Geometric frustration and pairing-order transition in confined bacterial vortices

Dense systems of active matter exhibit highly dynamic collective motion characterized by intermingled vortices, referred to as active turbulence. The interaction between these vortices is key to controlling turbulent dynamics, and a promising approach for revealing the rules governing their interaction is geometric confinement. In this study, we investigate the vortex-pairing patterns in confined bacterial suspensions as a model frustrated system in which a perfect antiferromagnetic state is prohibited. We find that three-body vortex interactions exhibit anomalous pairing-order transition from corotational vortex pairing to counterrotating patterns with frustration. Although an active matter system is in nonequilibrium, our theory based on bending energy accounts for significant features, including pattern transition and a shift of the transition point in frustrated systems. Moreover, the interplay between the chirality in collective motion and frustration in vortex pairing creates a collective rotational flow under the broad geometric conditions of a confined space. Our results show that frustrated vortex patterning promotes a geometric approach for arranging active turbulence in microfluidic systems.