Active matter systems comprise self-propelled particles that move on a substrate while leaving chemical trails that influence other particles through chemotaxis (e.g., slime-depositing bacteria). Orientational chemotaxis manifests as a torque that steers the particle toward the chemical gradient. As each particle is coupled to its own trail, the dynamics exhibits an instability: when the particle gently diffuses, it abruptly transitions to trajectories with a radius of curvature comparable to its own size, becoming apparently trapped. We argue that this trajectory instability occurs for any chemotactic coupling strength, either directly above some threshold or via an activated process below it.

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• Phase transitions in biological systems