Reinhard's Experimental Physics Letters (unpublished) 5/1996

Single Bubble Sonoluminescence HOWTO

What's this all about?

There are many papers about the theory of Single Bubble Sonoluminescence available, but exact descriptions how to produce it are rare. So if you already know about sonoluminescence and now want to reproduce it, this is the right place to look at. I tried to give a complete and detailed report of the steps towards SBSL. Any suggestions, supplementations, comments are welcome...

This experiment may be dangerous. Always think before you work. Working with vacuum or boiling water can cause explosions. High voltages are generated in this experiment. I will not be responsible for any injuries or damaged equipment. If you don't know what you are doing just don't do it at all. And: No, I am also not responsible for any thermonuclear accident you trigger.

Equipment (required):

Equipment (nice to have):

Set up the assembly:

  1. Prepare the transducers:
    They are polarized, so when connecting them in parallel watch out for the mark on the electrodes. Do not connect them antiparallel. Clean the electrodes of the transducer with a pencil eraser. Do not erase the mark. Put 3 dots of solder equidistantly on one electrode; This will give a better contact when glued to the flask. You may also connect 3 wires to the dots to have spares in case one breaks. Put one dot on the other electrode. Be careful soldering on the piezoceramic: Use a cold (250 degrees centigrade) soldering iron and work quickly to avoid reaching the Curie Temperature (see transducer datasheet; 320 degrees centigrade for PIC 155, but most of them got an even lower one). Use fine wires to reduce sound loss. Tin the wires and solder them on the dots. In the same way put a wire on each electrode of the pill-transducer. These wires should be really short before entering a coaxial cable. It's also possible to connect a thin coaxial cable directly to the pill-transducer.
  2. Glue the transducers to the flask:
    Clean the glass with aceton. Fix the drivers on opposite sides of the flask. Remember the marks on both transducers have to point into the same direction i.e. both towards the flask or both away from it. Use a quick-drying epoxy but watch out: Some of them shrink when they set, causing the glass to crack. If this happens:
  3. Fix the microphone transducer on the bottom of the flask.
    One final hint: You really have to glue the transducers. Don't try any clamp-press-or-something-stuff. It won't work.

  4. Attach the flask with the three finger clamp to the laboratory stand.
  5. select the coil(s):
    Measure the total capacity C of your drivers. A transducer makes quite a nice plate capacitor, so the following calculation may give you a hint, too:
    C=13.9 e (d^2)/h [pF/m]
    where d is the diameter, h is the thickness and e is the dielectric constant of a transducer (see datasheet of your transducers, 1700 for PIC 155). To match this capacity to the audio amplifier, you have to set up a serial-resonant circuit with a coil of the inductance:
    L=1/(C (6.28 f)^2)
    where f is the resonance frequency (about 26000 Hz for the used flask).
    Example: Two PIC 155 transducers with d=16 mm, h=8 mm, e= 1700 have a calculated capacity of C=756 pF. At a driving frequency of f=26 kHz the needed inductance is about L=50 mH.
    The inductance has to be variable. This is achieved by an adjustable core or by changing the distance between two (or more) coils wired in series. Last not least the coil has to be strong enough so it won't overheat.
  6. Connect the generator to the amplifier. Connect the coil in series to the driving transducers and the 1 Ohm resistor (current sense). The other two resistors make a 1:100 voltage divider at the input of the resonant circuit (voltage sense). Use coaxial cable where ever possible. Fix the cables to the laboratory stand to avoid wire breakage. The voltage of the resonant circuit may give you a shock, so don't touch it, or even much better: Insulate any exposed connection with suitable tape or varnish, especially if you want to produce Multi Bubble Sonoluminescence.
    SBSL assembly

Persuade one bubble to glow:

  1. Prepare degassed Water:
  2. Clean the flask thoroughly.
  3. Open your vessel with the degassed water. Pour the water into your flask letting it run down the glass. Be careful not to produce bubbles while filling. Fill it up to the neck, so the water is nearly spherical shaped.
  4. Find the acoustic resonance:
    By-pass the coil and set the sine generator to 26 kHz. Display the microphone signal on your oscilloscope. Now change the frequency slowly to find the maximum amplitude of the signal. There may be several local maximums around that frequency, but in most cases you'll have to select the absolute maximum (i.e. the highest peak). If you got a sweep generator, it might be helpful to display a 20-30 kHz sweep on the scope. The acoustic resonance appears as a some 100 Hz broad peak. If you found the resonance, remove the by-pass.
  5. Adjust the inductance:
    Display voltage and current on your oscilloscope so you can see the phase shift. Adjust the core/change the distance between the coils until the phase shift is zero. If you got a sweep generator, it might be helpful to display a 20-30 kHz sweep of the microphone signal on the scope. The electric resonance appears as a 1 kHz broad peak overlaying the acoustic resonance.
  6. Choose a low driving amplitude, e.g. 100 mV peak-to-peak on your oscilloscope.
  7. Watch the microphone signal: It shows a pure sinusoidal voltage. If ripples are on the signal, there are some bubbles in the flask. Leave them alone a minute or two with the driving voltage switched off. If they don't vanish, your water contains too much air; you have to degas it more carefully. Ripples may also indicate a poor contact between a transducer and the flask. The epoxy has to cover the whole transducer and no air may be between transducer and flask. Air between epoxy and flask appears as a reflecting area.
  8. Create a bubble:
    Extract some water with the syringe or the eyedropper. With the syringe let a drop fall on the surface of the water. This creates a bunch of tiny bubbles. Some of them will dissolve whereas others will drift to the center of the flask and unite. To see them, you have to light them from behind and look against a dark background. A broadened laser beam is really nice and helpful! Watch the microphone signal; Now there should be some ripples on the sinusoidal signal. The ripples are still visible if the bubble is to small to be seen without a laser; So watch the ripples, not the bubble. A high-pass filter improves their visibility. If you see no ripples, increase the voltage and try again. If the ripples quickly vanish, turn the voltage lower and try again.
  9. Slowly increase the driving amplitude. At a certain amplitude the bubble becomes instable and vanishes: No ripples are visible. Turn the voltage lower a bit and create a new bubble.
  10. This is the big moment:
    Darken the surrounding light and look at the center of the flask. You should see a tiny blue-white dot, like you captured a little star from the night sky.
  11. If no glowing dot is visible: The microphone signal has to give a stable image of the ripples on the scope. Change the driving voltage and perhaps the frequency a bit to achieve that. Increase the driving voltage just below the instability border. You may change the frequency, too, but it isn't that critical.

Once you succeeded, you should be able to reproduce SBSL without difficulties. With this basic setup you may start experimenting. Investigate the parameter range where SBSL is visible. Try other flasks, other liquids (putting glycerine into the water improves SBSL),...

Persuade many bubbles to glow:

  1. Fill the flask with water up to the neck.
  2. Find the acoustic resonance.
  3. Adjust the inductance.
  4. Increase the driving voltage until you hear a horrible screeching noise, which sounds like your flask is going to crack. Don't be surprised if it does... and don't touch any exposed connection. The sound is produced by cavitation, i.e. by imploding cavities in the water.
  5. Pay attention not to overheat the coil(s) or transducers.
  6. You need a very high driving amplitude (maybe some hundred volts). If your amplifier doesn't reach it: To double the voltage, feed the inverted signal to the second (stereo) amplifier channel and use the signal between the two live outputs.
  7. Darken your room completely. Adapt your eyes, which may take 15 minutes up to half an hour. Look at the flask or at least where you think your flask is (remember: the room is completely darkened). You will see a weak glowing in some areas of the water and sometimes flashes crossing the flask. This is the famous Multi Bubble Sonoluminescence.



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