Swollen lamellar phases between two solid walls : undulation forces and generation of defects

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Abstract
Forces between curved mica surfaces across a brine swollen lamellar phase (CTAB/hexanol/Brine) have been measured. The bilayers oriented spontaneously parallel to the mica surfaces. The force-distance curves show regimes where brine is continuously expelled from between the lamellae, and regimes in which bilayers are ejected over a few seconds. We have investigated the effect on the forces of moving in the phase diagram at constant bulk repeat distance, as well as along the dilution line. We found that the force necessary to squeeze out bilayers becomes smaller as the system approaches the transition to the bicontinuous phase (Lα/L3), i.e. on increasing the hexanol content. Dynamic effects are observed and may be explained in terms of hydrodynamic coupling of the bilayers with the solvent. They may produce surprising behaviors : on decompressing the phase, lamellae are systematically ejected. The equilibrium state of the system is discussed. The data is consistent with the 1/d2 profile calculated by Helfrich for the undulation forces [1]. Least square fits of the data to 1/d2 gives bilayer curvature modulus values of the order of 5.5 ± 3 kB T