Determination of Surface Anchoring of Nematic Liquid Crystals from Optical Response Measurements.

A sensitive new method has been developed to determine the azimuthal anchoring strength of nematic liquid crystals. The method consists of comparing measured values of the transmittance of a cell as a function of applied magnetic field with the results of calculations. The director configuration in the cell is determined by using a numerical relaxation method to minimize the free energy. The transmittance is calculated using a newly developed adaptive numerical method with local extrapolation to solve Maxwell's equations. The method makes no assumptions about the form of the anchoring energy, and requires only the knowledge of the refractive indices rm n_{e}, n_{o}, and of the elastic constant K_2. The method was used to determine the azimuthal anchoring strength of the liquid crystal 5CB on a rubbed polyimide layer as a function of rubbing strength. It was found that the azimuthal anchoring strength increases with increasing rubbing strength. Deviations from the Rapini-Papoular form were found for surface director angles greater than 0.4 radian for all rubbing strengths studied. The pretilt angle was also determined, and it was found that the pretilt angle also increases with increasing rubbing strength for the polyimide material used. Pretilt angles versus the number of layers of Langmuir-Blodgett films were also determined. The proposed method is also applicable to liquid crystal cells with deformations other than twist.