Mechanical stability in bendy straws
Flexible corrugated cylinders are useful for making reconfigurable objects such as collapsible bowls, medical tubing, and the joints of bendy straws. Previous work has explained the cylinders’ stability in the extended and collapsed states, but not in the emblematic bent state shown in the figure. For the cylinder to bend, each ridge along the curve must be collapsed along the inner radius of the curve and extended along the outer radius; it is not obvious that such a configuration should be stable. Now, Ryan Hayward at the University of Massachusetts Amherst, James Hanna at Virginia Tech, and their collaborators have identified a necessary condition for a stable bent state.
The researchers began by making test cylinders that varied the geometry of the creases. Some of the test cylinders were stable only in the extended state, whereas others could also be compressed into a stable collapsed state. However, unlike an everyday bendy straw, none of the test cylinders were stable in the bent state.
Hayward and coworkers cut open a bowl, a toy, and a straw that had stable bent states. They found that all three products naturally relaxed to a larger radius than before the cut, which indicated that the cylinders were prestressed during their production. The researchers replicated that added stress by removing a section from each test cylinder and reclosing it with a smaller radius. That alteration facilitated the elusive stable bent state.
The test cylinders did not have any internal stress from their fabrication, so they required external intervention to develop a stable bent state. On the other hand, cylinders made from a viscoelastic polymer that slowly deforms over time were able to acquire the necessary stress through being left in a collapsed state for a few hours. Leaving viscoelastic cylinders in an extended state reversed the process and thus showed that in addition to being reconfigurable, the objects can also be adaptable. (N. P. Bende et al., Soft Matter, 2018, doi:10.1039/c8sm01355a .)
