An experimental setup, based on the principles of atomic force microscopy (AFM), was used to measure directly the adhesion and rolling-friction forces between individual silica microspheres of radii between 0.5 and 2.5 μm. It showed that the linear dependence of the pull-off force on the particle radius is still valid for micron-sized particles. Rolling-friction forces between silica microspheres were measured for the first time by combining AFM methods and optical microscopy: They are $\sim 100$ times lower than the corresponding adhesion forces.

• Received 16 April 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.3328