Why do droplets merge

Drop catapult for clean surfaces

Some types of mushrooms use a surprising catapult effect to spread their spores around the area with water droplets. Researchers have now examined the effect of these so-called ballistospores more closely and have been able to simulate it with small plastic beads. As they report in the journal "Applied Physics Letters", the catapult effect can be used to clean any surface. Complex microstructures, which used to repel water from a surface and clean themselves, would no longer be necessary.

Drop catapult

"The catapult effect is powered by internally generated energy," says Chuan-Hua Chen from Duke University in Durham, USA. In order to be able to capture this with a camera, Chen and his colleagues distributed only a few millionths of a meter small spheres made of the plastic polystyrene on a surface. With the nozzle of an inkjet printer, they each squirted two tiny drops of water onto a bead. The two droplets melted together, releasing enough energy to fly away from the surface with the bead like a catapult launch.

In order to understand this effect in detail, the scientists carried out some computer simulations in addition to the experiment. They discovered that when the drops merged, the overall surface tension decreased. The energy released was sufficient to overcome the moment of inertia of drops and microspheres. The result: drops and globules jumped up from the surface of their own accord.

“In our experiments, neither the beads nor the surface had superhydrophobic, water-repellent properties,” says Chen. Therefore, he can now imagine using this effect to independently clean any surface. However, classic cleaning methods appear to be much simpler. It would be more interesting to optimize this catapult effect in order to specifically control the movement of microparticles in analysis devices, for example.