  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.
