Knifefish have a long doral fin that flutters back and forth to move the fish. Here's a video showing one in motion. Scientists at Northwestern University thought that it could serve as a useful basis for an underwater robot, and so studied its movement:
Planning for the robot -- called GhostBot -- began when graduate student Oscar Curet, a co-author of the paper, observed a knifefish suddenly moving vertically in a tank in MacIver's lab.
"We had only tracked it horizontally before," said MacIver, a recent recipient of the prestigious Presidential Early Career Award for Scientists and Engineers. "We thought, 'How could it be doing this?'"
Further observations revealed that while the fish only uses one traveling wave along the fin during horizontal motion (forward or backward depending on the direction on the wave), while moving vertically it uses two waves. One of these moves from head to tail, and the other moves tail to head. The two waves collide and stop at the center of the fin.
The researchers then created a computer simulation of the fish and designed a robot to duplicate its movements:
The group took the robot to Harvard University to test it in a flow tunnel in the lab of George V. Lauder, professor of ichthyology and co-author of the paper. The team measured the flow around the robotic fish by placing reflective particles in the water, then shining a laser sheet into the water. That allowed them to track the flow of the water by watching the particles, and the test showed the water flowing around the biomimetic robot just as computer simulations predicted it would.
"It worked perfectly the first time," MacIver said. "We high-fived. We had the robot in the real world being pushed by real forces."
Link via Fast Company