Abstract
This paper deals with the optical transmission characteristics and the dielectric effects of a nematic liquid crystal (MBBA) film subjected to an ultrasonic field, which is the counterpart of our previous work on the effects subjected to shear vibration. The experimental arrangement for the measurement is illustrated in Fig. 1. The changes of the dielectric constant and tan δ with the sound pressure are almost alike both at 25℃ and 32℃, which are however much smaller at 40℃ (Fig. 3). This is explained by the fact that the dielectric and conductive anisotropy is small near the transition temperature, that is, 43℃. The relative changes of the transmitted light intensity through the cell subjected to the sound pressure are shown in Figs. 5 (400kHz) and 8 (1MHz). A He-Ne laser is used as the light source together with a crossed Nicol system. Dielectric constant and tan δ as a function of the sound pressure are shown in Figs. 6(400kHz) and 9(1MHz). It is found that as in the case of application of shear vibration there exists a threshold sound pressure above which rapid changes not only for the transmitted light intensity but also for the dielectric constant and tan δ follow. They can all be explained by the same mechanism that the molecular alignment rotates under the radiation pressure of the incident sound. The threshold is almost inversely proportional to the film thickness as well as the frequency of the ultrasound, as seen from Fig. 10. Thus these tendency agrees well with that in Nagai's and Helfrich's theories. With increase of the sound pressure above the threshold, the transmitted optical intensity sharply increases and then fluctuation occurs alternately with maxima and minima. The dielectric constant increases while the tan δ decreases. In this range, the color of the liquid crystal film varies alternately from red to blue and then from blue to red with increasing sound pressure (Figs. 6 (a) and (b)). A parallel domain structure takes place with the vibratory displacement applied, however, does not appear in the present case. With further increase of the sound pressure the dielectric constant reaches saturation, and the transmitted optical intensity is decreased due to generation of flow in the film. This phenomenon is not so-called DSM but a flow along the layer (Fig. 6 (c)). On the other hand, the change of tan δ is not so simple as that. The tan δ is sharply increased with temperature, while the dielectric constant remains almost constant (Fig. 3).
