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Rubber research leads to rarely seen hemihelix

May 1, 2014

News

Materials

Materials Science

A serendipitous discovery by scientists experimenting with elastic strips has resulted in a shape rarely seen in nature – a hemihelix. The Harvard team was trying to make two-dimensional springs by taking two strips of rubber material of different lengths and stretching the shorter to reach the same length as the longer, and sticking them together. “We expected that these strips of material would just bend – maybe into a scroll,” said David R Clarke from the School of Engineering and Applied Sciences (SEAS). “But what we discovered is that when we did that experiment we got a hemihelix and that it has a chirality that changes, constantly alternating from one side to another.” The researchers – a group from Katia Bertoldi’s laboratory – tested differences in the width-to-height ratio of the silicone rubber strips, discovering that when the strip is very wide relative to its height, it produces a helix. Further measurements exposed a critical value of the aspect ratio at which the resulting shape transitions from a helix to a hemihelix with periodic reversals of chirality.

This image shows the sequence of operations that leads to the spontaneous creation of hemihelices and helices. Starting with two long elastomer strips of different lengths, the shorter one is stretched to the same length as the other. While the stretching force, P, is maintained, the two strips are joined side-by-side. Then, as the force is slowly released, the bi-strip twists and bends to create either a helix or a hemihelix. Credit: Jia Liu

The researchers believe that this behaviour had never been observed before because other polymers subjected to the mismatched strains would simply have broken. “We see deterministic growth from a two-dimensional state – two strips bonded together – to a three-dimensional state,” said graduate student Jai Lui. “The actual number of perversions, the diameter, everything else about it is entirely prescribed. There is no randomness; it’s fully deterministic. So if you make one hundred of these, they’ll always perform exactly the same way. “From a mechanical point of view you can look at these as different springs with very different behaviour. Simply by changing the geometry, you can design this whole family of springs with very different behaviour with predictable results,” said Bertoldi, Associate Professor of Applied Mechanics She said the next step is to fabricate these shapes and control them to see if they have any unusual properties, for example their effect on the propagation of light. Knowing precisely how to make the structures predictably and consistently may help scientists mimic geometrical features in new molecules what could lead to advances in modern nanodevices. The work has been published in PLOS One. Structural Transition from Helices to Hemihelices Helix