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DFD Practice Talks

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a) Maria Tatulea-Codrean Optimal swimming of multi-flagellated bacteria

An important characteristic of motile multi-flagellated bacteria is their variable number of flagella, with some bacteria having only one, while others have a few dozen. The number of flagella in a cell is difficult to control in experiments, but it can be changed easily in simulations. This has motivated several theoretical investigations into the link between the swimming of bacteria and their number of flagella [1,2]. How does the number of flagella affect the swimming speed and efficiency of a bacterium? We revisit this open question using slender-body theory simulations, where we include the full hydrodynamic interactions inside a bundle of parallel helical filaments that rotate and translate in synchrony. In contrast to previous studies, we incorporate the full torque-speed relationship of the bacterial flagellar motor [3]. This enables us to obtain novel and surprising predictions on the swimming speed of multi-flagellated bacteria. Our observations are relevant to bacteria with a small number of flagella, such as the model organism Escherichia coli, and we hope will inspire new experiments to address this question.

b) Steven (Pyae Hein Htet) Load-dependent resistive-force theory

The passive rotation of rigid helical filaments is the strategy employed by flagellated bacteria (and some artificial microswimmers) to swim at low Reynolds numbers. In his classical 1976 paper, Lighthill calculated, for the force-free swimming of a rotating helix with no load attached (e.g. with no cell body), the ‘optimal’ resistance coefficients that, in a local resistive-force theory, most closely reproduce predictions from the nonlocal slender-body theory. These resistance coefficients have since been used ubiquitously in the literature, regardless of whether the conditions under which they were originally derived hold. Here we revisit the problem in the case where a load is attached to the rotating helical filament. We show that the optimal resistance coefficients depend in fact on the size of the load, and highlight and improve upon the growing inaccuracy of Lighthill’s coefficients as the load increases. We also provide a physical explanation for the origin of this surprising load-dependence.

c) Weida Liao Artificial cytoplasmic streaming

Recent experiments in cell biology have probed the impact of artificially-induced intracellular flows and transport in cell division. Using focused light localised in a small region of the cell, a global thermo-viscous flow was induced inside the cell in these studies; this is known as focused-light-induced cytoplasmic streaming (FLUCS). Here we present an analytical, theoretical model of FLUCS . The focused light induces a small, local temperature change, causing a small change in the density and viscosity of the fluid locally. This heat spot translates along a finite scan path. We show that the leading-order instantaneous flow results from thermal expansion and depends linearly on the heat-spot amplitude. The net displacement of a passive tracer after a full scan period is quadratic in the heat-spot amplitude and is due to both thermal expansion and thermal viscosity changes. The far-field average velocity of tracers is a source dipole, showing excellent agreement with recent experimental data. Our quantitative model will enable future work on artificial cytoplasmic streaming.

This talk is part of the DAMTP BioLunch series.

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