Active contacts create controllable friction

Sliding friction between two dry surfaces is reasonably described by the speed-independent Amonton-Coulomb friction force law. However, there are many situations where the frictional contact points between two surfaces are "active" and may not all be moving at the same relative speed. In this work we study the sliding friction properties of a system with multiple active contacts each with independent and controllable speed. We demonstrate that multiple active contacts can produce controllable speed-dependent sliding friction forces, despite each individual contact exhibiting a speed-independent friction. We study in experiment a rotating carousel with ten speed-controlled wheels in frictional contact with the ground. We first vary the contact speeds and demonstrate that the equilibrium system speed is the median of the active contact speeds. Next we directly measure the ground reaction forces and observe how the contact speeds can control the force-speed curve of the system. In the final experiments we demonstrate how control of the force-speed curve can create sliding friction with a controllable effective viscosity and controllable sliding friction coefficient. Surprisingly, we are able to demonstrate that frictional contacts can create near frictionless sliding with appropriate force-speed control. By revealing how active contacts can shape the force-speed behavior of dry sliding friction systems we can better understand animal and robot locomotion, and furthermore open up opportunities for new engineered surfaces to control sliding friction.