Spontaneous reorientation from planar to homeotropic alignment in dual-frequency mixture doped with chiral dopant

• Published in: Phase transitions Vol. 87; no. 10-11; pp. 1138 - 1147
• Main Authors: Perkowski, Paweł, Mrukiewicz, Mateusz, Chojnowska, Olga, Piecek, Wiktor, Dąbrowski, Roman
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis 02.11.2014; Taylor & Francis Ltd
• Subjects: Cooling; Dielectrics; Dopants; dual frequency nematic; Heating; homeotropic and homogenous alignment; Orientation; Phase transitions; Planar structures; Reflection; selective reflection; Shutters; Spectra; Spectrum analysis; Temperature
• ISSN: 0141-1594; 1029-0338
• DOI: 10.1080/01411594.2014.953516

We investigate a dual-frequency (DF) nematic mixture by means of the dielectric spectroscopy. A chiral dopant has been added to the DF matrix. A pure DF matrix as well as three samples with increasing concentration of chiral dopant (DF*) have been prepared. Investigated samples have shown a selective reflection band, according to a helical pitch induced in proportion to the chiral dopant amount. Such system promises a great application potential as a selective light reflecting shutter or spectral filter due to nearly symmetrical switching times and defectless structures. When we put the non-doped DF mixture into homogenous (HG) oriented cell we found that planar orientation is permanent on both cooling and heating cycles. In the case of DF* chiral mixtures, the behavior changes substantially. One can observe that the oriented virgin planar structure of DF* within the HG cell starts to change its orientation to homeotropic one at different temperature. When heating is continued the homeotropic orientation in the HG cell is preserved up to the clearing temperature. Upon a subsequent cooling from the isotropic state, the homeotropic orientation is preserved within the whole mesogenic state. The effect of induction of the homeotropic state observed within HG cells is analyzed and discussed.