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Bubbles key to understanding surfaces, MIT researchers find

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The performance of microscopic and nano-scale devices could now be better predicted -- and improved -- thanks to MIT work with a substance familiar to any kid: bubbles.

In a communication published June 7 in Nature, the researchers explain how they used a raft of soap bubbles to simulate the behavior of atoms on and near the surface of a material when that surface comes in contact with another surface or object.

The work could be used to gain valuable insights into the mechanisms of atomic scale contact and wear (so-called "nano-tribology") and defect nucleation at surfaces, which have numerous applications in nanotechnology, including nano-electro-mechanical systems (NEMS), nanoindentation, and atomic force microscopy (AFM). Nanoscopic deformation and strength of surfaces is also of paramount importance to the creation of new materials in nanotechnology.

"As devices get smaller and smaller, understanding the phenomena of contact and defect nucleation at surfaces becomes more and more important," said Subra Suresh, the R. P. Simmons Professor and Head of the Department of Materials Science and Engineering. His co-authors are Andrew Gouldstone (MIT PhD 2001) and Krystyn J. Van Vliet, a graduate student in Materials Science and Engineering.

Scientists can measure various properties associated with how surfaces respond mechanically to being probed with nanoindenters, objects with tip sizes smaller than one-thousandth of the diameter of the human hair. However, "we cannot see how the atoms move and how defects form. Atoms are simply too small," said Suresh, who also holds an appointment in the Department of Mechanical Engineering.

About a year ago, Suresh, Gouldstone and Van Vliet were frustrated by their inability to conclusively explain certain results associated with indenting a few billionths of a meter into the surface of various metals. "Then we thought, 'why don't we create a layer of soap bubbles to simulate the atoms?' " Suresh remembers.

So the team created a raft of bubbles a single layer thick to represent an atomic layer of a material's surface. And since each bubble was nearly a million times larger than an atom, using a high-speed digital camera the researchers could monitor, in real time, what happened when they indented the surface from the side.

Although soap bubbles had previously been used to study deformation of bulk metals, this work constitutes the first attempt at the analysis of nano-scale deformation at surfaces.

They then compared data from the bubble simulation with data from nanoindentation of a real metal. "We found that the two sets of data matched, both qualitatively and quantitatively," Suresh said. Suddenly they had a macroscopic system that could represent behavior on the nanoscale.

With insights gained from the bubble model, they formulated a mechanistic theory for defect nucleation at surfaces during nanoindentation. They were also able to explain with remarkable accuracy the unusually high strength of crystalline surfaces subjected to nanoindentation.

The team has since used the bubble system to explore how atoms move -- and defects form -- for a variety of surface conditions. They have experimentally simulated, for the first time, the effects of atomic level surface roughness on defect nucleation at surfaces.

"For the first time, these bubble experiments give insight into the real-time behavior of atoms near the surface of a material during mechanical contact deformation," Suresh said. "Our ultimate goal is to use them to predict how defects will form on the nano-level, because such defects contribute to the performance of these surfaces and nanoscale devices." Suresh and his research group have already made considerable progress in developing fully three-dimensional, molecular dynamics computer simulations of nanoindentation in engineering materials, in collaboration with Sidney Yip, Professor of Nuclear Engineering, and Ju Li (MIT PhD 2001).

The work was supported by Defense University Research Initiative on NanoTechnology (DURINT) in the area of nanostructured materials, which is funded at MIT by the Office of Naval Research, and by a Department of Defense graduate fellowship to Van Vliet.

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