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.

Jordan Barrett wins NSF GRFP

Undergraduate Jordan Barrett has been awarded a National Science Foundation Graduate Research Fellowship. Congratulations, Jordan!

Splash-wrapping droplets in Science

A polystyrene sheet that is 1,000 times thinner than a human hair is impacted by a falling oil droplet. The end result is droplet encapsulated by a flexible sheet.

Thin elastic sheets make surprisingly good wrappers for liquid droplets: surface tension will spontaneously pull an ultrathin sheet around a droplet, all while making efficient use of the sheet (see it in this short video clip by Science Magazine). The wrapper can be used for a variety of tasks: it provides a strong barrier for protecting the liquid cargo, it can deform the droplet into predictable shapes, and it provides a platform for adding a chemical pattern. But creating many such droplets requires a rapid and scalable process. A new technique uses droplet impact on a floating polymer film to achieve a tidy wrapping in a fraction of a second. The experiments were carried out by Deepak Kumar and Joseph Paulsen at UMass Amherst, and the results are published here.

Lab YouTube channel

A new Paulsen Lab YouTube channel features video clips of liquid droplets and thin elastic sheets. Current content is from recent work done at UMass Amherst and UChicago, plus an interview by the Syracuse University College of Arts & Sciences. More clips coming soon!

Geometry-driven folding in PRL

A thin polymer film with an annular shape that is floating on water. As the surface tension pulling on the outer edge is lowered, the sheet forms wrinkles and then two folds. The sheet is 394 nm thick and 16 mm wide.

Wrinkles are all around us — on hanging curtains, the skin of drying fruit, or a surprised forehead. The more a material is squished, the deeper and taller the wrinkles become, until they collapse into a fold. Typically, this process depends strongly on the materials in question, for example the thickness of the skin, or the softness of the flesh underneath. However, we show that a wrinkle-to-fold transition may be affected only by the shape of the compressed object, rather than by any mechanical properties!

New postdoc

The group welcomes postdoctoral researcher Yousra Timounay. Yousra comes from the groups of Florence Rouyer and Elise Lorenceau at the Navier Laboratory in Paris, France. Welcome, Yousra!