Dynamics of Collective Flagellar Bundles through Macroscopic Scaled-up Model

E. coli swim by rotating a helical, rod-like propeller composed of multiple flagella bundled together. While the swimming behavior of flagellated bacteria has been widely studied, most existing research simplifies the flagellar bundle as a single rotating helix. However, accurately understanding how each flagellum coordinates within the bundle remains challenging due to complex hydrodynamic, elasto-hydrodynamic, and steric interactions occurring at nanometer scales. In this study, we explore the collective dynamics of multiple flagella by constructing a macroscopic model of a flagellar bundle. Using centimeter-scale rigid and flexible helical filaments rotated in high-viscosity silicone oil by mechanical motors, we replicate key characteristics of bacterial flagellar motors. We examine the flow fields generated by rotating flagella at various distances and phase differences and measure the torque of the bundle. By comparing the behavior of rigid and flexible helices, we aim to understand the role of flexibility in flagellar bundling. Our findings illuminate the hydrodynamic principles that govern bacterial flagella's collective dynamics, providing a crucial piece in understanding flagellated bacterial locomotion.