Abstracts Physics

Coherent soft x-ray dynamic light scattering from smectic-A liquid crystals

by Allen C. Price

We have used a coherent beam of soft x-rays to measure the thermally driven dynamic layer fluctuations in five different smectic-A liquid crystals (4O.8, 7O.7, 8CB, 8OCB and 10OCB). Our technique, soft x-ray dynamic light scattering (SXDLS), required the design, construction, and testing of (1) a double pinhole spatial filter, for extracting a coherent beam from an incoherent soft x-ray beam, (2) a liquid crystal oven for creating, maintaining, and manipulating our freely suspended liquid crystal films, and (3) high quantum efficiency, fast soft x-ray photon counting detectors. The dynamic time-scales of the fluctuations we measured range from 2 to 70 mus. These are the fastest time-scales ever measured using DLS with a coherent beam of x-rays – roughly five orders of magnitude faster than the fastest time-scales measured with hard x-ray DLS. We accomplished this by taking advantage of (1) the higher coherent fluxes available from soft x-ray sources, (2) the high quantum efficiency of our specially designed detector, and (3) the high scattering cross section associated with the Bragg peak in the smectic-A structure factor. We also developed a normalization scheme for dividing out the fast, periodic intensity noise inherent in synchrotron radiation sources.My collaborators, Andrzej Poniewierski and Robert Holyst, were able to work out the detailed microscopic theory for the dynamics of the finite-size smectic-A films which we studied. By simulating a Langevin equation based on their theory, I was able to calculate the form of the correlation function that we measured in our experiment, and thus allow a direct comparison between theory and experiment.The theory predicts a linear relation between the layer fluctuation decay time and the liquid crystal film thickness. This is confirmed by our experimental results. The theory also allows us to interpret our results as a measurement of the ratio of the surface tension, gamma, to the layer sliding viscosity, eta 3. We found eta3/gamma to be from 1 to 4 sec/m for all the liquid crystals studied. Using known values of surface tension, we determined eta 3 to be from 0.2 to 1 poise for all the liquid crystals studied.