Optical study of the molecular alignment in a nematic liquid crystal in an oblique magnetic field

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Abstract
The light intensity I transmitted through a homeotropically oriented nematic liquid crystal slab held between crossed polarizers, as a function of the angle θH of an applied magnetic field, is calculated for oblique light incidence. The presence of singular points in the I vs. θ H curve which are related to the elastic constants of the liquid crystal is demonstrated One of these points, found here for the first time, for small angles of incidence depends only on the ratio K33/Χ a, and allows a direct determination of this ratio. The comparison of the calculated curve I vs. θH with the experimental points taken in a sufficiently wide range of θH values, using K 11/χa, K33/Χa and An = ne— no as best fit parameters, allows a very precise determination of all these constants. The method turns out to be much more sensitive than the usual ones, mostly based on capacitance measurements and it is thus very suitable for determining the behaviour of the elastic constants when the temperature is changed, particularly near the transition point