LIQUID CRYSTAL SURFACES

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Abstract
The use of specular reflection of X-rays to study the structure of the liquid-vapor and liquid crystal-vapor interfaces along the direction normal to the surface is described. If RF(θ) is the theoretical Fresnel reflection law for x-rays incident on an ideal flat surface at an angle θ and R(θ) is the measured reflectivity from the true surface, the ratio R(θ)/RF(θ) is a measure of the electron density along the surface normal : i.e. [MATH] where ρ∞ is the electron density far from the surface, d/dz is the gradient of the x-y average of the electron density along the surface normal and Qz=(4π/λ)sin(θ). For the surface of simple liquids like H2O, the principal observation is that the electron density profile from the bulk to the vapor is dominated by thermally excited capillary waves. For both the isotropic phase and the nematic liquid crystal phase, structure is observed in R(θ) due to surface induced local smectic order. Depending on the material, layering transitions at the surface of isotropic phases are observed to be either sharp or gradual functions of temperature. The possible interpretation of this in terms of wetting and roughening effects will be discussed. On approaching a 2nd order nematic to smectic A transition, the number of layers diverge in a manner that can be described as critical absorption. Other examples to be discussed include the surface structure of a lytropic liquid crystal and the appearance of surface layers of the tilted hexatic smectic I phase on freely suspended films of a smectic C phase

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