Casimir Effect in Fluids above the Isotropic-Lamellar Transition

We study fluctuation-induced interaction in confined fluids above the isotropic-lamellar transition. At an ideally continuous transition, the disjoining pressure has the asymptotic form Π(d→∞)≈-Ck_BTq₀²/d, where d is the interwall distance, q₀ is the wave number of the scattering peak, and C = 1/4π in the strong anchoring limit. The long-rangedness is enhanced due to continuous distribution of soft modes in the q space. An unconventionally strong Casimir force with a range of several lamella thicknesses is realistic above the transition. We also find an oscillatory force profile near a surface-induced transition.