Turning Round Drops Square
A drop of water sitting on a surface minimizes its surface energy by assuming a round shape. Rafael Schulman and Kari Dalnoki-Veress of McMaster University in Canada wondered whether the same would be true for a drop sandwiched between two thin, stretchable films. Would it also form a round blob, or could it take on a wackier shape? In a series of experiments, the duo found that the drop can indeed develop a noncircular outline, such as a square, with its exact shape depending on the amount of tension in the two films. Dalnoki-Veress calls the results “counterintuitive” and “fun,” noting that the experiments were driven purely by curiosity and without any particular application in mind. That said, he imagines potentially using such a stretched and squeezed drop as a tiny lens with adjustable optical properties.
Dalnoki-Veress and Schulman placed a 30- to 300- -diameter drop of either glycerol or polyethylene glycol on an elastic film secured to a silicon substrate. They stretched a second elastic film along two orthogonal directions and lowered it over the drop. When the top film’s tension was equal in all directions, the initially round drop flattened into a pancake. When the film’s tension was greater along one direction than along the other, the drop assumed an oval shape. Finally, when they suspended the setup and stretched both the top and bottom films in orthogonal directions, the drop adopted a square outline.
For the suspended drops, the duo monitored the focal properties of the film-drop sandwiches using a laser. They found that flatter drops had longer focal lengths. The focal spot shape also varied, with square drops producing a cross-shaped pattern and oval drops creating a line.
This research is published in Physical Review Letters.
–Christopher Crockett
Christopher Crockett is a freelance writer based in Arlington, Virginia.