Julia Yeomans OBE FRS

Abstract:

We propose a theoretical model for a magnetically-actuated artificial cilium in a fluid environment and investigate its dynamical behaviour, using both analytical calculations and numerical simulations. The cilium consists of a spherical soft magnet, a spherical hard magnet, and an elastic spring that connects the two magnetic components. Under a rotating magnetic field, the cilium exhibits a transition from phase-locking at low frequencies to phase-slipping at higher frequencies. We study the dynamics of the magnetic cilium in the vicinity of a wall by incorporating its hydrodynamic influence, and examine the efficiency of the actuated cilium in pumping viscous fluids. This cilium model can be helpful in a variety of applications such as transport and mixing of viscous solutions at small scales and fabricating microswimmers.

Abstract:

© 2019, The Author(s), under exclusive licence to Springer Nature Limited. The locomotion of swimming bacteria in simple Newtonian fluids can successfully be described within the framework of low-Reynolds-number hydrodynamics 1 . The presence of polymers in biofluids generally increases the viscosity, which is expected to lead to slower swimming for a constant bacterial motor torque. Surprisingly, however, experiments have shown that bacterial speeds can increase in polymeric fluids 2–5 . Whereas, for example, artificial helical microswimmers in shear-thinning fluids 6 or swimming Caenorhabditis elegans worms in wet granular media 7,8 increase their speeds substantially, swimming Escherichia coli bacteria in polymeric fluids show just a small increase in speed at low polymer concentrations, followed by a decrease at higher concentrations 2,4 . The mechanisms behind this behaviour are currently unclear, and therefore we perform extensive coarse-grained simulations of a bacterium swimming in explicitly modelled solutions of macromolecular polymers of different lengths and densities. We observe an increase of up to 60% in swimming speed with polymer density and demonstrate that this is due to a non-uniform distribution of polymers in the vicinity of the bacterium, leading to an apparent slip. However, this in itself cannot predict the large increase in swimming velocity: coupling to the chirality of the bacterial flagellum is also necessary.