Rubber Band Distortions Lead To Discovery Of A New Shape
April 24, 2014

Rubber Band Distortions Lead To Discovery Of A New Shape

[ Watch the Video: Rubber Bands Help Scientists Discover New Shape ]

Brett Smith for - Your Universe Online

In a new report published in the open-access journal PLOS ONE, a team of Harvard University engineers have described the factors behind the creation of a hemihelix, a distorted version of a regular helix, from elastic rubber bands.

The Harvard team said these newly-described factors could inform the creation of new geometric shapes at the molecular level.

"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 study author Katia Bertoldi, associate professor of applied mechanics at the Harvard School of Engineering and Applied Sciences (SEAS).

Regular Helices are fairly uniform three-dimensional spirals, like a corkscrew to Slinky. A hemihelix is similar to a helix, but the route or handedness where the spiral turns, referred to as the chirality, varies or turns around regularly along the length of a hemihelix.

The Harvard team stumbled upon the hemihelix as they were trying to better determine if the detected 3D structures were arbitrarily occurring, or if particular aspects governed their formation. The study team stretched, joined and then released the rubber bands while observing the shapes they formed. Next, they numerically modeled and examined the shape-generating process.

"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 study author David R. Clarke, a materials scientist at SEAS.

By evaluating variances in the width-to-height ratio of the rubber bands, also known as the aspect ratio, the authors found that when a strip is wide in relation to its height, it forms a helix. Additional calculations showed that there may be a crucial value of the aspect ratio at which the form changes from a helix to a hemihelix, with intermittent reversals of chirality. The authors indicate that this phenomenon has not been detected in nature due to the fact that other types of materials would break when expanded this way.

"We see deterministic growth from a two-dimensional state—two strips bonded together—to a three-dimensional state," said study author Jia Liu, a graduate student in Bertoldi's group. "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 behavior,” said Bertoldi. “Some of them are very soft and then they stiffen up. Some are more linear. Simply by changing geometry, you can design this whole family of springs with very different behavior with predictable results."

The Harvard team said their findings are important for creating three-dimensional shapes from flat components.

"Intellectually, it's interesting—and we believe it is significant too," Clarke says. "There are a variety of complex shapes in nature that arise as a result of different growth rates. We stumbled quite by accident on a way to achieve fully deterministic manufacture of some three-dimensional objects."