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Measurement of the interactions between two ordering surfaces under symmetric and asymmetric boundary conditions.

With a surface forces apparatus we have measured the interaction between two surfaces immersed in the isotropic and the nematic phases of a lyotropic solution near its lamellar phase. A smectic ordering shows up near walls, giving a specific oscillatory force profile that is shown to be the sum of two contributions. The oscillations are the elastic response of the stack mechanically constrained by the confinement. The shape of the base line supporting the oscillations derives from the distribution of the smectic ordering between the two walls. In the case of fixed symmetric boundary conditions, the background is always attractive. However, under fixed asymmetric boundary conditions, the background turns out to be repulsive at short separations and becomes even repulsive at every separation when one of the two surfaces does not induce any order. \textcopyright{} 1996 The American Physical Society.