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Anisotropic and isotropic electroconvection

Explore the electrohydrodynamics of nematics with detailed models, material parameters, and boundary conditions for various alignments and combinations. Study the effects of flexoelectricity and transition to isotropic electroconvection. Conductive and dielectric modes analyzed.

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Anisotropic and isotropic electroconvection

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  1. Anisotropic and isotropic electroconvection Collaborators: L.Kramer, W.Pesch (Univ. Bayreuth/Germany and N.Eber (Inst. Solid State Phys./Hungary) H I. (anis.) II. (interm.) III. (isotr.) OR (low f) y x NR (high f) + + planar homeotropic homeotropic

  2. ELECTROHYDRODYNAMICS OF NEMATICS STANDARD - free energy density - balance of torques - equation of motion - incompressibility - equation of electrostatics -charge conservation MODEL (SM)

  3. Material parameters: Boundary conditions: planar or homeotropic Relevant: alignment + sign of a and a  8 combinations planar, a < 0, a > 0 anisotropic homeotropic,a < 0, a > 0 intermediate homeotropic, a > 0, a < 0isotropic IV. planar, a < 0, a < 0non-standard SM

  4. I. planar, a < 0, a > 0 anisotropic Ginzburg-Landau description works MBBA: - 0.13

  5. At threshold, increasing f (planar, a > 0, a < 0): OR NR n TW (non-stand.) DR

  6. II. homeotropic, a < 0, a > 0

  7. H drives between semi-isotropic and anisotropic • soft <-> patterning mode • direct transition to STC • AR-s • chevron formation • defect glide • 2 LP-s NR OR

  8. Homeotropic alignment (standard, semi-isotropic) (A.Rossberg, L.Kramer) OR NR theor. exp.

  9. III. Homeotropic, a > 0, a < 0 ( truly isotropic) Direct transition to isotropic EC

  10. Direct transition to EC -> SM

  11. Swift-Hohenberg eq. (W.Pesch, L.Kramer, B.Dressel) At onset: exp. theo. f nonlinear regime: hard squares soft squares : not reproduced

  12. IV. planar, a < 0, a < 0: no standard pattern (conductive)

  13. PR or oblique • nz= 0, no shadowgraph • ny (?) oscillates • Uc~ d, f • - qc is d indep. Experimental: Dielectric mode! (LK)

  14. I and II- conductive III and IV - dielectric

  15. 1. Dielectric mode for MBBA (planar, a < 0, a > 0) 2. Dielectric mode for MBBA (planar, a < 0, a < 0) - no pattern Flexoelectricity

  16. Flexoelectricity Effect on the roll angle, only for d.c. (only in conductive)

  17. 3. Dielectric mode for MBBA (planar, a < 0, a < 0) + flexoelectricity e1- e3= 1.34 e1+ e3= -7.84 finite threshold! obliqueness!

  18. 4. Dielectric mode for MBBA (planar, a < 0, a < 0) + flexoelectricity e1- e3= 2.68 e1+ e3= -7.84 (A.Krekhov, W.Pesch)

  19. planar, a < 0, a < 0: no standard pattern (conductive) • dielectric mode at low f • SM + flexoelectricity • why is DM more sensitive to flexo, than CM?

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