Behavior-dependent critical dynamics in collective states of active particles

¹ Fachbereich Physik, Universität Konstanz - 78464 Konstanz, Germany
² Department of Physics, Massachusetts Institute of Technology - Cambridge, MA 02139, USA
³ Department of Mathematics and Applied Mathematics, University of Cape Town - 7701 Rondebosch, Cape Town, South Africa
⁴ Max Planck Institute for Intelligent Systems - Heisenbergstr. 3, 70569 Stuttgart, Germany
⁵ 4 th Institute for Theoretical Physics, University of Stuttgart - Pfaffenwaldring 57, 70569 Stuttgart, Germany

^(a) clemens.bechinger@uni-konstanz.de (corresponding author)

Received: 12 April 2021
Accepted: 17 June 2021

Abstract

We experimentally 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.