MICRO-SCALLOP: Peer Fischer and his team at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, revisited Edward Purcell’s “scallop theorem” to design a robot that could potentially navigate the human circulatory system.ALEJANDRO POSADA/MPI-IS
The physicist Edward Purcell, who in 1952 shared a Nobel Prize for discovering nuclear magnetic resonance, later turned his attention to the movement of microorganisms through liquids. In a 1977 paper, Purcell put forth the now famous “scallop theorem.” Through a series of calculations, he showed that scallop-like, reciprocal opening and closing motions could not propel a microbe forward through a fluid, and concluded that flagella must rotate asymmetrically to allow bacteria to move (Am J Phys, 45:3-11, 1977).
In water, a Newtonian fluid in which a moving object does not affect viscosity, large beings such as humans can glide along, but microbes are limited by a low Reynolds number—the ratio of an object’s inertia to the viscosity of the fluid through which it’s moving. ...
