Liquid Crystals in Aqueous Ionic Surfactant Solutions: Interfacial Instabilities & Optical Applications
by Karthik Reddy Peddireddy
Date of Examination:2014-05-12
Date of issue:2014-06-02
Advisor:Dr. Christian Bahr
Referee:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Stephan Herminghaus
Referee:Prof. Dr. Sarah Köster
Persistent Address:
http://dx.doi.org/10.53846/goediss-4535
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Description:Doctoral thesis
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Abstract
English
This thesis presents the properties and applications of various liquid crystal (LC)-aqueous ionic surfactant systems. It begins with the study of micellar solubilization of different LCs in aqueous ionic micellar solutions. The analysis of a few hundred μm-sized LC droplets in either nematic or isotropic state (parent droplets) shows a linear decrease of the droplet size over time. During the entire micellar solubilization process, the material transport across a parent droplet’s surface exists in both directions. Numerous aqueous droplets (daughter droplets) start to appear in the parent droplet due to the transport of surrounding aqueous solution into it. The average size of the aqueous daughter droplets and their number density increase over time. But, no daughter LC droplets form in the surrounding aqueous solution due to the transport of LC molecules from the parent LC droplet into it. However, in special cases, depending on the interfacial tension between the parent droplet and its surrounding aqueous solution, the parent droplet may either expel tiny droplets into the surrounding aqueous medium or transform itself into a filament like structure. Furthermore, the material transfer across a parent droplet’s surface generates convective flows around it. In concentrated aqueous surfactant solutions, the convective flows are strong enough to propel the parent droplet itself. The parent LC droplets become completely solubilized and a clear aqueous solution is produced, provided the weight fraction of the LC in the system is not larger than a few percent. The properties of the equilibrium LC-aqueous systems are investigated through shear and small angle neutron scattering experiments. In the second part, the behavior of the smectic droplets in the aqueous micellar solution is presented. A smectic droplet spontaneously produces smectic filaments at the interface with an aqueous solution, which resemble the myelin figures observed in lyotropic lamellar systems. The three-dimensional structure of the smectic-A filaments is studied with the polarizing optical microscopy and the fluorescence confocal polarizing microscopy. The results show that the LC molecules are oriented perpendicular to the surface of the filament and the layers are wrapped around a central disclination line in a concentric cylindrical fashion. Further, the study on the growth and the stability of the smectic-A filaments shows that the freely floating smectic-A filaments are only transient structures and they either transform into discs or break into small smectic droplets. However, it is possible to stabilize the filaments at least for several months, either by confining them in square capillaries or by photo polymerization. In the last part, the optical applications of the smectic filaments are discussed. In the case of the smectic-A filaments, light guiding through the filaments and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the filaments are presented. Moreover, it is also shown that smectic-C* filaments act as photonic band-gap waveguides.
Keywords: liquid crystals; surfactants; liquid crystal emulsions; micellar solubilization; artificial swimmers; filaments; myelin figures; liquid crystal ropes; waveguiding; lasing