Colloidal rod suspensions are found in different biological systems and are widely employed in industry to tune the mechanical properties of composite materials. The elongated geometry of the particles and their orientation kinetics in imposed flows introduce rich visco-elastic rheological responses to the bulk material, which may lead to flow instabilities even under low Reynolds number conditions. The dynamics of the particles at the microscale are dictated by flow kinetics, particle shape, Brownian fluctuations, interactions with external fields, and interparticle interactions. It is known that hydrodynamic interactions (HI) play a crucial role in the rheology of different types of suspensions, such as spherical particles and emulsion droplets. However, the role of HI in the rheology of fiber suspensions is still not fully understood. To address this gap, we employ numerical simulations based on the Brownian Dynamics method to explore the influence of HI on the orientation kinetics and bulk rheology of semi-dilute colloidal rod suspensions. Our findings from this relatively simple model unveil a cascade effect, wherein the tumbling of a single rod induces a disturbance in the flow, subsequently triggering the tumbling of neighboring particles and increasing the bulk stress of the system.
