Effect of Different Layer Thicknesses and Boundary Conditions of Confined Liquid Crystal on Dynamics of Director Relaxat
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Effect of Different Layer Thicknesses and Boundary Conditions of Confined Liquid Crystal on Dynamics of Director Relaxation: Dynamic Light Scattering Investigations Sarmistha Basu and Fouad M. Aliev Department of Physics, University of Puerto Rico, San Juan, PR 00931-2343, USA ABSTRACT Dynamic light scattering (DLS) has been used to study the influence of boundary conditions (planar-axial and homeotropic-radial) and layer thickness of liquid crystal (8CB) confined to cylindrical pores on phase transitions and relaxation of director orientational fluctuations. For confined 8CB in the nematic phase two well-defined relaxation processes were observed for both axial and radial orientations of the liquid crystal. The first relaxation is qualitatively associated with bulk-like nematic director fluctuations. The second relaxation (with relaxation time slower than the first one) is most likely due to the fluctuations in the layers nearest the wall surface. In samples with radial boundary conditions, we observed the onset of smectic-A phase order formation on the pore wall even though the rest of the liquid crystal could be in the nematic phase. The influence of pore wall-liquid crystal interactions on the properties of the confined 8CB was stronger for radial boundary conditions of confined liquid crystal, than in the case of axial orientation.. The separation between the first and the second (slow) process was clearer for thinner layers and the amplitude of the slow process was greater for thinner layers. This suggests that the slow process was related to surface relaxation of director fluctuations. INTRODUCTION Liquid crystals (LCs) confined in porous media have been very useful [1] for exploring the fundamental physics of condensed matter as well in applications. Different aspects of the dynamic behavior of confined LCs can be investigated by photon correlation spectroscopy. Using this method it has been shown [2-8] that different kind of confinement strongly influences the dynamics of different fluctuations in LCs. Confinement has at least two impacts on a given material both of which are very strong for liquid crystals: the interaction of the material with the pore surface and the finite size effect. Difficult problems in studying the dynamics of confined systems include the separation of the role of random field effects from of finite size effects, and the influence of the pore solid surface on LC properties. Two important issues must be addressed in the study of confinement effects on LC properties: (1) boundary conditions (i.e. planar-axial or homeotropic-radial orientation of LC molecules on the surface of the solid pore walls of the matrices) and (2) the layer thickness (size of the confinement) of the LC inside cylindrical pores. The thickness of the LC layers formed on pore walls depends on the degree to which pores are filled with LC. This allows control the layers thickness over a wide range from 200 nm down to the diameter of single molecule, if monolayer is formed on the pore wall. We used cylindric
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