Active Matter (1003.2032v1)

Abstract: The term active matter describes diverse systems, spanning macroscopic (e.g. shoals of fish and flocks of birds) to microscopic scales (e.g. migrating cells, motile bacteria and gels formed through the interaction of nanoscale molecular motors with cytoskeletal filaments within cells). Such systems are often idealizable in terms of collections of individual units, referred to as active particles or self-propelled particles, which take energy from an internal replenishable energy depot or ambient medium and transduce it into useful work performed on the environment, in addition to dissipating a fraction of this energy into heat. These individual units may interact both directly as well as through disturbances propagated via the medium in which they are immersed. Active particles can exhibit remarkable collective behaviour as a consequence of these interactions, including non-equilibrium phase transitions between novel dynamical phases, large fluctuations violating expectations from the central limit theorem and substantial robustness against the disordering effects of thermal fluctuations. In this chapter, following a brief summary of experimental systems which may be classified as examples of active matter, I describe some of the principles which underlie the modeling of such systems.

Active Matter: A Comprehensive Overview

This paper by Gautam I. Menon offers an extensive examination of active matter, elucidating its diverse systems ranging from macroscopic entities such as shoals of fish and bird flocks to microscopic components like motile bacteria and cellular structures. Active matter comprises self-propelled units exploiting internal or ambient energy to perform work, exhibit non-equilibrium phase transitions, and resist thermal disorder, leading to unique collective behaviors.

The paper begins with an exploration of active fluids and develops a framework for understanding the collective motion of self-driven agents, highlighting the significance of both direct interactions and disturbances mediated by the surrounding medium. These interactions result in complex dynamical patterns and large fluctuations that defy conventional expectations, encouraging the formulation of new theoretical models.

Active matter is differentiated from other driven systems by its internal energy input. The motion and direction of each unit are dictated by intrinsic properties rather than external fields. This intrinsic drive leads to emergent behaviors observable in various experimental systems, which include bacterial suspensions, cytoskeletal filament interactions with molecular motors, fish schools, bird flocks, and vibrated granular rods. Each of these systems exemplifies large-scale, self-organized dynamics that stem from microscopic interactions.

The paper further explores hydrodynamic descriptions of these systems, abstracting them into polar or nematic order parameters for self-propelled particles. Emphasizing the role of momentum conservation across active particles, it outlines how emerging structures lead to unique rheological properties. Theoretical models predict fluctuating nematic orders and generic instabilities not present in passive matter. Such hydrodynamic frameworks also predict unconventional number fluctuations, anomalous viscoelastic behavior, and specific hydrodynamic instabilities that shed light on the behavior of living systems down to molecular levels.

In a parallel approach, active gels, a different class of active matter, are examined through the lens of non-equilibrium statistical mechanics. Here, systems are treated as viscoelastic gels driven by non-equilibrium forces. Active gels provide a paradigm for understanding cytoskeletal behavior in living organisms, with models that consider both constitutive equations and active stress generation within the medium.

The implications of this research extend beyond theoretical interest. Practically, understanding active matter might influence the design and manufacture of synthetic biomaterials and nano-devices that mimic biological capabilities. Theoretically, it aids in constructing comprehensive frameworks bridging microscale dynamics with macroscopic phenomena, emphasizing the potential of active matter to illuminate aspects of living systems in non-equilibrium conditions.

Future research may elaborate on these findings, exploring new experimental systems or refining theoretical predictions to better align with empirical data. As active matter remains a dynamically evolving field, continued dialogue among disciplines promises further innovation in both fundamental science and practical applications.