Elastic Film to Change Shape of Drops

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Researchers from McMaster University used two stretched elastic films to change the shape of a liquid drop

A round shape of a drop of water on a surface helps to minimize its surface energy. A team of researchers led by Rafael Schulman and Kari Dalnoki-Veress of McMaster University in Canada focused on the validity of the phenomenon for a drop placed between two thin, stretchable films. The team performed a series of experiments to determine whether the drop would form a round blob or take on a wackier shape. The team found that the drop can develop a noncircular outline that is similar to a square and its exact shape relies on the amount of tension in the two films.

According to Dalnoki-Veress, the results are counterintuitive and the experiments were driven purely by curiosity and without any particular application in mind. He also stated that such a stretched and squeezed drop can be used as a tiny lens with adjustable optical properties. The team placed a 30- to 300-μm-diameter drop of either glycerol or polyethylene glycol on an elastic film. The film was secured to a silicon substrate and a second elastic film was stretched along two orthogonal directions and was lowered over the drop. The team found that the initially round drop flattened into a pancake when the top film’s tension was equal in all directions. The drop assumed an oval shape when the film’s tension was greater along one direction than along the other.

The team later suspended the setup and stretched both the top and bottom films in orthogonal directions and found that the drop adopted a square outline. The team monitored the focal properties of the film-drop sandwiches with the help of a laser for the suspended drops. The team found that flatter drops had longer focal lengths and the focal spot shape also varied as the square drops produced a cross-shaped pattern and oval drops created a line. The research was published in the journal Physical Review Letters on December 13, 2018.

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Bill Carr is a Senior Editor at Plains Gazette, based in Austin. Previously he has worked for FOX Sports and MSNBC's "Morning Joe." he is a graduate of the Annenberg School at the University of Southern California. You can reach Bill via email or by phone