Scientists discover new shape using rubber bands

In a surprising discovery, Harvard scientists have stumbled across an entirely new shape - while playing with elastic bands.
While setting out to fabricate new springs to support a cephalopod-inspired imaging project, researchers came across the hemihelix, a shape rarely seen in nature.
Knowing precisely how to make the structures, predictably and consistently, may enable scientists to mimic the geometrical features in new molecules that could lead to possible advances in modern nanodevices, including sensors, resonators, and electromagnetic wave absorbers.
"Once you are able to fabricate these complex shapes and control them, the next step will be to see if they have unusual properties; for example, to look at their effect on the propagation of light," said Katia Bertoldi, associate professor of applied mechanics at the Harvard School of Engineering and Applied Sciences (SEAS).

The shape that Bertoldi and colleagues unexpectedly encountered is a hemihelix with multiple "perversions."
Helices are three-dimensional structures; think of a corkscrew or a Slinky toy.

Hemihelices form when the direction in which the spiral turns - known as the chirality - changes periodically along the length. The reversal in chirality is called a perversion.
The team was trying to make two-dimensional springs by taking two strips of rubber material of different lengths and stretching the shorter one to reach the same length as the longer one and then sticking them together, said David R Clarke, Extended Tarr Family Professor of Materials at SEAS.

"We expected that these strips of material would just bend-maybe into a scroll. 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," said Clarke.
Jia Liu, a graduate student in Bertoldi's group, tested differences in the aspect ratio - the width-to-height ratio of the rubber strips - and discovered that when a strip is very wide relative to its height, it produces a helix.
Further measurements showed that there is a critical value of the aspect ratio at which the resulting shape transitions from a helix to a hemihelix with periodic reversals of chirality.

Other classes of materials would simply break when stretched to the mismatched strains that the polymers endured - likely the reason this behaviour had never been observed before, researchers said.
"We see deterministic growth from a two-dimensional state - two strips bonded together - to a three-dimensional state," Liu said.
The findings appear in the journal PLOS ONE.