Microrollers Flow Uphill as Granular Media

TL;DR

This paper demonstrates that applying torque to microrollers in a dense granular bed causes them to roll uphill, creating a heap with a negative angle of repose, revealing novel collective behaviors in granular media.

Contribution

The study introduces an experimental system where torque applied via a magnetic field induces uphill rolling in microrollers, a behavior not observed in traditional granular flows.

Findings

Microrollers can roll uphill under torque, forming heaps with negative angles of repose.

Two regimes of particle mobility or fluidization are identified.

A simple cohesive granular model predicts the negative friction coefficient.

Abstract

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.