Physical Chemistry and Soft Matter, Wageningen University & Research, Wageningen, The Netherlands.
Soft Matter. 2018 Jul 11;14(27):5572-5580. doi: 10.1039/c8sm00402a.
We perform experiments on an active granular material composed of individually-driven, spinning disks confined within a circular arena. Small bumps at the outer edges of the disks provide a variable amount of interparticle coupling in the form of geometric friction. The disks each spin counter-clockwise, but undergo a transition in their collective circulation around the center of the arena, from a clockwise orbit to a counter-clockwise orbit, as a function of packing fraction φ. We identify that, unlike for vibrated granular gases, the particles' velocity distributions are Gaussian over a large range of φ. By fitting the speed distribution to a Maxwell-Boltzmann distribution, we identify a temperature-like parameter which is a universal function of φ; this parameter is also equal to the mean translational energy of the particles. We quantify the collective circulation via its solid-body-like rotation rate, and find that this is a universal function centered around a critical packing fraction. In addition, the ratio of orbital kinetic energy to spin kinetic energy is also a universal function for non-zero geometric friction. These findings highlight the important role of both the type of driving and the interparticle interactions (here, geometric friction) in controlling the collective behavior of active granular systems.
我们在一个由独立驱动、旋转的圆盘组成的主动颗粒物质上进行实验，这些圆盘被限制在一个圆形竞技场内。圆盘外边缘的小凸起以几何摩擦的形式提供了可变的颗粒间耦合量。圆盘均逆时针旋转，但随着填充率 φ 的变化，它们在竞技场中心的集体循环从顺时针轨道转变为逆时针轨道。我们发现，与振动颗粒气体不同，在很大的 φ 范围内，颗粒的速度分布呈高斯分布。通过将速度分布拟合到麦克斯韦-玻尔兹曼分布，我们确定了一个类似于温度的参数，它是 φ 的通用函数；这个参数也等于颗粒的平均平移能量。我们通过类似于刚体的旋转速率来量化集体循环，并发现它是一个以临界填充率为中心的通用函数。此外，轨道动能与自旋动能的比值对于非零几何摩擦也是一个通用函数。这些发现强调了驱动类型和颗粒间相互作用（这里是几何摩擦）在控制主动颗粒系统集体行为方面的重要作用。
