The identification of the sign and strength of disclinations in the schlieren (nucleated domain) texture of the nematic phase, by optical microscopy

The optical texture of the nematic phase, variously known as the schlieren, structure à noyuax or nucleated domain texture, was identified over a century ago as being an array of point singularities. When viewed between crossed polars, patterns of dark brushes radiate from each point nucleus. The sign and strength of each nucleus can be uniquely determined from the changes in the orientation of these brushes when either the sample or the crossed polars are rotated, from two formulae given by Chadrasekhar in 1977. However, these were given with little exemplification, and

have been largely overlooked. Consequently the majority of the discussions given in current literature are either, incomplete and confusing, or in some cases, incorrect. Here we provide a detailed explanation of the textures and their behaviour as viewed with the most commonly used experimental geometry (i.e. with a rotating sample and stationary polars).