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Pair Interaction Between Colloidal PartIcles in Nematic Liquid Crystal: A Finite Element Study

Mozaffari, Mohammad Reza | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41831 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Ejtehadi, Mohammad Reza
  7. Abstract:
  8. Structure of colloidal particles in anisotropic solvent with long-range orientational ordering, such as nematic liquid crystals, has attracted great attention in science and technology. The nematic director is distorted from its uniform orientation in the bulk due to anchoring on the surface of the colloidal particles. These elastic distortions create topological defects around the particles and induce long and short range anisotropic interactions between the particles. Depending on the anchoring type and the colloidal geometry, the particle-defect can induce a long-range dipolar-dipolar, quadrupolar-quadrupolar and/or dipolar-quadrupolar potential.Experimentally, the colloidal interactions in nematic liquid crystals are studied using optical or magneto-optical traps. Smalyukh et al.[Phys. Rev. Lett. 95, 157801 (2005)] have measured the angular and the radial components of the force between two particles with planar anchoring in a nematic liquid crystal as a function of the interparticle separation and the angle with the bulk nematic. They showed that the particles aggregate in chains directed at nearly 30 degree with respect to the nematic direction. Lapointe et al.[Science 326, 1083 (2009)] indicated that the elastic colloidal interactions in nematic liquid crystals are sensitive to the colloids shapes.Equilibrium director field around platelet equilateral polygonal colloids exhibit dipolar symmetry if they have odd number of the edges and quadrupolar symmetry if it is is even. This is giving rise to dipolar and quadrupolar elastic colloidal interactions, respectively.Studing the degenerate planar anchoring on the spherical particles is numerically interest and hard in comparison to the normal anchoring. Here, the interaction between two spherical colloidal particles with degenerate planar anchoring in a nematic media has been studied by numerically minimizing the bulk Landau–de Gennes free energy and the surface energy by the mean of a finite element method. The results indicate that the particles form an angle 28±2 degree with respect to the far-field nematic direction that is consistent with Smalyukh research.We have also investigated interactions between two long parallel prism particles when their long axes are perpendicular with far-field nematic direction. Using adaptive finite element method the 2D Landau–de Gennes free energy with strong planar anchoring has been minimized. In agreement with Lapointe’s results, interaction between two triangle prisms has dipolar symmetry while the square prisms show quadrupolar symmetry
  9. Keywords:
  10. Colloid ; Defect ; Anchoring ; Dipole ; Quadrupole

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