Sonoluminescence is a little-understood process by which light is produced in tiny bubbles. Unlike conventional luminescence which is caused by the emission of photons from a light source, this phenomenon occurs within bubbles suspended in a liquid.
This is achieved when sound with a high frequency is passed through water and causes a bubble to snap back and forth between areas of higher and lower pressure. This produces a flash of light when the bubble collapses.
The Origins of Sonoluminescence
In 1934 scientists H. Frenzel and H. Schultes were experimenting with acoustic waves to accelerate the darkening of a photographic plate when they observed luminous bubbles in a liquid.
They had no idea what was happening, but the inexplicable results piqued their interest. They continued to work on this phenomenon.
Now it is possible to keep a single bubble stable, so that it expands and collapses periodically, emitting a flash of light each time. It is also known that certain noble gases, such as xenon or argon, increase the brightness of these pulses.
The pulsations are driven by sound waves that are generated within a liquid. These acoustic waves travel into the center of the bubble, which causes it to collapse and creates cavitation (the formation of voids).
Single Bubble Sonoluminescence (SBSL)
Single Bubble Sonoluminescence (SBSL) technology allows scientists to produce regular repeated compression and light emission from a single collapsing bubble. These oscillating bubbles are often used to study how sound waves cause the radiation of light.
These bubbles are made from a mixture of water and air; the temperature within a sonoluminescing bubble varies between 10,000 and 100,000 K (17,540 to 179,541degF; 9,727 to 99,727degC). The light flashes are very short–35 to a few hundred picoseconds long–with peak intensities of the order of 1-10 mW.
The mechanism of SBSL is based on adiabatic heating of the bubble, which occurs primarily due to spherical symmetry and a large amplitude of displacement of the bubble surface. This compresses the bubble to very high temperature with little loss of heat.
Multi-Bubble Sonoluminescence (MBSL)
Multi-Bubble Sonoluminescence (MBSL) technology involves the cavitation of air bubbles in a liquid. The bubble is then irradiated with ultrasound to emit a flash of light.
The flash of light is accompanied by a rapid rise in temperature. The ionization of noble gas inside the bubble generates thermal bremsstrahlung radiation and free electrons that emit light.
A wide spectrum of MBSL is observed in water with different noble gases at varying acoustic intensities and water saturation levels. The spectral data are characterized by three distinct light-emitting morphologies: filamentous, bulbous and cone shaped.
Applications of Sonoluminescence
Sonoluminescence is an interesting phenomenon that has yet to be fully understood, despite being fairly simple to create. It can be found in some critters like shrimp, but it’s still not understood why it happens.
When a gas bubble is acoustically suspended in water and driven periodically at ultrasonic frequencies, it will collapse and emit light. This process is called sonoluminescence and was first observed in 1934 by H. Frenzel and H. Schultes at the University of Cologne, Germany.
Scientists are now able to study this phenomenon more effectively thanks to technology called single bubble sonoluminescence (SBSL). SBSL uses an acoustic standing wave that generates a pulse of light whenever the bubble compresses inside the wave.
The mechanism of SBSL remains unsettled, but it is possible that a thermal process is the primary cause of light emission in SBSL. Theories include bremsstrahlung radiation, collision-induced radiation and corona discharges, non-classical light, proton tunneling, electrodynamic jets, fractoluminescent jets (now largely discredited due to contrary experimental evidence), and others.