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 i nvestigated with a surface force apparatus (SFA). The measured interac tion between the two SFA surfaces immersed in the mesophase is periodi c, oscillatory, decaying, and discontinuous. A model is developed to a ccount for the specific confining geometry of the SFA. It is based on an array of concentric dislocation loops that allows a homeotropic ali gnment of the layers in a space of variable thickness. The elastic res ponse of the array at any strain accounts for the periodicity and the oscillatory nature of the force profile and also describes the parabol ic shape of the oscillations. The layer compressibility modulus can be extracted from the data with a good accuracy. However the elastic ana lysis fails to explain the attractive background on which the oscillat ions are superimposed at small separations. Two possible origins for t he attraction are discussed, but unfortunately a quantitative comparis on does not allow a definitive conclusion on the actual cause.