February 2000

News about Science, Technology and Engineering at Iowa State University

Earth's fiery finale
It may take some time, but Earth will see a fiery end to its existence, say two Iowa State University astronomers. Lee Anne Willson, an Iowa State University professor of physics and astronomy and George Bowen, professor emeritus of physics and astronomy, say new computer models of the later stages of the Sun's life show that it will expand and engulf our planet before it fades away as a white dwarf.

"Earth will get scorched as part of the process the Sun will go through as it transforms from being a red giant into a white dwarf," said Willson, who will present the research at the annual meeting of the American Association for the Advancement of Science, Feb. 17-22, in Washington, D.C. Most astronomers agree that as the Sun evolves over the next 5 thousand million years, it will become about twice as bright as it is today. However, when the center of the Sun runs out of fuel and the Sun becomes a red giant, it will grow much brighter and larger. At question is the maximum size the Sun will reach and whether Earth will be able to get out of the way before the Sun gets too big.

Willson and Bowen studied the mass loss of red giant stars and used that information to predict the fate of the Sun and its effects on the planets. Their computer models are able to predict in great detail the conditions that Earth and the other planets will encounter as the Sun ages. Bowen and Willson have found that the Sun's final mass loss process ramps up steeply only near the very end of the red giant stage, and therefore is over in a much shorter period of time, not giving Earth much of a chance to get out of the way.

"The Sun will expand and include Earth's orbit (a distance nominally of 93 million miles) before the Sun loses enough mass to let Earth move away," Willson said. "Most likely, the Sun will expand to hide the Earth for a few centuries and then retreat to reveal it, still mostly intact, at least once before the end." For more information contact Willson, (515) 294-6765; Bowen, (515) 294-7659; or Skip Derra, News Service, (515) 294-4917.

Simulations tell the story
Ames Laboratory theoretical physicists Cai-Zhuang Wang and Kai-Ming Ho have developed a technique for doing quantum molecular dynamics simulations of practical materials. The technique -- called tight-binding molecular dynamics (TBMD) -- is easier, faster and more economical than conventional first principle methods. TBMD is a computationally efficient means of studying the structures, dynamics and electronic properties of complex systems at the atomic level. It can accommodate changes in bonding within a material due to electronic excitation, a capability that most atomic simulation methods don't possess. Wang, Ho and their Ames Lab colleagues have successfully applied the technique to simulate how carbon atoms behave in different environments and under different circumstances.

Wanting to better understand what they were observing in their laboratory experiments on laser ablation of diamond, researchers in Iowa State University's mechanical engineering department approached Wang and Ho about doing TBMD simulations that would describe what was going on at the atomic level during diamond ablation with laser pulses of different durations. The TBMD simulations explained a curious lab finding -- a dramatic difference in diamond surfaces following nanosecond (1 x 10-9 second) and femtosecond (1 x 10-15 second) laser ablation. The TBMD simulations revealed that heat from the nanosecond laser pulses was transmitted to the atoms during the ablation, causing thermal melting and graphite formation on the surface. By contrast, because the ultrashort femtosecond laser pulse ejected the electrons into highly excited states while the atoms were still thermally cool, the ablation peeled off the atoms layer by layer from the diamond surface through a "non-thermal" mechanism, leaving a smooth diamond surface following the laser ablation.

Through their TBMD simulations, Wang and Ho demonstrated the fundamental difference between diamond ablation with laser pulses of different durations, which may have a significant impact within the microelectronics and cutting tool industries. For more information, contact Wang, (515) 294-6934, or Saren Johnston, Ames Lab Public Affairs, (515) 294-3474.

Big gains for nanoscale structures
A $450,000 three-year National Science Foundation grant was recently awarded to Pal Molian, an Iowa State University professor of mechanical engineering, to develop nanoscale structures by combining ultrafast lasers and nonlinear optics. Molian will use femtosecond (1 x 10-15 second) laser pulses to develop micro-scale materials that can be used to improve medical devices, such as catheters in drug-delivery systems, and microelectronics. The work is being done in collaboration with the Lawrence Livermore (Calif.) National Laboratories and Clark-MXR Inc., Dexter, Mich.

"Nanometer-sized holes will be fabricated in micropumps to precisely pump drugs into the body or even allow the drugs to crawl around the bloodstream repairing damaged tissue," said Molian. Other medical uses for nanoscale structures include allowing for precise measurement of blood in catheters, accurate monitoring of a patient's condition through biosensors and controlled passage of gas or liquid in membranes.

In the microelectronics industry, Molian sees ways to improve micro-electromechanical (MEM) systems developing new methods to remove polymer layers in silicon wafers. Normally these layers are removed chemically, which poses disposal problems. The laser technique, according to Molian, will allow step-by-step release of individual components from a single layer, eliminating the use of chemicals and speeding up the process. For more information contact Molian, (515) 294-2101, or Sunanda Vittal, Engineering Communications, (515) 294-6750.

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