TECH FRONT
are very cool, but we wanted to introduce more fl exibility,” said Pei Dong, a postdoctoral researcher who co-led the study with Rice graduate student Alin Cristian Chipara. “We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way. But we wanted the liquid to be stable instead of fl owing everywhere.” In SAC, tiny spheres of polyvinylidene fl uoride (PVDF) encapsulate much of the liquid. The viscous polydimethylsi- loxane (PDMS) further coats the entire surface. The spheres are extremely resilient, Lou said, as their thin shells deform easily. Their liquid contents enhance their viscoelasticity, a measure of their ability to absorb the strain and return to their original state, while the coatings keep the spheres together. The spheres also have the freedom to slide past each other when compressed, but remain attached. “The sample doesn’t give you the impression that it con- tains any liquid,” Lou said. “It’s more like a sugar cube that you can compress quite a lot. The nice thing is that it recovers.”
The polymer components begin as powder and viscous liquid, said Dong. With the addition of a solvent and con- trolled heating, the PDMS stabilizes into solid spheres that provide the reconfi gurable internal structure. In tests, Rice scientists found a maximum of 683% increase in the mate- rial’s storage modulus—a size-independent parameter used to characterize self-stiffening behavior. This is much larger than that reported for solid composites and other materials, they said.
C
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