MEASUREMENT OF THE LAYER COMPRESSIBILITY MODULUS OF A LAMELLAR MESOPHASE WITH A SURFACE FORCES APPARATUS

The lamellar phase of a lyotropic solution in a confined geometry is investigated with a surface force apparatus (SFA). The measured interaction between the two SFA surfaces immersed in the mesophase is periodic, oscillatory, decaying, and discontinuous. A model is developed to account for the specific confining geometry of the SFA. It is based on an array of concentric dislocation loops that allows a homeotropic alignment of the layers in a space of variable thickness. The elastic response of the array at any strain accounts for the periodicity and the oscillatory nature of the force profile and also describes the parabolic shape of the oscillations. The layer compressibility modulus can be extracted from the data with a good accuracy. However the elastic analysis fails to explain the attractive background on which the oscillations are superimposed at small separations. Two possible origins for the attraction are discussed, but unfortunately a quantitative comparison does not allow a definitive conclusion on the actual cause.