Crystallization and Motility-Induced Phase Separation in the Flow of Active Brownian Particles Around an Obstacle

The interaction between fluid flow and obstacles is a fundamental problem in fluid dynamics, exemplified by the Kármán vortex street and its Reynolds number-dependent instabilities. Although extensively studied through both Navier-Stokes simulations and classical molecular dynamics, the behavior of active matter under similar conditions remains largely unexplored. Using large-scale molecular dynamics simulations, we investigated active Brownian particles (ABPs) flowing past a fixed obstacle. We discover novel emergent patterns, including unexpected crystallization, arising from the interaction between hydrodynamic forces and active particle dynamics. The phenomena are governed by both Reynolds number and self-propulsion velocity, with crystallization driven by two distinct mechanisms: compression-induced ordering upstream of the obstacle and motility-induced phase separation in specific flow regions.