Aggregation-fragmentation and individual dynamics of active clusters

Abstract

A remarkable feature of active matter is the propensity to self-organize. One striking instance of this ability to generate spatial structures is the cluster phase, where clusters broadly distributed in size constantly move and evolve through particle exchange, breaking or merging. Here we propose an exhaustive description of the cluster dynamics in apolar active matter. Exploiting large statistics gathered on thousands of Janus colloids, we measure the aggregation and fragmentation rates and rationalize the resulting cluster size distribution and fluctuations. We also show that the motion of individual clusters is entirely consistent with a model positing random orientation of colloids. Our findings establish a simple, generic model of cluster phase, and pave the way for a thorough understanding of clustering in active matter.

Gravity induced formation of spinners and polar order of spherical microswimmers on a surface

“…Examples of this, from bacterial world, include the hydrodynamic stabilisation of rotating Volvox pairs [28] due to a interplay between sedimentation and hydrodynamic effects and the formation of vortex arrays [29] near confining surfaces, while guiding [30,31] and flow-induced phase separation [32] have been observed with artificial swimmers in confinement. Potential interactions such as phoretic [30,31,33,34], electrical [35,36], magnetic [37] in addition to hydrodynamic effects [29,32,38] can be used to create complex patterns and dynamic self-assembled structures.…”

“…In a typical experimental realisation of self-propelling colloids [33,34,39,40], the particles sediment at the bottom of the container and form a monolayer near the confining surface.…”

Sedimentation of self-propelled Janus colloids: polarization and pressure

“…Due to the huge reservoir of peroxide, activity can be considered as constant during each experiment. For each experiment we record movies of 5000 images @ 20 fps, for a total duration of 250 s. This experimental set-up, sketched in figure 1, was previously used to study the cluster phase [32] and the weak sedimentation limit [1] of active Janus colloids.…”

Taming polar active matter with moving substrates: directed transport and counterpropagating macrobands

“…As one of their main characteristics, these systems are intrinsically out of equilibrium allowing them to self-organize into new ordered and even functional structures. In synthetic active systems, such structures include dynamic clusters which dynamically form and break-up in low density Janus colloids [4][5][6][7][8] as well as laser driven colloids which spontaneously start to move ballistically (self-propel) when binding together [9][10][11]. Likewise, biological microswimmers form patterns such as vortices in bacterial turbulence [12][13][14][15], or swirls and microflock patterns in chiral active matter like curved polymers or sperm [16][17][18][19].…”