Raj defends PhD thesis

Raj De at his pole-climbing celebration, after successfully defending his thesis, “Geometric confinement of thin films: From crumple formation to curvature-induced propulsion”. Congratulations, Raj!

Writeup in Quanta Magazine

Different-shaped cutouts of spherical shells, which wrinkle when deposited on a bath of water. A new theory predicts the emergence of domains of wrinkles, which can be ordered (green lines) or disordered (white outlines).

Our group’s recent work on pattern formation in elastic shells was written up in Quanta Magazine. This work compares a new theory by Ian Tobasco (UIC) with experiments by Yousra Timounay and Graham Leggat (Paulsen Lab) and simulations by Eleni Katifori and Desislava Todorova (UPenn).

Softly Stamping a Thin Shell

A star-shaped cutout of a thin spherical cap, placed onto a pool of water in silico. The sheet forms a complex pattern of wrinkles to accommodate the change in geometry from a sphere to a plane. Image: Desislava Todorova, University of Pennsylvania. Cover Design: Amie Fernandez.

Our group has published an article in Nature Physics, featuring the experiments of former postdoc Yousra Timounay and undergraduate intern Graham Leggat, in collaboration with researchers at UIC and UPenn:

“Exact solutions for the wrinkle patterns of confined elastic shells.” Tobasco et al., Nature Physics 18, 1099 (2022).

This collaborative work was also highlighted by NewScientist and Penn Today.

First Lab Graduates!

Bottom: The Physics Department machine shop furnished this commemorative plaque, which Monica fastened to a wooden beam in the sub-basement of our building today. Her plaque joins a collection of many others like it, from previous dissertations by experimentalists in our department.

Monica Ripp and Jikai Wang gave successful PhD defenses this year. Monica’s dissertation, “Buckling Patterns and Mechanics of Thin Interfacial Polymer Films,” was defended on October 15, 2021. Jikai’s dissertation, “Self organization in models of cyclically-sheared suspensions,” was defended on April 2, 2021. Congratulations, Jikai & Monica!

Sculpting liquids with shells in PRL

A halftone image viewed through an ultrathin shell that has ‘sculpted’ an oil droplet to match its rest curvature. The curved interface brings this bird’s head into focus.

Our group recently had a manuscript published in Physical Review Letters, titled: “Sculpting Liquids with Ultrathin Shells”. Co-authors are postdocs Yousra Timounay and Mengfei He, former SU undergrad Lindsay Murphy, REU student Alex Hartwell, grad student Eric King, and collaborator Prof. Vincent Démery (ESPCI Paris). Our work shows how an ultrathin shell can “sculpt” a liquid interface into its own shape, through a mechanism that is much like the inflation of a stiff balloon.

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!