Michael Solway's Undergraduate Research
Sonoluminescence at Rutgers
During my last year and a half at Rutgers University as an undergraduate, I did a physics honors research project under Professor Haruo Kojima on the effect called sonoluminescence. It occurs when a sufficiently intense sound wave induces a small gas bubble in a liquid to collapse quickly and in the process emit a burst of light. One can also stabilize sonoluminescence by trapping the bubble at a pressure anti-node of a standing acoustic wave so that it repeatedly expands and contracts emitting light during each contraction. The standing wave must be at a resonant frequency dependent on the shape and size of the container holding the liquid.
Bubbles can be created in the liquid by cavitation. The sound waves are applied onto the container through piezoelectric transducers. The liquid, usually distilled water, must first be degassed by a pump and then gassed with a gas or combination of gases that the bubbles are intended to be composed of. The effect seems to require the presence of a noble gas such as helium, argon, or xenon. Atmospheric air contains about 1% argon, so it works with just air, in which case degassing is not needed.
After a bubble is trapped by the wave, the wave's amplitude is slowly increased to compress the bubble to a smaller average diameter. After some critical diameter, the bubble starts emitting the luminescence. If the bubble is compressed further, at another critical diameter the bubble destabilizes and completely dissolves back into the liquid. The typical diameter of the bubble when it sonoluminesces is about 1 micrometer.
Each light flash during bubble collapse is very short on the order of 100 picoseconds and has a peak intensity of 1-10 mW. Sonoluminescence can be seen by eye in a very dark room or chamber. Bubbles composed of a single noble gas emit a blackbody spectrum. During collapse, the interior gas of the bubble reaches very high pressure of ~10,000 atm and temperature of ~20,000 K. However, some argue that the core can reach much higher temperatures.
The mechanism of sonoluminescence is still unsettled. Some have proposed that it is a form of cold fusion, but this is highly debated.
During my project, I rebuilt the quartz container to be spherical as opposed to the old cylindrical one. Spherical symmetry allows for a cleaner resonant standing wave and easier optical measurements of the bubble properties through the container. I also made sure that the new setup works, and observed sonoluminescence using it with my own eyes (right picture below). Unfortunately, I ran out of time to begin collecting data. We wanted to create bubbles from a mixture of two noble gasses, and test how the intensity and spectrum of the emitted light compares with that of using either of the noble gases alone, and whether the two gases segregate in the bubble during collapse possibly caused by the accelerating bubble wall pushing the lighter of the two noble gases into the center faster than the heavier one. In the summer of 2007, I mentored a high school student who was helping me with the project.
Two Theoretical Projects on Dark Energy
at Indiana University Bloomington
In the first project, I tested cosmologies in which the three constituents of the universe (radiation, matter, and dark energy) interact with each other, meaning that one can transform into another. In particular, the dark energy density was held constant, the universe was confined to be flat, and two interactions were introduced, one between dark energy and radiation and the other between dark energy and matter, such that they related to the Hubble constant scale through the holographic principle. This possibility has not been ruled out. By numerically integrating the corresponding differential equations, I found that such cosmologies permit two realistic solutions given that the universe at present contains 30% matter, 70% dark energy, and no radiation as we believe it does. In one solution, the universe begins with no matter and 30% radiation, which eventually all transforms into 30% matter through the two interactions with dark energy, while the dark energy is held constant at 70% as prescribed. And in the other, a second shift follows with all the matter being transformed back to 30% radiation some point in the future.
In the second project, I analyzed different nonlinear dispersion relations of scalar fields trying to get the trans-Planckian tail modes to yield an equation of state close to that of dark energy (wΛ ≈ -1). I found that flattening out the tail modes, i.e. flattening out the exponential decay of the dispersion relation that approaches zero at large wave numbers, makes the pressure decrease compared to the energy density, and thus raises the equation of state closer to -1.