SONOLUMINESCENCE.

 

Sonoluminescence is the emission of light produced from small air bubbles, when the latter are exposed to an intense acoustic field.

Although sonoluminescence is a rather old phenomenon, a renewed interest on this subject has been triggered by recent papers suggesting that light are produced by the exceedingly high temperature achieved within the gas during the collapsing of the bubble.

The relevance of these findings did not pass unnoticed, as it showed that the acoustic energy can be effectively transferred to the gas within the bubble causing the temperatures to rise, as much as that in the sun or the stars.

 

SONOLUMINESCENCE EXPERIMENT AT ULL

Motivated by these findings, we decided to build a similar experiment in our Laboratory of Electrónica Básica in the Facultad de Física of ULL. To this end, we contacted Mr. Andrew Steer, at Imperial College, University of London, who, so generously, provided with all required information to set up our own experiment.

The experiment is based on an acoustic resonator, made out a 100 ml round flask. The sound is delivered to the flask by means of a couple of piezos, which are glued at opposite positions on the equator of the flask. A little piezo, glued down in the bottom, is used as a microphone to sense the acoustic level in the resonator.

See figure at the right.

 

 
 
 
Schematic of electronic instruments utilized in the experiment.

 

 

 

 

 

 

 

 

The picture at the right shows the resonator used in this experiment. One readily identify the flask, the two exciting piezos and the small one in the bottom.

Observe that wires were all curled in order to avoid excessive wearing at the solder points.

Also, water, previously distilled and degas, fills in up to beginning of the flask neck.

There seems to be no a single resonance frequency, but several frequencies at which the flask emits the typical sound of having achieved a high sound intensity. Nevertheless, one very distinctive appears around 25 kHz, others occur above 60 kHz.

It is found that the so called single bubble sonoluminescence (SBSL) occurs mainly in the 25 kHz resonance. In this case, sound intensity seems to be weaker and due to the fact that only one bubble is glowing, this is a difficult case to observe.

 

 

At higher frequencies, multiple bubble sonolum. (MBSL) are observed, and these are more intense a easier to see.

In the pictures below, one has several cases of MBSL. The light, bluish, is indeed faint and it requires the flask to be in a carefully darken place.

These pictures were obtained using a Minlota, Dynax 505si camera, with a Minolta AF50/1.4 lens and a Fujifilm 1600 ISO color film.

The exposure time ran from 10 up to 20 minutes and, sometimes, as in the picture above, right and down, left, we made a double exposure, i.e. a long one all darkened, superimposed to a second with camera in AUTO and the flask slightly illuminated. This helps one to see the flask, the position of the bubbles and the size of them.

It must be mentioned that emitting bubbles are indeed small and its size is not apparent to naked eye. They are rapidly oscillating, with an amplitude of the order of 1 mm, thus causing them to look blurred in the pictures.

 

ACKNOWLEDGEMENTS: to Dr.Williams Andrew Steer of University College London, for providing the informations requiered to build the Sonoluminescence experiment, and to Lic. Carlos de Jesús Aquino, from Universidad Nacional del Nordeste (Argentina), for mounting and "tuning" all electronic circuits necessary to have the experiment running.

 

 

 

Mario Jakas (2003)