A simple experiment using rubber bands and cups of water is putting a new twist on helical geometry. Traditional telephone cords are coiled into helixes that spiral clockwise or counterclockwise. By securing one end of the cord while twisting the other in the opposite direction of the spiral, a rainbow-shaped boundary called a perversion forms at the intersection of the opposing twists. A helix with one or more of these perversions is known as a hemihelix. Although hemihelixes with a single perversion are commonly found in meandering plant roots and wool fibers, hemihelixes with multiple perversions have never been seen until now. In an attempt to create a spring that replicates the light-bending properties of cuttlefish ink sacs, a team of researchers suspended two rubber strips of different lengths. Connecting the bottoms of the two strips to a cup of water, the shorter band stretched to the same length as the longer one. After gluing the two stretched strips together, the researchers gradually drained the water from the cup. As the bands retracted and twisted from the reduced strain, the researchers were shocked to see a hemihelix with multiple perversions form (as seen in the above video). The width-to-height ratio of the rubber bands determines the final form of the resulting helix, the team reports online today in PLOS ONE. The authors suggest that such shapes have not been made before because most materials simply break when put under the forces used in the tests. The team hopes their work inspires nanodevices and molecules that twist and transform from flat strips into predetermined 3D shapes on demand.
(Video credit: J. Liu et al., PLOS ONE 9, 4 [23 April 2014])