Motion of submerged particles on soft hair beds

We study the motion of spherical particles moving under gravity through viscous fluids on inclined surfaces coated with an array of flexible hairs. Experiments reveal that shorter, stiffer hairs accelerate particles compared to smooth surfaces, while longer, flexible hairs can significantly slow down or even trap the particles. This slowdown arises from the bending of the hairs, which creates a compliant contact region that resists motion. To interpret these observations, we develop an elastohydrodynamic theory that couples the particle motion to the fluid flow, bending deformation of hairs and the driving gravitational forces. For highly compliant systems, we find that viscous dissipation occurs primarily along the length of the deforming hairs in contact with the particle. We identify scaling laws relating particle velocity to the hair length, obtaining excellent agreement with experiments. Our findings underscore the importance of surface compliance in modulating mobility, and suggest strategies for using soft microstructured surfaces to control particle transport in bio-inspired environments.