Interplay between ordering and mobility in a system of self-propelled grainsNarayanan Menon*

Self-propelled particles can locally break rotational symmetry along two distinct axes.  The symmetry of the propulsion identifies one axis, and the other is the symmetry of the pair interaction, which could be for example, set by the shape of the particle.  In biological systems and in most synthetic systems, these axes are aligned.  A 2-dimensional vibration-fluidized system of macroscopic grains affords us a flexible model system for peeling these axes apart and exploring how ordering in a dense system of grains may be promoted or disrupted by self-propulsion. In-plane grain motion is driven by collisions with the floor and ceiling in the vertical direction. The collisional noise is rectified to produce directional motion in the horizontal plane by anisotropic features on the top and bottom surfaces.  Thus, the shape of the particle can be kept fixed, while the anisotropy of its motion is varied.  We show two examples using square tiles where this flexibility allows us to explore qualitatively new behaviour. First, we explore the phase behaviour of square grains in the presence of self-propulsion.  Orientational and translation ordering transitions in the dense phase are qualitatively altered as the activity of the grains is tuned.  To our knowledge, there are very few experimental systems in which the phase behaviour has been mapped out in this fashion.  Second, we show that the melting of a crystallite of grains in noisy conditions can be strongly affected by the relative orientation of the symmetry of the self-propulsion and the symmetry axes of the packing.  Endowing the grains with activity not only modifies typical time-scales of metling, but can introduce qualitatively new routes to melting that were not available in the corresponding equilibrium system. 

This work was done with Zhejun Shen and Lee Walsh.