Powered by Indico
v3.2.9
Zoom: https://stockholmuniversity.zoom.us/j/622224375
Functional collective states in living systems are thought to exhibit an optimal balance between stability and sensitivity to external perturbations. One way to achieve such behavior is that individuals adjust their response to neighbors so that the entire group operates near a critical point that separates distinct dynamical collective states. Since responses between individuals in living systems cannot easily be changed, evidence for critical behavior in groups of birds, insects, and also micron-sized protozoa, has mainly been inferred from how their properties vary as a function of group size or density. Here we demonstrate critical collective behavior in a system of active colloidal particles (APs), achieved through variations of their mutual interactions using feedback control. At the transition between a swarm and a swirl we observe an explicit bifurcation dynamics of the rotational order parameter and a critical slowing down, i.e., a growth of the relaxation time by almost one order of magnitude. Additional signatures of critical dynamics, including hysteresis in presence of symmetry-breaking particle interactions, and a maximum of the susceptibility, are measured and characterized in terms of a theoretical model which is based purely on the time-reversal symmetry of the order parameter. Our results suggest that collective dynamics can be tuned not only via group size but also by small behavioral changes between group members.
1. T. Bäuerle, R. C. Löffler, C. Bechinger, Formation of stable and responsive collective states in suspensions of active colloids. Nature Comm. 11, 1-9 (2020).
2. F. Lavergne, H. Wendehenne, T. Bäuerle, C. Bechinger, Group formation and cohesion of active particles with visual perception-dependent motility. Science 364, 70-74 (2019).