Abstract In recent interesting experiments, Peng et al. ((2015) PRL, 134, 176202) have shown that the static friction coefficient in a spherical contact drops of a factor close to 2 over 3 decades of increase of normal load, converging to a dynamic friction coefficient. The difference is larger than what commonly attributed in dry metals. They have interpreted this with a numerical boundary integral contact calculations involving many asperities using two input parameters (a static and a dynamic friction coefficient from AFM experiments at nanoscale). However, we show that similar drop with normal load is also expected from the theory of "Griffith" or "JKR" friction (Ciavarella (2015) J Mech Phys Solids 84: 313–324) which has the advantage of being a simple analytical theory and also of being closely connected with friction laws used commonly today in geophysics. Further, it also uses two input parameters, and requires no numerical solution of the rough contact problem for the sphere.
M. Ciavarella (Wed,) studied this question.