  Under partial confinement, the motion of colloidal particles is restricted to
a plane but their dynamics is influenced by hydrodynamic interactions mediated
by the unconfined, three--dimensional flow of the embedding fluid. We
demonstrate that this dimensionality mismatch induces a characteristic
divergence in the two--dimensional collective diffusion coefficient of the
colloidal monolayer. This result, independent from the specific interparticle
forces in the monolayer, is solely due to the kinematical constraint on the
colloidal particles, and it is different from the known divergence of transport
coefficients in purely two--dimensional fluids.
