MEASUREMENT OF THE INTERACTIONS BETWEEN 2 ORDERING SURFACES UNDER SYMMETRICAL AND ASYMMETRIC BOUNDARY-CONDITIONS

With a surface forces apparatus we have measured the interaction betwe en two surfaces immersed in the isotropic and the nematic phases of a lyotropic solution near its lamellar phase. A smectic ordering shows u p near walls, giving a specific oscillatory force profile that is show n to be the sum of two contributions. The oscillations are the elastic response of the stack mechanically constrained by the confinement. Th e shape of the base line supporting the oscillations derives from the distribution of the smectic ordering between the two walls. In the cas e of fixed symmetric boundary conditions, the background is always att ractive. However, under fixed asymmetric boundary conditions, the back ground turns out to be repulsive at short separations and becomes even repulsive at every separation when one of the two surfaces does not i nduce any order.