A water droplet encapsulated within a spin-coated polystyrene film while immersed in silicone oil. Image: UMass Amherst/Joseph Paulsen.
A water droplet encapsulated within a spin-coated polystyrene film while immersed in silicone oil. Image: UMass Amherst/Joseph Paulsen.

Materials scientists seeking to encapsulate droplets of one fluid within another often use molecules like soap or micro- or nanoparticles to do it. An alternative approach is to take advantage of capillary action to wrap a droplet in a thin sheet. However, because it takes some force to bend a sheet around a drop, there were thought to be limits on what can be accomplished by this process.

Now, experimental and theoretical physicists and a polymer scientist at the University of Massachusetts Amherst have teamed up to use much thinner sheets than before to overcome these limits, allowing them to produce a new class of wrapped shapes. Such wrapping techniques could be used to contain toxic or corrosive liquids, to physically isolate a delicate liquid cargo or to shrink-wrap drops. Details of this work appear in Nature Materials.

The team is made up of experimental physicists Narayanan Menon and postdoctoral researcher Joseph Paulsen, theoretical physicists Vincent Démery, Benjamin Davidovitch and Christian Santangelo, and polymer scientist Thomas Russell.

Paulsen devised a process in which a circular flat sheet made from spin-coated polystyrene is placed on a drop, which is completely wrapped by the sheet as the droplet's volume is gradually decreased by withdrawing fluid with a thin straw. Small-scale wrinkles and crumples allow the sheet to curve around the droplet as it wraps.

Surprisingly, using a very thin skin to wrap a drop leads to non-spherical shapes, whereas one might have imagined that the sheet would simply conform to the spherical shape of the drop. "These non-spherical shapes are reminiscent of foods in which a filling is wrapped inside a sheet of pastry or dough, such as a samosa, an empanada or a dumpling," says Menon.

The theorists developed a general model that explains "all the observed partially and fully wrapped shapes purely geometrically, independent of material parameters, in a regime of thickness that often occurs in nature and is easily achieved in technological settings."

"Wrinkles, fold and crumples are challenging to understand on their own, let alone when they interact in a highly-curved geometry. However, we show that the essence of the wrapping process can be understood without describing any small-scale features," the authors point out. Paulsen adds: "We've shown that for very thin sheets, you can ignore the complicated small-scale features and still predict the overall three-dimensional shape of the wrapping."

Three other interesting findings have also come out of this work, which was funded by the Keck Foundation. First, when ultrathin sheets are used as wrappers, they spontaneously select a method of wrapping that wastes the least amount of material in wrapping up a given volume of fluid. "This corresponds to satisfying the goal of everyone who has wrapped a gift using the least amount of wrapping paper possible," he says.

Second, energies at the droplet-wrapper interface and the mechanical properties of the sheet are irrelevant in the new model, which allows greater functionality, the authors point out. Greater functionality in this case means that if you want to use a sheet with different properties, say with a different color, chemistry or something with holes on it, this process is not disrupted, the physicist explains.

Finally, complete coverage of the fluid can be achieved without special sheet designs, the researchers say. "Special sheet designs are possible, but if you are trying to do this on a large scale, then it is tedious to make sheets that are cut in some complicated way so they can fold up easily,’ says Menon. “Thin enough sheets automatically wrinkle and fold in such a way that you don't need to cut them up."

"We expect our findings to be useful in applications where a liquid cargo needs to be protected in a solid barrier,” adds Paulson. “Our main focus was on shape, but we expect these wrapped droplets to have interesting mechanical properties as well."

This story is adapted from material from the University of Massachusetts Amherst, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.