MIT researchers use bubbles to reveal nano-scale secrets

The performance of microscopic and nano-scale devices could now be better predicted and improved thanks to work done with bubbles by researchers at the Massachusetts Institute of Technology.

The researchers have used a raft of soap bubbles to simulate the behaviour 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).

‘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.

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.

One year ago, Suresh and colleagues Andrew Gouldstone and Krystyn J. 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.

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 is said to constitute 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,’ said Suresh. This meant that they had a macroscopic system that could represent behaviour 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 the high strength of crystalline surfaces subjected to nanoindentation.

The team has since used the bubble system to explore how atoms move — and how 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.