Lyotropic and Thermotropic Lrquid Crystals Under Shear Flow
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LYOTROPIC AND THERMOTROPIC LRQUID CRYSTALS UNDER SHEAR FLOW
E. B. Sirota*, C. R. Safinya', R. J. Piano%, C. Jeppesen"* and R. F. Bruinsma** Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801 Department of Physics, University of California, Los Angeles, CA 90024
ABSTRACT We describe x-ray scattering studies of the nematic and smectic-A phases of the thermotropic liquid crystal 8CB and the L, and L3 (lamellar and sponge) phases of the lyotropic liquid crystal system: SDS, dodecane, pentanol and water. In the thermotropic system a series of orientational states of the nematic director is observed before entering the smectic-A phase where the smectic density wave and nematic director are oriented normal to the shear plane at high shear rates. In the lyotropic system lamellar fluctuations in the sponge phase (L 3 ) become oriented by shear and in the lamellar phase (La) at high shear the layers orient preferentially along the velocity-gradient direction, in highly anisotropic cylindrical multilamellar liposome-like structures.
INTRODUCTION We have used high resolution synchrotron x-ray scattering techniques to study the non-equilibrium steady-state structures of complex fluids under shear flow in a sealed and temperature controlled Couette cell. We will discuss experiments on both thermotropic and lyotropic systems which show interestingly different behavior. The experiments were primarily carried out at the National Synchrotron Light Source on the Exxon beam line X-IOA using 8 KeV radiation and a linear position sensitive detector. The shear cell design and experimental details are discussed elsewhere [1,2].
Mat. Res. Soc. Symp. Proc. Vol. 248. @1992 Materials Research Society
170
THERMOTROPIC NEMATIC TO SMECTIC-A PHASE TRANSITION The simplest liquid crystalline phases are the nematic (N) and smectic-A (SmA) phases [3]. In the nematic phase, the molecules' long axes A point, on average, along the director n. The N-SmA transition corresponds to A the onset of a mass density wave along n. At equilibrium, the pretransitional SmA fluctuation clusters, which are measured by x-rays, grow in the nematic phase as the N-SmA transition is approached and are ultimately responsible for the ordered SmA phase with the anisotropic correlation lengths ý,, and • diverging at the transition TNA [4]. According to dynamical scaling [5], the fluctuation relaxation time r - t3/2 where t is an effective correlation length for critical fluctuations; thus, i-t which drives the system away from equilibrium should increase substantially near the N-SmA transition. Our coordinate system in real space is defined such that (v) and (Vv) are along the x- and y-axes, and the neutral direction parallel to the cylinder axis is along the z-axis. In reciprocal space, qx, qy, and qz, are parallel to v, V v, and the neutral directions.
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Fig. 1) Three simple orientations for n in the nematic phase
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