This page gives a brief outline of the physics behind sonoluminescence and the method used to achieve it.
Sonoluminescence is the process by which acoustic energy is turned into light. The method used in this project to achieve sonoluminescence is as follows; A standing acoustic field is set up across a spherically symmetric body of liquid using a signal generator, amplifier and LCR circuit. A small bubble of gas is placed into the field and, because of the pressure gradients, becomes trapped in the antinode. Since the bubble has a finite size there is a pressure difference across it when it is in the field. This pressure difference causes the bubble to expand and contract in time with the rarefactions and compressions of the field. The expansive phase of the oscillation is retarded by atmospheric pressure but the compressive phase is accelerated by it. If the drive of the acoustic field is sufficient the speed of the bubble walls during collapse are several times that of sound. When the minimum radius of the bubble is reached there is a sudden deceleration of the walls and light is emitted in a pulse that lasts abot (50-300)ps. This minimum radius is very nearly the Van der Waals limit. This is repeated for every acoustic cycle and results in picosecond flashes of light being given out every 30ms with an error in their regularity of 40ps. Sonoluminescence is highly dependant on experimental factors; the concentration of gas in the water, the size of the bubble, composition of the gas in the bubble and the temperature of the liquid must all be within certain bounds for sonoluminescence to be observed
Water is the substrate that most readily emits light so the first step to achieving sonoluminescence is to create a suitable sample. As a control to the experiment distilled water must be used. To get a suitable gas level in the water it is necessary to steadily boil it for approximately 11 minutes. The 100ml of water should be degassed in two equal parts to stop splashing whilst the water is boiling. The flask must be sealed with a rubber bung immediately after the heat is removed to prevent gases being reabsorbed. The water should then be left to cool to room temperature and preferably cooled even further in a fridge before the experiment is conducted.
Piezo-electric transducers (P.Z.T.s) are used to create the acoustic field. These are ceramic devices that expand when a potential difference is imposed across one way and contract when a voltage is imposed across the other way. They need a voltage of around 700V to work efficiently. If subjected to an A.C. potential they will expand and contract rapidly. If they are glued to the side of the sonoluminescence flask and wired in parallel they will cause the flask to vibrate thus producing the required acoustic field.
The P.Z.T.s although wired parallel to each other form part of a series L.C.R. circuit in which they act as the capacitors. This L.C.R circuit is needed to produce the large voltage needed to drive the P.Z.T.s.The variable inductor is a coil of enamelled copper wire with a ferrite rod at its core. The resistance is provided by a 1W resistor. A frequency generator that operates in the 24-27 kHz range drives the L.C.R. circuit. A frequency meter monitors the produced frequency. A 30W audio power amplifier then amplifies the power of this signal in order that the maximum electrical resonance can be achieved.
A third P.Z.T. is used as a microphone and must be glued to the bottom of the sonoluminescence flask. This microphone is used to measure the acoustic field produced in the flask. The output of the microphone and of the amplifier are measured using both inputs on a cathode ray oscilloscope.
The L.C.R. circuit and the frequency generator used in this project are the original pieces of equipment created by A. Steer. The experimental set-up for sonoluminescence is shown in figure 2.
The sonoluminescence flask needs to be thoroughly cleaned before starting the experiment. It is recommended that it is washed with warm water and washing up liquid first. Followed by a thorough rinsing in cold water and then finally in distilled water. This removes any dust or contaminants that might effect the achieving of sonoluminescence.
The flask is held by means of a three-finger clamp on a retort stand. It must be held as lightly as possible so as not to cause any damping of the acoustic wave. Water is then poured into the flask, held at 45°, as gently as possible so as dissolve as little gas into the water as possible.
Now the system must be tuned. To find the acoustic resonance of the flask it is necessary to adjust the signal generator until the maximum peak to peak voltage is shown across the oscilloscope from the output of the microphone P.Z.T. The electrical resonance of the L.C.R. circuit is found by changing the inductance of the coil. To do this the ferrite core is moved very gently until the maximum peak to peak voltage is shown on the oscilloscope on the output across the inductance coil.
Once resonance is achieved the system is ready for the insertion of the bubble. A pipette is used, which also needs to be cleaned according to the method used for the flask. The pipette is filled with distilled water and a small drop is released from it into the flask as gently as possible. The impact of the drop causes bubbles to form in the flask. Some of these bubbles coalesce and are drawn to the antinode of the acoustic field. It may take several attempts to maintain the bubble here. In order to view the bubble it is best the room must be dark and the flask backlit with a red light.
Once a stable bubble is captured, small adjustments in the inductance and the acoustic field should bring about sonoluminescence.
Once sonoluminescence can be achieved with ease and regularity attempts can be made to analyse the spectra of light that the bubbles emit. The spectrum can only be directly observed to a wavelength of 2eV as water absorbs photons with a higher energy than this. His can be overcome by adding a fluorescent dye to the water that absorbs the light at U.V. wavelengths and re-emits it at visible wavelengths. An example of this type of dye is Sodium Fluorescein. The observable spectrum has been shown to have no characteristic peaks and has been compared to both blackbody and Brehmstrahlung spectra. The process by which this light is produced is still a mystery. The Two strongest models that have been proposed are the hotspot model and the jet model. In order that a solution may be found experiments have to be devised that will find the complete spectrum of light, the internal temperature of the bubble at its minimum radius and other factors which may offer some clues. It has been shown that the luminosity of sonoluminescence is increased if the bubble is doped with heavy inert gases or if the temperature of the liquid is decreased.It has also been shown that volatile solutes quench the sonoluminescence.
The only liquids that have produced sonoluminescence are pure water, aqueous solutions and ethanol at -115° C. Other liquids with similar bubble forming properties e.g. viscosity and density may be able to produce sonoluminescence but have not yet been observed.
SBSL