19 Juillet – Thesis defense - Lachlan Alexander

14 h Amphi - CRPP (Pessac) - videoconference

Bacterial microswimmers as active particles in colloidal liquid crystals.

The study of self-propelling active particles has brought impetus to nonequilibrium statistical physics through the discovery of new phenomena which led to an expansion of fundamental theory. Active matter systems can display complex dynamics, such as spatiotemporal self-organisation and collective motion, solely from localised motility and simple particle interactions. Crucially, activity plays a large role in the behaviour of living organisms. Being exceedingly common and having been optimised for swimming by evolution, bacteria provide us with ideal systems of Brownian active particles. Their motility plays a key role in many aspects, from infections to the structure of their environments. In recent years, there have been an increasing number of studies into how they behave in complex fluids, such as in molecular liquid crystals and DNA matrices. A key result of these works is the control of swimming orientation by the nematic director. However, bacteria have scarcely been tested in media containing colloidal particles with similar length scales. Using Bacillus subtilis and Escherichia coli, both well-established species of biological swimmers, we determine how these microswimmers change their behaviour in isotropic liquid and liquid crystalline suspensions of fd viruses, widely used in soft matter as model semi-flexible monodisperse rod-like colloidal particles. We discover that, in the isotropic phase, Bacillus subtilis swims with speeds up to three times that of when no viruses are present. This enhancement occurs despite an increase in viscosity and elasticity of the surrounding medium and still occurs in the early nematic phase, where bacteria swim along the rod orientation. We also show how the phase properties of fd virus suspensions, such as the isotropic-nematic interfacial tension, affect the behaviour of our swimmers and demonstrate the effect the one-dimensional periodic layers of the smectic phase has on bacterial velocities. This work serves as an example of how small differences in seemingly similar swimmers and the relative size of the components in complex systems can lead to large differences in behaviour. We foresee that our results will prompt investigations with more varied swimmers and colloidal rod-like particles which could eventually be implemented to separate, direct and optimise specific swimmers. Our results could also shed light on the currently unknown aspects of bacterial swimming mechanics, specifically in flagellar bundling dynamics, via possible virus-flagella interactions.

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