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Modern thin-film-transistor-driven twisted nematic (TN) products on the market require thin cell gaps below the value to satisfy the condition of the Gooch-Tarry first minimum in transmission for fast response times and wide viewing angles. For the first time to our knowledge, we have developed a simple experimental method based on a new configuration of out-of-plane cell rotation and analyzed our data by extended Jones matrix method to obtain accurate pretilt angles for such thin TN cells where the well-accepted configuration of the out-of-plane cell rotation method analyzed by Birecki and Kahn [The Physics and Chemistry of Liquid Crystal Devices (1980), p. 115] was insufficient to address.

We have developed a general approach to perform direct measurements of the pretilt angles from 0° to 75° in hybrid-aligned nematic (HAN) liquid-crystal cells whose cell gaps can also be accurately determined with the help of known pretilt angles. In this paper, we have used a Zeeman laser system to measure the angular-dependence phase retardation of the HAN cells and MATLAB mathematical software to carry out theoretical calculations and fit the measured data to derive the pretilt angles. In general, pretilt angles adjacent to opposite substrates of a HAN cell are different. Our measured pretilt angles of the HAN cell were in good agreement with the measured pretilt angles of two accompanying homogenous cells whose alignment methods were the same as applied to opposite substrates of the HAN cell, respectively. The advantage of direct measurement is easily applicable to measure the pretilt angles of aged HAN cells.

We have demonstrated the capabilities to catch the signals of induced electromotive force (EMF). The characteristics of EMF signals are analyzed systematically. Influences of volumes, sizes and travelling velocities of ferrofluids on the magnitudes and wave spans of the signals are presented. In addition, simulations of the induction fields are carried out to provide the physical insights of the induced EMF signals. The signals can be potentially applied as diagnostic characteristics in the measurements of travelling velocities or sizes of moving ferrofluid drops or slugs.

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