Elasto-Viscous Dynamics of Spherical Particles on Flexible Hairy Substrates

We investigate the motion of spherical particles moving under gravity on inclined beds of flexible hairs, focusing on the role of hair flexibility in modulating particle velocity. Experiments demonstrate that rigid hairy substrates allow faster particle motion compared to smooth surfaces. When the hairs are flexible, however, we find that the particle motion may be either faster or slower than on a smooth surface. In particular, motion is more rapid on beds with short hairs, while it is much slower for longer, more flexible hairs. We rationalize these behaviors by developing a model that accounts for the effects of the resistance to flow through the porous hair beds, the bending deformation of the hairs, and elasto-viscous contact forces. Our findings provide insights into the design of bio-inspired systems for controlled particle transport in complex environments.