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

Frustrated wrinkle patterns in PNAS

A complex pattern of wrinkles on a centimeters-wide thin polymer shell. Islands of order are visible in the form of nearly-parallel wrinkles.

We are all familiar with the wrinkled texture of a raisin or a candy wrapper. Studying the arrangement of wrinkles in polymer films can lend insight into these and other materials that wrinkle, from textiles to biological tissues to synthetic skins. To address the complexity of wrinkle patterns, we asked the following basic question: “What happens when the direction of wrinkling is in direct conflict with the preferred wavelength?” Our answer is a quantitative framework for understanding the mesoscale organization of groups of wrinkles in such situations, with analogs to liquid crystals and superconductors. Links: PNAS, arXiv

New postdoc

The group welcomes postdoctoral researcher Mengfei He. Mengfei comes from the Nagel lab at the University of Chicago. Welcome, Mengfei!

Review paper on material memories

A suspension of plastic spheres can remember multiple driving amplitudes that were applied to it, but forgets most of them in the steady state. This behavior occurs also in charge-density wave conductors and a model of worn grass between park benches. 

Despite the ubiquity of memory formation in condensed matter, there is presently no overarching framework for classifying memory behaviors. This review article considers memory formation across a broad range of systems (memories in rocks, rubber, glasses, and amorphous solids; memories in magnetic systems; echo phenomena; shape memory; associative memory; etc.), with an eye towards developing unifying conceptual underpinnings for material memories. Such a study of memory formation provides a setting for exploring some of the most fascinating aspects of history-dependence, dynamics, and information storage in far-from-equilibrium systems.

How much can a grassy path remember?

 

States and transitions in a simple model of a grassy path with park benches along it. The model has surprisingly rich memory behaviors, including learning, forgetting, and a stabilizing effect of noise.

We typically think of memory as the stuff stored in hard drives or our brains, but memory effects abound in a wide range of materials. Rubber and rocks can remember the largest load that was applied to them, glasses may remember a temperature where they were aged, and shape-memory alloys can recall a programmed shape. One approach to building a broader understanding of memory formation in matter is to construct and study simple models where memories may be written, stored, and retrieved. This article shows how a model of worn grass between park benches can produce a peculiar memory behavior that has been observed in the motion of electrons in a special kind of conductor, and in the flow of solid grains in viscous liquids.

Jessica Stelzel wins NSF GRFP

REU student Jessica Stelzel has been awarded a National Science Foundation Graduate Research Fellowship. She will be starting her PhD at Johns Hopkins University in the Fall, working to create regenerative biomaterials with Dr. Hai-Quan Mao. Congratulations, Jessica!

Review paper on wrapping

Wrapping air: A balloon constructed by G. I. Taylor to test his prediction for the shape of an axisymmetric parachute, circa 1919 (public domain).

Many objects are wrapped in thin sheets to contain, protect, or conceal the contents, from foil-wrapped candy to high-altitude helium balloons to a liquid droplet sealed inside a polymer film. This review explores the mechanics and geometry of thin elastic wrappers, which are essential to a wide variety of applications in mechanical, chemical, and aerospace engineering, and have spurred basic questions in soft matter physics and mathematics. The manuscript will appear in Annual Review of Condensed Matter Physics in March 2019; see the advance online edition here or the arXiv version here.

Self-organized hyperuniformity in Nature Communications

Structure factor, S(k), for a cyclically-sheared suspension of particles that are sedimenting under gravity. At high sedimentation speed (left image), nothing remarkable occurs. At low sedimentation speed (right image), density fluctuations vanish at long lengthscales.

Suspensions appear in a wide range of industrial settings, and dispersing particles in a uniform manner throughout a fluid is an important challenge. We studied how shear can be used to control the spatial distribution of particles that are settling under gravity in a viscous liquid. We discovered that at sufficiently low sedimentation speeds, extremely homogeneous mixtures are automatically obtained, without any fine tuning of the driving. See the paper here.