Deformable sheets are ubiquitous in nature and industry across a vast range of scales, from graphene to metal foil to the earth’s crust. A sheet that is much thinner than it is wide will more readily bend than stretch. This simple property leads to a wide variety of wrinkling and stress-focussing behaviors when a sheet is compressed, poked, crumpled, or twisted.
We recently studied how a thin sheet wraps itself around a liquid drop. Whereas thicker sheets spontaneously wrap by long-wavelength bending [Py et al., Phys. Rev. Lett. 98, 2007], thinner sheets deform more readily by forming small-scale wrinkles and stress-focussing patterns. Although predicting the wrapping shape involves very complex mechanics, we have shown that it reduces to a simple geometric problem for very thin sheets. In the end, the wrapper forms a three-dimensional shape that maximizes the enclosed volume for a given area of the sheet. In this way, a thin film on a drop is similar to a stuffed food like a calzone, which maximizes its filling while using minimal dough. But thin sheets on droplets are remarkable in that they find this shape all on their own.
Wrapping liquids in elastic sheets could be useful in applications where a liquid cargo needs to be protected in a solid barrier, or in creating droplets with particular shapes or mechanical properties.
Curvature-induced stiffness and the spatial variation of wavelength in wrinkled sheets. JD Paulsen, E Hohlfeld, H King, J Huang, Z Qiu, TP Russell, N Menon, D Vella, & B Davidovitch. Proceedings of the National Academy of Sciences, USA (2016). pdf link bibtex
Splash-wrapping synopsis by Science Magazine (2018). link
Supplementary videos from the paper: Optimal wrapping of liquid droplets with ultrathin sheets, Paulsen et al., Nature Materials 14, 1206 (2015). link
Wrapping a drop with a flat sheet. Gallery of Fluid Motion, APS Division of Fluid Dynamics Meeting (2014). link