APS News

Sonoluminescence, Applications Featured at SCCM Meeting

New studies of the phenomenon of sonoluminescence, as well as industrial and medical applications of shock wave physics, were among the highlights of the biennial international meeting of the APS Shock Compression of Condensed Matter Topical Group, held 13-18 August in Seattle, Washington. Intended to promote an understanding of fundamental shock physics, to highlight current research in the many related fields, and to discuss the application of various aspects of shock physics, the conference provides an international forum for technical exchange among scientists, engineers, academic professionals, users, and industrialists.

Sonoluminescence. Sonoluminescence is a phenomenon which involves the conversion of acoustical energy to optical energy arising from the nucleation, growth, and collapse of gas-filled bubbles in a liquid. The process of generating light appears to be extremely rapid and represents a remarkable -degree of energy concentration, i.e., as high as 12 orders of magnitude. Recent experimental -advances have pro-duced a single acoustically levitated sonoluminescent air bubble with unanticipated temporal and spectral properties, which are inconsistent with existing models of sonoluminescence.

According to Lawrence Crum of the Applied Physics Laboratory in Seattle, Washington, the most plausible explanation of the origin of the extremely short bursts of light emitted from the bubble is that an imploding shock wave is generated within the bubble during the final stages of collapse. "If this shock wave does indeed exist, then preliminary computations of the final temperatures and pressures that would occur in the bubble as a result of the symmetric implosion of the shock wave indicate temperatures in excess of a million degrees and pressures on the order of megabars," he said.

Industrial and Medical Applications. The growing understanding of shock physics phenomena has led to numerous industrial applications. M. Sanai of SRI International discussed two recent applications at the SCCM Meeting: a method of assessing explosion safety in industrial plants, and a bomb-resistant baggage container for wide body aircraft.

Explosion safety usually employs computer simulation techniques to model the details of the detonation of energetic materials. Using a general load-damage analysis technique known as the pressure-impulse method, Sanai and his colleagues have developed a PC-based computer algorithm that incorporates a continually expanding library of load and damage curves.

Sanai also described SRI's development of a patented hardened luggage container that can protect the aircraft from a terrorist bomb hidden inside the baggage. The container relies on blast management and debris containment provided by a flexible flow-through blanket woven from threads made with a strong lightweight material, such as Spectra or Kevlar. "When an explosion occurs, the mitigation blanket expands into a nearly circular shell that contains the flying debris while directing the flow into the adjacent containers," said Sanai. "This redirectioning of the flow and slow venting of the detonation products dissipate the explosive energy without damaging the structural shell of the aircraft."

In addition, promising new medical therapies are being developed which use concentrated energy deposition via lasers and ultrasound. According to Livermore's M.E. Glinsky, an understanding of stress wave propagation and material response is vital in order to design these new techniques for maximum efficiency and minimal damage. He discussed ongoing experiments to explore such issues as the structure of several generic hard and soft composite tissues, and the impact of thermal destruction on their structural integrity.

Shock Temperature Measurements of Liquid Hydrogen. Scientists at Livermore have developed a new model to determine the final pressure and -volume of double-shocked liquid deuterium and hydrogen, after a series of measurements produced temperatures as much as 40 percent lower than predicted by theory. According to N.C. Holmes, the lower temperatures are a result of a fraction of the available shock energy being used to dissociate some of the hydrogen molecules. In the new model, molecular dissociation energy decreases with volume, and subsequent double-shock experiments, which reach higher densities than equivalent-pressure experiments, were in overall agreement with this hypothesis.

Shock Wave Properties of Brittle Solids. Using planar impact methods and velocity interferometry diagnostics, scientists at Sandia National Laboratories have performed extensive experiments in large-amplitude, nonlinear wave-profile measurements which manifest the shock strength and equation-of-state properties of brittle solids. They also provide controlled, shock-induced motion histories that validate existing computational models.

According to Sandia's D.E. Grady, these high-resolution shock-profile data are providing a window into the physics of emerging understanding of the compression and deformation behavior of high-strength brittle solids. The shock wave data suggests that both crystalline plasticity and brittle fracture may play important and interconnected roles in the dynamic failure process.

Numerical Anomalies Mimicking Physical Effects. Numerical simulations of flows with shock waves typically use finite difference shock capturing algorithms, which give a shock an artificial width proportional to the cell size in order to generate the entropy increase that occurs across a shock wave. But numerical anomalies can occur when shock waves interact or a shock wave is formed. Examples include strong shock reflected from a rigid wall, and errors that occur when a shock impacts a material interface.

According to R. Menikoff of Los Alamos National Laboratory, this class of anomalies can be explained by the entropy generation that occurs in the transient flow when a shock profile is abruptly formed or changed. When a shock wave has a profile, its position can be defined to match the mass of an idealized continuous shock. However, "In a shock interaction, the shift in energy of the wave profiles between the incoming and outgoing shocks leads to an entropy anomaly in the spatial region where the incoming wave profiles overlap," said Menikoff.

Time-Resolved Molecular Changes. Researchers at Washington State University (WSU) have conducted time-resolved Raman scattering experiments to probe molecular changes in shocked materials. According to G.I. Pangilinan, a good understanding of the molecular mechanisms governing shock-induced chemical decomposition in condensed energetic materials is fundamental to the field of detonation science, as well as to the development of new energetic materials.

To this end, Pangilinan and his colleagues made time-resolved Raman spectroscopy measurements that enabled them to monitor in real time the molecular changes associated with the onset of a chemical reaction in shocked nitromethane. Raman spectra of nitromethane at high peak pressures exhibited irreversible changes indicative of chemical reaction. It appears that the activation energies of the initial reaction steps are lowered and this impacts the microscopic models of decomposition of the material.

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Editor: Barrett H. Ripin