Impacts of poroelastic spheres

Understanding the physical mechanism governing the impact of soft porous solids on rigid surfaces is at the basis of the development of shock-absorbing materials. Traditionally, the static contact of such soft solids with the substrate is well described by Hertz contact theory. However, their rebound behavior can only be explained by invoking a variety of dissipation mechanisms. Here, we study experimentally the impact on rigid surfaces of different soft porous solids saturated with liquid: hydrogel balls and liquid-saturated foam balls. We find that the general rebound mechanism is governed by the interplay of three “wet” dissipation mechanisms, depending on the material: capillary adhesion, viscous dissipation in a liquid film between the ball and the substrate, and poroelastic dissipation. While the first two dissipations are known, here we introduce the theory for poroelastic dissipation originating from the porous flow inside the ball, as it is the main rebound mechanism for saturated foam balls. The understanding of this dissipation mechanism with its dependence on both permeability of the porous solid and liquid viscosity can open the way towards engineering a new generation of shock absorbers and cushions.