Synopsis: Nonlinear Forces Explain Elastomer Ridges

A new theory that incorporates nonlinear properties of rubber-like materials correctly describes the shape of the ridge that forms when the material is strongly deformed by an object.

Place a droplet of water on a thin sheet made from a rubber-like material called an elastomer and the material deforms. A depression develops under the drop and a ridge rises around its edge. Researchers have long known of the behavior, but they lacked a theory to accurately describe the ridge’s shape. Julien Dervaux at Paris Diderot University and colleagues have now developed such a theory by accounting for nonlinear properties of the elastomer. Their theory could aid in the creation of technologies made from rubbery materials such as “soft” robots and medical implants.

More than a century ago, physicists predicted how an elastomer sheet should deform under a highly concentrated force, such as that exerted by an object placed on top of the sheet. But the theory remained unconfirmed because of difficulties in creating and measuring these localized forces. That changed six years ago when researchers made the first detailed observations of the phenomenon. The measurements and theory, however, didn’t match, raising questions about the 100-year-old theory’s validity. This mismatch has since been observed in other experiments, and recent attempts to explain the data have failed.

Dervaux and colleagues solved this problem by developing a theory that accounts for the nonlinear elastic properties of elastomers. Earlier work neglected these properties, which meant that previous theories only applied to small deformations, not the large ones observed in experiments. By including these nonlinear properties, the team predicted the ridge shapes measured in recent experiments. They also found that the calculated forces at the ridge have the same form as those predicted around defects in hard crystals, despite elastomers’ lack of order at the molecular scale. Thus, they say, elastomers could provide a controllable platform for studying defects in metals or other crystalline materials.

This research is published in Physical Review Letters.

–Katherine Wright

Katherine Wright is a Senior Editor of Physics.


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