Sonoluminescence William Thomas Spring 2007

Overview Discovery What is sonoluminescence? Types of sonoluminescence –MBSL –SBSL Apparatus Stability and Dynamics Next time…

Discovery In 1933 Marinesco and Trillat were studying the affects of ultrasonic waves on the development of a photographic plate in a liquid. They discovered that fogging of the plate occurred and incorrectly determined the cause to due to ultrasonic waves violently mixing the reactants accelerating the processes of reduction in the plate. In 1934 Frenzel and Schultzes discovered that the fogging was due to sonoluminescence.

What is it? Sonoluminescence is the emission of light from a bubble (in a fluid) that has been excited by sound of sufficient intensity. During the tensile portion of the pressure variation, induced by the sound wave, the bubbles grow Subsequent compression forces the bubbles to rapidly collapse and emit light.

Sonoluminescence From left to right : bubble, slow expansion, quick and sudden contraction, emission of light

Sonoluminescence F. Ronald Young, Sonoluminescence, CRC Press, New York, 2005

Types of Sonoluminescence Multi-bubble sonoluminescence (MBSL) was discovered as noted earlier and was studied but interest waned as years passed because of the inability to precisely measure parameters affecting sonoluminescence. In 1989 single-bubble sonoluminescence (SBSL) was achieved by Gaitan and Crum and allowed for the precise measurement of variables affecting sonoluminescence.

Stability Factors that affect the stability of a bubble include: –Buoyancy (rise) –Diffusion of gas out of the bubble (dissolve) –Surface tension (contract) –Gas pressure (expand) –Bjerknes forces (sound pressure gradient) –Rayleigh-Taylor instabilities (geometric)

Dynamics The approximate motion of a bubble can be modeled by the Rayleigh-Plesset equation: Where R is the radius of the bubble as a function of time t, eta is the viscosity, gamma is surface tension, rho is liquid density, and P is pressure. The Rayleigh-Plesset equation is derived from the compressible Navier-Stokes equations.

Apparatus

Next time… In the next presentation I plan to go over a couple of the theories that try to explain where exactly the light in sonoluminescence is coming from. Examples are: –Hot Spot Theory – Bremsstrahlung –Shock Wave Theory –Quantum Radiation Theory –Proton-Tunneling Model

Sonoluminescence what is making the bubbles light up… William Thomas Spring 2007

Overview Questions from last time –Predictive models –Concentration of energy Why is it so hard to explain sonoluminescence? Proposed theories that attempt to explain the origins of the light generation –Electrical Microdischarge Theory –Hot Spot Theory – Bremsstrahlung –Proton-Tunneling Model Suggestions for future work Some pictures if time allows

Predictive models As for predictive models there are several models that predict many different aspects of sonoluminescence including bubble geometry, temperature distribution within the bubble, and the spectra of the light emitted from the bubble. More on these a little later…

Concentration of energy The concentration of energy is justified thusly, the average acoustic energy given to an atom of the liquid is where rho is the density of the liquid and v is the velocity amplitude produced by the sound wave.

Concentration of Energy If the wavelength of the sonoluminescence is 200 nm the energy of a photon is Which is about 12 orders of magnitude larger than the acoustic energy afforded an atom

Why is it so hard to explain sonoluminescence? Sonoluminescence is poorly understood because the spatial extent of the event is on the order of a micron and the time scale is a only few nanoseconds. MBSL makes this even more difficult because of the large number of randomly growing and collapsing bubbles.

Electrical Microdischarge In several papers spanning from , Margulis proposed that bubbles larger than resonant size coalesce from smaller bubbles at a driving frequency between 10 and 200 Hz (this phenomena has been observed). The light event is pre-empted by a smaller bubble forming off the side of the larger bubble joined by a neck.

Electrical Microdischarge The bubbles begin to separate and the charges of the large bubble become concentrated on the smaller bubble. As a result, a positive charge remains on the large bubble and a negative charge collects on the smaller bubble and this gives rise to a discharge and light.

Electrical Microdischarge This model helps explain the observed phenomena but its complicated dynamics are not reproduced in SBSL and has been criticized by Suslick (1990) and Lepoint- Mullie et al. (1996). Margulis suggested that MBSL is a result of discharge and SBSL is a result of thermal glowing.

Hot Spot Theory Hot spot theory has been proposed by several people including Srinivasan and Holroyd. In 1961 they suggested that the sonoluminescent light emitted is from adiabatic heating of the bubble and they found that it is well modeled by a black body at about 8800K.

Hot Spot Theory The following graph shows the spectral distribution for oxygen-saturated water, a typical SL spectrum. The solid line is a theoretical curve for a black body at 8800K.

Hot Spot Theory Further studies on the hot spot theory suggest that it may be a combination of black body and Bremsstrahlung radiation. There are several types of Bremsstrahlung radiation but it is basically the deceleration of a charged particle, i.e. an electron, by another charged particle, i.e. an atomic nucleus.

Hot Spot Theory In sonoluminescence it is suggested that the air of the bubble becomes fully ionized by the acoustical compression and this gives rise to the Bremsstrahlung radiation. One issue with this model is that the temperatures required for Bremsstrahlung radiation wavelengths below 180 nm are a order of magnitude greater than that ever observed in a bubble and there is no concrete evidence for these temperatures

Proton-Tunneling Model In 1998 Willison suggested that the light emitted is due to a large number of current impulses that occur as water around a bubble goes through a phase transition. The phase transition is caused by a sudden pressure change that occurs as the bubble reaches its minimum size.

Proton-Tunneling Model Water molecules, which have very strong dipole- dipole interactions, during the phase transition move to new positions and change their orientations. Classically, these water molecules are thought to rotate into their new positions but because the protons in the water molecule are light enough, and the potential barriers are small enough and the distances they need to travel are only 2.75 A (the distance between the cores of the oxygen atoms), the protons can tunnel into their new positions.

Proton-Tunneling Model One component of the tunneling protons during phase transition involves them moving 0.75 A between the oxygen atoms as shown below. This event exchanges the covalent and hydrogen bonds and flips the electric dipole moments of the water molecules. So as the proton tunnels to the right, the electron distribution of the molecules move the left which amplifies the apparent current impulse. The proton tunneling and corresponding electron current are thought to be the most important current contributions for the observed emissions.

Suggestions For Future Work Ruuth et al. (2002) recommend coupling the bubbles internal molecular dynamics to the wall velocity to obtain a model of bubble motion and internal dynamics. They suggest this could be done by the coupling of the Euler and Navier-Stokes models for the surrounding fluid. This could be used to determine the dynamics through the point of minimum radius. They also suggest that other collapsing bubble geometries be explored which may increase the temperatures inside the bubble. This could possibly be used to induce deuterium-deuterium fusion.

Some Pictures Temperature distribution inside a collapsing bubble filled with 1 million argon atoms. The bubble is 4.5 micrometers, the driving frequency 26.5 kHz, and the pressure amplitude is 130kPa. Total time = 106 ps

Another Picture

References Lawrence A. Crum, ”Sonoluminescence”, Physics Today, F. Ronald Young, “Sonoluminescence”, CRC Press, Boca Raton, M.A. Margulis, I.M Margulis, “Peculiar Properties Of Light Emission From Cavitation Bubbles In Acoustical Field”, Session of the Russian Acoustical Society, Moscow,