Active contacts control sliding friction.

Sliding friction between two dry surfaces is commonly described by the speed-independent Amonton-Coulomb friction force law. However, there are many situations where multiple frictional contact points between two surfaces are "active" and each can move at a different relative speed. Here, 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 sliding friction 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.