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We have investigated the entropic interactions between lamellar membranes and spherical colloidal particles using a small-angle neutron scattering (SANS) technique. By adding colloidal particles between lamellar sheets, the first lamellar peaks in SANS profiles become intense and the second and higher order Bragg peaks begin to appear, indicating that the membrane fluctuations are suppressed by the colloidal particles. We estimate the interlamellar interaction potential in the presence of the colloidal particles from the layer compressibility obtained by the SANS profile analysis and propose a phenomenological free energy model based on the restriction of membrane fluctuations. By further addition of the colloidal particles, the lamellar membranes transform to prolate micelles. In order to release the strong frustration due to the restriction of membrane fluctuations, the surfactant assemblies change the morphology from the two dimensional sheets to the one dimensional prolate micelles. [ABSTRACT FROM AUTHOR]