Colloquium: Ron Pindak

    Thursday, November 10, 2016 - 3:30pm - 5:00pm
    Meinel 307

    Using Complementary Optical and Synchrotron X-Ray Techniques to Understand the Complex Liquid Crystalline Ordering in Layered Phases of Tilted Molecules


    Liquid crystal molecules have found wide spread applications in electro-optical devices due to their optical and dielectric anisotropy.  The layered (smectic) liquid crystal phases are particularly interesting because they can be prepared as free-standing films as thin as a single molecular layer or thick films with near-perfect layer alignment. If the molecules are tilted with respect to the layer normal, then their optical anisotropy provides a method of viewing liquid crystalline orientational order as well as orientational textures that are indicative of varying degrees of positional or interlayer ordering. The availability of high-brightness X-ray sources, tunable over a wide energy range, has enabled the direct determination of complementary structural information, even in films a few molecules thick. What is less commonly appreciated is that resonant X-ray scattering provides a direct probe of orientational structures in liquid crystals with nanometer-scale periodicities that cannot be readily observed with visible light. We have applied this technique to study interlayer orientational ordering in ferro-, ferri-, and antiferro-electric tilted liquid crystal layered phases in well-aligned free-standing films as well as in a device cell geometry, that is, sandwiched between two thin glass plates with an ITO conductive layer and a rubbed polymer alignment coating. We have studied both linear and bent-core molecules. The bent-core molecules, even when non-chiral, can develop chiral structures that are ferro- or antiferro-electric.  They also exhibit a number of modulated phases.  I’ll provide an overview of the field highlighting the most important discoveries and open issues for the future.

    Speaker Bio(s): 

    As Complex Scattering Program Manager in the Photon Science Division, Dr. Ronald Pindak is responsible for a suite of beamlines that cover a wide range in momentum transfer and energy so can be used to measure the structure and dynamics of hard, soft, and biological materials. Among the NSLS-II beamlines, the strength of the complex scattering beamlines is the ability to measure complex materials with a structure that encompasses multiple length scales and exhibits equilibrium or non-equilibrium kinetics over multiple time scales. A goal of the Complex Scattering Program is to study complex materials as they are being synthesized, processed, or operated in devices with real-time analysis and feedback to intelligently optimize the synthesis, process, or device performance. The NSLS-II Complex Scattering Program beamlines include ID-10: Inelastic X-ray Scattering (IXS), ID-11: Coherent Hard X-ray Scattering, BM-11: Complex Materials Scattering (CMS), and ID-12: Soft Matter Interfaces (SMI).As a Senior Scientist at Brookhaven National Laboratory, Ron conducts research focused on understanding the physics underlying the self-assembly of liquid crystalline materials, the nature of their phase transitions, and changes in structural ordering at interfaces. He also applies model-independent phase retrieval techniques to determine structural ordering and concentration profiles at buried interfaces in epitaxial thin films.