Report on Sonoluminescence Research
Christina Hung - Commack High School
This project was an investigation into the parameters that would induce the bluish glow of sonoluminescence. A few of the parameters that were investigated included the amount of dissolved gas left in the water, frequency to induce Sonoluminescence which is also the resonant frequency, voltage or amplitude of signal and the effect of the water level on resonance.
Sound and Light. Two completely different concepts, both depending on wavelengths and frequencies, with one being four times faster than the other. However, these two concepts are intertwined in that of sonoluminescence. Sonoluminescence, which was first discovered in the early 1930’s, occurs when a bubble, surrounded by water, is compressed by sound waves which causes the bubble to collapse and produce bursts of light. In other words, it is an ultrasonically driven gas bubble in liquid that emits light under its collapse. There are several types of sonoluminescence under research right now though the two most prominent are SBSL (Single bubble sonoluminescence) and MBSL (multiple bubble sonoluminescence). Research concerning the concept did not prosper until the late 1980’s though it was discovered nearly 50 years before.
The discovery of sonoluminescence was made in 1934 by German physicists H. Frenzel and H. Schultes at the University of Cologne when they witnessed the glow of sonoluminescence after exciting water with sound waves. Further research became inconclusive after the sound waves created clouds of bubbles that grew, collapsed and luminescenced in an unpredictable manner.
Single Bubble Sonoluminescence (SBSL) occurs when only one bubble is trapped by the nodal waves causing a single beam of light. Discovered in the 1990’s, it is much easier to characterize and research than several bubbles at the same time. Bubbles are manually injected into degassed water. The injected bubble will rise, held in place by the sound waves. The frequency needed for sonoluminescence is just beyond the range of human reception which prevents auditory problems at around 26 kHz. The bubble concentrates the acoustic energy by a factor of 1 trillion which means that although the sound wave is several centimeters in length (usually 3-4 cm), the emission of light is only a few nanometers. The radius of the bubble starts as a few microns which expands as the acoustical pressure drops to about 50 microns. This effect causes the inside of the bubble to become a vacuum because of the dispersing gas molecules which also begins the process of collapse. Size changes from 50 microns to .5 microns, where it stops shrinking due to the trapped gases. Sonoluminescence can be witnessed when the bubble collapses. The bubble maintains its shape which optimizes energy concentration. The same cycle can be observed several times because the .5 bubble radius will expand with the next sound wave cycle.
Although the glow seems to be continuous, it has been shown to be rapid flashes of light lasting several picoseconds each instead. This was first discovered by Seth Putterman of U.C.L.A. by using a photomultiplier tube which indicated that the flashes lasted for about 50 picoseconds. It was also determined that the time between flashes were about 35 picoseconds, which never varied for more than 40 picoseconds. Sonoluminescence is the only known method of producing picoseconds of light without the use of complex, expensive lasers. In 1991, Robert Hiller discovered that the majority of the light emitted from the Sonoluminescence belonged to the ultraviolet spectrum.
It has been shown that water is the only surrounding liquid that produces SL under standard conditions. There are two theories as to why water is the only liquid that can be used for SL. The first is its parametric instability which is when the "bubble wall oscillates, distorting the bubble’s shape", which also leads to bubble collapse and the second is the Rayleigh-Taylor instability. The Rayleigh-Taylor instability occurs when the bubble is at its smallest radius and it causes the bubble to burst due to surrounding water bubbles. The parametric oscillations allows the bubble to adjust its size which results in some, but limited protection in accordance with the Rayleigh-Taylor instability. The bubble, however, will collapse if the outside atmosphere reaches 1.15 atms. Liquids other than water do not allow the oscillations to occur as quickly which allows the pressure from surrounding bubbles to cause its premature collapse. This is primarily due to the fact that the other liquids are more viscous or adhering to the bubble wall than water is. If the surrounding liquid is extremely viscous, it prevents the oscillations from occurring which is why water is used. Heavy water (deuterium based), has been experimented with, surprisingly with success. However, there are differences found in the peak wavelengths of the sonoluminescence.
One of the more prevalent theories concerning sonoluminescence is called the Shock wave theory. This theory stemmed from the transient bubble theory which associated the transient cloud of cavitating bubbles with the phenomenal temperatures inside the bubble (see above paragraph concerning adiabatic heating). In this model, the collapse of the bubble causes chemiluminescence which involves the dissociated molecules to recombine, emitting light. However, this model on transient cavitation does not fully explain the ultraviolet spectrum emitted by a single bubble. First, the temperature calculated inside a bubble would be about 10,000 k while the actual temperature was about 72,000 kelvin. The shock wave theory extends the adiabatic heating and states that the speed of collapse of a single synchronized bubble is so fast and symmetrical that it launches a spherical shock wave into its interior. As the shock wave with the radius Rs closes in on the focal point, the amplitude and speed increases rapidly. Therefore, the equation of hydrodynamics becomes: Rs = Atb. In the equation, A is a constant, t is time measured from the moment of focusing when Rs = 0 and b is 0.7 for air. For every shock wave, a Mach number is associated with it. The Mach number is determined by the ratio of shock velocity to the ambient speed of sound. Since the temperature of the area behind a shock wave is usually higher than in front, the ratio of the two temperatures is proportional to the square of the mach number. However, when the shock wave implodes, focusing on the center, the Mach number approaches infinity, which results in extraordinary heating and phenomenal temperatures. Once the wave hits the center and explodes outwards, the molecules that were behind the shock wave are once again in front of it which causes the already high temperatures to raise even higher or another factor of the square of the Mach number. Temperatures, therefore, are remarkably high, though they are limited by the stability of the shock wave. If this theory is proven, it could be significant in the study of controlled fusion or inertial confinement fusion. There is a high correlation between the inertial confinement fusion and sonoluminescence. If the sonoluminescent shock can remain stable down to the radius of 20 nanometers, the region would be able to reach temperatures necessary for fusion to occur. Some obstacles, however, include the instability of the shock wave and thermal diffusion and radiation damping. From the shock wave theory, it can be theorized that the interior of the bubble can get as hot as that of the sun.
SL requires an oscilloscope, a sound generator and a stereo amplifier. A glass spherical 100 ml flask willed to the neck with water can serve as a resonator which is the cavity where the sound will be trapped to drive the bubble. Piezoelectric transducers are cemented onto the flask and used as small speakers. They are powered by an audio generator which creates the sound and an amplifier. Bubbles are introduced into the center which are drawn to the node of the wave. However simple it may sound, finding the correct resonance of the flask and matching it with the resonance of the sound wave can be difficult. First, the vibration of the filled flask has to reach its resonance. Resonance can be explained as the frequency that will cause the object to respond most intensely. However a 100 ml pyrex spherical flask of 6.5 cm in diameter (similar to the one we used) had the resonant frequency at 25 kilohertz. Three ceramic piezoelectric transducers are needed; two to create the acoustic wave and one to act as a microphone to monitor the sound of the collapsing bubble. This set up is attached with wires to an oscilloscope which depicts the output voltage of the amplifier and the current through the drivers. Also attached to a flask is an audio amplifier and function generator which is two separate units. A sonoluminescent bubble occurs most successfully in water that has been distilled and degassed which is the process of taking the air out of water. The sound waves in the flask must be in their second mode in order for the bubble to be drawn into the center and sonoluminescence to occur.
After being bequeathed the sonoluminescence setup with the PZTs and microphone circuit completed, one of the first items I decided to investigate was the effect of the water level in the flask as opposed to the resonant frequency. In order to do this, I filled the flask with enough water to create a perfect sphere. This I labeled my "original" or "control". I began at 23 kHz and worked my way up to 40 kHz in .2 kHz intervals. After, I began to change the level of the water in the flask by adding and subtracting drops of water.
At about this time, we found that the microphone at the bottom of the flask broke. This meant that the microphone had to be replaced. There were several available alternatives. The first alternative was from a buzzer from Radio Shack. The standard buzzard contained a small round, ceramic disc that acted as a microphone. The ceramic microphone was attached to a small plate of brass which refused to be removed though we used acetone and other means in hopes of dissolving the glue between the two. Instead, the microphone was replaced by one that was ordered from Channel Industries, a company that sells transducer kits specifically for sonoluminescence experiments.
The next step was to determine how much gas should remain in the flask of water. If there was too much or too little gas, it would cause the bubble to collapse and dissolve. In order to do this, we implemented a system to measure partial pressure of the water vapor or air in the flask. It was determined that the water temperature was approximately 6.8 degrees Celsius which corresponded to a partial pressure of 7.4 Torr. This meant that we wanted to pump the gas out of the water as close to that amount as possible. The pump is a standard pump relying on temperature gradients. There are two valves- an air valve and an actual pump. The contraption is first cooled with liquid Nitrogen. This is followed by the vacuum seal onto the flask, connecting the pump and flask. The gauge is opened to allow the pump to start emitting air as the air valve is closed to create a vacuum. This continues until there are few bubbles remaining and the pp is close to 7.4 Torr.
After pumping for a few more moments, we had to determine the correct resonant frequency of the flask. The first resonance was around 24 kHz while the second one was higher- at around 26 kHz. Upon comparing the oscilloscope readouts, it was determined that there was a sharper resonance at 26 kHz. Therefore, we experimented by injecting bubbles into the water. If the bubble did get pulled into the center, it would show a distinct ripple pattern on the oscilloscope. If the bubble remained suspended in the center for more than 30 seconds, there was a possibility that they might glow. However, most bubbles tended to just remain suspended while others simply dissolved.
Through this investigation of sonoluminescence, we have discovered that there is a subtle shift in the resonant frequency upon changing the volume of the flask. We have also discovered that the partial pressure is only a rough estimate of the amount of gas present and needed for SL. In fact, chance plays a large role in it since each bubble injected releases large amounts of air back into the water. Lastly, the most important result was that the resonant frequency was approximately 2642.42 Hz. This is variable with the temperature of the water and the amount of the water in the flask.
I would like to thank Dr. Metcalf for the resources he provided me in addition to the opportunity to explore such a fascinating field of study. I would also like to acknowledge Dr. John Noe for the patience and dedication he put into helping me understand my project, and congratulate him on his observation of the sonoluminescent light.
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