Friction anisotropy in the sliding motion of polymer microspheres on a compliant rippled surface

We have investigated the sliding motion of PMMA microspheres elastically driven on a rippled polyvinyl siloxane surface for different values of normal load, scan velocity, and substrate temperature. The spheres were rubbed both parallel and perpendicular to the ripples, and the resulting friction was found to be almost constant and, respectively, to vary in a stick-slip fashion with time. The average friction value was also enhanced in the perpendicular direction, which we attribute to a larger adhesive force established at the end of the slip phase. In both cases, the friction was also found to increase linearly with increasing load, consistently with the predictions of the Persson contact theory, and to increase logarithmically with increasing scan velocity and decrease with increasing temperature according to the Eyring's reaction rate theory. The stability of this simple system suggests its possible implementation as a basic unit for artificial tactile sensors.