Liquid crystal structures & order parameters
Liquid crystalline ordering is intermediate between the short-range order in ordinary fluids and the true 3D long-range order in solid crystals. Liquid crystal structures are thus unique in that they combine aspects of fluid short-range order with aspects of solid-like long-range order. All liquid crystals exhibit at least long-range orientational order (of rod- or disc-shaped molecules, micelles or nanoparticles) giving rise to the macroscopic anisotropy of their physical properties. In addition to long-range orientational order, smectic and columnar liquid crystals exhibit certain degrees of 1D or 2D long-range translational order. As in all soft matter systems, the energies involved in the various degrees of liquid crystalline ordering are in the order of thermal energy. Substantial fluctuations are thus inherent to all liquid crystal structures and the quality of liquid crystal ordering has to be quantified by certain order parameters. In the understanding of liquid crystal structures, the knowledge of order parameters and correlation lengths is as important as the purely geometric features (such as its dimensions and symmetry) of these structures.
Our group continuously contributes to the structural characterization of new liquid crystalline materials, thermotropic as well as lyotropic liquid crystals. Our main experimental tools in these investigations are X-ray diffraction experiments, often complemented by optical and spectroscopic methods. We further contribute to the advancement of methods, which allow the extraction of liquid crystal order parameters from X-ray or spectroscopic experiments. In 2008 we presented a simple experimental assessment of smectic translational order parameters and in 2013 we revealed the absolute amplitude of molecular tilt fluctuations in smectics from 2D X-ray diffraction experiments.
Orientational fluctuations near the smectic A to smectic C phase transition in two “de Vries”-type liquid crystals
D. Nonnenmacher, S. Jagiella,Q. X. Song, R. P. Lemieux, F. Giesselmann, ChemPhysChem 14, 2990-2995 (2013). DOI: 10.1002/cphc.201300358
The 2D-correlated structures of a lyotropic liquid crystalline diol with a phenylpyrimidine core
J. R. Bruckner, D. Krueerke, J. H. Porada, S. Jagiella, D. Blunk, F. Giesselmann, Journal of Materials Chemistry 22, 18198-18203 (2012). DOI: 10.1039/c2jm32868b
Orientational order parameters in liquid crystals: A comparative study of x-ray diffraction and polarized Raman spectroscopy results
A. Sanchez-Castillo, M. A. Osipov, F. Giesselmann, Physical Review E 81, 021707 (2010). DOI: 10.1103/PhysRevE.81.021707
Simple experimental assessment of smectic translational order parameters
N. Kapernaum, F. Giesselmann, Physical Review E 78, 062701 (2008). DOI: 10.1103/PhysRevE.78.062701
Orientational order in smectic liquid-crystalline phases of amphiphilic diols
F. Giesselmann, R. Germer and A. Saipa, Journal of Chemical Physics 123, 034906 (2005). DOI: 10.1063/1.1943972
On the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order
J. P. F. Lagerwall*, P. Rudquist, S. T. Lagerwall, and F. Giesselmann, Liquid Crystals 30(4), 399 - 414 (2003).