[hal-01099412] Flexibility of bacterial flagella in external shear results in complex swimming trajectories

Many bacteria use rotating helical flagella in swimming motility. In search for food or migration towards a new habitat, bacteria occasionally unbundle their flagellar fil-aments and tumble, leading to an abrupt change in di-rection. Flexible flagella can also be easily deformed by external shear flow, leading to complex bacterial trajecto-ries. Here we examine the effects of flagella flexibility on the navigation of bacteria in two fundamental shear flows: planar shear and Poiseuille flow realized in long channels. On the basis of slender body elastodynamics and numer-ical analysis, we discovered a variety of nontrivial effects stemming from the interplay of self-propulsion, elasticity, and shear-induced flagellar bending. We show that in pla-nar shear flow the bacteria execute periodic motion, while in Poiseuille flow they migrate towards the center of the channel or converge toward a limit cycle. We also find that even a small amount of random reorientation can in-duce a strong response of bacteria leading to the overall non-periodic trajectories. Our findings exemplify the sen-sitive role of flagellar flexibility and shed new light on the navigation of bacteria in complex shear flows.



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