Kinky business: watching atoms wiggle.

What's more annoying than a bump in a rug? Step on it, and it shifts beneath your feet but doesn't flatten out.

At the molecular level, the surfaces of ductile materials were thought to harbor similar structures. Heat a sheet of metal or silicon enough to soften its brittleness, bend it ever so slightly, and tiny kinks, or dislocation lines, could form on the curved surface, looking somewhat like a series of speed bumps.

Until recently, this analogy-devised to explain how soft materials bend-had never found observational support. Now, John C.H. Spence, a physicist at Arizona State University in Tempe, and his colleagues have developed a technique-called forbidden reflection electron microscopy-to watch such kinks form and move on a flexible surface.

"We've been able to see a kink at atomic resolution, watch it move, and determine how fast it moves in response to a given pressure," Spence said this week at a meeting of the Materials Research Society in San Francisco. "Until now, no one's actually observed these structures in motion." Although researchers have assumed that the motion of these kinks controls the strength of ductile materials, Spence says, "it's all been theory. No one's seen it happening."

Robert Hull, a materials scientist at the University of Virginia in Charlottesville, agrees. "This is the first time that anyone has directly seen kinks moving in a material. That's really quite remarkable." According to the new observations, it is the speed with which those bumps grow and move that controls how easily the material yields to pressure. The softer and more pliant a material is, the more easily the kinks form and roll along. In contrast, a brittle material resists forming kinks and stays stiff in the face of pressure.

As a surface bends, the kinks look like "strands of spaghetti zipping around," says Spence. "The million-dollar question is how fast those dislocation lines move in response to a given pressure and what impedes their motion." Working with physicists Helmut Alexander and Harold R. Kolar at Arizona, Spence's team applied pressure to a hot strip of silicon under the gaze of an atomic-resolution electron microscope that they adapted to detect electrons bouncing off the bumps that form as the material bends. Ordinarily, scientists scan an ordered crystal lattice of atoms to study the material's regularity. Certain reflections are referred to as "forbidden" because they never are produced by these lattices. Spence's group adjusted the microscope to capture such forbidden reflections, which arise from the irregularities caused by the pressure wave moving through the silicon. The new observations help to explain, for instance, why adding impurities to a material can strengthen it. Pure aluminum is "almost too soft to be useful," Spence says. "But if you add impurities, it becomes stiffer."

Spence has observed that impurities block kinks from forming and moving, so the material won't bend easily. "If we can block the motion of dislocations, then a material can be strengthened," he says. "One might want to do this by introducing obstacles on the atomic scale." Although the aim of these experiments is to understand the behavior of ductile materials, Spence says, the new technique may help in designing flexible alloys less likely to fracture under intense heat or stress-like the conditions that can cause jet engine turbine blades to fail.