Clusters, Waves, and Force Chains in Fire-Ant Collectives: Emergent Behavior in Out-of-Equilibrium Particulate Systems

Abstract

Unlike other out-of-equilibrium systems, active matter is held far from equilibrium by energy input at the single particle level. The field includes a wide range of systems, but the most familiar examples, including flocks of birds and schools of fish, are biological. Despite the prevalence of biological systems, most of the important experimental work over the last two decades has examined synthetic systems with relatively simple particle interactions that closely approximate theoretical models. In this thesis, we examine fire ants, a biological system with complicated social interactions, and compare their behavior to expectations from active matter theory. We find surprising evidence of two hallmark aspects in active matter, collective-motion and motility-induced phase separation. Then, we demonstrate that the ants propagate a new type of nonlinear solitary wave in 2D columns, in which the ants activate and deactivate as the wave passes them, indicating the importance of time-dependent activity in future models of active matter. We also compare 3D columns of ants to columns of fluids and passive grains. Our results show that activity is not enough to fully wash out the granular nature of the ants. Finally, we turn to a synthetic system, vibrated polar disks, to examine the nature of the collective-motion phase transition in finite systems and find that the phase transition is qualitatively different than in the infinite-size limit. We argue that, in contrast to equilibrium systems, finite size effects are likely important in predicting the behavior of most practical active matter systems. Altogether, our work shows that the universal predictions of active matter theory are robust to the nature of particle interactions and proposes simple new directions for future theory to encompass more complicated active matter systems.