Hydrodynamic Trapping of Tracer Particles Induced by Spatially Correlated Noise in Thermal Equilibrium

Abstract: We investigate the fluctuating Stokes equation in the presence of spatially correlated noise -- characterized by a single length scale -- while preserving thermal equilibrium through the corresponding fluctuation-dissipation relation. Numerical simulations reveal that a tracer particle's mean-squared displacement (MSD) depends nonmonotonically on the ratio $\ell/r$ between the correlation length $\ell$ and the particle radius $r$: the MSD decreases as $\ell$ approaches $r$, reaches a minimum near $\ell\simeq r$, and rises again for $\ell > r$. The initial suppression stems from transient trapping, where sluggish momentum diffusion at short wavelengths confines the particle. When $\ell>r$, the particle hops between traps and eventually recovers normal diffusion ($\text{MSD}\propto t$) consistent with the fact that momentum diffusion crosses over to the standard Laplacian at long wavelengths. We further illustrate that spatial correlation introduces nonlocal momentum diffusion in the hydrodynamic equations, reminiscent of the slow dynamics in glassy and other disordered systems.