Swimming spheres

The gliding motion of jellyfish, eels, and other sea creatures is not just captivating to watch in an aquarium. Understanding the mechanics of swimming can, for example, help life scientists explicate biological functions and engineers design tiny robots for targeted drug delivery. Now, Daphne Klotsa of the University of North Carolina at Chapel Hill and colleagues from the University of Nottingham in the UK have carried out a detailed observational and simulation study of an extraordinarily simple type of artificial swimmer: two spheres attached by a spring and placed in a saltwater bath. The researchers caused the bath to vibrate vertically, both to provide energy that the swimmer could convert into translational motion and to allow for control of the streaming Reynolds number—loosely speaking, the ratio of inertial to viscous forces acting on the swimmer. For low Reynolds number, the experimental team observed oscillations of the spheres, but no center-of-mass...