Dynamical swirl structures powered by microswimmers in active nematics

Abstract

Active nematics, in their pure form, have demonstrated a plethora of dynamic and steady-state behaviors, including large-scale dynamic structures, collective flows, and intricate multi-spatial temporal dynamics. This complexity further increases in the presence of external polar agents. We investigate active nematics interspersed with polar microswimmers, akin to active apolar cells infused with active impurities (microswimmers). Our comprehensive numerical study reveals that varying the microswimmers’ motility induces a novel spatiotemporal state in the active nematics backdrop. This state is marked by macroscopic swirl-like structures and a reduction in the overall order of the active nematics. Interestingly, this state emerges at intermediate motility levels, where microswimmers form local clusters and exhibit coherent motion. However, at higher motility levels, the swirls become less coherent, and microswimmer clustering intensifies. We show that the effect of the polar microswimmers on active nematics can be interpreted as a spatiotemporally correlated colored noise on active nematics, which promotes bend instability in active nematics, leading to the observed swirling dynamics. Our findings indicate that the spatiotemporal states are highly sensitive to the microswimmers’ motility, offering potential avenues for pathogen identification based on known motility characteristics. © 2025 Elsevier B.V.