Sonoluminescence experiment: sound into light

Sonoluminescence is a little-understood phenomenon whereby light is emitted by tiny bubbles suspended in a liquid subjected to intense acoustic fields. The aim of this work was to construct apparatus to enable the observation of single-bubble sonoluminescence, to investigate its basic properties, and leave a kit and instructions to form the basis of a future final-year undergraduate experiment. It was found that despite the apparent simplicity of the setup, to obtain successful and repeatable sonoluminescence required great care in the selection and tuning of system components, and a good degree of patience. The precision and stability of the signal generator was found to be particularly critical, and (if not building your own as I did) a modern digital piece of instrumentation is highly recommended. The widely-reported increase in bubble brightness at low temperatures was readily confirmed, and a simple Mie scattering arrangement configured to monitor the bubble size gave results consistent with those already published.ContentsIntroductionDetailed MethodPreparation of flaskElectrical circuitWater preparationTuning upTrapped-bubble behaviourViewing the glowing bubbleInterpretation of the microphone signalTemperature-dependence of glow brightnessSummary of findings and typical figuresMeasuring the bubble-size: Mie ScatteringConclusionSuggestions for future studyAcknowledgementsAppendixIntroductionSonoluminescence was first observed in an ultrasonic water bath in 1934 by H. Frenzel and H. Schultes at the University of Cologne, an indirect result of wartime research in marine acoustic radar. This early work involved very strong ultrasonic fields and yielded clouds of unpredictable and non-synchronous flashing bubbles, now termed “multi-bubble sonoluminescence”. Such a chaotic phenomenon did not lend itself to detailed scientific investigation. Study of sonoluminescence then made little progress until 1988, when D. Felipe Gaitan succeeded in trapping a stable sonoluminescing bubble at the centre of a flask energised at its acoustic resonance – single-bubble sonoluminescence (SBSL). However their interest soon waned, and the research was subsequently taken up by Dr S. Putterman et. al., at UCLA, California.Putterman pursued SBSL, published numerous papers, and established many of the characteristics which are now taken for granted. Once per acoustic cycle, coincident with a sharp decrease in bubble size, bluey-white light is emitted in a brief flash shorter than 100picoseconds in duration, with incredible regularity. Despite the results that have been obtained, the actual mechanism by which sound is converted to light remains elusive, not least because of the difficulty in measuring the conditions inside a pulsating bubble whose diameter is measured in micro-meters. It is generally agreed that the adiabatic compression of the bubble leads to very high interior temperatures, but beyond that, shocks, plasmas, ionisation and photo-recombination, Bremsstrahlung radiation, and even fusion are all hotly-debated possible explanations.In Scientific American, February 1995, Putterman published an introductory overview paper on sonoluminescence together with a practical guide in the “Amateur Scientist” section of the same issue. By making the subject accessible to a wider audience, interest escalated dramatically, and given the apparent ease with which sonoluminescence could be obtained, many university groups attempted it. A revised and more detailed version of their “Amateur Scientist” guide can be found on the World Wide Web at http://www.physics.ucla.edu/~hiller/sl/, and is maintained by Robert Hiller, a student of Putterman.Within the Physics Department here at UCL, sonoluminescence was offered last year (1996-97) as a final-year undergraduate project. Unfortunately the quest for the glowing bubble proved fruitless for those involved… a story which has been echoed by several other groups around the world. So, I was “commissioned” by the department to get the experiment up and running over the summer, ready for next year.I too used the Scientific American article as my primary guide, but successfully “saw the light” within a couple of weeks. I found the article to be quite correct as far as it goes but that, probably due to the constraint of brevity, it fails to emphasise some significant details as much as might be desired. Combined with the somewhat optimistic impression of the ease with which sonoluminescence can be obtained, this may explain others’ failed attempts and subsequent discouragement. I attribute my success to extensive experience in hands-on practical work and electronics, and ready access to a variety of electronic bits and pieces from home!Photo 1: Success… that elusive glow! (small bluey-white dot at centre of flask)Detailed MethodPreparation of a flask for use as a sonoluminescence vesselWe scoured glassware catalogues for suitably narrow-necked 100ml round-bottomed flasks. Finally we chose a d

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