Freely Suspended Liquid Crystalline Films

Author : Sunil K. Sharma
Publisher :
ISBN 13 :
Total Pages : 157 pages
Book Rating : 4.:/5 (319 download)

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Download or read book Dynamic Light Scattering Studies of Layer Fluctuations in Freely Suspended Smectic Liquid Crystal Films written by Sunil K. Sharma and published by . This book was released on 2007 with total page 157 pages. Available in PDF, EPUB and Kindle. Book excerpt: Smectic liquid crystalline films or membranes have been widely studied by using the hydrodynamic theory and dynamical scattering methods, but most studies have been confined to either frequency domain light scattering measurements above ~1GHz, which probe sound waves in a limit where damping is a weak effect, or by low frequency photon correlation spectroscopy, performed in the time domain below 10 MHz (i.e., for times) and usually designed to study overdamped fluctuations of the smectic layer orientation when inertia is negligible. The interesting crossover regime, where inertia, damping, and elastic forces all contribute significantly in the hydrodynamic description, has posed a long standing challenge to scattering experiments. In this dissertation we present an extension of the optical photon correlation spectroscopy (PCS) technique down to ~100 picosecond resolution that fully closes the historical gap between frequency and time domain scattering techniques. We study smectic films from which we can determine, for optical wavevectors 10^4-10^5/cm, both the propagation frequency and damping rate for undulatory motion of thin free standing smectic films (a few tens of molecular layers), as well as the relaxation rates associated with overdamped layer dynamics of bulk films (thousands of layers thick), by measuring the time correlation function of the intensity of light scattered from thermal motion of the film surfaces. We find that the standard hydrodynamic model for incompressible, isothermal smectic films provides a good overall framework for quantitative analysis of our correlation data in both under and overdamped regimes for films of common cyanobiphenyl liquid crystals in the SmA phase. In particular, the main elastic properties - the film surface tension and compression elastic constant - are found to be reasonably consistent with values obtained from other, more invasive techniques. However, there are significant aspects in which the standard model appears incomplete. First, in relatively thin films where the dominant scattering comes from nearly pure undulation modes of the film, the effective layer sliding viscosity is more than an order of magnitude less than values reported in bulk samples, typically measured between substrates. On the other hand, in a very thick film, we obtain a result, from purely overdamped dynamics, which basically agrees with quoted bulk values of the layer viscosity. Second, in thinner films, we detect an additional, overdamped mode not expected in the light scattering derived from the standard hydrodynamic theory. We propose that this mode is associated with consequences of a higher degree of order on the film surfaces. We have also carried out preliminary surface light scattering experiments on layer motion in thin smectic-C and hexatic-B films. The dynamical parameters obtained from fits to these data show only modest variations through the SmA-SmC and SmA-HexB transitions. The additional, overdamped mode is revealed to be clearly hydrodynamic (relaxation rate ~ square of wavevector) over the range probed.