  Within the framework of a discrete Gaussian model, we present analytical
results for the interaction induced by a lamellar phase between small embedded
colloids. We consider the two limits of particles strongly adherent to the
adjacent membranes and of particles impenetrable to the membranes. Our approach
takes into account the finite size of the colloids, the discrete nature of the
layers, and includes the Casimir-like effect of fluctuations, which is very
important for dilute phases. Monte Carlo simulations of the statistical
behavior of the membrane-interacting colloids account semi-quantitatively,
without any adjustable parameters, for the experimental data measured on silica
nanospheres inserted within lyotropic smectics. We predict the existence of
finite-size and densely packed particle aggregates originating from the
competition between attractive interactions between colloids in the same layer
and repulsion between colloids one layer apart.
