Finite-size enables preferential sampling by rods in cellular flow

Anisotropic, finite-sized particles, common in environmental and industrial flows, exhibit complex dynamics not seen in small spherical particles. Their shape introduces orientation-dependent forces, and their finite size affects how they experience the flow field. These geometric effects alone can lead to behaviors like preferential sampling, even in the absence of inertia. We find that particles can preferentially sample a flow field given the combination of just three ingredients: finite particle size, anisotropic particle shape, and a nonlinear flow field. To investigate preferential sampling, we simulate inertialess finite-sized rods using a slender body theory framework in a 2D cellular flow field: the Taylor Green vortex. We observe clear preferential sampling that increases with rod length, and find that rods over sample linear regions of the flow and undersample nonlinear regions. We additionally analyze their trajectories from a dynamical systems point of view, finding that these rods exhibit quasi-periodic or chaotic trajectories depending on their lengths and initial positions. We then relate this analysis to their preferential sampling behavior.