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Collaboration explores materials at extreme conditions
To better understand how high-strain-rate plasticity evolves, researchers from Lawrence Livermore and the University of Oxford are combining very large-scale molecular dynamics simulations with time-resolved data from laser experiments of shock-wave propagation through metals. A strong shock produces an unusual number of line defects, or dislocations, within a metal’s crystalline lattice. Plastic deformation occurs when a high number of dislocations moves, changing the metal’s mechanical properties such as its strength, ductility, and resistance to fracture and cracking.
The research team, which is led by Livermore scientist Eduardo Bringa, used atomistic molecular dynamics to simulate a strong shock wave in a metal with preexisting dislocation sources. The team studied the nucleation and motion of dislocations relaxing the large strain behind the shock front. An abrupt impact causes dislocations to multiply too rapidly, becoming entangled before they can move far enough to completely relax the uniaxial strain. When pressure increases gradually, fewer dislocations are generated. However, they are more effective at relieving the strain because they can move freely for a longer period.
Dislocation activity behind a shock wave has not been measured directly. However, Bringa’s team has proposed that dynamic x-ray diffraction experiments, similar to those being conducted by Livermore physicist Hector Lorenzana and collaborators, could provide indirect information on such dislocation activity. The team’s research, which is described in the October 1, 2006, issue of Nature Materials, will help scientists assess material properties and performance under extreme conditions, such as an automobile crash or an explosives detonation. Says Bringa, “Experiments and simulations make a powerful pair for exploring uncharted, even unimagined regimes of material dynamics.”
Contact: Eduardo Bringa (925) 423-5724 (ebringa@llnl.gov).

New method examines the function of cell membranes
A collaboration involving researchers from Lawrence Livermore, Stanford University, and the University of California (UC) at Davis has developed a method that can directly test for the existence of lipid rafts in cellular membranes. The cell membrane is composed primarily of a fluid bilayer of lipids. Previous research indicated that this lipid bilayer was a passive carrier for the proteins that perform the active work of the membrane.
The model being tested by the collaboration suggests that, instead, lipids are organizing the functional proteins of the cell membrane. “This is a very elegant hypothesis, under which the subtle affinities that certain lipids have for each other make the cell membrane self-organizing,” says Livermore physicist Peter Weber.
To develop the method, the research team formed model cell membranes on silicon chips, induced the formation of tiny lipid-raft-like gel domains, and freeze-dried the membranes. Then using NanoSIMS, the Laboratory’s high-resolution secondary ion mass spectrometer, the scientists detected gel domains that measure about 100 nanometers laterally and 5 nanometers thick.
The team’s work appears in the September 29, 2006, issue of Science. Results from this research could help scientists develop methods to short-circuit a virus attack on cells, to characterize structures within biological pathogens, and to increase the sensitivity and flexibility of biological sensors.
Contact: Peter Weber (925) 422-3018 (weber21@llnl.gov).

Marine experiment tests detection capability
In August 2006, Livermore scientists participated with the Naval Postgraduate School (NPS) of Monterey in a series of experiments aimed at detecting, identifying, and interdicting nuclear materials in open waters. The experiments, which were conducted in San Francisco Bay, involved participants from several federal agencies, including the departments of Defense, Energy, and Homeland Security, as well as military representatives from several nations.
Marine-enforcement first responders face an enormous challenge attempting to screen cargo inside the endless stream of containers that enter a major facility such as the Port of Oakland. Many commercial products, including smoke detectors, radiant signs, and even bananas, emit radiation, and a shipment of such items can cause radiation detectors to alarm. A false-positive alarm slows commerce unnecessarily and increases product costs. Successful interdiction requires not only modern technology but also coordinated operations and effective communications among many agencies.
To test detection capabilities and agency preparedness, NPS conducted an exercise in which a vessel entering the Port of Oakland required inspection because it was emitting signs of ionizing radiation. For this exercise, the Alameda County Marine Enforcement Agency provided the operations center and the boarding vessel, and a boat operated by the Oakland Police Department played the target vessel. Coast Guard officers—or in some cases Laboratory researchers acting as Coast Guard officers—boarded the vessel to take readings with portable radiation detection instruments. Those readings were immediately relayed to scientific experts at other locations. Their results were radioed back to the boarding vessel for use by first responders on the scene.
“Experiments of this sort are iterative,” says participant Bill Dunlop, a physicist in Livermore’s National Security Office. “We find out what works well, what needs improvement, and what’s unsuccessful. The next exercise will incorporate improvements from the lessons learned this time.”
Contact: Bill Dunlop (925) 424-4462 (dunlop1@llnl.gov).

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UCRL-52000-06-12 | December 8, 2006