Emergent behaviour in active systems: the role of disorder

Minimal models of active Brownian colloids consisting of self-propelled spherical par- ticles with purely repulsive interactions have recently been identified as excellent quanti- tative testing grounds for theories of active matter and have been the subject of extensive numerical and analytical investigation. These systems have a rich phase diagram, forming active gases, liquids and solids with novel mechanical properties and exhibiting behaviour such as motility induced phase separation. A particularly interesting phenomenon can be found if we introduce noisy aligning interactions. By varying the density of the sys- tem or the intensity of the noise one can switch between a disordered phase where the particles move randomly and independently and a flocking state where the velocities of the particles are aligned. In this work we study the effect that disorder has on this flock. In particular, we consider what happens if a fraction p of the particles does not expe- rience the aligning interaction or if the particles have to flock through a medium with obstacles. By carrying out extensive molecular dynamics simulations we show that even a very small fraction of such ”dissenters” can have a dramatic effect on the whole system and, indeed, that the flocking can be destroyed for a very low value of p.Non UBCUnreviewedAuthor affiliation: Syracuse UniversityPostdoctora