A PHOTOALIGNMENT OF LIQUID CRYSTALS: TECHNOLOGICAL AND SCIENTIFIC ASPECTS

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Ferro-nematics and Their Outlook
John West
together with
A. Jakli and A. Glushchenko Liquid Crystal
Institute, Kent, USA Yu. Reznikov, O. Buchnev,
V. Reshetnyak, O. Tereshchenko, E.
Ouskova Institute of Physics, Kyiv, Ukraine
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Concentrated Liquid Crystal Suspensions
P.G. de Gennes, 1970
R.Eidenschink, 1981
Suspension of ferromagnetic particles
Suspension of aerosil
Traditional liquid crystals colloids are highly
concentrated suspensions with Cparticles
30-60 or suspensions with rather big (gtgt?m)
particles. It disturbs the liquid crystal
orientation and drastically changes its optical
properties. Ensembles of these particles and
defects can form complex structures.
Yu.Reznikov, 2001
J.West, 2000
Suspensions of micro-spheres
Suspension of alkali halide
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Preparation of Our Suspension
1. Milling of the ferro-electric
material. Crystalline particles (1 µm) are
ground in the presence of a surfactant for gt 100
hours in a micro-mill.
2. Fractioning the suspension in a liquid
carrier. A homogeneous fraction of the suspension
is segregated in a column where particles of
different size separate due to the gravity forces.
3. Suspending particles in a liquid
crystal. Particles covered with a surfactant are
dispersed in the isotropic phase of a liquid
crystal.
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Basic Characteristics of Our Suspension

• The resulting suspension is transparent and
  produces excellent homogeneous orientation
• Particle size lt 200 nm
• Distance between the particles ? 0.5 ?m
• Clear points difference
• Pretilt angle practically the same

The suspensions has essentially the same elastic
and anchoring properties as the pure liquid
crystal.
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Low Concentration LC Suspensions

• The particles are small enough that they do not
  disturb the liquid crystal orientation but
  produce a macroscopically homogeneous structure
  D ? 1?m (the anchoring with the liquid crystal is
  small)

- The particles are large enough to maintain
their intrinsic macroscopic properties D ? 10nm
- Concentration of the particles is small enough
to prevent sedimentation and aggregation C ?
10-3
- Concentration of the particles is large enough
affect the electro-optic properties of the LC
suspension C ? 10-4.
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Ferroelectric Particles in a Nematic Matrix

• Particles are oriented along the director of the
  liquid crystal because of anistropies in their
  shape and electric and magnetic properties.
• There is no preferred orientation of the
  permanent dipole of the particles ( -).
• The distance between the particles is less than
  the liquid crystal coherence length.

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Ferro-electric LC Suspension in a DC-field
DC-field breaking of a -symmetry due to the
linear torque a sensitivity to the sign of an
AC-electric field should appear.




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Linear Electro-Optical Response
Kang and Rosenblatt Phys. Rev. E 53, 2976 (1996)
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Ferro-electric LC Suspension in AC-field
AC-field. Both LC and particles are oriented
along the field. Dielectric torque Suspension
should reveal strong dielectric response.

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Decrease of the Freedericksz Transition Voltage
Reznikov et al. Appl. Phys. Lett., 82(12), 1917
(2003)
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Dielectric Properties of the Suspension
Ouskova et al. Liquid Crystals (2003)
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Dielectric Properties of Ferro-electric LC
suspensions
V. Reshetnyak, Mol.Cryst.Liq.Cryst, 2004
Dielectric function depends on the spontaneous
dipole moment d and an anisotropy of the particle
shape,
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WHAT IS THE ORIGIN OF THE EFFECT? There is a
dilemma DIPOLE MOMENT or ANISOTROPY of the SHAPE?
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No Jump !!!
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Chain model
O.V.Kuksenok, Phys.Rev.E, 54), R5198 (1996)

K 10-11N E 1V/?m a 0.5?m, ?particle
103 ?LC 10
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Where we are now...

• Diluted ferroelectric suspensions reveals unique
  properties and promise wide applications for
  tailoring and improvement of LC materials
• Suspensions are stable enough.
• Suspensions have a good optical quality.
• Suspensions have the same elastic and anchoring
  characteristics as the LC matrix.
• Suspensions posses enhanced and fast dielectric
  response.
• Suspensions reveal a linear response to the
  electric field

The Nature of the Effects Needs Further Studies!
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Ferroparticles in cholesteric liquid crystals
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Ferroparticles in Stressed Liquid Crystals
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Suspension Benefits

• Lower the driving voltage of all types of
  electro-optic affects
• TN
• STN
• IPS
• etc.
• Increase the order parameter of LC phase
• Introduce ferroelectric behavior into nematic and
  related phases

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Conclusion
Because of the anchoring and sahpe interaction
with the liquid crystal, dispersed particles
share their macroscopic properties with the
nematic matrix. Therefore low concentration
suspensions optically appear like a pure liquid
crystal but possesses unique properties,
intrinsic to the dispersed particles. One more
example diluted ferro-magnetic nematics
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Magnetically-Induced Alignment of a
Ferro-Magnetic Nematic Suspension
1. Milling of ferromagnetic material. Crystallized
particles of the Fe3O4 (1-µm) were milled with
the surfactant (to prevent particles
coagulation) during 100 hours. 2. Fractionation
of the suspension in a liquid carrier. A
homogeneous fraction of the resulting suspension
was segregated in a column where particles of
different size were separated by the gravity
forces. 3. Mixing the particles with a liquid
crystal. Obtained sub-micron particles Fe3O4
(1.5 wt ) covered surfactant were added to the
5CB.
There two kind of particles those ones dispersed
in the bulk and those that adsorbed at the
substrates surface
LC 5CB with Fe3O4 particles covered with a
surfactant
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Magnetically-Induced Alignment of Ferro-Nematic
Suspension
1. Cells preparation A symmetric cell was filled
with the suspension in isotropic phase and cooled
down. The ITO surface of the substrates does not
provide a homogeneous liquid crystal alignment
throughout the whole cell. 2. Magnetically
induced orientation The cell was put into a
magnetic field of H 1.25 kGs. In this way
homogeneous planar alignment of FNS along the
magnetic field was achieved. 3. Re-orientation
of FNS at magnetic field application The cell
with magnetically induced planar alignment can be
re-oriented at magnetic field application (H gt 1
kGs) to a new direction.
H 0
H 1?? 0
H 2 ? 0
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Experimental Set-up and Procedure
Combined cell A combined cell was filled with the
suspension in the isotropic phase and cooled
down. The reference substrate was covered with
rubbed polyimide, which provided a strong planar
alignment of a liquid crystal.
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Ferromagnetic LC Suspension
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Magnetically-Induced easy axis
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Magnetically-Induced Alignment
1. Application of a weak magnetic field to the
cell with the ferro-suspension induces an easy
orientation axis with a weak anchoring energy on
glass surface. 2. Varying the direction of the
magnetic field can change the easy axis
orientation. 3. The magnetically-induced
alignment of the ferro-LC suspension is caused by
adsorption of the ferro-particles on the test
surface and their manipulation with magnetic
fields