Ordered Micro-Particle Structures in a Liquid Crystal: Formation and Physical Properties
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Ordered Micro-Particle Structures in a Liquid Crystal: Formation and Physical Properties Ke Zhang, Anatoliy Glushchenko, and John L. West Liquid Crystal Institute, Kent State University, Kent OH 44242 ABSTRACT Ordered colloids are of great scientific and practical interests. Liquid crystals offer enhanced ways of producing and stabilizing these complex structures. We therefore studied the rheological and electro-rheological properties of the structured colloids as a means of probing this stabilization. We found that the mechanical properties of the colloids and stability of their 3D structures can be controlled by the particles size and distribution. In addition, when an electric field is applied, we observed an increase in the apparent viscosity with saturation at high electric fields. This effect depends on the shear rate and temperature. The results are also compared with the published data for the viscosity measurements of pure liquid crystals and isotropic colloids. While we are only beginning to understand the details of these complex colloids we expect they will find a wide variety of applications. INTRODUCTION We focused our research on micro-particles dispersed in liquid crystalline matrices. The anisotropic liquid crystal phase produces new effects not observed in colloidal suspensions in isotropic liquids, where the interaction between the particles may be explained by a combination of the long-range Coulomb repulsion and short-range Van der Waals attraction 1. Particles dispersed in the liquid crystal phase produce deviations in the liquid crystal director, n. Because of the surface anchoring of the liquid crystal at the particle surface, these dispersed particles introduce defects in the liquid crystal phase. In order to minimize the free energy of the system, particles will share defects and will tend to reside at defect planes between liquid crystal domains. The particle surfaces can also induce a gradient in the magnitude of the nematic order parameter in their neighborhood, leading to an attractive short-range interaction. There can also be interactions arising from the restriction of thermal fluctuations in the director field by the particle surfaces. The resulting distribution of the particles in an anisotropic fluid depends on the interplay between the described factors. On the other hand, if a redistribution of the particles is formed somehow, it will impose new physical properties to the system, including electro-optical and mechanical. Terentjev, et al. 2-4 recently explored small (150 and 250nm) polymeric particles dispersed in a nematic liquid crystal. The initial colloid dispersed homogeneously in the isotropic phase formed a cellular structure upon cooling into the nematic phase. The cellular structure is comprised of domains of practically pure nematic phase surrounded by walls of densely packed particles. This long-lived rigid cellular structure enhanced the mechanical properties of the systems, increasing the nearequilibrium (low-frequency) storage modulus by several orders of
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