Some Comments on Hydrogel and Cartilage Contact Mechanics and Friction

We discuss the origin of the low friction observed for some elastically soft materials, such as hydrogels and the human cartilage. When a soft elastic solid is squeezed against another solid in a fluid, if the nominal contact pressure is high enough, after long enough contact time the contact area percolates, resulting in islands of confined fluid. For charged surfaces, with charges of equal sign, the osmotic pressure in the area of real contact may be large enough to keep the surfaces separated at nanometer separation. In this case the solid contact pressure (here the osmotic pressure) in the area of real contact will be nearly independent of the external load. The finite surface separation in the area of real contact results in a very small (breakloose) friction force even after long time of stationary contact.