Ju Li of Ohio State University and his colleagues used quantum mechanics to model the behavior of one-atom-thick layers of both aluminum and copper. Specifically, they studied a process known as pure shear strain, in which a layer of atoms slides over a second layer of atoms. The reliability and durability of very small electronic devices, in which temperatures fluctuations often cause materials to expand or contract, depends in part on how their components react to the effects of shear strain. The researchers determined that two layers of copper atoms typically slide over each other quite smoothly. But aluminum layers don't slide and instead hop across one another, the team found. The scientists suggest that so-called directional bonding, in which atoms on neighboring layers share electrons (see image), could be responsible for the observed movement. Such bonds are often found in ceramics and semiconductors, but aren't usually present in malleable metals like aluminum. According to Li, "this could mean that aluminum behaves more like ceramics in certain ways than anyone had previously thought." At the atomic level, aluminum was also 32 percent stronger than copper, according to the team's simulations. "We know copper is three times heavier than aluminum, and significantly stiffer than aluminum under normal conditions," Li says. "But when we looked at large shear strains, aluminum won hands down."