Hydrodynamics-driven phase-locking and collective motility of sessile active dumbbells†

Collective motion is a phenomenon observed across length scales in nature – from bacterial swarming and tissue migration to the flocking of animals. The mechanisms underlying this behavior vary significantly depending on the biological system, ranging from hydrodynamic and chemical interactions in bacteria to mechanical forces in epithelial tissues and social alignment in animal groups. While collective motion often arises from the coordinated activity of independently motile agents, this work explores a novel context: the emergence of collective motion in systems of non-motile active agents. Inspired by the oscillatory shape dynamics observed in suspended cells such as neutrophils and fibroblasts, we model active dumbbells exhibiting limit-cycle oscillations in shape as a minimal representation of such systems. Through computational simulations, we demonstrate that hydrodynamic interactions between these dumbbells lead to three key phenomena – a density-dependent transition from sessile to collective motion, hydrodynamics-induced phase separation, and synchronization of oscillatory shape changes. We have explored the role of hydrodynamic interactions on these emergent properties of sessile active dumbbells. These results underscore the critical role of hydrodynamic coupling in enabling and organizing collective behaviors in systems lacking intrinsic motility. This study lays the groundwork for future investigations into the emergent behavior of active matter and its implications for understanding cell motility, tissue dynamics, and the development of bio-inspired materials.