New York: Dolphins and whales may attract a lot of attention when they leap dramatically out of the water. But aquatic animals thousands of times smaller, such as plankton, are accomplished jumpers, too.
Understanding their acrobatics could help improve engineering processes, like oil refining and wastewater treatment, that rely on controlling the interaction of small particles with air-water interfaces, scientists believe. Sunghwan Jung from Virginia Tech has released a new study focused on the jumping behaviour of copepods — small teardrop-shaped plankton near the bottom of the aquatic food chain that can sometimes vault out of the water to escape predators.
The study showed that the copepod’s velocity is the primary factor determining whether it can break the surface or not, and suggests that these copepods may be the smallest creatures capable of leaping out of the water into the air. For a moving body to break the air-water interface, it must have enough inertia to overcome not only gravity but also the water’s surface tension — the attractive force between molecules that makes the surface elastic.
Surface tension is what allows liquids to form droplets and keeps lightweight bugs like water striders from sinking. For larger animals like dolphins and fish, the surface tension is negligible compared to their own mass, so gravity is the primary consideration. But for tiny creatures like plankton, “there is a huge energy cost to deform the surface”, according to Jung. Creatures below a certain size will simply stretch the surface and bounce back instead of breaking through. For water, this critical length is around three millimetres — about the size of a copepod.
But only certain species of copepods can actually jump out of the water. What makes the difference? To answer that question, Jung and his co-authors simulated leaping plankton by setting up a spring — harvested from a retractable pen — at the base of a tank of water, and launched spherical beads towards the surface. A high-speed video camera tracked the beads’ movement, allowing the researchers to monitor which ones broke the surface.
The experiments showed that the particle’s velocity is the most important factor: the faster a particle flew towards the surface, the more likely it was to break through water surface. Other parameters, like the angle of contact and the particle’s surface characteristics did not matter as much. The study was published in the journal Interface.
