Polymer Reptation: Entangled Chain in a Tube

Long polymer chains in a melt or concentrated solution cannot pass through one another. That single topological constraint — no crossing — is enough to turn ordinary diffusion into de Gennes' reptation: each chain is trapped in a tube-shaped cage traced out by its neighbours, and can only escape by sliding lengthwise along its own contour like a snake. This simulator renders one tagged chain as a bead-spring polymer moving through a field of fixed obstacle points, switches between entangled (tube-confined) and unentangled (free Rouse) dynamics, and tracks the centre-of-mass mean-squared displacement live on a log-log plot so you can watch the diffusion exponent change with chain length, mesh density and time.