Dynamical and conformational behavior of a polymer in a crowded solution

We investigate the structure and dynamics of a polymer in a fluid containing mobile spherical colloidal crowders of radius R. We compare and contrast the behavior with Langevin dynamics (LD) and lattice Boltzmann molecular dynamics (LBMD), the latter incorporating long-range hydrodynamic interactions. Both the colloid size relative to the monomer radius r and the volume fraction ϕ are varied to determine how crowding modifies polymer behavior. Increasing volume fraction induces polymer compaction, with the mechanism strongly dependent on the size ratio R/r. Small colloids primarily modify the short-wavelength polymer conformation, causing self-avoiding-walk-like behavior to persist to shorter length scales, whereas large colloids reduce the effective long-wavelength Flory exponent, indicating a degraded solvent quality consistent with a confinement-blob picture. Polymer diffusion exhibits distinct behavior in LD and LBMD. In LD, diffusion decreases rapidly and depends strongly on R/r; a phenomenological scaling involving ln(1 + R/r) captures this size dependence, and additional scaling with Rg reduces scatter, indicating polymer-scale correlations induced by crowding. In contrast, LBMD diffusion follows an effective-medium-like exponential dependence on concentration, governed by hydrodynamic coupling. Rouse-mode analysis identifies three regimes: scaling breakdown at low volume fraction, Zimm-like behavior at intermediate density in both LD and LBMD, and, at high density, hydrodynamic screening in LBMD with confinement-dominated dynamics in LD.