Colloidal adsorption at fluid interfaces: Regime crossover from fast relaxation to physical aging

The adsorption of a colloidal particle at a fluid interface is studied theoretically and numerically, documenting distinctly different relaxation regimes. The adsorption of a perfectly smooth particle is characterized by a fast exponential relaxation to thermodynamic equilibrium where the interfacial free energy reaches the global minimum. The short relaxation time is given by the ratio of viscous damping to capillary forces. Physical and/or chemical heterogeneities, however, can result in multiple minima of the free energy giving rise to metastability. In the presence of metastable states we observe a crossover to a slow logarithmic relaxation reminiscent of physical aging in glassy systems; sufficiently close to equilibrium the slow relaxation becomes exponential. The long relaxation time is determined by the Kramers escape rate from metastable states. Derived analytical expressions yield quantitative agreement with molecular dynamics simulations and recent experimental observations. This work provides new insights on the adsorption of colloidal particles with surface microstructure.