Symmetry-reversals in chiral active matter

Abstract

We perform experiments on an active granular material composed of individually-driven, spinning disks confined within a circular arena. Small bumps at the outer edges of the disks provide a variable amount of interparticle coupling in the form of geometric friction. The disks each spin counter-clockwise, but undergo a transition in their collective circulation around the center of the arena, from a clockwise orbit to a counter-clockwise orbit, as a function of packing fraction φ. We identify that, unlike for vibrated granular gases, the particles' velocity distributions are Gaussian over a large range of φ. By fitting the speed distribution to a Maxwell-Boltzmann distribution, we identify a temperature-like parameter which is a universal function of φ; this parameter is also equal to the mean translational energy of the particles. We quantify the collective circulation via its solid-body-like rotation rate, and find that this is a universal function centered around a critical packing fraction. In addition, the ratio of orbital kinetic energy to spin kinetic energy is also a universal function for non-zero geometric friction. These findings highlight the important role of both the type of driving and the interparticle interactions (here, geometric friction) in controlling the collective behavior of active granular systems.

Inertial delay of self-propelled particles

“…Our observations demonstrate the profound influence of inertia on the long- and short-time dynamics of self-propelled particles. Considering the relevance of inertia 30 , our model is applicable to various systems, such as levitating 38 , 39 and floating 40 granular particles and dusty plasmas 41 . It is straightforward to extend the model to elongated particles 5 , 11 , 12 , 14 , 37 , 42 and it was shown numerically that collective motion of rod-like particles is well described by similar equations of motion 37 .…”

On the fluctuation-dissipation relation in non-equilibrium and non-Hamiltonian systems

“…However, the observed different time scales are not necessarily related to an underlying complex dynamics 65 . The general issue of effective temperature has been the subject of recent reviews 16,18 and is an open line of intense research, with applications for instance in granular [66][67][68] and active matter [69][70][71][72][73][74][75][76][77][78][79][80][81][82] (see also Section III D below). Here, we focus on a few examples in the context of athermal systems.…”

Robust boundary flow in chiral active fluid

“…, 14] and sperm cells [4,15] near surfaces, and magnetotactic bacteria in rotating fields [16,17]. Artificial chiral active systems have also been developed, such as colloids [18][19][20][21][22][23][24], millimeter-scale magnets [25,26] and rotating granular particles [27][28][29][30]. Multiple numerical and theoretical studies on chiral active fluid have been carried out [27,[31][32][33][34][35][36][37][38].…”