Adhesion between weakly rough beads.

Cohesion effects are of prime importance in powders and granular media, and they are strongly affected by the roughness of the grain surface. We report measurements of the adhesion force between surfaces of Pyrex having a nanometric roughness, with a surface force apparatus. The two surfaces are immersed in liquid n-dodecane. The adhesion force measured is much smaller than expected in the case of smooth surfaces. We find that the adhesion force depends on the maximal load that has been applied on the surfaces, but does not depend on the time during which they have been in contact. We propose a model of plastic deformation of the small asperities in a macroscopic Hertz contact which is in good agreement with the experimental data.