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Self-Assembly of Colloidal Particles in Liquid Crystal

Eskandari, Zahra | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 46461 (04)
  4. University: Sharif University of Technology
  5. Department: Physics Science
  6. Advisor(s): Ejtehadi, Mohammad Reza; Telo da Gama, Margarida
  7. Abstract:
  8. A nematic colloid is a mixture of microscopic particles dispersed in a nematic liquid crystal (NLC), which is an anisotropic fluid with long-range orientational order. This long-range order might be perturbed due to different reasons, such as confinement between flat or curved surfaces which leads to frustration of the nematic director in some singular points or lines called ”topological defects”. The nucleation and the structure of these defects emerge from the elastic properties of NLC and leads to unique behavior of nematic colloids. The interaction between colloidal particles in simple fluids are mainly excluded volume interactions, electrostatic interactions, van der Waals interactions, entropic interactions, etc., which are mostly isotropic and short-range; whereas in nematic colloids, long-range structural interactions enable the self -assembly of such colloids into one-, two- and three-dimensional ordered structures.Theoretical and experimental studies, as well as computer simulations, have shown that when a spherical particle with perpendicular boundary conditions is immersed in a NLC host, two configurations with either a point defect of hyperbolic hedgehog or a
    Saturn ring around the particle are stable, while for a particle with tangential boundary conditions, two point defects, called ”boojums”, are nucleated in the poles of the particle. The particle with a hyperbolic hedgehog is an elastic dipole, while the particle with Saturn ring or boojums are elastic quadrupoles, due to their structural symmetry. Obtaining the director field configuration analytically is not straightforward due to the highly nonlinear are nature of the equations in such systems, and to reach this aim, anzats analogous to electrostatics are applied. Recently, many experimental and numerical studies have been done to investigate the details of the interaction between colloidal particles in a NLC and to find the stable structures. Here, we investigate the interaction between the colloidal particles, as well as their interaction with the walls of the cell, considering several geometries and boundary conditions at the surface of the particles and the wall, by minimizing Landaude Gennes’ free energy using the finite element method. We have shown that elastic quadrupoles with different boundary conditions can form a two-dimensional square lattice, in agreement with the experimental observations, while the boojum particles can only form one- dimensional chains which are either oblique (with an angle of 30) or parallel to the far field director, depending on the elastic properties of the NLC. We have also shown that by changing the geometry of the wall, as well as the boundary conditions at the surface of the particle and the wall, one can control their interaction, either improving the repulsion or changing it to attraction, and hence investigate the possibility of constructing self-assembled ordered structures
  9. Keywords:
  10. Topological Defect ; Landau De-Gennes Energy ; Self Assembly ; Nematic Colloids

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