  The self-organization of active particles is governed by their dynamic
effective interactions. Such interactions are controlled by the medium in which
such active agents reside. Here, we study the interactions between active
agents in a dense non-active medium. Our system consists of actuated spinning
(active) particles embedded in a dense monolayer of passive (non-active)
particles. We demonstrate that the presence of the passive monolayer alters
dramatically the properties of the system and results in a reversal of the
forces between active spinning particles from repulsive to attractive. The
origin of such reversal is due to the coupling between the active stresses and
elasticity of the system. This discovery provides a new mechanism for the
interaction between active agents in complex and structured media, opening up
new opportunities to tune the interaction range and directionality via the
mechanical properties of the medium.
