Director and polarization fluctuations in suspensions of ferroelectric nanoparticles in nematic liqu

1
Director and polarization fluctuations in
suspensions of ferroelectric nanoparticles in
nematic liquid crystals

• Alenka Mertelj1, Martin Copic1,2, Olexander
  Buchnev3, and Yuri Reznikov3
• 1J. Stefan Institute, Ljubljana, Slovenia
• 2Dept. of Physics FMF, University of Ljubljana,
  Jadranska 19, 1000 Ljubljana, Slovenia
• 3Institute of Physics, Ukrainian Academy of
  Sciences, Ukraine

2
Outline

• Nematic liquid crystals
• Composites
• Suspensions of ferroelectric particles
• Dynamic light scattering results
• Discussion
• Conclusions

3
Nematic liquid crystals
n director S ltP2(cos q)gt

• NEMATIC
• long range orientational but no long range
  positional order
• anisotropic e, c
• ea10
• optically uniaxial with optical axis n
• ne-no0.2
• 5 viscosity coefficients

• ISOTROPIC
• no long range positional and orientational order
• isotropic e, c
• 1 viscosity

4
Elastic deformations
Three types of deformation gt three elastic
constants K1, K2, K3
Bend, K3
Splay, K1
Twist, K2
Ki ? S2
5
Composites

• addition of small amount (1 or less) of
  nanoparticles can change physical properties of
  host medium significantly
• in liquid crystals additives can change
• order parameter (n, S)
• elastic constants, viscosities
• response to external fields, light
• phases and phase transition temperatures
• the changes can be local defects or/and
  collective changes of order parameter S
• nanoparticles (lt 50 nm) do not cause the
  formation of the defects

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Ferroelectric nematic suspensions

• Nematic liquid crystal mixture LC 18523, TNI55
  oC
• Ferroelectric particles
• BaTiO3 with Ps 0.26 C/m2 , TCurie 123 oC,
  TNI,susp65 oC
• Sn2P2S6 (SPS) with Ps 0.14 C/m2 , TCurie 66
  oC, TNI,susp62 oC
• Concentration 1 wt, particle size 10 - 30 nm
• Orientation homeotropic and planar, cell
  thickness 12 microns
• The suspensions look the same as pure LC, no
  defects or texture is visible.

7
Dynamic light scattering (DLS)

• Orientational fluctuations are fundamental
  hydrodynamic excitations of the director field
• in bulk overdamped plane waves with relaxation
  rates
• Bend mode
• In DLS experiment relaxation rates are measured,
  i.e., the ratio between elastic constant Ki and
  effective rotational viscosity ?i.

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Results - Effect of electric field
The effect is comparable with dielectric
measurements where ea increases by a factor of 2
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Results - Bend mode
10
Results - Twist mode
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Results - A new mode
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Discussion

• Our results can be explained by assuming that the
  internal electric field due to the ferroelectric
  particles couples to the nematic order parameter.
• The field at the particle surface is very large,
  of the order of (assuming particles are
  separated)

Estimated average field Eint cE 107 V/m c
concentration of the particles (In LC displays
typically E 105 V/m) The electric field
increase the magnitude of the order parameter S
and, consequently, the dielectric anisotropy, the
elastic constants and, the orientational
viscosities.
13
Coupled director and polarization fluctuations
Fast additional mode can be explained as the
collective mode where the nematic director and
spontaneous polarization of the ferroelectric
particles fluctuate in a counterphase
14
Conclusions

• Inclusion of ferroelectric nanoparticles also
  strongly influences the dynamics of orientational
  fluctuations.
• Main effect is probably due to increase in S.
• An optic mode with spontaneous polarization of
  the nanoparticles Ps fluctuating against n
  appears. Its existence proofs that the particles
  are collectively coupled to the nematic host.
• No signal observed from the translational motion
  of the particles, so the particles are either
  charged and separated or aggregated in chains
• Ref Phys. Rev. E 76, 011702 (2007).