University of Minnesota
School of Physics & Astronomy

Academic Calendar

Thursday, June 10th 2010
1:30 pm:
Ph. D. Thesis Defense in Physics 157
Speaker: Shun Wang, University of Minnesota
Subject: Investigating the orientational order in liquid crystals
This is the public portion of Shun Wang’s Thesis Defense

My thesis is composed of two projects. The first one is the investigation of a reversed phase sequence, which subsequently leads to the discovery of a novel Smectic-C liquid crystal phase. The 10OHFBBB1M7 (10OHF) compound, shows a reversed phase sequence with SmC_d4 phase at a higher temperature than the SmC phase. This phase sequence is stabilized by moderate doping of 9OTBBB1M7 (C9) or 11OTBBB1M7 (C11). To further study this unique phase sequence, the mixtures of 10OHFBBB1M7 and its homologous have been characterized by optical techniques. In order to perform the resonant X-ray diffraction experiment, we have added C9 and C11 compounds to the binary mixtures and pure 10OHF. In two of the studied mixtures, a new smectic-C liquid crystal phase with six-layer periodicity has been discovered. Upon cooling, the new phase appears between the SmC_alpha phase having a helical structure and the SmC_d4 phase with four-layer periodicity. The SmC_d6 phase shows a distorted clock structure. Three theoretical models have predicted the existence of a six-layer phase. However,our experimental findings are not consistent with the theories.

The second project involves the mixtures of liquid crystals with different shapes. The role of different interactions in stabilizing the antiferroelectric smectic liquid crystal phases have been a long standing questions in the community. By mixing the antiferroelectric smectic liquid crystal with achiral liquid crystal molecules with rod and hockey-stick shapes, we see distinct different behaviors. In the case of the mixtures of chiral smectic liquid crystals with rod-like molecues, all the Smectic-C* variant phases vanish with small amount of doping. However, the hockey-stick molecule is much less destructive compared to the rod-like molecule. This suggest that the antiferroelectric smectic liquid crystal molecules may have a shape closer to a hockey-stick other than a rod.

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