The electrical untwisting of a homeotropically aligned smectic C* liquid crystal

This article is concerned with the application of an electric field along the planar layers of a finite sample of ferroelectric smectic C^* liquid crystal with strong smectic anchoring at the boundaries. It will be seen by use of continuum theory that above a certain critical field strength it is energetically favourable for the helix to unwind such that the c-director becomes aligned perpendicular to the applied field throughout the sample. This problem is similar to that of untwisting the helical structure of a cholesteric liquid crystal [1], [2]; the differences lie in the symmetry and polarization of the smectic C^* phase. Comparison of this work with that detailed in [2] will show that these two factors lead to significant differences in the analysis. The effects of sample width on the minimum field strength required for this untwisting are also examined and an expression is gained for the critical field strength in a large sample. In addition, director reorientation due to a reversal of the applied field is discussed.