Inertial effects of self-propelled particles: From active Brownian to active Langevin motion

Abstract

Active particles which are self-propelled by converting energy into mechanical motion represent an expanding research realm in physics and chemistry. For micron-sized particles moving in a liquid ("microswimmers"), most of the basic features have been described by using the model of overdamped active Brownian motion. However, for macroscopic particles or microparticles moving in a gas, inertial effects become relevant such that the dynamics is underdamped. Therefore, recently, active particles with inertia have been described by extending the active Brownian motion model to active Langevin dynamics which include inertia. In this perspective article recent developments of active particles with inertia ("microflyers") are summarized both for single particle properties and for collective effects of many particles. There include: inertial delay effects between particle velocity and self-propulsion direction, tuning of the long-time self-diffusion by the moment of inertia, effects of fictitious forces in non-inertial frames, and the influence of inertia on motility-induced phase separation. Possible future developments and perspectives are also proposed and discussed.Invited perspective article for The Journal of Chemical Physics. arXiv:1910.13953v1 [cond-mat.soft]

Nonmutual torques and the unimportance of motility for long-range order in two-dimensional flocks

“…In the classic models of flocking [1,2] and much of the later literature [3] each agent is assumed to adjust its direction of motion to the mean of its neighbors including itself, plus a random error. It has recently become clear [4,5] that, on time and lengthscales relevant to observations on real bird flocks, the inertial dynamics of this reorientation must be explicitly taken into account, via a classical spin angular momentum on which the aligning interaction acts as a torque [6]. This inertial effect was shown [4,7,8] to give rise, on intermediate length scales [9], to turning waves reminiscent of those predicted for inertial flocks in fluids [10] or rotor lattices [11].…”

Active phase field crystal systems with inertial delay and underdamped dynamics

“…To be specific, instead of the traveling crystal state we find a coexistence of high and low density phases which are continually reorganized by directed particle fluxes which are induced by activity and modified by an interplay of structure forming interactions and inertial convection. This example demonstrates the relevance of our underdamped active matter model [23] for future works in the currently opening realm of underdamped active matter research [4]. Specifically, the emergence of motility induced phase separation in active systems is predicted to depend on the time scale of inertial motion, while high and low density phases have different kinetic temperatures [22].…”

“…Therefore, we now briefly report on the case of general underdamped dynamics which includes convection for the active PFC system eqs. (4).…”

“…Many-body systems composed of self-propelled, interacting particles are widely studied in many variations (for reviews, see [1][2][3][4]). Physically, the particles correspond to, e.g., biological or artificial microswimmers [5][6][7][8], cell colonies [9][10][11], or protein filaments [12].…”

“…However, there are also systems like flocks of birds [16,17] or artificial robots [18][19][20] where inertia cannot be neglected. Therefore, recent works study the consequences of inertial effects in active systems [4,21,22].…”