Nano Lett. http://doi.org/5fz (2015)
Most artificial micro- and nanoswimmers are designed to mimic natural flagella and are composed of a spherical head attached to a chiral helix, which through an actuation mechanism can rotate in one direction and propel the system in solution. Peer Fischer, Alexander Leshansky and colleagues have now worked out the optimal geometry for a nanoswimmer to achieve the highest velocity.
Contrary to common intuition, a long chiral tail is not necessary to swim fast. In fact, there are two opposing forces to be considered. On the one hand, a long tail increases the viscous drag, that is, the resistance the system encounters by the surrounding water as it moves along. On the other, a short helical tail has little torque to generate much linear motion. The optimal geometry therefore lies somewhere in between. In particular, the researchers calculate, and experimentally verify, that, when actuated by an external stimulus such as a rotating magnetic field, the fastest nanoswimmers have a tail that is only around one full helical turn long and a length-to-head-radius ratio of about 5.
Leshansky and colleagues — who are based at the Israel institute of Technology, the University of Stuttgart and the Max Planck Institute in Stuttgart — explain that this is a different regime than that found in bacterial flagella, which are usually much longer. This is because the bacterial flagella are actuated by an internal mechanism of chemical energy transduction and optimized to maximize power rather than speed.