A simple method to determine the pretilt angle of nematic guest-host liquid crystals

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Abstract
The phase retardation-extremum method offers a fast and simple way to measure the pretilt angle of nematic liquid crystal (NLC) cells. Due to optical absorption of the dissolved dyes, such a method is not suitable to measure the pretilt angle of a guest-host (GH) NLC cell. We have obtained a simple and fast method to determine the pretilt angle of a GH NLC cell from the incident angle corresponding to the minimum transmission intensity of a polarized light passing through the GH cell. Using this simple method, we have measured the pretilt angles of GH NLC cells with different dye concentrations, and compared them with the pretilt angles of the corresponding host NLCs measured by the retardation-extremum method. Excellent agreement between the two methods was observed with a dye concentration of not more than 1%. Within our experimental errors, dye concentration of up to 5.8% (by weight) seems not to change the pretilt angle of the GH NLC from that of the corresponding host NLC.