There’s a simple reason you can’t walk on water: Humans are so big that the force of gravity overcomes the so-called surface tension of water, making us sink. But for tiny creatures, surface tension—the force created when water molecules cling together—becomes dominant, allowing insects and other small animals to walk effortlessly over ponds and other liquid bodies. For a long time, most biologists ignored the air-water interface. But a growing number of studies are revealing how that interface—and surface tension itself—shapes a lot of the living world.
Take water striders. Using dyes and high-speed video, researchers have learned how the long-legged insects skate so effortlessly along the surfaces of ponds. By vigorously rowing along the surface, striders create swirls that help propel them forward, all without rupturing the water surface. Striders can even take off and land on water without breaking through—as in this video, slowed down 20 times.
Credit: David Hu and John Bush, Georgia Institute of Technology
Now imagine you have to bury a seed, but lack a trowel or even the fingers to dig a hole. Surface tension can solve that problem as well. A few seeds have long projections called awns that coil and uncoil with the changing humidity. In this video, the awn of a succulent plant, Pelargonium carnosum, is initially wet and straight, but it coils as it dries. (The movie plays 30 times faster than the actual process.)
Credit: Ho-Young Kim and Wonjong Jung
The balance between surface tension and gravity even affects how animals pee. In larger animals, urine exits a big enough tube in the body that surface tension doesn’t matter too much, and a steady stream gushes out. But with rats and other tiny creatures, urine exits their much tinier urinary tubes as single drops, pushed out one by one. The last drop can linger 10 minutes before falling, as in this video of a rat, goat, cow, and elephant urinating.
Credit: Patricia Yang and David Hu, Georgia Institute of Technology
Physicists have a pretty good understanding of how surface tension arises. The clingy water molecules attempt to minimize their connections with other types of molecules. So when something deforms the water surface, the displaced water molecules work to return to their minimum-energy configuration—unless the intruder itself attracts water molecules, in which case the water clings like glue.
For more on the many ways surface tension affects plants and animals, and even people, see “Water’s Tough Skin” in this week’s issue of Science.