We observe prewetting films of 8CB (4'-n-octyl-4-cyanobiphenyl) spreading at room temperature on silicon wafers by ellipsometry and x-ray reflectivity. Ellipsometry indicates the formation of a nondense monolayer spreading in front of a 45-A-thick film. X-ray reflectivity, performed using a ribbon geometry for the liquid crystal (LC) reservoir, allows us to determine the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar monolayer with tilted molecules. We show that the thickness of the bilayer is equal to the smectic periodicity in the bulk material and that the tilt angle of the molecules in contact with the solid surface is close to 60 degrees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using x-ray reflectivity because it induces a modulation of the electron density along the normal to the surface. Furthermore, a study of the ellispometric profile of a drop heated in the nematic phase, where we observe a complete spreading of the LC, shows the complex structuration of the LC close to the solid interface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific interaction between the trilayer and silicon wafer.

The wetting properties of 8CB ( 4(')-n-octyl-4-cyanobiphenyl) on silicon wafers have been studied with scanning polarization force microscopy (SPFM). Layer-by-layer spreading of 8CB droplets is observed. With the help of the surface potential mapping capability of SPFM, we found that the molecular dipole of the first monolayer of 8CB is parallel to the surface. A layer of nearly vertical molecular dimers on top of the monolayer has an associated surface potential of 40 mV, which is attributed to a distortion of the dimer. The dimer distortion propagates to the subsequent smectic bilayers, producing an additional 7 mV potential increase in the second layer, 2 mV on the third, and approximately 1 mV on the fourth.

Chiral smectic A liquid crystal materials of the de Vries type (with molecules tilted relative to the layer normal) exhibit analog field-induced (electroclinic) optic axis rotation accompanied by an increase in birefringence. We identify two such de Vries smectic A* materials and use them to develop and test models for these characteristic electro-optic effects. These materials also exhibit colossal analog field-induced optic axis rotation in the lower temperature smectic C* phase, a consequence of polarization charge stabilization, and of polarization screening of the applied field in the liquid crystal.

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