Symmetry-reversals in chiral active matter†,Soft Matter

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Symmetry-reversals in chiral active matter†
Soft Matter ( IF 2.9 ) Pub Date : 2018-05-28 00:00:00 , DOI: 10.1039/c8sm00402a
Marcel Workamp _{1,

2,

3,

4} , Gustavo Ramirez _{5,

6,

7,

8,

9} , Karen E. Daniels _{8,

9,

10,

11} , Joshua A. Dijksman _{1,

2,

3,

4}

Affiliation

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.

中文翻译：

手性活性物质中的对称逆转†

我们对由单个驱动的旋转盘组成的活性颗粒材料进行了实验，这些盘被限制在圆形区域内。圆盘外边缘的小凸起以几何摩擦的形式提供了可变数量的粒子间耦合。该盘每次旋转逆时针，但经过在各地舞台的中心集体流通的转变，从顺时针轨道逆时针轨道，作为包装分数的函数φ。我们确定的是，与用于振动颗粒的气体，所述粒子的速度分布是高斯在大范围的φ。通过拟合速度分布麦克斯韦-玻尔兹曼分布，我们确定的温度状参数这是一个普遍的函数φ; 该参数也等于粒子的平均平移能。我们通过其类似固体的旋转速率来量化集体循环，发现这是一个以临界堆积分数为中心的通用函数。另外，轨道动能与自旋动能之比也是非零几何摩擦的通用函数。这些发现凸显了驱动类型和颗粒间相互作用（此处为几何摩擦）在控制活性颗粒系统的集体行为中的重要作用。

更新日期：2018-05-28

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