A Macroscopic Model for the Collective Dynamics of Bacterial Flagellar Bundles

Flagellated bacteria such as E. coli swim by rotating a helical rod-shaped propeller composed of a bundle of multiple flagella. Although the swimming of flagellated bacteria has been extensively studied, most existing studies treat the flagellar bundle as a single rotating helix. Understanding the precise coordination of flagellar dynamics within a bundle is still challenging, due to the complication associated with the interplay of hydrodynamic, elasto-hydrodynamic, and steric interactions at nanometer-scale distances. Here, we experimentally investigate the collective dynamics of multiple flagella by constructing a macroscopic scale model of a flagellar bundle. Specifically, we rotate centimeter-scaled helical filaments in high-viscosity silicone oil with mechanical motors mimicking the features of bacterial flagellar motors. We examine the flow fields induced by rotating flagella at various distances and phase differences and measure the torque of the bundle. Our study sheds light on the hydrodynamic principles governing the collective dynamics of bacterial flagella, placing a crucial missing piece in the puzzle of the locomotion of flagellated bacteria.