Introduction
Sonoluminescence is the phenomenon in which sound is directly converted into light energy. Here an air bubble trapped within a liquid in a flask when exposed to sound waves emit light.This is because the waves generated in the liquid by the sound compresses the bubble suddenly to such an extent that the air within the bubble compresses and temperature of the gas increases enormously which is visible to us in the form of a bluish star.
So this increased tempereture can be utilized for nuclear fusion, and obtain energy!!!
Sonoluminescence was first
observed in an ultrasonic water bath in 1934 by H. Frenzel and H.
Schultes at the University of Cologne, an indirect result of wartime
research in marine acoustic radar. This early work involved very strong
ultrasonic fields and yielded clouds of unpredictable and
non-synchronous flashing bubbles, now termed "multi-bubble
sonoluminescence". Such a chaotic phenomenon did not lend itself to
detailed scientific investigation. Study of sonoluminescence then made
little progress until 1988, when D. Felipe Gaitan succeeded in trapping
a stable sonoluminescing bubble at the centre of a flask energised at
its acoustic resonance - single-bubble sonoluminescence (SBSL). However
their interest soon waned, and the research was subsequently taken up
by Dr S. Putterman et. al., at UCLA, California. Putterman pursued
SBSL, published numerous papers, and established many of the
characteristics which are now taken for granted. Once per acoustic
cycle, coincident with a sharp decrease in bubble size, bluey-white
light is emitted in a brief flash shorter than 100picoseconds in
duration, with incredible regularity. Despite the results that have
been obtained, the actual mechanism by which sound is converted to
light remains elusive, not least because of the difficulty in measuring
the conditions inside a pulsating bubble whose diameter is measured in
micro-meters. It is generally agreed that the adiabatic compression of
the bubble leads to very high interior temperatures, but beyond that,
shocks, plasmas, ionisation and photo-recombination, Bremsstrahlung
radiation, and even fusion are all hotly-debated possible explanations.
In Scientific American, February 1995, Putterman published an
introductory overview paper on sonoluminescence together with a
practical guide in the "Amateur Scientist" section of the same issue.
By making the subject accessible to a wider audience, interest
escalated dramatically, and given the apparent ease with which
sonoluminescence could be obtained, many university groups attempted
it. A revised and more detailed version of their "Amateur Scientist"
guide can be found on the World Wide Web at http://www.physics.ucla.edu/~hiller/sl/, and is maintained by Robert Hiller, a student of Putterman.
Within the Physics Department here at UCL, sonoluminescence was offered
last year (1996-97) as a final-year undergraduate project.
Unfortunately the quest for the glowing bubble proved fruitless for
those involved... a story which has been echoed by several other groups
around the world. So, I was "commissioned" by the department to get the
experiment up and running over the summer, ready for next year.
I too used the Scientific American article as my primary guide, but
successfully "saw the light" within a couple of weeks. I found the
article to be quite correct as far as it goes but that, probably due to
the constraint of brevity, it fails to emphasise some significant
details as much as might be desired. Combined with the somewhat
optimistic impression of the ease with which sonoluminescence can be
obtained, this may explain others' failed attempts and subsequent
discouragement. I attribute my success to extensive experience in
hands-on practical work and electronics, and ready access to a variety
of electronic bits and pieces from home!
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Detailed Method
Preparation of a flask for use as a sonoluminescence vessel
We scoured glassware catalogues for suitably narrow-necked 100ml
round-bottomed flasks. Finally we chose a distillation flask [Griffin
and George part FHD-230-030G], which happened to be ordinary
soda-glass. As supplied, its very long neck had an extra tube
protruding from the side; we truncated the neck just below this to
leave about 7cm of parallel-side neck. The flask was thoroughly
washed, using hot water and Fairy Liquid (the original, not Lemon
variety), and rinsed. The final rinse was with cold distilled water.
Three piezoelectric transducers, as described by Hiller, were supplied
to us by Channel Industries Inc. The two larger transducers, 20mm
diameter by 4.4mm thick (and with an 8mm diameter hole through the
centre) were used as the drivers, and the smaller transducer, 6mm
diameter by 2mm thick, was used as a microphone.
Lightweight
hook-up wire (7/0.2mm) was cut into ~10cm lengths to make the leads for
the driver transducers, and the ends stripped of insulation and
"tinned" with solder. To reduce mechanical stresses on the connections
the leads were coiled, by winding them around the inside tube of a
biro. Three very small (approx. 2mm dia., 0.5mm high) dabs of solder
were then applied to each silvered surface of the transducer, just
inside the outer circumference. The exposed part of one end of each
lead was cut down to a mere 2mm, and then held over a solder dab on the
transducer and gently pushed down with a soldering iron for a second or
so, to make the connection. Soldering was carried out quickly as the
silvering on such transducers is liable to be destroyed if too much
heat is used. It was also discovered that a mild electric shock can be
received from the transducer during thermal expansion/contraction
following soldering!
Transducers are polarised - applying a DC
voltage one way round causes the ceramic to expand, while opposite
polarity results in contraction. To ensure all transducers in an
application can be wired in phase, they are supplied with one side
marked with a red cross. By convention I wired the red sides of the
transducers (with red wires) to the signal, and the reverse (with black
leads) to the ground. The solder connections described were small
enough that they didn't touch the surface of the flask when the
transducer was held against it. Photo 2 illustrates the connection to
the transducers.
Photo 2: Wired and mounted transducer
For the small microphone transducer, the lightest grade screened audio
cable was used (about 2mm outside dia.). About 4cm of outer sleeving
was stripped off, and the inner covered wire and screen separately
coiled. The end of the inner wire was prepared as above, and soldered
to the red side of the transducer using just one tiny solder dab. The
screen was soldered to the reverse.
These more rugged connections (compared to Hiller's design) didn't have any adverse effects on the acoustics.
Figure 1: Flask, showing location of transducers
Judged by eye, target positions for the transducers were marked on the
flask using an OHP pen. The small transducer was to go at the very
bottom of the flask, and the two larger ones diametrically opposite
each other on a horizontal axis through the centre of the flask -- see
figure 1.
Araldite Rapid was mixed, and a sufficient coating
applied to the first transducer site on the flask. Using a three-finger
clamp, the flask was then gripped with that site facing downwards. The
transducer was placed on a paper towel on the side of a
individual-portion breakfast-cereal packet (a "springy" support!) and
working swiftly, Araldite was applied to its face. Finally the flask
was lowered on to the transducer and left for 20minutes to set.
For safety reasons, the red sides of the transducers, later to be connected to the several hundred volt drive signal, were glued towards the glass where they cannot be touched.
Once the first transducer had stuck, fresh Araldite was mixed and the
procedure repeated for the next driver transducer, then again for the
microphone.
Electrical circuit
Stable trapping of an air bubble at the centre of a flask requires an
acoustic standing wave in the water, so the flask must be excited at
its natural resonance. When filled with water, a 100ml round-bottomed
flask resonates at approximately 25kHz. Piezo-ceramic transducers
are the standard "loudspeaker" devices for ultrasonics but, unlike
hi-fi speakers, they require several hundred volts to drive them and
draw very little current. Because the piezo transducers behave
electrically like capacitors, a suitable inductor can be wired in
series with them to form a tuned circuit, which "matches" them to the
loudspeaker-output of an audio amplifier -- see figure 2. The inductor
trades current for voltage, and while presenting a low impedance to the
amplifier (output swing around 40Vp-p), generates the necessary 700V or so across the transducers.
Figure 2: Piezo transducer drive circuit. The section shown in red operates at high voltage.
Figure 3: Physical wiring diagram.
The two drive transducers were wired in parallel by soldering the two
red wires together, and the two black wires together. Those junctions
were each soldered to the core of separate 1metre lengths of standard
50ohm coaxial cable, and the joint covered with sleeving for safety. In
each case, the screening of the 50ohm cable was cut away where
it emerged from the outer covering. Both the 50ohm cables, and the
microphone cable, were firmly clamped to the retort stand using cable
ties, to prevent tension on the connections to the transducers. BNC
plugs were fitted to the other ends of the two cables, and plugged into
to a junction unit linking in the 1ohm resistor and the inductor -- see
figure 3. The core of the cable to black wires connects to the 1ohm
resistor (current-sense), and the core of the coaxial cable to red wire
to the inductor.
By keeping all the cable screens at ground
potential in this way, a BNC lead from a scope could be inserted at any
point in the circuit using a T-piece without causing short-circuits.
The use of screened cables is essential to minimise radiation of 25kHz,
which might otherwise interfere with the low level signals involved in
measurements.
Given the capacitance of the transducers and the
frequency at which we operate, an inductance of around 30mH is required
to form a resonant circuit. The inductor needs to be made of thick
enough wire to take the required current, but not so thick that it
becomes physically huge, which might also lead to a fall in
performance. Our "best" inductor consisted of a partially-filled 500g
plastic spool of 0.5mm diameter (24swg) enamelled copper wire, tuned by
sliding a ferrite rod (usually sold as MW/LW radio aerial) up and down
the centre of the spool. It was found that to prevent power loss to
eddy currents, owing to the large fringing field, the coil had to be
sited well away from any conducting object - particularly that the axis
of the coil did not intercept any nearby metal.
The wiring and
coil were measured to have a DC resistance of about 10ohms (bypassing
the transducers of course), and present an impedance of about 100ohms
to the amplifier when tuned to resonance. It is this impedance which
determines the upper limit on the power that can be transferred to the
transducers given a finite voltage swing from the amplifier. In
practice, we found that a mere 1Watt of power needed to be transferred
to the drive circuit to achieve sonoluminescence.
Signal Generator
Although Hiller implies that any lab signal generator will do, we found
some to be useless, and most general purpose instruments frustrating at
the least. It is vital that the amplitude of the sinewave output does
not jump about as the frequency is varied, and that the frequency
varies smoothly, also without jumping about. To find the resonances of
the flask, it is essential that the frequency can be set to within 30Hz
or better at 25kHz. Common [old-fashioned, analog]*
signal generators [of the type you find in Universities]* typically
sweep a decade of
frequency in one revolution the knob, making such precise adjustments
difficult.
The generator needs to be stable, not drifting too much with time. Our
first
success was obtained with an off-the-shelf lab generator, but very soon
I
decided to build my own instrument. It sweeps about 24-27kHz in one
turn of
the knob, and is stable to within a few Hertz after having been on for
a short
while. The frequency was measured by connecting the separate
square-wave output of the signal generator to a digital frequency
counter. Note: With some waveforms and signal levels, frequency
counters are liable to report wildly incorrect readings. Occasional use
of a scope to check the readings are reasonable is mandatory!
* If I'd used a modern digitally-synthesised signal generator such as the
Fluke/Philips PM5138A (the luxuries I now get working in an Industrial R&D lab!),
then I probably would have avoided a lot of problems! - December 2004
Preparation of the water
Air bubbles can only be driven to sonoluminescence in water which has
substantially less than the usual amount of dissolved air. Previous
researchers have reported that a partial pressure of around 150mmHg,
one fifth of atmospheric pressure, is ideal. Water can be "degassed"
either by boiling, or by evacuating the space above it in a sealed
flask. We adopted the former method, being simpler - though less
controllable. A 100ml quickfit flask was just-over half-filled
with distilled water, then heated in an electric mantle at full power,
and maintained at a rolling boil for 15minutes. After switching off the
heat a rubber bung was gently placed into the neck of the flask before
leaving it to cool. As the water cools the bung is sucked in and leaves
a vacuum above the water, preventing more gas dissolving back in.
Sometimes cooling was accelerated by holding the bulb of the sealed
flask under running cold water, and gently swirling its contents. With
the bung kept in place, the water can remain degassed for many days if
required. The procedure was repeated with a second 100ml flask, so that
a quantity of over 100ml of de-gassed water was prepared.
For
the experiment, the sonoluminescence flask is held in a 3-finger clamp.
If the flask contained water from a previous run, then that was poured
away by lifting and tilting the entire stand (not just the flask, which
could lead to breakage of the wires or connections). There was a
tendency for the water to gain dust, fluff, broken glass... etc., so
the last of the water was always swirled out to remove any debris.
Holding the stand at about 45 degrees, the freshly-degassed,
room-temperature water was poured into the sonoluminescence flask,
letting it run down the side. The flask was filled to the top of the
spherical part; I tried to fill so that the lowest part of the meniscus
is in a position to form a continuation of the spherical shape, but a
millimetre or two either side didn't seem too critical.
Tuning up
With the flask now full of water, it is necessary to tune the signal
generator to the best resonance of the flask. Having settled on an
acoustic resonance, the inductor is then tuned to maximise the drive to
the transducers. The microphone transducer was plugged into the
oscilloscope input. As confirmation that all is well, if the scope
sensitivity was turned up (and a fairly low sweep rate selected), the
trace would jump if the flask was gently tapped.
The
amplifier, signal generator and frequency counter were wired up and
turned on. Before proceeding, the signal generator was allowed to
stabilise - with my generator about 5minutes was found to be ample.
If the ferrite-rod was in the coil (inductor), it was removed at this
point, before tuning up. Care was taken not to drop the rod as they are
very fragile.
With the scope timebase on 10µs/div, and
sensitivity on 0.1V/div., the gain control on the amplifier and/or
oscillator was adjusted to get a reasonable trace on the scope. It was
sometimes useful to also monitor the direct output from the amplifier
on the second channel on the scope (5V/div) to ensure the waveform was
not clipping (as it might if the levels are set too high).
Sweeping through the range of my signal generator (23-26.5kHz),
acoustic resonances were located by watching for maximal amplitudes of
the microphone signal. One was usually found near 25.5kHz and another
at 25.1kHz at room temperature. It is not unusual for the waveshape
from the microphone to look skewed or distorted between acoustic
resonances. Confirmation that an acoustic resonance has been found is
obtained by momentarily touching or gently squeezing the flask - the
microphone signal level drops appreciably as the vibrations are damped.
Having located the strongest acoustic resonance, the ferrite
rod was placed in the coil and slowly moved in and out to find
electrical resonance, which further maximises the signal from the
microphone. Because this increased the drive considerably, it was
necessary to change the scope sensitivity to 1V/div.
Trapped-bubble behaviour
With experience, it became much easier to trap a bubble and make it glow. Patience was essential, particularly at the outset.
Back-lighting was used to make the tiny bubbles visible: a small torch
was placed behind the flask, directed towards its centre.
With the electronics tuned up, the drive amplitude was adjusted to obtain a microphone signal somewhere between 1 and 7Vp-p. With no bubble present, the trace on the scope should be a clean sine wave.
Using a narrow, clean pipette, a small amount of water was withdrawn,
the pipette tip lifted above the water surface, and the contents gently
dropped back into the flask. This action induces some bubbles, and with
practice, a force sufficient to generate just one or two bubbles can be
judged.
At the instant the bubbles are injected, the
microphone signal usually decreases in amplitude and fluctuates in
waveform for a few seconds, before an equilibrium is established.
The general behaviour of the bubble at different sound levels and dissolved gas content is summarised in figure 4.
Figure 4: Dependence of bubble characteristics on sound intensity and dissolved-gas content.
Variations in the threshold levels were observed between runs, with the
upper SL level lying anywhere from 3V to 7V. There are inherent
practical difficulties in obtaining an accurate record of the form
above - when does "a long time to dissolve" become "does not dissolve"?
Repeatedly injecting bubbles will invariably lead to increase in the
dissolved-gas content, perturbing the very system we are trying to
measure. Despite these difficulties, the chart gives a good indication
of the main characteristics, namely:
If the sound level was high, say anywhere above the upper dancing threshold, then injected bubbles were found to "stream" very rapidly to the flask centre. Above the upper SL threshold they just disappeared at the centre, while below the threshold a bubble remained after the streaming. At lower sound levels the bubbles take a more leisurely drift to the centre. Whenever the trapped bubble was lost, or could not be seeded, the sound level was adjusted, and another bubble injected. A typical description of bubble-behaviour in intermediately degassed water would run as follows:
"A level of 1 to 1.5 volts p-p on the microphone was sufficient to trap the bubble at the centre of the flask. At around 2.7 volts the bubble began to dance about the centre, spanning a distance of a couple of millimetres. With more power still, the bubble became slightly larger and "fuzzy". At 4Vp-p the jittering started to subside, and between 4.1V and 4.5V the bubble was very small, and stable. At the uppermost end of this range, the bubble visibly glowed if the room was dark. At about 4.5V the bubble suddenly disappeared altogether."
Viewing the glowing bubble
With a small stable
bubble, trapped by a sound level just below the upper threshold at
which the bubble can exist, the room was blacked out and the backlight
switched off. It was found to be helpful if there was still just enough
room light to be able to make out the flask. A faint bluey-white glow,
like starlight, was seen from the bubble in the centre of the flask -
sonoluminescence! If the drive level was cautiously increased, the glow
from the bubble could be made brighter, but too great an increase
caused the bubble to re-dissolve and be lost. When lost, the drive
could be backed off a fraction, and a new bubble dropped in. After
stabilising for a second or so, the new bubble glowed as before. Under
all circumstances, best sonoluminescence was observed when the flask
was clamped as loosely as possible in the stand.
Interpretation of the microphone signal
When the amplitude is in the "fuzzy, jittering bubble" region or
higher, the microphone trace on the scope will normally have some kind
of ripple superimposed -- see figure 5. Depending on the amplitude of
the ripple however, it may only be apparent as a mild distortion of the
underlying sinewave. 
Figure 5: Ripples seen on the microphone signal when a trapped bubble is present.
The strength of the ripple depends on the exact driving frequency used,
and the tightness of grip of the flask in the stand - the loosest grip
gives the brightest SL bubble and largest ripple. The ripple is highly
characteristic and moves along the fundamental waveform as the drive
amplitude is adjusted. It moves rightwards (later in time) as the drive
is increased, and when the bubble is glowing at its brightest, at the
upper sonoluminescence threshold, the ripple has a peak at the centre
of the crest of the main wave and also causes the trough of the wave to
flatten out. The ripple appears to be a harmonic resonance of the
flask, excited by the shock wave of the collapsing bubble. Hiller et al
(private correspondence) have described placing a microphone in the
water right next to the bubble and observing merely an instantaneous
"click" at the point of collapse. When the drive is increased too far
the bubble is lost and the scope trace returns to a pure sinewave --
see figure 6.
Figure 6: Ripples disappear with the bubble if the amplitude is driven too high.
If the scope trace is not steady then something is wrong, and the bubble won't glow.
In this case, or just to increase the brightness further, fine
adjustment of the drive frequency (up to 30Hz either way) may help.
Maximising the amplitude of the ripples on the trace can be a useful
guide.
Temperature-dependence of bubble brightness
A bucket of dry ice was positioned on its side above the flask in such
a way as to allow the cold gas and cooled air to flow around it. This
method enabled the flask to be cooled down to nearly freezing by a
non-contact means, thus not changing the acoustics. It was observed
that the frequency of the acoustic resonances in the flask decrease
with temperature, being about 300Hz lower near freezing point. In
our particular flask, the 25.5kHz resonance was best at room
temperature while below about 16C the 25.1kHz resonance became far
better. [The temperature of the water was measured by inserting a
digital thermometer probe into the water - but it was found that the
probe gave bizarre readings while the sound was on.]
Qualitatively, it was observed that the bubble got much brighter when
the water was cooled. Subjectively it also appeared (when back-lit)
that the bubble was larger than at room temperature, though this was
not confirmed.
Summary of findings and typical figures
Water: distilled, boiled for 15minutes then sealed and left to cool under own vacuum. Transducer drive: of the order of 700Vp-p swing required.
Inductor specification:
around 30mH. Partially-used 500g spool of 0.5mm diameter enamelled
copper wire, tuned by sliding a ferrite rod up and down the centre of
the plastic spool.
Microphone output: typically between 3 and 7Vp-p for sonoluminescence. [Better guide than drive levels]
Frequency generator minimum spec.: tuneable to within 30Hz at 25kHz. Stable to within 30Hz over ten minutes or more. The output level must
remain constant during adjustment of frequency, and the adjustment must
be smooth with no jumping about. Watch it on a scope! My experience is
that the typical generator found in an undergraduate lab is barely up
to the task, though my initial success was with such an instrument.
Highly desirable: Tuneable/stable to 10Hz or better. Use a modern
digitally-synthesised signal generator (such as the Fluke/Philips PM5136 or PM5138A. Agilent also seems to have a good series of function generators (although I have no personal experience of these))
and save yourself a lot of trouble!
Flask clamp:
the strength of the acoustic resonance is sensitive to the way the
flask is mounted - with our three finger clamp we found that the loosest
grip gave the strongest resonance and brightest sonoluminescent light.
A clamp with cork 'grips' seems to work much better than one with
rubber-covered fingers.
Flask resonance: the acoustic
resonant frequency of the flask decreases with temperature, falling by
about 300Hz from room temperature to near freezing. I actually found
two resonances, at 25.5kHz and at 25.1kHz - the former was stronger at
room temperature, and the latter much better below 16C. The two
resonances are probably specific to our flask.
Flask temperature:
the water-filled flask has a fairly large thermal mass, so
temperature-induced resonance variations are quite slow.
Sonoluminescence is however much brighter at the lower temperatures -
below about 10C is really quite bright. Much above 20C it becomes much
harder to see the glow.
Bubble size: bubbles reach an
equilibrium size for a given set of parameters. Other researchers have
established that a sonoluminescing bubble cycles its size between about
50microns and 0.5microns with each acoustic wave.
Flash: Putterman et al report that the upper limit on the flash duration is 100ps, and at room temperature around 106 photons are emitted in each.
Measuring bubble size - Mie scattering
Mie scattering describes the interaction of light with an object whose
dimensions are comparable to its wavelength. Integrated over a
sufficient angular region, the scattered intensity is proportional to
the cross section of the object. Hence by shining a light beam on one
of our tiny bubbles, its radius can be monitored by measuring the
amount of light scattered in a given direction. Towards the end of
my time on the experiment I built an op-amp amplifier for a BPX65
high-speed photodiode, and encased the diode and amplifier in a small
die-cast box for electrical screening. With a solid-state laser beam
focused on the bubble using a 2" focal length eye-glass, scattered
light was collected by a 1" focal-length lens and focused onto the
photodiode. Output from the diode amplifier was fed to an oscilloscope,
giving a trace of scattered light intensity. With the well-focused
beams, optical alignment was quite tricky, and results correspondingly
variable. Nevertheless, it proved the idea and equipment worked, and
excellent (qualitative) plots of bubble radius vs. time were obtained
-- see figure 7. These results were completely consistent with those
published by Hiller et al. With more time, careful setting up, and
calibration, quantitative measurements could be taken, and
relationships of bubble-size against water temperature, gas content, or
drive level etc. could be investigated.
Figure 7: Oscilloscope traces showing: 1 the signal from the microphone attached to the bottom of the flask, and 2
photodiode signal for laser light scattered by the bubble (relating to
bubble radius). I would like to emphasise that the Mie scattering
results shown are obtained from a single acoustic cycle, and they are real data - not some theoretical best fit!!!
Conclusion
It was greatly encouraging to witness sonoluminescence relatively soon
after commencing work on this project. After the initial success, the
drive arrangements and signal generator were rapidly developed to
improve the control and stability of the bubble. At first, the
behaviour of the bubble seemed temperamental, but over the following
weeks a 'feeling' for the setup was gained, particularly the
recognising of characteristics due to different dissolved-gas
concentrations and flask grip. Eventually, most phenomena were fitted
into a wider picture and became explicable and reproducible.
Consistent with published results, the brightness of sonoluminescence
was seen to increase as the drive level was increased, and as the water
was cooled. Regrettably, but for largely logistical reasons, a
photomultiplier or similar device for quantitatively measuring bubble
intensity was not employed in this study. Despite its simplicity, the
Mie scattering configuration set up in the final days of the project
gave extremely promising results, with plots of bubble size against
time being in almost exact agreement with those of Hiller et al., save
for an absolute scale. With a little more care in setting up and
alignment, and overcoming the problem of the bubble changing its
position in the flask as the acoustic drive is increased, the
measurements could be made quantitatively and more easily. This would
enable the relation of bubble-size to water temperature, as well as
drive-level, to be investigated.
Altogether I am confident
that the requirements of the project brief have been excelled, with a
reliable sonoluminescence apparatus constructed, the initial problems
overcome, major characteristics verified -- and a great deal of
expertise gleaned! I await with interest the results of subsequent
studies.
Courtesy:
Source: http://www.techmind.org/sl/
©1997-2005 William Andrew Steer
andrew@techmind.org
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