Emergence and melting of active vortex crystals

Abstract

Melting of two-dimensional (2D) equilibrium crystals, from superconducting vortex lattices to colloidal structures, is a complex phenomenon characterized by the sequential loss of positional and orientational order. Whereas melting processes in passive systems are typically triggered by external heat injection, active matter crystals can self-assemble and melt into an active fluid by virtue of their intrinsic motility and inherent non-equilibrium stresses. Emergent crystal-like order has been observed in recent experiments on suspensions of swimming sperm cells, fast-moving bacteria, Janus colloids, and in embryonic tissues. Yet, despite recent progress in the theoretical description of such systems, the non-equilibrium physics of active crystallization and melting processes is not well understood. Here, we establish the emergence and investigate the melting of self-organized vortex crystals in 2D active fluids using an experimentally validated generalized Toner-Tu theory. Performing hydrodynamic simulations at an unprecedented scale, we identify two distinctly different melting scenarios: a hysteretic discontinuous phase transition and melting through an intermediary hexatic phase, both of which can be controlled by self-propulsion and active stresses. Our analysis further reveals intriguing transient features of active vortex crystals including meta-stable superstructures of opposite spin polarity. Generally, these results highlight the differences and similarities between crystalline phases in active fluids and their equilibrium counterparts.

Supplementary videos available at https://youtu.be/p_br0pkBEVs https://youtu.be/n-2Yue-WFWc and https://youtu.be/jQZlmNoSzFs

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Cited by in corpus (7)

• Dark photon vortex formation and dynamics
• Intermittency, fluctuations and maximal chaos in an emergent universal state of active turbulence
• Mesoscopic Simulations of Active-Nematics
• Ising-like critical behavior of vortex lattices in an active fluid
• Helical Flow States in Active Nematics
• Lagrangian Manifestation of Anomalies in Active Turbulence
• Flocking turbulence of microswimmers in confined domains