Phase transitions in liquid crystal + aerosil gels.
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Phase transitions in liquid crystal + aerosil gels.

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Published .
Written in English

Subjects:

  • Physics Theses.

Book details:

Edition Notes

ContributionsBirgeneau, Robert J. (supervisor).
The Physical Object
Paginationxv, 166 p. :
Number of Pages166
ID Numbers
Open LibraryOL20158072M

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Abstract. We have studied the effects of quenched random disorder created by dispersed aerosil nanoparticle gels on the nematic to smectic- A (N- SmA) and smectic- A to reentrant nematic (SmA -RN) phase transitions of thermotropic liquid-crystal mixtures of hexyloxycyanobiphenyl (6OCB) and octyloxycyanobiphenyl (8OCB). High-resolution ac calorimetry has been used to study the nematic to smectic-A (N-SmA) phase transition in the liquid crystal octylcyanobiphenyl (8CB) confined in aligned colloidal aerosil gels. Low density thixotropic gels formed by dispersing aerosil nanoparticles in a liquid crystal LC+gel, although origi-nally explored for potential applications 4–6, provide ex-cellent systems for the study of quenched disorder 7–9. The focus of this work is on the nematic N –smectic-A SmA transition, and the N-SmA critical behavior of Cited by: 6. The limiting 8̅ S5 smectic-A correlation length follows a power-law dependence on the aerosil density in quantitative agreement with the limiting lengths measured previously in other smectic-A liquid crystal gels. The smectic-A to smectic-C liquid crystalline phase transition is altered fundamentally by the presence of the aerosil gel.

The present work focusses on a different liquid crystal— 8OCB—having dispersed in it the same type of aerosil over a comparable range of silica densities as the well-studied 8CB1aerosil system. This liquid crystal has several impor-tant differences from the closely related 8CB. The liquid crystal 8OCB has stronger smectic and nematic. nematic to smectic-A (N–SmA) phase transition for the study of QRD. There are three silica systems that are the focus of this paper: a colloidal gel of aerosil particles dis-persed in the liquid crystal (denoted as 8CB + aerosil), a fused silica gel of aerogels immersed in 8CB (denoted as 8CB + aerogel), and a randomly interconnected con-.   It is known that the liquid crystalline smectic-A phase has geometric defects, called focal conic domains, which can be used as gradient-index microlenses. Cholesteric (chiral nematic) phases also have topological defects with a central symmetry and a singularity at their center. We explore a weakly chiral system in which both types of defects can be present in the same material at different. to characterize the effects of a nano-colloidal dispersions of aerosils in the phase transitions of several liquid crystals. The aerosil (SIL)ismadeof70˚Adiam-eter SiO2 particles coated with hydroxyl (-OH) groups. The coating allows the SIL particles to hydrogen-bond together, to form a very low density gel in an organic solvent.

High-resolution ac and nonadiabatic scanning calorimetric as well as optical turbidity studies have been carried out on the isotropic to nematic (I-N) phase transition of the liquid crystal 4'-trans-butylcyanotrans-heptyl-1,1'-bicyclohexane (CCN47) containing a dispersed colloidal gel of aerosil particles. Four CCN47+aerosil samples having silica densities of , , , and 0. moments and no cholesteric phase. The fundamental question of the origin of the phases is still not resolved (Lagerwall et al., ). x4 covers studies of the phase transitions of liquid crystals confined in gels. Early experiments were carried out for the N–SmA transition . The term phase transition (or phase change) is most commonly used to describe transitions between solid, liquid, and gaseous states of matter, as well as plasma in rare cases. A phase of a thermodynamic system and the states of matter have uniform physical a phase transition of a given medium, certain properties of the medium change, often discontinuously, as a result of the.   This indicates the existence of tricritical point at the I-SmA phase transition line. Ramazanoglu et al. studied the I-SmA phase transition in liquid crystal-aerosil gels. They showed that the liquid crystal-aerosil gel retains only short-range SmA order deep into the SmA phase .