MEASUREMENT OF THE INTERACTIONS BETWEEN MACROSCOPIC SURFACES INDUCING A 1ST-ORDER PHASE-TRANSITION - THE SPONGE LAMELLAR TRANSFORMATION

The interactions between two macroscopic walls immersed in an isotropic symmetric sponge phase (L3) at different volume fractions, PHI, were studied with a surface force apparatus. At temperatures well below the lamellar (L(alpha))/L3 bulk transition, the force-distance profile is weakly oscillatory with a period that is twice the correlation length (xi) of the L3 phase measured from SAXS. The oscillations are superimposed on an exponentially attractive background with an order correlation length of 2-4 cell sizes of the sponge structure. When the temperature is raised, a first-order phase transition to a lamellar structure can be induced for separations below a threshold. Another oscillatory interaction results which has a period that is twice the reticular distance for an L(alpha) phase of similar PHI. In addition, the thickness of the induced lamellar film increases (reversibly) with temperature. The compressibility modulus extracted from the oscillatory interaction is consistent with that expected for lamellae stabilized by undulation forces.