Shah Rohan, Gravish Nick
Department of Mechanical and Aerospace Engineering, University of California, San Diego, CA 92093.
Proc Natl Acad Sci U S A. 2025 Jul 15;122(28):e2501169122. doi: 10.1073/pnas.2501169122. Epub 2025 Jul 9.
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.
两个干燥表面之间的滑动摩擦通常由与速度无关的阿蒙顿 - 库仑摩擦力定律来描述。然而，在许多情况下，两个表面之间的多个摩擦接触点是“活跃的”，并且每个接触点都可以以不同的相对速度移动。在这里，我们研究了一个具有多个活跃接触点的系统的滑动摩擦特性，每个接触点都具有独立且可控的速度。我们证明，尽管每个单独的接触都表现出与速度无关的摩擦，但多个活跃接触点可以产生与速度相关的可控滑动摩擦力。我们在实验中研究了一个旋转圆盘，它有十个与地面摩擦接触的速度可控的轮子。我们首先改变接触速度，并证明平衡系统速度是活跃接触速度的值。接下来，我们直接测量滑动摩擦力，并观察接触速度如何控制系统的力 - 速度曲线。在最后的实验中，我们展示了如何通过控制力 - 速度曲线来产生具有可控有效粘度和可控滑动摩擦系数的滑动摩擦。令人惊讶的是，我们能够证明通过适当的力 - 速度控制，摩擦接触可以产生接近无摩擦的滑动。通过揭示活跃接触点如何塑造干滑动摩擦系统的力 - 速度行为，我们可以更好地理解动物和机器人的运动，并且进一步为控制滑动摩擦的新型工程表面创造机会。
