  It has been recently shown that a colloidal monolayer, e.g., formed at a
fluid interface or by means of a suitable confining potential, exhibits
anomalous collective diffusion. This is a consequence of the hydrodynamic
interactions mediated by the three-dimensional (3D) ambient fluid when the
particles are confined to reside on a two-dimensional (2D) manifold. We study
theoretically and with numerical simulations the crossover from normal to
anomalous diffusion as the particles are, in real systems, confined by a 3D
external potential and thus have the possibility to fluctuate out of the 2D
manifold, thus forming actually a quasi-monolayer.
