Sonoluminescence: The Ultimate Water Hammer Confronts a New Generation

Mark Valentine, Electrical Engineer

Single bubble sonoluminescence (SBSL) is a process by which a nearly invisible bubble of gas is acoustically levitated and made to oscillate in a fluid medium, synchronously emitting a very short pulse of light—on the order of 50 picoseconds—with each acoustic cycle. This experimental technique was developed in 1990 by Dr. Felipe Gaitain.

I first learned about SBSL from a description in Don Lancaster's "Hardware Hacker" column in Electronics Now magazine. That column was published when the first commercially available blue LEDs were still an exotic and expensive technology, and ultraviolet LEDs were unheard of. Though not as relevant today, Don's suggestion that SBSL could serve as a new source of blue and UV light made a lasting impression on me that would resonate a year later with the February 1995 issue of Scientific American, which featured a contributed article on the SBSL research efforts of Dr. Seth J. Putterman.

In "The Amateur Scientist" column of that same issue, there was also a set of instructions for reproducing SBSL , which led to a flood of interest and activity in the field, such as the documented effort of Dr. William Andrew Steering. There's even a sonoluminescence kit available, the SL100B (Fig. 1), developed by Dr. John Koromenos. My first direct experience with achieving SBSL was with the Physics Club at Kansas State University in 1996 using an earlier version of that system, the SL100.

Modifications to the original SBSL apparatus as described in that Amateur Scientist article were the basis for several independent research efforts that produced some interesting results. The experiments can be broadly categorized into three groups. These are variations in the composition of the surrounding liquid medium, variations in the acoustical driving waveform, and variations in the acoustical properties of the resonator. Experiments that combine any two or all three categories are also possible.

My favorite paper dealing with variations in the surrounding medium was written by an Oklahoma high school student nearly ten years ago. This paper may have been the first documented instance of SBSL occurring in a solution formed from a solid solute. As if that weren't enough, the light produced by SBSL in the resulting solution was pink, not blue. This is Amateur Science at its best!

Often, the acoustical drive signal in a sonoluminescence apparatus is a pure sine wave. Research has shown that drive signals with separate harmonic components can increase the light emission by as much as 300%.

When SBSL began to emerge as a popular topic, various resonator shapes appeared. For example, the resonator that appeared in Putterman's feature article in Scientific American was cylindrical with transducers at the ends. However, the apparatus described in the Amateur Scientist article used a spherical flask, with transducers mounted on opposite sides of the flask along its equator. A “stethoscope” transducer for detecting the acoustical signature of the bubble was centrally mounted on the flask's underside.

Yet another resonator design is found in the SL100B sonoluminescence kit (Fig. 1). In my opinion, this system is especially well suited for experimentation, since the geometry of the resonator and, thus, the shape of the surrounding liquid medium can be much more carefully controlled. Different resonator shapes can be tested using the acoustical drive resonator horn included as a component in the kit.

One interesting result I obtained with the SL100 system occurred when I placed a uniform layer of cleaned BB's at the bottom of the resonator and then operated the apparatus according to standard procedure. I observed that even though the finish on the BBs partially dissolved in the distilled water and turned it cloudy, the modified system sustained a bubble for a few minutes that was positioned very close to the bottom of the resonator near the layer of BBs. Apparently, multiple spherical wave fronts of sound were being generated as the BBs reflected impinging sound. The resulting interference was probably creating multiple pressure antinodes, and a bubble must have been formed and trapped in an antinode particularly close to the layer of BBs. Had I used spherical glass beads, this effect would have lasted longer, since the water would not have been contaminated as it was with the metal BBs. Marbles could have been used, but since they are large, the matrix of antinodes produced by the sound they would have reflected probably would have been less intricate than that from the layer of BBs, resulting in fewer antinodes where a bubble could be trapped.

An interesting feature of the vertical geometry of the resonator included with the SL100 or SL100B system is that it enables two or three SBSL bubbles to be driven simultaneously (see Fig. 2). This leads to the interesting possibility of an experiment that fits into the first category. Specifically, it may be possible to study how fast certain chemicals diffuse through water in a sound field by watching the effect on each bubble as the diffused agent progresses from the top of the resonator to the bottom.

One of the reasons SBSL is a frequent subject of physics research is that the mechanism that produces it is still unknown. Perhaps SBSL is a special case in a class of systems that some advanced theory in the future may explain, just as general relativity explained the perihelion orbit of the planet Mercury. Even if that is the case, it will be helpful to probe the intricacies of SBSL more deeply to aid in the development and confirmation of such a theory. General Relativity itself grew from Special Relativity, and the result from the now famous Michelson-Morley Experiment may have influenced Einstein's thinking as he formulated the latter, as suggested in Philipp Frank's “Einstein: His Life and Times.”

Because SBSL is such an incredibly violent phenomenon, it is difficult to study. However, it may be possible to broadly apply the techniques for studying other violent subjects in nature to SBSL research. Two techniques that have proven remarkably effective in other areas are the use of probes and the use of reflected energy.

Most folks living in Kansas have seen a tornado from the outside (which is probably the best vantage). However, a new generation of probes is on the verge of capturing views from inside a tornado . While there's a great deal of interesting footage of tornadoes, researchers are turning to new types of RADAR to probe tornadoes more deeply. This will allow them to analyze the inner structures and hopefully learn how tornadoes form and, thus, how to more accurately predict when and where they will occur.

Aside from the moon, the brightest object in the night sky is Venus. Yet its proximity to the sun has created an opaque atmosphere that blankets an inhospitable surface. The best available images of Venus may be recently enhanced photos originally produced by a Russian Venera probe, one of many that successfully landed on Venus in the mid 1970s and early 1980s. While the topography of Venus has been hidden to visual instruments outside its atmosphere, RADAR operating from orbiting satellites has provided detailed maps of its surface. Galileo would have loved seeing these images.

Returning to SBSL, one might loosely compare a dynamic sequence of an SBSL bubble collapsing to the shadow of Venus traveling across the sun or a tornado dropping out of the sky . These phenomena are all quite intriguing to behold and reveal general properties, but they also reveal that other techniques are needed to reveal the detail and dynamics of surface and sub-surface features of the oscillating bubble in SBSL.

One method for obtaining high resolution data for SBSL might be through the use of reflective acoustic microscopy . This is the use of high frequency sound that can not only reveal surface details but can also provide data from beneath the surface of the target sample. Normally, the subjects studied with acoustic microscopy are static, and the rapid motion of a bubble undergoing SBSL will certainly present a new set of difficulties. However, the challenges of probing a micron-sized bubble oscillating at 20 kHz might be addressed by this method, which can resolve sub-micron features using sound in the gigahertz range.

But how could a probe small enough to be placed inside a collapsing bubble be built? To address this question, an appeal must be made to nanotechnology. Recent advances in nanotechnology have resulted in the ability to create relatively sophisticated mechanical structures only 4 nanometers wide. It may be possible to make structures that are perhaps 400 nanometers wide, with supporting projections that uniformly extend a few additional micrometers. If such a structure could be placed inside a bubble undergoing SBSL—admittedly a feat in itself—it might be possible to influence the initially spherical shape of the collapsing air-water interface. One probe could be made to have a uniform central structure, such as that of a buckyball , and another probe could have a non-uniform structure at its center resembling a red blood cell. The observed effects of these probes on a collapsing bubble might shed light on whether or not the bubble remains spherical as it collapses during SBSL, and whether or not this has any relevance to the emitted light.

Sonoluminescence is now the center of much controversy and thus much media attention. A great deal of this controversy is due to the lack of detail and repeatability with which the phenomenon can be probed. What has been established so far is that the wavelength spectrum of the emitted light is quite broad. Also, the jitter and duration of the emitted light pulses are fantastically small. In fact, in one U.S. patent (5,659,173) a “broad spectrum picosecond light pulser” is suggested as a potential application for SBSL. Now that I think about it, I recall reading about a similar idea a long time ago.