Nanotech stress test

Just as you might paste a piece of paper across a crack in a wall to reveal whether the crack is growing over a period of weeks or months when the paper tears, so researchers in the US have developed the high-tech equivalent for detecting microscopic cracks in engineering structures and materials. The scientists at Berkeley Lab and the University of California Berkeley have developed tiny light emitting, four-armed nanocrystal quantum dots that are embedded in a polymer film. The film pasted on to a structure could reveal the development of microscopic cracks before structural failure becomes inevitable.

The core of the team's "tetrapods" fluoresce when any of their four feet, or arms, are twisted or bent under tensile or compressive strain. If the structure is under such stresses sub-microscopic cracks might form from which bigger cracks develop leading ultimately to macroscopic failure. The team has carried out preliminary tests to show that tetrapods can cycle more than 20 times without losing their ability to reveal stress to a portable, fluorescent spectrometer. The system could be used to reveal problems in steel beams, aircraft components to reveal incipient cracks just a 100 nanometers long.

"This is the length scale at which cracks develop, which is when you want to catch them, well before the material fails," explains Shilpa Raja. Details are reported in the journal Nano Letters [SN Raja, Nano Lett., (2016), 16(8), 5060-5067; DOI: 10.1021/acs.nanolett.6b01907]. "Our approach could also be a big step toward self-healing smart materials. The tetrapods could be coupled with nanoscale repair particles to form a material that senses local stress and then repairs itself," adds Raja.

To develop the technique, the scientists started with a polymer widely used in airframes and other structures. They mixed tetrapod nanocrystals into the polymer and cast slabs of the mixture. They could then mount these slabs in a tensile tester and under laser exposure they could measure fluorescence and mechanical stress at the same time. "This is a low-cost fabrication technique, and it resulted in the best optomechanical agreement between fluorescence and mechanical tests sensed by a nanocrystal in a film," Raja explains. The four arms of the tetrapods make them very sensitive to stress each acting as an antenna sensing the immediate environment and essentially amplifying forces experienced by the device and transferring it to the fluorescent core. The frequency of fluorescence correlates with stress levels.

The team points out that the same technology might even be used to detect the presence of cancer cells in a biopsy sample as malignant cells commonly have increased stiffness relative to healthy cells.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".