Controlling Interactions in Active Matter Systems

It has been demonstrated in a 2D colloidal monolayer composed of active spinning ferromagnetic and passive colloids that an ultra-long range attractive interaction emerges between active colloids. This interaction was induced by the activity of the spinning particles and mediated by the elasticity of the passive colloidal monolayer. We demonstrate through experiments and simulations that the range and dynamics of this emergent attractive interaction can be tuned or reversed by changing the mode of activity or the composition of the passive monolayer. With a 3D Helmholtz-coil like apparatus we can change the mode of activity from spinning to a top-like motion. These tops exhibit a similar attractive interaction at long-range, however they repel at distances less than four particle diameters, unlike the spinners. The tops also effectively anneal the passive monolayer into almost perfect crystal grains while the tops occupy sites on the grain boundaries effectively functioning as dislocation sources. Additionally, by doping the passive colloidal monolayer with small concentrations of passive particles of different sizes the monolayer behaves more elastically. This increases the range of the interaction but the dopants impede dislocation motion so the rate of attraction decreases.