Microscopic droplets shrink-wrapped in thin elastic sheets take on the telltale half-moon shape of a curry puff or an empanada. Such structures could one day replace chemical surfactants in soap, since the sheets make for a stronger barrier to encase dangerous or delicate liquids. In a new paper in Nature Materials, Syracuse University physicist Joseph Paulsen reported on his latest experiments in soft condensed matter, aka the science of "squishy things". He was interested in learning more about how drops of water wrapped in very thin elastic sheets (a thousand times thinner than a human hair) behave at the microscopic scale — in this case, sheets that bend very easily, offering little to no resistance.
Initially Paulsen expected that the wrapped droplets would look like spheres. This makes intuitive sense, since most bubbles take on a spherical shape, thanks to the phenomenon of surface tension, a force that arises from molecular attraction. A bubble, in essence, is a volume of air encased in a very thin liquid skin. The greater the surface area, the more energy that is required to maintain a given shape. So a bubble will seek to assume the shape with the least surface area: a sphere.
Instead, Paulsen found that, although they started out spherical, as they collapsed under the thin sheets, his shrink-wrapped mini-droplets formed half-moons, looking far more like a curry puff than a dew drop:
Curious as to why, he teamed up with some colleagues in physics and mathematics. Together they discovered that these kinds of shapes are more efficient than a sphere, at least for these easily-bendable sheets at the microscale. The half-moon geometry covers the water droplet in such a way that less of its area is exposed to the outer liquid.
And even though the sheets fold, crumple and wrinkle in unique patterns as they collapse around the droplets, this didn't seem to have much effect on the final curry puff shape. Nature is all about efficiency, plain and simple.
Paulsen, Joseph D. et al. (2015) "Optimal wrapping of liquid droplets with ultra thin sheets," Nature Materials 14: 1206-1209.
[Via Nanowerk News]
Images and video: Paulsen et al/Nature Materials.