Optical properties – observing defects
A further property of nematic liquid crystals when viewed using polarised light microscopy is the appearance of schlieren brushes; these are the distinctive dark cross shapes that appear throughout the image below.
The centre of a cross is in fact a disclination in the liquid crystal, the surrounding dark regions occurring where the orientation of the crystals is parallel to either the polariser or analyser.
In order to work out which type of cross corresponds to which type of disclination we therefore need to think about the orientation of the local directors relative to a given set of crossed polars. This is shown for four different disclinations below:
A further property of disclinations in nematic liquid crystals is that when one of the polarisers is rotated the schlieren brushes appear to rotate themselves; furthermore disclinations with opposite signs can be differentiated by the fact that their brushes appear to rotate in opposite directions. This is demonstrated in the video below: