Elastic sheets can spontaneously wrap themselves around liquid drops in a process often called “capillary origami”. Ultrathin sheets are especially suitable for encapsulating liquids, but the mechanics of these sheets are very complex, involving wrinkles, crumples and folds. “We have shown that for very thin sheets, you can ignore these complicated small-scale features and still predict the overall 3D shape of the wrappers,” explains team member and lead author of this study Joseph Paulsen.

To wrap droplets, the researchers placed a thin circular sheet of polystyrene (with a radius of 1.52 mm) on the surface of a water drop immersed in silicone oil. They then withdrew fluid from the drop until the sheet covered the entire surface. “This process, starting with a large drop and slowly reducing in size, leads to a variety of 3D shapes,” says Paulsen. “Some are rotationally symmetric, so they look the same from any side view, while others are punctuated by folds or are even polygonal.

When the sheet completely covers the drop, we get a triangular wrapping, like a samosa, or a folded-over shape, like a calzone or an empanada,” he tells nanotechweb.org. “This wealth of wrapped and partially wrapped shapes, all originating from a circular sheet, was very surprising for us at first.”

According to the team members, experimental physicists Narayanan Menon and Paulsen, theoretical physicists Vincent Démery, Benny Davidovich and Christian Santangelo, and polymer scientist Thomas Russell, the result could come in useful in applications where a liquid cargo needs to be protected inside a solid barrier. “Our main focus was on shape,” says Paulsen, “but we expect these wrapped droplets to have interesting mechanical properties as well. Our finding suggests that the pressure inside these droplets is very small when they are almost completely wrapped up, so they should be very soft when pressed against a wall, for example.”

The researchers, reporting their work in Nature Materials doi:10.1038/nmat4397, say they are now busy studying different shapes, other than circular ones, for the initially flat elastic sheets.