Structures and Self-Organization at Liquid Crystal Interfaces: Surface Ordering and Anchoring
von Xunda Feng
Datum der mündl. Prüfung:2013-04-15
Betreuer:Dr. Christian Bahr
Gutachter:Prof. Dr. Christoph F. Schmidt
Gutachter:Prof. Dr. Stephan Herminghaus
EnglischThis dissertation concerns research on the structures and self-organization at liquid crystal interfaces laden with surface-active molecules. Of particular interest are the surface ordering phenomena occurring at interfaces formed between liquid crystals and a second fluid or between liquid crystals and air. A major part of the research has been dedicated to exploring the influence of surface-active molecules on the surface-induced pretransitional order and surface anchoring behavior at interfaces of liquid crystals. First, smectic surface layering transitions and nematic prewetting behavior have been investigated at surfactant-laden interfaces between isotropic liquid crystal phases and a water/glycerol phase whose polarity can be systematically varied by the water-to-glycerol ratio. It is shown that the polarity of the immiscible liquid phase can strongly influence the surface coverage of the surfactant molecules and hence the surface ordering behavior of liquid crystals. Second, surface properties of liquid crystal/air interfaces can be manipulated by the adsorption of a special kind of surface-active molecules, i.e., semifluorinated alkanes. The absorbed layer (also called the Gibbs film) of the semifluorinated alkane C18H37−C12F25 undergoes at the liquid crystal/air interface a structural phase transition from a dilute state at higher temperatures to a dense state at lower temperatures. The structural phase transition causes an anchoring transition from homeotropic to planar of the liquid crystals at the air interfaces. If this transition occurs in the isotropic temperature range of the liquid crystals, it destroys the smectic surface order or changes the orientation of the nematic surface order. This dissertation further describes in situ AFM measurements on the self-organized structures formed in the Gibbs film of C18H37−C12F25 at the liquid crystal/air interface. It is revealed that the Gibbs film of C18H37−C12F25 is not featureless but consists of ordered hexagonal surface micelles which are similar to the Langmuir films formed by semifluorinated alkanes.
Keywords: Gibbs films; surfactant; phase transition; AFM; liquid crystals; wetting; surface ordering and anchoring; semifluorinated alkanes