Engineering Clustering, Isotropy, and Flocking in Microswimmer Systems

Hydrodynamically interacting microswimmers can show interesting collective phenomenon such as self-organization, flocking, pattern formation etc. Our recent work (Thambi & Uspal, Phys. Rev. Fluids, in press), suggests that two simple design parameters, the asymmetry of interfacial actuation (δ) and the particle aspect ratio (Γ), can shape the collective motion of microswimmers by modulating active hydrodynamic interactions. Here we map the δ-Γ plane, and study the emergent collective behaviour. We identify mainly three regimes in the δ-Γ plane: (i) clustering, where n-particle dynamical fixed points become stable and pull swimmers together to form clusters; (ii) isotropic, where positions and orientations of swimmers remain random; and (iii) flocking, where swimmers align and move as a group. Within the clustering regime, we observe a critical phase transition to a dynamically arrested, absorbing state that exhibits Class I hyperuniformity, and we quantify its critical exponents and the universality class. A field-theory analysis yields an analytic boundary where the isotropic state loses stability, predicting the onset of flocking. Together, these results show how microscopic design through δ and Γ controls macroscopic behaviour. The phase map offers simple rules for selecting swimmer properties to either promote uniform dispersion, trigger flocking, or engineer stable clusters.