String Theory Group

Julia Yeomans
University of Oxford, UK

Swimming and scattering at low Reynolds number

Because of their size bacteria and fabricated microswimmers swim at low Reynolds number, a regime where the effect of hydrodynamic interactions can be appreciable and counterintuitive. This is equivalent to humans trying to move in a very viscous liquid like treacle. Inertia is unimportant: once the swimmer ceases to move it stops instantly. The Stokes’ equations, which govern the zero Reynolds number limit, are invariant under time reversal and hence to move at all the microswimmer must have a swimming stroke which is irreversible in time. The current interest in microswimmers has been fuelled by advances in nanotechnology which have led to novel experiments aimed at fabricating microswimmers and micropumps. I will describe research using analytic and numerical approaches to model swimming at low Reynolds number. We are interested in understanding the velocity fields of the swimmers and its dependence on the symmetry of the swimming stroke, the form and relevance of hydrodynamic interactions between swimmers, and the interplay between Brownian motion and directed swimming. Traditionally, scattering experiments have played an important role in elucidating the interactions between physical objects. Nowadays, modern experimental techniques are allowing us to track the motion of individual microorganisms suggesting that it will be possible in the near future to systematically study the hydrodynamic interaction forces generated by algae, bacteria or artificial microswimmers through suitably designed biophysical scattering experiments. In anticipation of this we compare the tracer motion induced by an externally driven colloid to that generated by various model swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories which reflect the hydrodynamic properties of the microswimmer.