An Investigation of Medium Variance in Sonoluminescence.

Matthew D. Kline
Westmoore High School

Abstract

Sonoluminescence is the process by which sound waves are utilized to resonate an air bubble suspended in a medium with the resulting oscillations causing cavitation of the bubble wall producing an emission of visible light. Here, the effects of varying the medium were researched and analyzed by the Rayleigh-Plesset equation and parametric instability. Distilled water was used as a control and trials were run with salt (NaCI) and Glycerin (CH2OHCHOHCH2OH) solutions in water castor and olive oil, methyl alcohol and Potassium Chloride (KCl) solutions. It was found that the non-aqueous mediums were not suitable for Sonoluminescence, while the glycerin increased the bubble's stability. The salt solutions were inadequate but the potassium chloride trials demonstrated that solid state solutes did not inhibit Sonoluminescence. No suitable mediums were found for X-ray analysis.

Introduction

Sonoluminescence is a new area of study, reopened in the 1980's after 50 years of nearly complete stagnation. It is the phenomenon that occurs when sound waves of the correct frequency pass through a liquid medium and create intense light. In the 1920's & 30's, chemists working with loudspeakers designed for the research and development of sonar systems during World War 1, discovered an interesting phenomenon: a strong sound field could catalyze reactions that take place in an aqueous solution. Reinhard Mecke, a German scientist of the University of Heidelberg, commented to his colleagues that the amount of energy required for a chemical reaction is the same as that needed to excite the emission of light from an atom. Soon afterward, H. Frenzel and H. Schultes discovered Sonoluminescence in a bath of water excited by acoustic waves. They explained their findings by means of frictional electricity of bubble cavitation. They concluded their paper by saying they had better things to do. (1 )

In the late 1980's the resurgence of interest in SL (abbreviation for Sonoluminescence) generated a race to deduce the cause and intensity of the produced light. Chemists Kenneth Suslick and Edward Flint made light-emitting vapor bubbles by sending ultrasonic waves through liquid hydrocarbons such as dodecane. By examining the spectrum of the released light they showed that the vapor reached temperatures up to 5000 degrees Celsius. Suslick, along with Doktycz also created particle collisions with this method. They caused through acoustic cavitation particles of zinc powder to melt together. (2,3)

However, these bubbles formed erratically making precise study difficult. In 1988, Lawrence Crum and D. Felipe Gaitan found a method to make a single bubble that achieved acoustic cavitation.(4) Seth J. Putterman and his associates at UCLA homed in on these bubbles and discovered four qualities.(1 ,5)

1)Each bubble expands to 50 micrometers and shrinks to less than two.

2)The flash of light was too fast to measure. At most, it lasted 50 trillionths of a second. It also appeared to be in synchronization with the soundwaves.

3)The temperatures of single bubble SL reached at least 72,000 degrees Celsius!

4)While the bubble begins its collapse according to conventional theories of fluid motion, something different must account for the light.

Trying to explain just what happens when the bubble flashes has kept several theorist busy for the past few years. Lawrence Crum says there are probably ten different models right now. One theory is that the collapse of the bubble touches off supersonic shock waves that collapse with the bubble and then expand outward.

The production of the light is much more varied however. Some researchers think the gas becomes a plasma. Fromm hold and Atchley propose that the light could come from collisions between neutral molecules. This theory is supported by Putterman's findings that noble gases dramatically enhance the bubbles intensity. Lepoint argues that the bubble's oscillations inject tiny jets of liquid carrying electric charge into the bubble. Hickling, proposes that high pressure causes the water to freeze and the light comes from cracking the ice. This theory is generally not agreed with because SL has been shown in other mediums besides water, and water follows different freezing rules than other liquids. Nobelist Julian Schwinger offered a theory that the bubble's radiation could come from a subtle quantum effect involving electrons.(1)

With only ballpark figures for the bubble's smallest radius and the timing of the flashes, and with indirect measurements of temperatures, there isn't much experimental evidence to support any one hypothesis. One restraint is that scientists don't know whether or not the bubbles emit X-rays, evidence of high temperature. Because water absorbs X-rays, it is futile to detect them from outside the flask.(6) Water is used in nuclear reactors to absorb the radiation released during fusion. This property, while a boon for safety, complicates SL research. Another difficulty encountered is that SL as to now has only been produced in water and a couple of hydrocarbons.(7) Other liquids disrupt the process by a number of methods.(8,9)

1)Density of the liquid causes air bubbles to be too buoyant for the sound waves to capture.

2)The viscosity of the liquid creates instability on the surface of the bubble, and there are not enough alternate forces to counter-balance.

3)The Rayleigh-Plesset instability, occurring whenever gas is strongly accelerated into a liquid, with the bubble with radius R(t), when p is the air pressure, P0 is the ambient pressure, Pa(t) is the pressure of the acoustic field, v is the kinematic viscosity of water, and cw is the speed of sound in water.

4)Parametric instability, arising when the bubble is overwhelmed by the oscillations.

Within the past decade, many papers have been presented to the Physical Review Letters for approval and publishing, and thus many names like Michael Brenner, Bradley Barber, Seth Putterman, Lawrence Crum, and Robert Hiller have come to head the papers of the leading edge of physics. Their works include doping the bubbles with noble gases, measuring the conditions at which SL seems to occur with the most intensity, measuring light scattering in order to discover the dynamics of the collapse of the bubble wall, spectral analysis of the emissions, and isotopic effects in heavy water. (8,10,-15)

The area of variability that most affects SL is the variance in the medium that SL is attempting to occur in. Changing the temperature of the medium for example changes the stability of the bubble. It is thus the purpose of this research to investigate what alters SL when various mediums are used. This avenue of research could provide a liquid that produces greater intensity of SL, or a medium that produces X-rays and doesn't contain them absorb them within the medium.

Methodology

The machine in Lawrence Livermore National Laboratory known as Nova costs around one-hundred million dollars and is the size of a gymnasium. It was built to study nuclear fusion, one of the alleged uses for SL. It can be studied however on a desktop for under two-hundred dollars. Unlike Nova, the process is not difficult and accomplished through brute force, just exacting and requiring finesse.(16)

The apparatus assembled in this experiment was a scaled down version of Seth J. Putterman's original apparatus.(1,17) First, three piezoelectric transducers were purchased. from Channel Industries, Inc., 839 Ward Drive, Santa Barbara, California. Two were fifteen millimeters in diameter and used as drivers. The third was six millimeters and used as a microphone. Using a pencil eraser, the oxide covering on the silver sides of each transducer was removed. Using 36 gauge wire, three leads were quickly soldered on the positive side of each driver and the negative side of the microphone. Three leads were used to insure a planar surface, and to act as backups in case of breakage of the primary lead. The transducers were then attached to a 200 millimeter Pyrex spherical boiling flask with a diameter of seven centimeters, by five minute quick-drying epoxy. The drivers were placed positive side in on opposite sides of the flask and the microphone was placed negative side in on the bottom. A short lead of the same 36 gauge wire was soldered to the outside of each transducer. The drive transducers were then wired in parallel so they would expand and contract simultaneously. The wires were then connected to coaxial cables to reduce electrical cross-talk between components. The flask was then suspended by a laboratory stand.

A function generator with a maximum range of 100,000 Hz was wired into an audio amplifier, which in turn drove the transducers. A One-Mega-Ohm resistor was then wired in series with a Ten-Kilo-Ohm resistor, and then wired to then audio amplifier and grounded. The drive transducers act as capacitors, so to drive them an inductor must be wired in series with them. The inductance is chosen so that it is electrical resonance with the piezoelectric capacitance at the appropriate resonant frequency. The drivers have a capacitance of 2.3 nanofarads, so the required inductance is about 23 millihenrys. Thus three inductors summing up to 23 millihenries were wired in series. A one-ohm resistor was wired in series after the transducers, then grounded in the audio amplifier. The completed circuit is shown in the circuit diagram.

A two channel oscilloscope was used to determine the correct inductance, found when the current's and voltage's patterns were in phase. From the circuit it can be seen where the voltage and current measurements were taken. By adjusting the distance between the inductors, the correct inductance was found. The microphone transducer was then hooked up directly to the high-impedance input of the oscilloscope. The completed apparatus when assembled appears bellow.

Because a sonoluminescent bubble can only be created in liquids where the naturally dissolved air is removed, a 500 milliliter Pyrex Erlenmeyer flask with an airtight stopper, a hollow tube, and rubber tubing was filled with 300 milliliters of distilled water. The water was slowly warmed then kept at a rolling boil for 15 minutes upon a laboratory hot plate. The hot plate was then turned off and the flask was immediately removed and set upon a heat resistant surface. Upon removal of the flask, the rubber tubing was clamped shut tightly. The flask and the water was then allowed to cool, while keeping the clamp shut, thereby stopping the inflow of air and creating a strong vacuum.

After cooling, the flask was decorked and the water was quickly poured into the resonator flask, stopping at the neck of the flask, making the volume of water approximately spherical. Doing so introduces air but brings it to about l/5 atmospheric concentration, the correct level for SL. The medium must be resonated at the resonant frequency, which is the speed of sound in the medium divided by the diameter of the medium. The glass flask and the epoxy causes this value to be 10%-15% higher. The resonant frequency therefore was set at 23,000 Hz, +/- 1000. The frequency generator was set at 23,000 Hz. An eyedropper was then used to create bubbles inside the flask. The frequency and amplitude were adjusted to capture a cavitating air bubble and produce SL. Readings were taken visually, upon analyzation of the bubble, and by the oscilloscopes variance in the wave pattern. The process was repeated using sine, square, and triangle wave patterns. To aid in observing the produced light, a 120 volt neon laser was used to illuminate the flask for contrast. Further repetitions utilized other mediums: Three salt/water solutions, three water/glycerin solutions, olive oil, methyl alcohol, castor oil, and a potassium chloride/water solution.

Results and Discussion

The information gathered from the 100% water medium was used as the control, as its characteristics have been well documented, and the relative density of water is 1.000 gram per cubic centimeter. The medium resonated at 23,000 Hz, and produced a visible bubble that could be seen only under darkened conditions. It appeared first as a bright white, then faded to a faint purple before the bubble dissolved into solution over a period of 15 seconds.

Sl in distilled water

The water/glycerin mixtures provided data that was characteristic of soap bubble life extension. In many applications, glycerin is added to make bubbles last longer, such as shampoo and toy bubble solutions. Glycerin's density is 1.260. The 1:3 mixture did not produce a sonoluminescencing bubble, but resonated 24,500 Hz. The air bubbles that were introduced were simply too buoyant for the sound field to capture, obviously because the glycerin raised the density of the medium too high The 1:10 mixture resonated at 24,000 Hz, and produced a bubble that was captured by the sound field, yet never obtained SL. This bubble however, did not dissolve into solution for 37 seconds. Logical analysis dictates that the density was low enough to support capture, yet the viscosity was too high to permit the oscillations required for SL. However, the extended length of the bubble's existence provides that glycerin strengthens the bubbles walls and slows the degeneration of it and the diffusion of air into solution. The 1:30 mixture resonated just over 23,200 Hz, generated a bubble that quickly achieved SL, giving off a bright blue light, and remained oscillating, not entering the solution for approximately 26 seconds. This data supports the previously hypothesized analysis of the 1:10 mixture, with the addition of a oscillating and luminescent bubble show that glycerin does not impede SL.

Sl in 1:30 glycerin/water mixture

Methyl alcohol resonated just under 22,000 Hz. The introduced air bubbles coalesced into the center but quickly disintegrated. Therefore, because the density of methyl alcohol is .810 in relation to water's density, it is followed that SL was not achievable in these condition because of parametric instability and the Rayleigh-Plesset equation. The oscilloscope displayed a jump in wave pattern that quickly disrupted, more evidence of instability in a medium with lower viscosity. Olive oil resonated about 22,500 Hz and provided the same analyzation of data though the disruption of the bubble was slightly slower, again, probably because of a low density, .918. Castor oil, with its density of .969 resonated a little lower than 23,000 Hz. The sound field captured bubbles sluggishly, yet held them in place before disintegration for about 8 seconds. SL was not achieved however in this medium, probably because of a lack of stability when amplitude was increased, caused by bubble wall weakness based upon viscosity.

Example of flask with no SL

The three salt/water (NaCI/H2O) mixtures of 100% saturation, 25% saturation, and 5O% saturation all failed. They each resonated at different and variating frequencies ranging from 25,000 Hz to 23,500 Hz. Bubbles formed and coalesced in each trial, yet SL was not apparent visually or upon examination of the wave patterns. The density of such solutions are typically not much in excess of 1.051 and usually revolve around 1.025, which is close to distilled water. Therefore, the unresponsiveness could be due to the presence of a solid (NaCI).

The water/potassium chloride solution resonated at 23,000 Hz and produced a faint pink bubble. In contradiction to the previously stated unresponsiveness of solid/liquid solutions, it could now be argued that the molecular mass and solubility of the solute determines the ability for SL production. This is the first production of SL in a medium with a solid solute.

SL in Potassium Chloride

All of the mediums that produced SL had an absorption coefficient that would halt the emission of X-rays outside the flask. Therefore, no high spectral analysis steps were initiated for results would have been undoubtedly negative.

Conclusion

Sonoluminescence, being a new branch of science is theoretical as often as it is experimental. Lack of direct observation, inefficiency of measuring devices, and the inability to take direct measurements from inside the bubble present scientists with more questions whenever one is answered. Isolation of the effects the medium places on the process will quickly eliminate some of the theoretical possibilities and allow concentration upon more pressing matters. The addition of glycerin clearly increased the stability of the bubble, and in some cases the intensity of the light. Perhaps it is not the density of the medium that associates with SL, but the cohesiveness of the material. The elasticity is usually proportional to the density. Therefore a higher elasticity could increase the degree of SL. The absence of SL in the Water/Salt trials clearly indicates that density is NOT all that matters. The density of these revolves around that of water, yet the absence of SL shows that solid additions to the solutions halted the sonoluminescencing effect. This is in contradiction however with the KCI trial which clearly indicate that SL can be achieved with a solid solute. It is now known that while some solid state solutes cause parametric instability, the presence of others might be a welcome addition. The possibility that the oils and alcohol are not sufficient for SL is not ruled out by this experiment. Superior equipment (stronger and wider sound field) might yield these mediums plausible. It should also be taken into consideration that the organic compound structures themselves might break down and cause parametric instability.

Further Research

Additional experimentation and research can be classified into two categories: external and internal variances. The latter include but are not limited to increasing intensity by solution variation, investigating whether or not the effect of the KCI upon SL extends to all solid state matter, introducing a gas into the solution before experimentation, searching for a pure inorganic substance that SL can occur in, and finding a medium that does not impede X-rays to such an extent to make readings devoid of results. The final example is of crucial importance, as it would dictate in which direction this field would travel.

The external variances, are much more difficult to analyze because they in themselves are not completely understood. Magnetic and electrical fields, when passed perpendicular through the medium might alter the process, stop, or enhance it. Research along these avenues might result in nuclear engines and power sources such as those only now seen in science fiction films. An aspiration for inspired scientists would be to determine different ways to probe the phenomenon, such as a probe to detect temperatures and pressure inside the bubble at the point of luminescence. The application of such data could result in a safe method for waste disposal. In the future, SL could be responsible for a furnace as hot as the sun, that provides power from trash, and breaks down biological contaminants.

Acknowledgements

I would like to thank the following people for their aid in my endeavor. Dr. Leonard Feuerhelm and the Oklahoma Christian University of Science and Arts natural science department for allowing me the use of their equipment and laboratory, and for Dr. Feuerhelm's advice; my science teacher Mr. Bradley O. Brauser, for pushing me to do my very best and then making me pick my stick up and keep running; Mrs. Janice Willingham for her chemical advice and loan of equipment, and for teaching me well; my peers for supporting my efforts and reviewing it with appropriate scrutiny. and my parents for putting up with this for one last time, and helping me keep my sanity.

Resources .

Addendum


Table 1


Relative Density




Table 2


Frequency of Resonance in Hz




Table 3


Levels of Sonoluminescence