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Wednesday, May 7, 2014

Explaining the hemihelix

If you’ve ever had a child or been a child, I guarantee you are familiar with the hemihelix. It’s the shape your slinky takes on when the coils suddenly switch to the opposite direction and you can’t make the thing squeeze neatly together any more. That kink in the middle changes the slinky from a helix to a hemihelix.

You can find hemihelices with just the one direction change, or ‘perversion’ in nature. Roots will sometimes spiral in one direction and then switch to going around the other way as they penetrate the soil. If you want a spring with multiple perversions, you’ll have to create it in the lab, and that’s exactly what Harvard University scientists, led by Jia Liu, did.

The researchers played with elastomer strips to figure out what properties (width and length) and what energies were required to make the rubbery strips switch from helix, to single-perversion helix, to multiple perversion helix. If your strips have the right cross-section, you can get cool shapes like the ones below.

Top: regular helix
Middle: a hemihelix with one perversion marked by an arrow
Bottom: a hemihelix with multiple perversions
The scale bar is 5 cm for each image.
Credit: Jiangshui Huang

Did I say play? This is serious science folks. 

Liu, J., Huang, J., Su, T., Bertoldi, K., & Clarke, D. (2014). Structural Transition from Helices to Hemihelices PLoS ONE, 9 (4) DOI: 10.1371/journal.pone.0093183.

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