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.
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.
Undergraduate Jordan Barrett has been awarded a National Science Foundation Graduate Research Fellowship. Congratulations, Jordan!
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.
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! Continue reading “Geometry-driven folding in PRL”
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!
The Paulsen Group was recently awarded a “Doctoral New Investigator” grant from the ACS Petroleum Research Fund, for a project titled: “Hyperuniform Dispersal of Non-Brownian Particles in Viscous Liquids”. See a video profile here. Continue reading “Grant from American Chemical Society”