scallop theorem

Etymology
From an explanation of a principle of fluid dynamics given by E. M. Purcell.

Noun

 * 1)  A theorem which states that to achieve propulsion at low Reynolds number in simple (i.e. Newtonian) fluids, a swimmer must deform in a way that is not invariant under time-reversal.

Usage notes

 * E.M. Purcell. "Life at Low Reynolds Number", American Journal of Physics vol 45, p. 3-11 (1977)