Science and Technology Materials science Stronger when stricken
科技 材料科学 越敲越结实
A new material that gets stiffer when it is stressed
ONE of the valuable properties of bone is that when it endures repeated stress it responds by becoming denser and stronger.
A living material can do that. A non-living one cannot.
It has no way of adding the extra matter needed to provide the extra density. But it would help engineers a lot if non-living stuff could at least stiffen in response to stress—and that may now be possible.
Brent Carey, a graduate student at Rice University in Texas, thinks he has found a way to make it happen.
Mr Carey made his discovery when he was testing the properties of a material made of carbon nanotubes (cylinders of carbon atoms a few billionths of a metre across) and a rubbery polymer called polydimethylsiloxane.
He created this composite by growing a forest of nanotubes using hot hydrocarbon gases and an iron catalyst, and then filling the space between the tubes with the polymer.
The surprise came when he discovered how his new material responded to repeated stress.
He found this did not cause any of the damaging fatigue that would be expected.
Indeed, his initial inspection suggested the stuff was actually growing stiffer.
Fascinated by this result, he took his finding to his supervisor, Pulickel Ajayan, and they assembled a team to study the new material.
They gave the composite a real workout. They compressed it five times a second for a week. That caused its stiffness to increase by 12%.
Moreover, the effect showed no sign of abating,
which led them to suspect that if it were exposed to more stress it would grow stiffer still.
Why this happens is still a mystery.
Mr Carey and his colleagues report in the American Chemical Society's journal Nano that heating the new material did not eliminate the response.
This suggests that the self-stiffening is not the result of chemical changes in the polymer, which can usually be undone by heat.
The researchers do have one lead, though. Because of the regular alignment of the nanotubes, they were able to stress the material from various directions.
They found that when the direction of stress was at right-angles to the tubes, it stiffened by 5.9%.
When it was in the direction in which tubes were pointing, the increase was only 4.3%.
What that means is still unclear, but it may be the key to understanding the phenomenon—and thus being able to replicate it with other ingredients.