Orientation Dynamics of Rigid Fibers in a Microfluidic Burgers-like Vortex

Fiber suspensions are common in biological and environmental flows and are widely used in industrial applications. Fiber transport and orientation dynamics are affected by interactions with the surrounding fluid and strongly depend on the nature of the flow. The complexity of realistic flows, which are often heterogeneous or time-dependent, hinders a full understanding of fiber dynamics. In this study, we combine microfluidic experiments, theory and numerical simulations to investigate the orientation dynamics of rigid neutrally buoyant fibers in a well-controlled model system, a streamwise stationary vortex at moderate Reynolds number. Despite the three-dimensional nature of the flow, the orientation dynamics are remarkably simple: the fiber orientation is accurately described by Jeffery equations coupled with the Burgers-vortex model over a large range of particle Reynolds number. We show that fibers undergo uniform precession about the vortex axis driven by fluid vorticity while simultaneously aligning with the latter due to strain in the vortex core. These two motions are decoupled, with the alignment timescale determined by the local strain rate and the fiber aspect ratio. Finite particle size and inertia induce weak deviations from the base flow streamlines while leaving the orientational dynamics largely unaffected. These results establish a simple framework for understanding the behavior of elongated particles in stretched vortex flows, which constitute key building blocks of turbulence.