LIQUID CRYSTAL SURFACES
Le Journal de Physique Colloques
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(O) is the theoretical Fresnel reflection law for x-rays incident on an ideal flat surface a t an angle 0 and R(8) is the measured reflectivity from the true surface, the ratio R(B)/RF(B) is a measure of the electron density along the surface normal: i.e. where p, is the electron density far from the surface, a/Jz is
... he surface, a/Jz is the gradient of the x-y average of the electron density along the surface normal and QZ=(4dX)sin(8). For the surface of simple liquids like H20, 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(8) due to surface induced local smectic order. Depending on the material, layering transitions a t 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. Introduction: The unique character of liquid crystalline phases has facilitated a large number of theoretical and experimental studies on the statistical physics of condensed matter.(l,a) One of the earliest observations from this effort was that the molecular order near surfaces can often be strikingly different from the order in the bulk far from the surface~.(3-~) The purpose of this paper is to review those surface effects that have been studied by x-ray scattering Article published online by EDP Sciences and available at http://dx.