すべり振動によるネマチック液晶の光学的効果と誘電的効果

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This paper deals with the optical transmission characteristics and dielectric effect of nematic liquid crystal (MBBA) films when a vibratory shear wave is applied. The experimental arrangement for the measurements is illustrated in Fig. 1. The relative change of the transmitted light intensity as a function of the shear wave displacement is shwon in Fig. 2-4, for the case when (A) H_e-N_e laser is used as the light source with a cross nicol system, (B) unpolarized light from a H_e-N_e laser, (C) incandescent light with a cross nicol system and (D) unpolarized incandescent light. It is found in these optical characteristics that there exists a threshold displacement for vibratory excitation above which molecular orientation occurs. The effect is similar to that observed for the application of magnetic or electric fields. These values are almost constant, independent of the film thickness (66μm, 105μm and 153μm). A theory is developed based on the continuum elasticity theory, with which the experimental results can be explained with respect to this threshold-thickness relation. The theory also states that f・x should be constant, where f and x are the vibratory frequency and displacement, respectively. However, this relation did not hold for the experimental results, which is probably due to the fact that the liquid crystal is not simply of a linearly elastic fluid as assumed in the theory. For a further increase in the vibratory displacement, a complex transmission characteristic due to a retardation effect is observed. A parallel domain structure appears in the liquid crystal film, in which its space frequency increases with displacement (Fig. 9(a) and (b)). Then celluar motion is observed and finally DSM is acheived (Fig. 9(c) and (d)). Once DSM is generated, the transmitted optical intensity decreases linearly with the logarithm of the displacement. The intensity decay rate is rather low for the sample of 153μm thickness (Fig. 10(a)), while the intensity declines sharply for the other cases. In this range, vortices often appear in the liquid crystal film (Fig. 10(d)), which produce an abnormal effect on the transmitted light intensity. For comparision, the optical effect by the application of AC and DC electric fields is also examined (Figs. 5-7). The optical effect of the vibration is found to be similar to the latter case, though the domain structure as well as the DSM are not exactly the same (Fig. 11). It is also found that the dielectric constant and the tan δ of the liquid crystal change when a vibration is applied. Their relative change as a function of the vibratory displacement is shown in Fig. 8. The dielectric constant increases with displacement above the threshold, while tan δ decreases. This can be explained well by the process of rotation of the molecular orientation, together with the negative dielectric anisotropy of MBBA. The continuing increase of the dielectric constant is also observed over the range of the domain structure, until it is saturated with the generation of DSM, while tan δ has a minimum value.
社団法人日本音響学会の論文
1975-02-01

すべり振動によるネマチック液晶の光学的効果と誘電的効果

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