微粒滚子作为颗粒介质向上坡流动
Microrollers Flow Uphill as Granular Media
AI总结：
本文通过旋转磁场对致密微粒滚子施加扭矩，实验实现颗粒向上坡滚动并形成负休止角堆积，发现其宏观集体行为具有纯耗散颗粒系统的运动学特征。
摘要：
将沙子倒入容器中，只有靠近顶面的颗粒会移动。与薄流动层中颗粒平动和转动能量相关的集体运动，通过多体相互作用以摩擦的形式迅速耗散。或者，考虑如果对每个单独的颗粒施加扭矩，颗粒床会发生什么。在本文中，我们展示了一个实验系统，在致密的微粒滚子床中，通过旋转磁场在构成单元层面上施加扭矩。最终结果是颗粒向上坡滚动，形成一个具有负休止角的堆积。我们识别出与体相中颗粒的移动性或流化程度相关的两种不同状态。近表面流动层的测速分析表明，这些响应性颗粒的集体运动规律与体相颗粒流动的标度方式相似。一个包含内聚力的简单颗粒模型准确预测了表观负摩擦系数。与模仿热力学原理的活性或响应性颗粒的表现不同，该系统产生的宏观集体行为具有纯耗散颗粒系统的运动学特征。
英文摘要：
Pour sand into a container and only the grains near the top surface move. The collective motion associated with the translational and rotational energy of the grains in a thin flowing layer is quickly dissipated as friction through multibody interactions. Alternatively, consider what will happen to a bed of particles if one applies a torque to each individual particle. In this paper, we demonstrate an experimental system where torque is applied at the constituent level through a rotating magnetic field in a dense bed of microrollers. The net result is the grains roll uphill, forming a heap with a negative angle of repose. Two different regimes have been identified related to the degree of mobility or fluidization of the particles in the bulk. Velocimetry of the near surface flowing layer reveals the collective motion of these responsive particles scales in a similar way to flowing bulk granular flows. A simple granular model that includes cohesion accurately predicts the apparent negative coefficient of friction. In contrast to the response of active or responsive particles that mimic thermodynamic principles, this system results in macroscopic collective behavior that has the kinematics of a purely dissipative granular system.
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