When a raindrop hits a leaf, it flattens for a moment, then gathers itself and bounces off. But what if the drop isn't quite liquid, but jelly-like, like a gummy candy? Scientists took exactly such drops — made of hydrogel, a material that is 90% water but holds its shape thanks to a polymer network. They threw these drops onto different surfaces and tracked them with a camera capturing thousands of frames per second and a sensor measuring impact force.
It turned out that the droplet's behavior depends on how fast it's thrown. At slow impact (like dropping it from a few centimeters), water leaks out, forming a "leg"—a thin film that spreads across the surface separately from the main droplet. The droplet itself sticks to the surface and freezes into a pancake shape. At fast impact (like a vigorous throw), water doesn't have time to escape, and the droplet behaves like an elastic ball: it compresses, then tries to spring back. Surprisingly, in this case, the degree of flattening doesn't depend on how much the surface likes or dislikes water.
The scientists also figured out why such droplets almost never bounce, even when hitting a water-repellent surface. The reason is that polymer chains from the droplet latch onto the surface like microscopic anchors, preventing the droplet from lifting off. A bounce only happens if the droplet's elasticity is high enough to overcome this "stickiness." This knowledge could help create materials that stick better or, conversely, bounce better — for example, for 3D printing or coatings.
Hydrogel is the same material used to make soft contact lenses. Its elasticity can be tuned by varying the number of polymer strands, and the scientists used this to vary droplet properties by thousands of times.
In the movie Flubber, a stretchy green substance bounces around a room, barely losing energy. Real hydrogels are more modest, but studying their impacts brings us closer to creating materials with controllable stickiness and bounce.