Microscopic nonreciprocity drives emergent dynamic phase of living crystals

Active matter systems can exhibit non-equilibrium phases under broken symmetries. Non-reciprocal interactions arising from broken action-reaction symmetry are hypothesized to be prevalent in natural systems, which calls for experimental platforms capable of modulating these interactions. To address this, we leverage on the new discovery of living chiral crystals composed of starfish embryos, and investigate mixtures of starfish embryos at two distinct developmental stages. Our observations reveal that the binary mixture undergoes a dynamic phase marked by spontaneous collective chiral movement. Moreover, extended observations over several hours indicate a transition from this dynamic state to a static crystalline phase. To elucidate these phenomena, we introduce a coarse-grained theoretical framework that accounts for both the emergence of the dynamic phase through imbalanced hydrodynamic force interactions and the exceptional phase transition driven by variations in embryo number densities. Collectively, our research provides a robust experimental platform for exploring emergent phases and phase transitions in active matter systems.