Driven Shear Flow in Biological Magnetoactive Fluids

Active fluids made of powered suspended entities spontaneously give rise to patterns and flows. Yet, how the swimmers activity can be harnessed by external cues into coherent macroscopic flows remains a question of biological and applied relevance. Here, we use magnetotactic bacteria, which respond both to chemical and magnetic signals, to control the onset of global scale shear flows. Shaping positional and orientational orders with external fields allows the force dipoles of each swimmer to combine into a finite active stress. Incorporated in a magnetoactive model, this driving contribution quantitatively captures the observed flows in steady state, while the time-dependent response to rotating fields unveils additional complexity.