The European Physical Journal E ( IF 1.8 ) Pub Date : 2021-03-23 , DOI: 10.1140/epje/s10189-021-00038-5 Sarah Mohammadinejad 1, 2, 3 , Damien Faivre 4, 5 , Stefan Klumpp 1, 2
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Abstract
The swimming of bacteria provides insight into propulsion and steering under the conditions of low-Reynolds number hydrodynamics. Here we address the magnetically steered swimming of magnetotactic bacteria. We use Stokesian dynamics simulations to study the swimming of single-flagellated magnetotactic bacteria (MTB) in an external magnetic field. Our model MTB consists of a spherical cell body equipped with a magnetic dipole moment and a helical flagellum rotated by a rotary motor. The elasticity of the flagellum as well as magnetic and hydrodynamic interactions is taken into account in this model. We characterized how the swimming velocity is dependent on parameters of the model. We then studied the U-turn motion after a field reversal and found two regimes for weak and strong fields and, correspondingly, two characteristic time scales. In the two regimes, the U-turn time is dominated by the turning of the cell body and its magnetic moment or the turning of the flagellum, respectively. In the regime for weak fields, where turning is dominated by the magnetic relaxation, the U-turn time is approximately in agreement with a theoretical model based on torque balance. In the strong-field regime, strong deformations of the flagellum are observed. We further simulated the swimming of a bacterium with a magnetic moment that is inclined relative to the flagellar axis. This scenario leads to intriguing double helical trajectories that we characterize as functions of the magnetic moment inclination and the magnetic field. For small inclination angles (\(\lesssim {20^{\circ }}\)) and typical field strengths, the inclination of the magnetic moment has only a minor effect on the swimming of MTB in an external magnetic field. Large inclination angles result in a strong reduction in the velocity in direction of the magnetic field, consistent with recent observations that bacteria with large inclination angles use a different propulsion mechanism.
Graphic abstract
中文翻译：
趋磁细菌的斯托克斯动力学模拟
抽象的
细菌的游动提供了对低雷诺数流体动力学条件下的推进和转向的深入了解。在这里，我们讨论趋磁细菌的磁控游泳。我们使用斯托克斯动力学模拟来研究单鞭毛趋磁细菌（MTB）在外部磁场中的游动。我们的 MTB 模型由配备磁偶极矩的球形细胞体和由旋转电机旋转的螺旋鞭毛组成。该模型考虑了鞭毛的弹性以及磁力和流体动力相互作用。我们描述了游泳速度如何依赖于模型参数。然后，我们研究了场反转后的掉头运动，发现了弱场和强场的两种状态，以及相应的两个特征时间尺度。在这两种情况下，掉头时间分别由细胞体的转动及其磁矩或鞭毛的转动决定。在弱场状态下，转动由磁弛豫主导，U 形转动时间与基于扭矩平衡的理论模型大致一致。在强场状态下，观察到鞭毛的强烈变形。我们进一步模拟了具有相对于鞭毛轴倾斜的磁矩的细菌的游泳。这种情况导致了有趣的双螺旋轨迹，我们将其描述为磁矩倾角和磁场的函数。对于小倾角（ \(\lesssim {20^{\circ }}\) ）和典型场强，磁矩的倾角对 MTB 在外部磁场中游动的影响很小。 大倾角会导致磁场方向的速度大幅降低，这与最近观察到的大倾角细菌使用不同的推进机制是一致的。
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