles always move randomly and collide
with the wall or each other. Many believed that the flashes
of light are created during the collisions; similar those
created when two piece of stones are rubbed against each other.
Also, it is difficult to perform detailed studies on these
randomly moving bubbles. However, a breakthrough occurred
in 1993 when Prof. Putterman was able to suspend a single
light emitting bubble in a setup similar to the one shown
in Figure 1. This experiment put to death the hypothesis that
the observed light is created by friction.
As the single bubble created by Prof. Putterman is stationary,
detailed properties of this light emitting bubble can be studied.
Now, it is known that the radius of the seemingly motionless
bubble is in fact undergoing periodic changes. The period
of these changes is the same as the imposed sound wave used
to suspend the bubble. The maximum radius of the bubble can
be as large as a few tens of microns while the minimum radius
of the bubble is almost zero. The average size of the bubble
is a few microns, visible to the naked eyes. Light is emitted
only once every cycle when the radius of the bubble is at
its minimum for a period of about 50 pico-seconds. Since the
dim light we see from the bubble is the average value of the
emitted light intensity during one cycle, one can image how
strong the light intensity is during the short period in which
light is actually emitted. Another surprising property of
the bubble is that the light emitted has spectrum (characteristics)
similar to what is known as black
body radiation. If an object is emitting light
following the law of black body radiation, the temperature
of the body can be inferred from the measured spectrum. The
light emitting from burning charcoals in a hot furnace is
close to the black body radiation. The colour of the charcoals
will change from red to yellow and finally to blue when the
temperature of the furnace is getting higher and higher. There
is an old Chinese idiom which uses the blue color of a burning
furnace to mean a very high temperature. If the black body
radiation model can be used to fit the measured characteristics
(spectrum) of the light emitted from the single bubble, the
temperature inside the bubble is found to be extremely high;
higher than that of the sun! It is known that energy and light
are created by nuclear reactions in the sun. With this in
mind, it is not difficult to understand why scientists are
looking for sign of nuclear reactions in this light emitting
bubble.
The phenomenon of producing light from sound is called sonoluminescence.
The basic mechanism of sonoluminescence by itself has already
been generating heated debates and the claim of neutrons emitting
from the bubble will definitely create more controversies.
Since the energy associated with sound waves is usually many
times smaller than that of light, a commonly raised question
is: how can the bubble convert the energy carried by the sound
waves into light in such a large scale? At present, a probably
over-simplified explanation is that during the very short
period when the radius of the bubble is nearly zero, the bubble
concentrates all its mechanical energy to a very small point
and thus raises the temperature of this point to such an extent
that photons (light) are created. This process is similar
to the ignition of a piece of paper by using a magnifying
glass under the sun. The magnifying glass must focus the light
from the sun to a very small point on the paper in order to
ignite the paper. But no matter how good the magnifying glass
is, we know that it cannot be used to generate temperature
as high as the sun. How can the bubble concentrate so much
energy? How small is the bubble? These simple questions are
still waiting for satisfactory answers. All these questions
can only be settled when the physics of the bubble is well
understood. Although the evidence of nuclear reaction is controversial,
we are sure that this light emitting bubble will continue
to generate excitements in the future.
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