Number 72, March 23, 1992 by Phillip F. Schewe and Ben Stein|
BIOMIMETICS is the study of the structure and production of natural materials such as human bone and spider's silk for the purpose of designing useful artificial materials. At the March APS meeting in Indianapolis last week, Ilhan Aksay of the University of Washington described a technique called "ceraming," which involves replacing the cellulose fiber in wood with a ceramic polymer and in some cases metals. Special processing techniques can harden these composites in a matter of days rather than the millions of years it takes for petrified wood to form, and the resulting materials are, depending on the type of wood originally used, 50-90% harder than petrified wood samples and 20-120% stronger.
MAGNETOTACTIC BACTERIA , described at the March Meeting by Richard Frankel of Cal Poly, produce particles such as magnetite (Fe3O4) and greigite (Fe3S4). Protein-containing membranes in the bacteria line up between 10 and 20 of these particles, forming a permanent magnet sensitive to the Earth's magnetic field and allowing the bacteria to quickly locate oxygen regions conducive to their survival. In what Frankel called a "masterpiece of permanent magnetic engineering," the bacteria produce particles over a narrow size range centering around 500 angstroms. Frankel said that in the future, this process may be duplicated in order to produce materials such as yttrium-barium-copper-oxide particles, the raw materials for high-temperature superconductors.
CHEMICAL REACTIONS ON REDUCED-DIMENSIONALITY SURFACES , where diffusion processes are limited, do not conform to the classical laws of chemical kinetics. For example, when reactants meet on spatially-constrained (sometimes fractal) surfaces like microscopic pores or capillaries, they can, under certain non-equilibrium conditions, come to order spontaneously: instead of mixing, like reactants will aggregate while unlike reactants segregate. Convection currents, usually the predominant means for chemical mixing, have no room to form on these confined surfaces. At the March APS meeting, Raoul Kopelman of the University of Michigan described the first experimental evidence for this effect. In his experiment, he added two chemicals of contrasting colors to opposite ends of a gel-filled tube. When the chemicals met in the center and attempted to react on the gel surface, they formed two segregated regions, with the reaction rate between the chemicals actually diminishing in time. In addition to being understirred, chemicals on low-dimensional surfaces have a smaller probability of straying from their original positions and encountering atoms of different species. Kopelman says that reactions on low-dimensional surfaces occur in such processes as photosynthesis and photocatalysis, and play a role in population biology models and some theories of structure formation in the early universe.
THE TOKAMAK PHYSICS EXPERIMENT (TPX) may become the next principal magnetic-confinement fusion device now that plans for the $1.8-billion Burning Plasma Experiment (BPX) have been dropped. The $400-million TPX would not reach ignition temperatures or densities (which were planned for PBX), but would aim to sustain 1000-second plasma burning periods. (Science, 6 Mar. 1992.)