Spectroscopic Effects on Single Bubble

Spectroscopic Effects on Single Bubble Sonoluminescence from Fluorescent Silicon Nanoparticles Benjamin C. Masters Department of Physics, University of Illinois at Urbana-Champaign ABSTRACT This paper discusses the effects on adding Fluorescent Silicon Nanoparticles to Single Bubble Sonoluminescence. The proposed effects involve the translation of UV light emanating from the SBSL source into visible light. In order to measure such changes, spectroscopic methods were employed. In addition, the concept of electric field presence in the SBSL chamber is also explored. I. Background and Introduction The birth of Sonoluminescence occurred earlier in the century, and like many scientific findings, was discovered accidentally. In 1934 at the University of Cologne in Germany, 2 professors, H. Frenzel and H. Schultes, exposed a photographic plate to water while acoustically vibrating this water. The original experiment was geared toward the research and development of Sonar. Upon removing the photographic plate from the water, the professors noticed spots in random places. After further investigation, it was determined that the source of these light pulses that appeared on the photographic plate, had been from cavitating bubbles within the water. These cavitating bubbles had been created and destroyed from the sound waves themselves. This effect became known as Multi-Bubble Sonoluminescence (MBSL). It was postulated later that the amount of light emitted corresponded to the amount of energy required to ionize the gas in the bubbles. After this, Sonoluminescence was placed on the proverbial backburner, and forgotten for decades. It was not until 1989 at the University of Mississippi that a grad student, Gaitan, and his professor, Crum [1], decided to recreate this effect, but instead Gaitan looked for a way to simplify the process. He achieved this, by producing a single bubble himself within the water, and trapping it with a standing wave. Having trapped the bubble, he was

free to make observations and measurements. Gaitan had succeeded in the next step toward solving this mystery, Single-Bubble Sonoluminescence (SBSL). In March 2002, at Oak Ridge National Laboratory in Tennessee; Taleyarkhan[2] announced that SBSL could be used in order to achieve fusion. His setup was similar, however, instead of water, Taleyarkhan used Deuterated acetone. The results implied that there was an increased level of neutron emission, as well as an increased level of tritium decay radiation. These two consequences led them to believe that they had in fact achieved fusion in a controlled manner. There was a great deal of debate over whether or not the results were accurate, and was compared to the Pons-Fleishman experiment of the previous decade. Later that year, in July 2002, an experimentalist at UIUC found results concluding that the Taleyarkhan experiment had no validity[3]. Prof. Suslick of the Chemistry department had performed similar experiments and determined that fusion from SBSL was simply not possible, in that the required temperatures had not been achieved. The temperature measurement was based on the frequency of light emitted, and, according to Suslick, the temperature was far too low for fusion. Instead, he claimed that the resulting energy was from chemical reactions alone. In Suslick’s estimation, Taleyarkhan and Co. had mistaken increased levels of radiation from

contaminant background radiation at Oak Ridge. This problem was certainly not solved by any stretch of the imagination. To date, no theory posed by anyone has had enough credibility to agree completely with anyone else’s theory or experiment.

II. Method 1. SBSL Aparatus and Procedure The Sonoluminescence apparatus contains two basic parts: the degassing device and the Sonoluminescence equipment. First there is a discussion on the construction of both, and then the procedure and function of each in turn. The degassing device makes use of a set of valves all connected in such a way as to permit the controlled degrassing and regassing The setup included a 3 horsepower pump for lowering the pressure in the degassing vessel, a stir plate for agitating the water while degassing, a dry ice bath for cooling the water and a gas bottle that contained either air or a noble gas, depending on the preference. A set of valves labeled with letters connected all of these elements together in a network that made it possible to connect any number of pieces together while isolating the rest. There is also a digital pressure gauge attached to monitor degassing progress. The degassing procedure is as follows. Approximately 250-300 mL of distilled water is poured into an Erlenmeyer flask, known as the degassing chamber. The pump is turned on, and allowed to remove a significant amount of air from the degassing vessel for a period of 5 - 7 minutes. While this occurs, a stir plate continuously stirs the water in the vessel to aid in the removal of trapped bubbles throughout the flask. In addition, the flask itself is placed in a rubber container which also holds crushed dry ice. The purpose of the dry ice is to lower the temperature, which also makes the quantity of the dissolved gas in the water to decrease. As a limit, the pump must cause the degassing vessel to lower the pressure to a maximum of 5 mm Hg. Anything above this will not cause

sufficient degassing, and with the given pump, such a pressure unobtainable without the aid of the dry ice as a cooling mechanism. After degassing, the pump is turned off, and the flask left under its own vacuum briefly while the valve to the pump is closed, and the valve to gas bottle is opened. The gas bottle contains a gas, in this case air. Once the pressure is brought to approximately 150 mm Hg, the valve to the gas bottle is closed. The regassing need only occur for 2 minutes or less. Finally, the valve leading to the outside atmosphere is opened, bringing the flask back to ambient pressure. The setup of the Sonoluminescence apparatus is fairly simple, and can be summed up by a simple diagram. First, 2 DC power supplies are connected in series, each providing 25 V, with their current dials set at maximum. A digital multimeter was connected to the power supplies together, such that sum of their voltages could be measured. The power was connected directly to the Sonoluminescence PZT driver. Next, a function generator is connected to both the PZT driver, as well as an oscilloscope on the trigger connector. The PZT driver is then connected to the Sonoluminescence chamber on the high voltage ouput. On the low voltage output, the amplifier is connected to the first channel of the oscilloscope. Finally, the chamber is connected to the second channel of the oscilloscope. The first trace of the oscilloscope then is a monitor for the output of the PZT driver, while the second is a pickup for the chamber. The SBSL chamber is a 100 mL LabGlass flask. The reasons for using this shape are mainly due to its high degree of symmetry, which is favorable in this experiment. Given its shape, it is relatively simple to derive the equation to find the resonant frequency of the flask. Given that: (1.1) v= fλ and

d = 2r

(1.2)

we can impose the condition:

d=λ

(1.3)

which is the fundamental resonant frequency of the flask. Rearranging these three equations, we derive:

f =

v 2r

(1.4)

This can be further derived to include the volume of the flask, which is simpler to measure than the internal radius. Given:

4 Vol = π r 3 3

(1.5)

this can be rewritten in terms of r:

⎛ 3 ⎞ r = ⎜ Vol ⎟ ⎝ 4π ⎠

1/3

(1.6)

and using equation 4, we find that:

f =

v ⎛ 3 ⎞ 2 ⎜ Vol ⎟ ⎝ 4π ⎠

1/3

(1.7)

With a volume of 100 mL, and a speed of sound in water of about 1500 m/s, the resulting resonant frequency is 26040 Hz. In reality, since the speed of sound in actually based on temperature, and the speed of sound in glass need be included for coupling purposes, this frequency is mostly likely incorrect. As a reference, most trials indicated a resonant frequency in the 28kHz regime. However, it serves as a good starting point. The drive transducers need to be diametrically opposed and positioned tangent to the surface. The two drive transducers were connected in parallel, soldered so that the ground (black) was facing away from the flask, and the drive (red) was facing toward it. The drive had three connections, not only for redundancy, but also because the three spots of solder made it easier to position the transducer tangent to the spherical surface. The centers of the transducers were placed exactly halfway above the bottom of the flask. To do this, exactly 50 mL of water was added, and then a line was drawn around the flask at that height. This made the positioning of the drive transducers much easier. There is also one more transducer placed at the bottom of the flask, used as a microphone to monitor the flask.

2. Silicon Nanoparticles It is well known that there is a substantial amount of light emitted in the UltraViolet spectrum, beyond human vision range. Since water is a liquid which is highly absorbent of UV light it became obvious that to measure this UV light accurately, it would have to be first changed into visible light. The solution to this was to find an intermediary “translator” from UV to visible, known as a Fluorescent dye. Unfortunately, the fact that Fluorescein does not fully dissociate in water prevents the sound from oscillating the bubble symmetrically. As an alternative, experiments were performed with the use of Silicon nanoparticles, provided by Prof. Nayfeh of UIUC. These particles have a similar function in that they transform UV to Visible light, although the mechanism is different. These particles were removed from their original solvent and added to the degassed water. Since these particles are smaller than the clumps of Fluorescein, they were in effect “acoustically thin”, such that sound waves would pass through these particles with minimal loss. 3. Electric Field Presence An experiment in electric field measurement was also done with this setup. The goal of this experiment was to determine the presence or absence of an electric field in the water during sonoluminescence. To accomplish this, a small wire, which was exposed at its tip, was connected to a small charge amplifier to sense any electric fields. The signal from the amplifier was sent to an oscilloscope for observation. The presence of an electric field means a great deal to the explanation of SBSL. The cause for this field may arise from the brief alignment of some or all of the water molecules when vibrated by the acoustic signal. If this is true, then the electrons from the some of these molecules could be completely removed, partially ionizing the water. The electrons, now in the presence of a strong radial electric field would be acceler-

ated toward the center to the location of the bubble. Upon arrival, the electrons would be significantly slowed down by the change in medium, causing the emission of Bremstrahlung radiation. Another possibility is similar to the first, in that a radial electric field occurs and electrons are accelerated, however, if the velocity of the electrons exceeds the velocity of light in that medium, the result would be Cherenkov radiation. Both of these theories can be furthered by the exploration of electric fields in the water. III. Results and Discussion

to determine the relationship, resonant frequency was plotted as a function of temperature.

FIG 1: Graphs of both interior flask temperature and resonant frequency as a function of time. Linear trend-lines superposed over data

1. SBSL Observations There were many aspects of SBSL that were derived and verified from the experimentation with this setup. First and foremost, is that the environment in which the Sonoluminescence chamber is kept, has a serious effect on the ability to commence SBSL, as well as maintain it. Several papers have described the importance of temperature of the medium, and this was found to be a constraint that needed to be dealt with in order for long term measurements to be possible. Accordingly, the chamber was placed with a small refrigerator and kept at near freezing temperatures. The water itself was cooled to near freezing during the degassing process, but this ensures the maintenance of that temperature. Given the earlier equation for the resonance of the flask, it is readily apparent that since the velocity of sound in water is dependent upon the temperature, then the resonant frequency also has dependence,

f =

v v(T ) → 2r 2r

(1.8)

Although the velocity of sound in water is known at various values experimentally, there was no formal theoretical function to describe this. In an effort to predict the resonant frequency as a function of temperature, the temperature and resonant frequency data that was acquired was plotted vs. time. Then,

FIG 2: Graph of resonant frequency as a function of interior flask temperature. Linear trend-line superposed over data.

Another important discovery was the dependence on humidity. In the lab in which the apparatus resides, the average humidity is upwards of 60%. The problem with this is very simple. As cooling occurs within the refrigerator, the humidity in the air condenses onto the exterior of the flask. This is problematic for three main reasons. First, water on the exterior of the flask causes the spherical symmetry to be broken. The exterior water molecules themselves vibrate, causing a noise component to the microphone signal. This in effect is simply the damping of the drive signal. The resonant frequency no longer exists where it would, had there not been condensation. Second, the transducers epoxied to the surface of the flask have electrical leads to send and receive the ultrasonic signal. Water in the presence of the electrical contacts is hazardous, as it could cause a short circuit. In doing this, the signal could be severely weak-

ened or destroyed altogether. In addition, the potential for electric shock for someone working near the flask while seeding a bubble is increased. Finally, humidity prevents the clear observation of SBSL. In order to monitor the bubble while the door to the refrigerator is closed, a CCD camera was placed inside and connected to a television monitor for observation. As the light output of SBSL is inherently very weak, adding the obstacle of an uneven layer of water causes aberrations, focusing problems, and a darkening of the image. Given these problems, it was decided to find a method to lower the humidity within the refrigerator. The result was a dewar filled with liquid nitrogen that was resistively heated to pump nitrogen gas into the refrigerator, thus making the air inside “dry”, and hindering the effects of the high humidity.

it would be possible to extract a normalized set of data points over the visible spectrum. This would then yield a rough estimate for the actual nanoparticle-doped spectrum. The reasoning behind the pursuit of this experiment is not simply to gain a more complete understanding of the light emission in the UV regime. Such data can be fitted to a blackbody curve, possibly proving or disproving the theory that SBSL is a blackbody phenomenon. This can also be applied to determine the validity of the existence of Bremstrahlung radiation, again possibly proving or disproving one of the currently considered theories. Future plans are to find a less intrusive and more accurate method of measuring the SBSL spectrum, with and without the presence of the nanoparticles.

2. Silicon Nanoparticle Results

What was observed was a trace similar to that of what is observed from the microphone PZT. The same sine wave at the drive frequency was apparent, as well as the small ripples which accompany it. It should be noted at this time that the PZT drivers themselves act as parallel plate capacitors and thus have an electric field associated with them. In the original configuration, the ground of the transducer was chosen to be the surface facing away from the flask, as is the convention. A drawback from is that the electric field from the PZT driver is oriented inward, toward the center of the flask. To eliminate this, we installed a polarity switch between the drive amplifier and the PZT drivers. Upon switching the polarity, the sine wave at the drive frequency was significantly weakened, and the ripples became much more apparent. Since the electric field and sound measurements appeared similar, they were drawn simultaneously on a 2-trace oscilloscope. The drive frequency was varied to show any differences between the electric field and sound traces. It soon became apparent that for the original polarity configuration, the ideal conditions for SBSL observation was at the point when the electric field and sound had approximately zero phase difference.

The addition of the fluorescent nanoparticles created a very strange effect, in that the emitted light was no longer predominantly the bluish-white light from pure Sonoluminescence, but instead the light also had some characteristics of the color of the Silicon Nanoparticles, in this case orange. This new color implied that the light emitted from the bubble was indeed in part UV, and that the use of the particles could help explain this part of the spectrum. Since water has a high absorption in the UV range, most of the light that was emitted in this regime would not make it past the water. This meant that it the actual intensities of the UV light were unknown. The Silicon Nanoparticles can be used in order to further explore this problem. Due to time constraints, spectroscopy measurements could not be performed in order to determine the actual chromatic change in light emission from the bubble. However, the method for acquiring this data was originally going to be through the use of a Photomultiplier Tube and several narrow bandpass optical filters. By measuring the intee light with and without the filter, plus considering the quantum efficiency of the phototube,

3. Electric Field Observations

To verify this, we assumed that for the inverted drive polarity, that the opposite would be true, i.e. that the electric field and sound signals would be completely out of phase for ideal SBSL observation. After some testing, this postulate was proven valid. In the case of both normal and inverted polarity, the phase from the electric field signal varied significantly with drive frequency, while the sound signal did not. For small changes, it is reasonable to assume that the sound signal phase is fixed, and as a result, the phase between the sound and electric field signals is merely the measured phase between them. Another result was from exploring the time location of the light flash with regards to the microphone signal, as well as the electric field signal. In this experiment, a Photomultiplier Tube was used to sense the light flash.

FIG 4: Oscilloscope trace of the microphone response signal as well as the PMT signal.

Based upon the oscilloscope pictures taken with the microphone signal with the phototube, and the electrical signal with the phototube, the light flash occurs at or near a zero crossing of the electric and acoustic signals. IV. Conclusions This is where you summarize your findings and reiterate the important points a reader should glean from your paper. Again, see the handout for a detailed description.

FIG 3: Oscilloscope trace of the charge preamplifier response signal as well as the PMT signal.

V. Acknowledgments I would first like to thank Profs. S. Errede and M. Nayfeh for giving me the opportunity to combine two areas of physics. I thank them for having me admitted to the REU program, for their time, patience, advice and help. I am honored to have them as my mentors, and hope to become as good a scientist as they. Many thanks to Jack Boparai for giving me the opportunity to pursue an experiment originally considered fruitless. Thanks for his limitless patience, willingness to help, and constant support, without which I may have given up. Thanks to the REU program, supported by NSF Grant PHY-0243675, for experiences that have been invaluable as a student and researcher. Thanks to Prof. Clegg for the use of the band-pass optical filters for spectroscopy measurments.

VI. References [1] Crum, Lawrence, "Sonoluminescence," Physics Today, September 1994, p. 22-29 [2] Taleyarkhan, R. P., C. D. West, J. S. Cho, R. T. Lahey, Jr., R. I. Nigmatulin, and R. C. Block “Evidence for Nuclear Emissions Dur-

ing Acoustic Cavitation” Science 2002 March 8; 295: 1868-1873. (Research Articles) [3] Suslick, Kenneth S. and Yuri T. Didenko “The energy efficiency of formation of photons, radicals and ions during single-bubble cavitation” Nature 418, 394 - 397 (25 Jul 2002) (Letters to Nature)