DROPS of liquid are usually round, but聽they don鈥檛 have to be. Researchers sandwiched drops of glycerol between stretched elastic films to see what shapes they could make聽鈥� and were surprised to produce a square.
Rafael Schulman and Kari Dalnoki-Veress at McMaster University in Canada started with a thin film lying flat on a silicon surface, and deposited a droplet around 100 nanometres in diameter on top. Then they placed a second film over the droplet. When the tension in the top film was equal in all directions, the droplet鈥檚 outline was circular, viewed from above. From the side, the droplet looked like a tiny dome sitting on a flat surface.
When they stretched the top film in聽one direction before placing it on the droplet, the liquid took on an oval shape, elongating in the same direction as the film.
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Finally, they stretched the bottom film in one direction and the top film in聽another, at 90 degrees to the first. Now the trapped droplet took on a square shape with slightly rounded corners.
鈥淲e were surprised to see this,鈥� says Dalnoki-Veress. 鈥淭he lowest energy state for a liquid droplet is a round spherical cap, typically. That鈥檚 why a droplet on a spider web is round, that鈥檚 why a droplet on a leaf is聽a little spherical cap. But here you have an example of nature creating a聽structure which is completely counter-intuitive.鈥�
Now that we understand how to聽manipulate liquids into shapes between soft films, we can make nearly any shape, symmetrical or not, by playing with the tension of the films, says Dalnoki-Veress.
This method could be used to make聽arrays of small lenses that focus light in unusual ways. When the researchers shone a laser through the square droplet, for example, they found that it made a cross-shaped pattern.
Physical Review Letters
