Anisotropic and isotropic electroconvection

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


planar
homeotropic
homeotropic
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ELECTROHYDRODYNAMICS OF NEMATICS
STANDARD
- free energy density - balance of torques -
equation of motion - incompressibility - equation
of electrostatics -charge conservation
MODEL



(SM)
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Material parameters
Boundary conditions planar or homeotropic
Relevant alignment sign of ?a and ?a ? 8
combinations

• planar, ?a lt 0, ?a gt 0
  anisotropic
• homeotropic, ?a lt 0, ?a gt 0 intermediate
• homeotropic, ?a gt 0, ?a lt 0 isotropic
• IV. planar, ?a lt 0, ?a lt 0
  non-standard

SM
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I. planar, ?a lt 0, ?a gt 0
anisotropic
Ginzburg-Landau description works
MBBA - 0.13
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At threshold, increasing f (planar, ?a gt 0, ? a lt
0)
OR
NR
n
TW (non-stand.)
DR
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II. homeotropic, ?a lt 0, ?a gt 0
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H drives between semi-isotropic and anisotropic

• soft lt-gt patterning mode
• direct transition to STC
• AR-s
• chevron formation
• defect glide
• 2 LP-s

NR
OR
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Homeotropic alignment (standard, semi-isotropic)
(A.Rossberg, L.Kramer)
OR
NR
theor.
exp.
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III. Homeotropic, ?a gt 0, ?a lt 0 ( truly
isotropic)
Direct transition to isotropic EC
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Direct transition to EC -gt SM
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Swift-Hohenberg eq.
(W.Pesch, L.Kramer, B.Dressel)
At onset
exp.
theo.
f
nonlinear regime hard squares
soft squares

not reproduced
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IV. planar, ?a lt 0, ?a lt 0 no standard
pattern (conductive)
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• PR or oblique
• nz 0, no shadowgraph
• ny (?) oscillates
• Uc d, f
• - qc is d indep.

Experimental
Dielectric mode! (LK)
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I and II- conductive III and IV - dielectric
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1. Dielectric mode for MBBA (planar, ?a lt 0, ?a
gt 0)
2. Dielectric mode for MBBA (planar, ?a lt 0, ?a
lt 0) - no pattern
Flexoelectricity
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Flexoelectricity
Effect on the roll angle, only for d.c. (only in
conductive)
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3. Dielectric mode for MBBA (planar, ?a lt 0, ?a
lt 0) flexoelectricity
e1- e3 1.34 e1 e3 -7.84
finite threshold!
obliqueness!
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4. Dielectric mode for MBBA (planar, ?a lt 0, ?a
lt 0) flexoelectricity
e1- e3 2.68 e1 e3 -7.84
(A.Krekhov, W.Pesch)
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planar, ?a lt 0, ?a lt 0 no standard pattern
(conductive)

• dielectric mode at low f
• SM flexoelectricity
• why is DM more sensitive to flexo, than CM?