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Séminaire de Yuri Reznikov

Measurements of a Giant Cholesteric Pitch in Weakly Chiral Nematic Liquid Crystals


Measurements of a Giant Cholesteric Pitch in Weakly Chiral Nematic Liquid Crystals

Yuriy Reznikov

Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine

A method for detection of extremely low chirality and precise measurements of an extra large cholesteric liquid-crystal (LC) pitch is described. The idea is based on the orientation of the disclination line in a LC q-cell composed with one substrate with linear in plane alignment and the opposing substrate with circular alignment. The unique sensitivity of the disclination orientation to the value of the cholesteric LC pitch and thickness of the cell enables visualization of thickness variations in the LC cell with the accuracy of better than 10 nm. The measurement of the azimuth deviation of the disclination line from the direction parallel to the unidirectional substrate alignment determines the cholesteric pitch with values in the centimeters range. The method provides an effective means to study the effects of ultra-low chirality in liquid crystal compositions. In this study we found crossover between two types of concentration dependence of the chiral pitch. This crossover is to the point where a continuous spiral ordering in a nematic matrix forms.