Twisted sheets in PRL

A wrinkled film offers a reservoir of material that can be deployed at a later time. Taking this view of wrinkles, we show that a thin cylindrical shell offers a novel mechanical response that we call a “tunable locking material”. By manipulating the shell from its boundaries through axial compression and rotation, we can select the twist angle at which it locks—manifesting as a dramatic increase in the torque upon further rotation. We construct a simple geometric model that can predict the locking angle and locking stiffness, in excellent agreement with experiment. See the paper here.

Buckled balloons in PNAS

Our group published a new manuscript in PNAS, titled: “Cross-sections of doubly curved sheets as confined elastica”. We report experiments and numerics by postdoc Mengfei He, which test a new theory developed with collaborator Vincent Démery (ESPCI Paris). We discovered how the complex three-dimensional shapes of buckled membranes can be predicted by modeling each of its cross-sections as a string in an effective confining potential. Our results could aid the design of inflatable structures, or provide insight into biological patterns.

Featured in Physics Today

Our group’s work on pattern formation in elastic shells was written up in Physics Today. It solves the general problem of how a floppy buckled sheet chooses the directions for its wrinkles. The experiments were carried out by undergrad Graham Leggat and postdoc Yousra Timounay in the Paulsen lab, in collaboration with simulators Eleni Katifori and Desislava Todorova at UPenn. They show stunning agreement with a new theory by Ian Tobasco (UIC). The scientific paper by this team is here.

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