Elasto-viscous interactions of rigid beads moving on soft hairy beds

We study the motion of spherical beads moving due to gravity through viscous liquid on an inclined bed of flexible hairs. We experimentally study the effect of varying bead size and density, and the elastic stiffness and geometry of the hairs. Our experiments show that the beads move faster over stiff hairy surfaces than on smooth surfaces. By contrast, the situation is reversed with flexible hairs, which significantly slow down the beads compared with both rigid hairs and smooth substrates. Our experiments reveal that this slow down is associated with the bending of the hairs, which is coupled to the motion of the beads, underscoring the crucial role of hair elasticity. Furthermore, for beads of the same size, those with greater density are slowed down by a proportionately greater fraction. We organize and rationalize these findings using physical arguments that involve the gravitational driving force, viscous stresses due to flow, and the bending elasticity of the hairs. The ability of hairy substrates to control the motion of nearby particles holds promise for practical applications.