This study investigates the diffusive motion of a semirigid fiber immersed in a dense granular packing. Experiments are conducted in an index-matched medium composed of water-saturated hydrogel beads, which is sheared by means of a rotating paddle. In the quasistatic regime, we first characterize the self-diffusion of individual grains and show that it follows established scaling laws for granular diffusion. We then analyze the motion of a single fiber for various imposed shear rates and for different fiber and grain dimensions. We show that the fiber undergoes a slow and steady settling through the medium, with a sedimentation velocity that decreases as the fiber length increases relative to the grain size. After removing this mean settling motion, we find that the residual vertical displacement of the fiber exhibits a diffusive behavior. The effective diffusion coefficient decreases with increasing fiber length and is primarily governed by the ratio of fiber length to grain diameter. From these results, we derive an empirical relationship for the fiber diffusion coefficient in granular flow, expressed as a function of both fiber and flow properties.

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• Diffusion
• Granular flows
• Granular mixing
• Granular fluids
• Granular materials