An introduction to the statistical physics of active matter: motility-induced phase separation and the “generic instability” of active gels

Abstract

Abstract. In this work we review some statistical physics techniques to coarse grain active matter systems, writing down a set of continuum fields which track the evolution of macroscopic fields such as density, momentum, etc. While the method can be applied in general, we will focus here on two simple and by now well-studied, active matter examples. First, we will consider motility-induced phase separation, the phenomenon by which a concentrated suspension of self-propelled particles spontaneously separates into a dense and a dilute phase. Second, we will review the so-called "generic instability" of active gels, which refers to the nonequilibrium phase transition between a quiescent and a spontaneously flowing phase in a concentrated suspension of rodlike active particles. For both these cases, we also outline recent developments in the literature.

Physics of compartmentalization: How phase separation and signaling shape membrane and organelle identity

“…Classical theories of phase separation rely on concepts of equilibrium statistical mechanics, such as the free energy, that in some cases can be used to approximately describe also non-equilibrium dissipative processes during their relaxation toward an equilibrium state. However, many of the inner workings of living cells are intrinsically nonequilibrium, since they are continuously driven by external and/or internal forces, and can involve transport phenomena and enzymatic processes in which individual molecular components constantly and irreversibly consume and dissipate energy, a defining feature of ‘active’ matter [19] , [33] , [34] . Such intrinsically out-of-equilibrium processes permit the existence of a larger variety of stationary states than those realizable in close-to-equilibrium conditions [15] , [35] .…”

Microswimmers – From Single Particle Motion to Collective Behavior

“…Generic aspects, specifically the foundations of hydrodynamic interactions at low Reynolds numbers and the general principles of non-equilibrium statistical mechanics and collective motion, are covered in a few minireviews [1][2][3].…”

Onsager's variational principle in active soft matter

“…Active matter represents a novel type of nonequilibrium systems that contain a large number of self-propelling particles or creatures moving in fluids or more complex environments. [1][2][3][4][5][6][7][8][9][10] The self-propelling units are considered to be active in the sense that they are capable of continuously converting fuel or chemical energy (stored internally or in the ambient) into directional motion or mechanical work. Active matter containing selfpropelled units is ubiquitous in biology and in many artificial systems.…”

“…2,5 Over the last decade, several soft matter systems have been revisited with a focus on this point of view. 2 The physical understanding for the emergent structures and behaviors of active soft matter has been rapidly growing, [1][2][3][4][5][6][7][8][9][10] with particular attention paid to dry active matter, 3,18 active polar fluids, 42 active nematics, 43 active gels, 44 and active membranes. 45 Theoretically there have been two major approaches to the study of active soft matter: particle-based models 3,[6][7][8]26,46 and continuum phenomenological models.…”

“…2 The physical understanding for the emergent structures and behaviors of active soft matter has been rapidly growing, [1][2][3][4][5][6][7][8][9][10] with particular attention paid to dry active matter, 3,18 active polar fluids, 42 active nematics, 43 active gels, 44 and active membranes. 45 Theoretically there have been two major approaches to the study of active soft matter: particle-based models 3,[6][7][8]26,46 and continuum phenomenological models. 1,2,[4][5][6]14,15,43 In particlebased models, the active units are usually modeled as selfpropelled particles with fixed or variable speed and random orientation moving in an inert background, following the seminal work of Vicsek et al 47 It provides a straightforward approach to the study of active soft matter with an emphasis on the order and fluctuations rather than the forces and mechanics.…”