Minimal Models of Active Matter with Attraction: Phase Behavior and Kinetics

Abstract

Physicists' fascination of the phenomena of collective motion/behavior that we often see in nature on many scales (from fish to bacteria and down to intra-cellular units) has led to the emergence of the field of active matter. An active system is a collection of many interacting self-driven active particles capable of converting stored energy into systematic movement. They are out-of-equilibrium systems characterized by the interplay of noise, activity, and interactions which give rise to a wealth of novel phases. The simplest model consisting of self-propelled disks with purely repulsive interactions, exhibits surprising behavior; it phase separates into a dense fluid phase and a gas phase. This phenomena of phase separation in the absence of attractive interactions is termed motility-induced phase separation (MIPS). It is controlled by the Péclet number (Pe), which characterizes the persistence of self-propelled motion. Recent advances in understanding the novel phases of active matter are based on the concept of swim or active pressure that measures the strength of propulsive forces. Another contribution is termed passive pressure which measures the direct contribution of interaction forces. In this thesis, we add a layer of complexity by studying the phase behavior of active particles with attraction. When the attraction strength is much larger than the self-propulsion, this results in clustering of particles. We study the kinetics of cluster-cluster aggregation by measuring the time-evolution of the domain size and the number of particles in a cluster as a function of time. We identify three regimes: in the first low Pe regime attraction forces dominates leading to clustering. An intermediate regime in which neither of the two forces is strong enough to produce macroscopic phase separation. And a high Pe regime where phase separation is expected. Finally, we attempt at characterizing the reentrant behavior through a pressure equation of state calculation.