Abstract
We demonstrated previously that the temperature of a sandwich-type liquid crystal cell with unignorable electrode resistivity could be electrically increased as a result of dielectric heating. In this study, we take advantage of such an electro-thermal effect and report on a unique electric-field approach to the formation of uniform lying helix (ULH) texture in a cholesteric liquid crystal (CLC) cell. The technique entails a hybrid voltage pulse at frequencies f1 and, subsequently, f2, which are higher and lower than the onset frequency for the induction of dielectric heating, respectively. When the cell is electrically sustained in the isotropic phase by the voltage pulse of V = 35 Vrms at f1 = 55 kHz or in the homeotropic state with the enhanced ionic effect at V = 30 Vrms and f1 = 55 kHz, our results indicate that switching of the voltage frequency from f1 to f2 enables the succeeding formation of well-aligned ULH during either the isotropic-to-CLC phase transition at f2 = 1 kHz or by the electrohydrodynamic effect at f2 = 30 Hz. For practical use, the aligning technique proposed for the first time in this study is more applicable than existing alternatives in that the obtained ULH is adoptable to CLCs with positive dielectric anisotropy in a simple cell geometry where complicated surface pretreatment is not required. Moreover, it is electrically switchable to other CLC textures such as Grandjean planar and focal conic states without the need of a temperature controller for the phase transition, the use of ion-rich LC materials, or mechanical shearing for textural transition.
Highlights
Cholesteric liquid crystals (CLCs) constitute a class of soft photonic crystal, with a self-assembled, helically molecular configuration in one-dimension
When the cell is electrically sustained in the isotropic phase by the voltage pulse of V = 35 Vrms at f 1 = 55 kHz or in the homeotropic state with the enhanced ionic effect at V = 30 Vrms and f 1 = 55 kHz, our results indicate that switching of the voltage frequency from f 1 to f 2 enables the succeeding formation of well-aligned uniform lying helix (ULH) during either the isotropic-to-CLC phase transition at f 2 = 1 kHz or by the electrohydrodynamic effect at f 2 = 30 Hz
No matter how the ambient temperature is, the cell temperature can be increased to an appropriate value by optimizing the frequency and the amplitude of the leading component of the hybrid voltage pulse
Summary
Cholesteric liquid crystals (CLCs) constitute a class of soft photonic crystal, with a self-assembled, helically molecular configuration in one-dimension. Without undergoing the temperature-cooling process, voltage-generated ULH alignment has been obtained in planar-aligned CLC cells by providing an extra mechanical force to induce shear flow in the meantime [14,15,16], using ion-rich positive nematic host to permit electrohydrodynamic flow by low-frequency voltages [17,18], controlling voltage conditions precisely to induce the nematic-to-CLC structural transition [19], optimizing the pretilt angle as well as the anchoring energy [20], or by designing a tri-electrode configuration [21,22]. As confirmed experimentally and theoretically, this is caused by the pseudo-dielectric relaxation from the non-ideal cell geometry with finite conductivity in indium-tin-oxide (ITO) electrodes rather than the intrinsic molecular rotation Based on this electro-thermal effect, we propose, in this work, to form ULH structures by applying a hybrid pulse with a fixed amplitude and frequencies f 1 and f 2. The processes of forming ULH alignment by the treatment of designated hybrid pulse are manifested
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