Want to know the tension in a stretched membrane or thin film? Spraying it with liquid droplets might give you the answer. Rafael Schulman from McMaster University, Canada, and colleagues have shown that otherwise spherical drops resemble ellipses when they are sprayed on a film in which the tension is greater in one direction than in the other. The finding suggests that droplets could map the tension in a film much like iron filings trace the field from a magnet.
The group suspended an elastic polymer film such that it had a uniform tension and then sprayed the film with glycerol drops. Imaging from above, they observed that the drops assumed near-perfect circular shapes. But when the team stretched the film to induce an anisotropic tension, drops that were subsequently sprayed onto the surface looked more like peanut M&M’s—fatter along one axis than the other. Specifically, each drop’s long axis was lined up with the direction in which the film had been most strained, that is, the direction of higher tension.
Based on a side view of the drop-film profile, Schulman’s group and his colleagues at the ESPCI Paris were able to calculate the local tension in the film using an existing model, which they modified to incorporate anisotropic tension. This enabled them to map both the direction and magnitude of stresses at each point in the film. And unlike other tension-measurement approaches, theirs left the film intact.