Surface interactions can cause freely suspended thin liquid crystalline filmsto form phases different from the bulk material, but it is not known whathappens at the surface of thick films. Edge dislocations can be used as amarker for the boundary between the bulk center and the reconstructed surface.We use noncontact mode atomic force microscopy to determine the depth of edgedislocations below the surface of freely suspended thick films of4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline B phase.3.0 +/- 0.1 nm high steps are found with a width that varies with temperaturebetween 56 C and 59 C. Using a strain model for the profile of liquidcrystalline layers above an edge dislocation to estimate the depth of thedislocation, we find that the number of reconstructed surface layers increasesfrom 4 to 50 layers as the temperature decreases from 59 C to 56 C. This trendtracks the behavior of the phase boundary in the thickness dependent phasediagram of freely suspended films of 7O.7, suggesting that the surface may bereconstructed into a smectic F region.
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