Magazine

Introduction


The American Physical Society held its annual March meeting in Los Angeles the week of Mar. 21. As usual, it was a monster gathering, with some 6,300 talks ranging from hydrogen energy to bioengineering and new forms of steel. Here are some highlights.

Fuel cells promise up to a tenfold boost in battery life in laptops, cell phones, and the like. But they're expensive, especially the membrane. It's the magic component that produces electricity by separating electrons from a hydrogen-rich fuel such as methane.

Now, a University of Illinois group has built a membrane-free alkaline fuel cell. Its design is sheer simplicity: Just pipe methanol and oxygenated water alongside each other, so a chemical reaction at the parallel surfaces can liberate electrons. What stops the methanol and water from mixing and upsetting the process? It's called laminar flow. The two liquids emerge from a Y-shaped conduit and meet in a center channel so narrow -- roughly 1 millimeter wide -- that each liquid's surface tension keeps it intact. "It's like Aquafresh toothpaste," says team leader Paul Kenis, a chemical engineer. A prototype may be ready in three years.

A tuning fork's pitch depends on the size of its tines. With really tiny tines, you can figuratively play molecular music -- as a team from the California Institute of Technology has done. Physicist Jack Ya-Tang Yang told the APS meeting that the team has weighed an extremely small mass -- just a few dozen xenon atoms -- spraying them on silicon-carbide tines a mere 100 nanometers long. The added weight noticeably slows the tines' vibrations, and the mass that was applied is then calculated from the frequency change. So how few atoms can the tuning fork weigh? About 30, or a mass of seven zeptograms (seven trillionths of a nanogram). Detecting such minuscule quantities is essential for drug research, toxin detection, and more.

Those gates at the airport aren't terribly secure. X-ray systems that inspect luggage aren't safe to use on people. Even if they were, they might not spot plastic explosives. And magnetic gateways that do screen passengers can't detect a ceramic knife.

So-called terahertz rays can find both, even in a shoe's sole or heel. T-rays, which lie between microwaves and infrared light on the electromagnetic spectrum, see through fabrics and leather but don't penetrate or harm human tissues.

Two companies, Picometrix in Ann Arbor, Mich., and TeraView in Cambridge, England, are trying to develop a terahertz people scanner. Currently, T-ray images are extremely grainy and need extra processing to produce sharp pictures. Still, T-rays do have a fast-alarm capability. When they hit a certain type of matter -- say, plastic explosives -- a specific wave pattern bounces back. Such spectral signatures, TeraView says, can quickly determine if a suspicious material is present, and further processing could pinpoint its location.

-- In physicist Robert Westervelt's lab at Harvard University, researchers play a kind of chess with individual cells. The cells are tagged with micron-size magnetic beads. Then they're floated in fluid above a grid of computer-controlled magnetic coils. The cells are moved by clicking on the computer screen to turn the coils on and off. One goal of this chess game is to find the best pattern of moves for assembling the cells into an artificial skin.

-- What makes a droplet splash? Lei Xu at the University of Chicago says it's partly the density of the surrounding air. Reduce the air pressure to four-fifths of normal, and a drop of alcohol will flatten smoothly as it lands on dry glass -- no splash whatsoever. Understanding this might help reduce splashing in high-end inkjet printing, among other things.


Toyota's Hydrogen Man
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