Seth Putterman

Defining the unknowns of sonoluminescence

Abstract As the intensity of a standing sound wave is increased the pulsations of a bubble of gas trapped at a velocity node attain sufficient amplitude so as to emit picosecond flashes of light with a broadband spectrum that increases into the ultraviolet. The acoustic resonator can be tuned so that the flashes of light occur with a clocklike regularity: one flash for each cycle of sound with a jitter in the time between flashes that is also measured in picoseconds. This phenomenon (sonoluminescence or “SL”) is remarkable because it is the only means of generating picosecond flashes of light that does not use a laser and the input acoustic energy density must be concentrated by twelve orders of magnitude in order to produce light. Light scattering measurements indicate that the bubble wall is collapsing at more than 4 times the ambient speed of sound in the gas just prior to the light emitting moment when the gas has been compressed to a density determined by its van der Waals hard core. Experiments indicate that the collapse is remarkably spherical, water is the best fluid for SL, some noble gas is essential for stable SL, and that the light intensity increases as the ambient temperature is lowered. In the extremely stable experimental configuration consisting of an air bubble in water, measurements indicate that the bubble chooses an ambient radius that is not explained by mass diffusion. Experiments have not yet been able to map out the complete spectrum because above 6 eV it is obscured by the cutoff imposed by water, and furthermore experiments have only determined an upper bound on the flash widths. In addition to the above puzzles, the theory for the light emitting mechanism is still open. The scenario of a supersonic bubble collapse launching an imploding shock wave which ionizes the bubble contents so as to cause it to emit Bremsstrahlung radiation is the best candidate theory but it has not been shown how to extract from it the richness of this phenomenon. Most exciting is the issue of whether SL is a classical effect or whether Plancks constant should be invoked to explain how energy which enters a medium at the macroscopic scale holds together and focuses so as to be emitted at the microscopic scale.

Observation of nuclear fusion driven by a pyroelectric crystal.

While progress in fusion research continues with magnetic and inertial confinement, alternative approaches—such as Coulomb explosions of deuterium clusters and ultrafast laser–plasma interactions—also provide insight into basic processes and technological applications. However, attempts to produce fusion in a room temperature solid-state setting, including ‘cold’ fusion and ‘bubble’ fusion, have met with deep scepticism. Here we report that gently heating a pyroelectric crystal in a deuterated atmosphere can generate fusion under desktop conditions. The electrostatic field of the crystal is used to generate and accelerate a deuteron beam (> 100 keV and >4 nA), which, upon striking a deuterated target, produces a neutron flux over 400 times the background level. The presence of neutrons from the reaction D + D → 3He (820 keV) + n (2.45 MeV) within the target is confirmed by pulse shape analysis and proton recoil spectroscopy. As further evidence for this fusion reaction, we use a novel time-of-flight technique to demonstrate the delayed coincidence between the outgoing α-particle and the neutron. Although the reported fusion is not useful in the power-producing sense, we anticipate that the system will find application as a simple palm-sized neutron generator.

Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape

Relative motion between two contacting surfaces can produce visible light, called triboluminescence. This concentration of diffuse mechanical energy into electromagnetic radiation has previously been observed to extend even to X-ray energies. Here we report that peeling common adhesive tape in a moderate vacuum produces radio and visible emission, along with nanosecond, 100-mW X-ray pulses that are correlated with stick–slip peeling events. For the observed 15-keV peak in X-ray energy, various models give a competing picture of the discharge process, with the length of the gap between the separating faces of the tape being 30 or 300 μm at the moment of emission. The intensity of X-ray triboluminescence allowed us to use it as a source for X-ray imaging. The limits on energies and flash widths that can be achieved are beyond current theories of tribology.

Toward a hydrodynamic theory of sonoluminescence

For small Mach numbers the Rayleigh–Plesset equations (modified to include acoustic radiation damping) provide the hydrodynamic description of a bubble’s breathing motion. Measurements are presented for the bubble radius as a function of time. They indicate that in the presence of sonoluminescence the ratio of maximum to minimum bubble radius is about 100. Scaling laws for the maximum bubble radius and the temperature and duration of the collapse are derived in this limit. Inclusion of mass diffusion enables one to calculate the ambient radius. For audible sound fields these equations yield picosecond hot spots, such as are observed experimentally. However, the analysis indicates that a detailed description of sonoluminescence requires the use of parameters for which the resulting motion reaches large Mach numbers. Therefore the next step toward explaining sonoluminescence will require the extension of bubble dynamics to include nonlinear effects such as shock waves.

Effect of Noble Gas Doping in Single-Bubble Sonoluminescence

The trillionfold concentration of sound energy by a trapped gas bubble, so as to emit picosecond flashes of ultraviolet light, is found to be extremely sensitive to doping with a noble gas. Increasing the noble gas content of a nitrogen bubble to about 1% dramatically stabilizes the bubble motion and increases the light emission by over an order of magnitude to a value that exceeds the sonoluminescence of either gas alone. The spectrum also strongly depends on the nature of the gas inside the bubble: Xenon yields a spectral peak at about 300 nanometers, whereas the helium spectrum is so strongly ultraviolet that its peak is obscured by the cutoff of water.

Resolving the picosecond characteristics of synchronous sonoluminescence

The resolution with which the synchronous picosecond flashes of acoustically generated light can be measured has been improved. The flash widths are now found to be considerably less than 50 ps and the jitter in the time between flashes can also be substantially less than 50 ps. The flashes of sonoluminescence appear to turn off very sharply without ringing or after pulsing.

Theory of non-propagating surface-wave solitons

An incompressible inviscid fluid contained in a channel in a gravitational field admits soliton-like disturbances where the velocity potential depends upon all three coordinates as well as time, yet its centre of mass can be at rest. These solitons were recently discovered by Wu, Keolian & Rudnick. The calculations are carried out with the multiple-scales approach. Consequences of mass conservation and radiation are discussed.

Theory of long wavelength acoustic radiation pressure

A general formulation of the radiation force exerted on an object by a long wavelength sound field is developed in terms of an asymptotic multipole expansion. In various limits, the results of King [Proc. R. Soc. London, Ser. A 147, 212 (1934)], Gorkov [Sov. Phys. Dokl. 6, 773 (1962)], and Yosioka and Kawasima [Acustica 5, 167 (1955)] are recovered. In addition, the existence of a spectrum of monopole and dipole radiation force resonances is demonstrated which occur when the bulk modulus of the object is small compared to that of the surrounding medium. Near standing wave resonances, the bulk viscosity is shown to play an essential role in determining the force. For traveling waves, the bulk viscosity can lead to the dominant contribution to the radiation pressure even in the limit of zero frequency. For soap bubbles, the leading‐order contribution to the radiation pressure is shown to be quadrupolar.

Sonoluminescence: nature’s smallest blackbody

The transduction of sound into light through the implosion of a bubble of gas leads to a flash of light whose duration is delineated in picoseconds. Combined measurements of spectral irradiance, Mie scattering, and flash width (as determined by time-correlated single-photon counting) suggest that sonoluminescence from hydrogen and noble-gas bubbles is radiation from a blackbody with temperatures ranging from 6000 K (H(2)) to 20,000 K (He) and a surface of emission whose radius ranges from 0.1 microm (He) to 0.4 microm (Xe) . The state of matter that would admit photon-matter equilibrium under such conditions is a mystery.

Acoustic positioning and orientation prediction

A method for use with an acoustic positioner, which enables a determination of the equilibrium position and orientation which an object assumes in a zero gravity environment, as well as restoring forces and torques on the object, of an object of arbitrary shape in a chamber of arbitrary configuration. An acoustic standing wave field is established in the chamber, and the object is held at several different positions near the expected equilibrium position. While the object is held at each position, the center resonant frequency of the chamber is determined, by noting which frequency results in the greatest pressure of the acoustic field. The object position which results in the lowest center resonant frequency, is the equilibrium position. The orientation of a nonspherical object is similarly determined, by holding the object in a plurality of different orientations at its equilibrium position, and noting the center resonant frequency for each orientation. The orientation which results in the lowest center resonant frequency is the equilibrium orientation. Where the acoustic frequency is constant but the chamber length is variable, the equilibrium position or orientation is that which results in the greatest chamber length at the center resonant frequency.