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

The interactions between two macroscopic walls immersed in an isotropi c symmetric sponge phase (L3) at different volume fractions, PHI, were studied with a surface force apparatus. At temperatures well below th e lamellar (L(alpha))/L3 bulk transition, the force-distance profile i s weakly oscillatory with a period that is twice the correlation lengt h (xi) of the L3 phase measured from SAXS. The oscillations are superi mposed on an exponentially attractive background with an order correla tion length of 2-4 cell sizes of the sponge structure. When the temper ature is raised, a first-order phase transition to a lamellar structur e can be induced for separations below a threshold. Another oscillator y interaction results which has a period that is twice the reticular d istance for an L(alpha) phase of similar PHI. In addition, the thickne ss of the induced lamellar film increases (reversibly) with temperatur e. The compressibility modulus extracted from the oscillatory interact ion is consistent with that expected for lamellae stabilized by undula tion forces.