Direct observations of dislocations in thermotropic smectics using freeze-fracture replication

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Abstract
Modern rapid-freezing methods followed by freeze-fracture replication techniques are ideally suited to allow the direct visualization of the three-dimensional structure and defects of thermotropic smectic and cholesteric liquid crystalline phases with resolution approaching molecular dimensions. Cholesterol nonanoate was quench frozen from near the smectic-cholesteric transition to reveal extensive, well defined smectic layers distorted by a high density of screw and edge dislocations. The screw dislocations typically had Burgers vectors of a single layer and commonly organized as twist walls. This is suggestive that the twist present in the cholesteric phase induces a residual twist in the smectic phase incompatible with perfect layering. Screw-edge dislocation loops were also commonly observed, also organized into twist walls. These twist walls have been suggested as being responsible for the breakdown of smectic ordering leading to the cholesteric phase