From wrinkles to crumples in PRX

Smooth wrinkles and sharp crumples on an air cushion used for shipping.

Consider for a moment the smooth wrinkles on our skin and the sharp creases on a crumpled ball of paper. These two kinds of surfaces look and feel quite different, and you might wager that the difference comes down to the material itself: skin makes “wrinkles” and paper makes “crumples”. We found that this reasonable guess is actually wrong. By squeezing and inflating plastic and rubber sheets in a variety of experiments, we discovered how to turn wrinkles into crumples and then back. What’s more, we found that crumples are rather general features — nature uses this “building block” to help sheets contort in a lot of geometrically-tricky situations. So understanding the physics of a birthday balloon can teach you things that are important for designing deployable satellites or understanding ripples in a cell membrane. Links: PRX, APS Physics Magazine

Poking a thin film in Soft Matter

An 8-cm-wide floating polymer film indented by a force probe.

Thin sheets are easily bent and twisted into different shapes while staying within the linear elastic response of the material. Think of rolling up a scientific poster: large displacements occur with relatively small forces and virtually no damage to the material. Such geometric nonlinearities complicate the relationships between forces, deformations, and material properties for any slender material, from textiles to polymer capsules to flagella. We map out the surprisingly rich mechanical response of a floating polymer film to indentation, using experiments, simulations, and theory. Our geometric approach provides a new tool for understanding the mechanics of sheet-laden interfaces in general settings. Links: Soft Matter, arXiv