Structure and dynamics in active matter systems

Active matter systems are made of self-propelling particles and make ideal ground for studies of out-ofequilibrium phenomena. The self-propulsion is fueled by continuous drawing of energy from the environment at the single-particle scale. Examples of such systems are common, covering a wide range of lengths, from molecular level biological systems to large societies of animals. It is, however, not necessary that the constituents should only be living objects. These systems often exhibit fascinating patterns and dynamics. Understanding such collective phenomena is of importance from fundamental as well as practical points of view. Despite conceptual difficulties, [4–7] e.g., with respect to the definition of temperature, significant advancement has been made at the theoretical level. In the domain of nonequilibrium statistical mechanics, there exist simple models that are helpful in capturing interesting experimental facts. A primary interest in the area of active matter is in the understanding of fundamental aspects of nonequilibrium-phase transitions. Studies along this direction are motivated by observations of structures and dynamics in assemblies like a colony of bacteria or a flock of birds, [1,27] resembling those concerning phase transitions in passive matter systems. Model dynamical systems having Vicsek-like simple velocity alignment rules or possessing active Brownian particles (ABP) as constituents have been useful in realizing many of the observed structural and dynamical features spanning wide scales of length and time. In the passive case anomalous behavior of various thermodynamic and transport properties, [28,38,42,44] upon approach of the state point toward the criticality, have been quantified experimentally and are captured by combinations of analytical theories and computer simulations. A major drive in this area has been in the understanding of universality in the exponents of power-law singularities. Analogous questions have been asked in the context of kinetics of phase transitions as well. In this area, following quench of a homogeneous system, inside the coexistence region, as the new equilibrium is approached, investigations have focused on learning universality in the evolution dynamics. There the primary interest has been in the exponent for the power-law time dependence of average size of domains that are either rich or poor in constituents of a specific type. Among other interests in this sub-branch two important ones are in understanding scaling properties associated with pattern and aging in the coarsening systems. Both critical and coarsening phenomena are of importance in the active matter context as well and have been receiving attention. In critical phenomena there exist debates on universality. Unlike for the passive systems here the universality may be weak and different types of transport, e.g., presence or absence of hydrodynamic interactions, [40,45] may change critical properties that will remain unaltered in the passive case. Coarsening phenomena in this category of systems started getting systematic attention only recently. Apart from the aspects related to phase transitions, it is important to understand structural and dynamical properties, under varying environmental conditions, even in the normal regions of the phase diagrams, in the steady state situations. This special issue contains theoretical articles that address problems belonging to both the categories, by exploiting state-of-the-art analytical and computational techniques. In addition to being of fundamental nature, several of the contributions have strong practical importance. The “perspective” article by Binder and Virnau provides a good discussion on phase transitions and universality. These authors introduce critical phenomena, as well as kinetics of phase transitions, from modern