tine porenta mentor: prof. dr. slobodan Žumer januar 2010
TRANSCRIPT
SeminarIntroduction in liquid crystalsBasics of flexoelectricityTheoryNumerical methodRadial nematic-filled sphere
Radial nematic-filled sphere with point-like defect
Radial nematic-filled sphere with hedgehog defect
Conclusion
Introduction in liquid crystalsMaterials with properties most useful for
different applications in the modern worldLiquid oily materials made of rigid organic
moleculesIn proper temperature region they can self
orginise and form a mesophases between the liquid and solid state
Mesophases are characterized by orientational and positional order of the molecules
Nematic phase is the least ordered phase influenced only by long-range order but no positional order.
Long-range order is the phenomenon that makes liquid crystal unique
They are typically highly responsive to external fields
In confined geometries opposing orientational ordering of different surfaces can lead to formation of regions, where orientation is undefined -> defects.
Defects can be either point-like or lines. The average of the molecules are
described as a director n. Director is aporal, meaning the orientation n an –n are equivalent.
Degree of order: Orientational fluctuations of the molecules are defined as an ensemble average of the second Legendre polynomials S = <P2(cos )>
The director and the nematic degree of order can be joint together in a single tensorial order parameter defined as
By definition Qij is symmetric and traceless. Its largest eigenvalue is nematic degree of order S and the corresponding eigenvector is the director n
Phenomenological Landau – de Gennes (LdG) total free energy is used to incorporate liquid crystal elasticity and possible formation of defects:
)3(2 ijjiij nnS
Q
LC
jiijkijkijjiij
LC k
ij
k
ij
VQQCQQBQQAQ
Vx
Q
x
QLF
d 4
1
3
1
2
1
d 2
1
2
Electric field couples with nematic through a dielectric interaction with induced dipoles of the nematic molecules. Within the LdG framework, the electric coupling is introduced as an additional free energy density contribution
where ij is defined as
|| and are dielectric constant measured parallel and perpendicular to the nematic director
LC
iijd VEEF d 2
1j0
ijijij QS |||| 3
22
3
1
From piezoelectricity to flexoelectricityPiezoelectricity is the ability of some materials to
generate an electric field or electric potential in response to applied mechanical stress.
The effect is closely related to a change of polarization density within the material's volume.
The internal stress in this materials is proportional to electric field inside.
Stress tensor is defined as
ETijij u
F
,
ETi
j
j
iij x
u
x
uu
,2
1
Electric displacement field is then
where i,jk tensor rank three with symmetry i,jk = i,kj . If tensor is known, piezoelectric properties are entirely determined
In liquid crystals exist phenomenon similar to piezoelectricity that occurs from the deformation of director filed
wiht coefficients e1, e3 10-11 As/m
ijjkijiji EDDi
,0
))(()( 31 nnenneED iiji
The total macroscopic polarisation induced by deformation of liquid crystal is introduced by using a nematic degree of order
where Gijkl is a general fourth rank coupling tensor, which incorporates flexoelectric coefficients e1 and e3
For simplicity -> one constant approximation Gijkl=G
The corresponding free energy:
l
ijijkli x
QGP
LC i
ij VEx
QGF d j
Numerical methodNumeric relaxation method was developed to
calculate the effect of flexoelectricityElectric potential and the profile of the
nematic order parameter tensor are alternatively computed, until converged to the stable or metastable solutions
Cubic mesh with resolution of 10 nmStrong anchoring on boundaries is assumed
The total free energy is miminized by using Euler-Lagrange algorithm
Electric potential is calculated from Maxwell’s equations in an anisotropic medium
LC ji
ij
LC jiij
LC
jiijkijkijjiij
LC k
ij
k
ij
Vxx
QG
Vxx
VQQCQQBQQAQ
Vx
Q
x
QLF
d
d 2
1
d 4
1
3
1
2
1
d 2
1
0
2
02
0
ji
ij
jij
i xx
QG
xx
ijijij QS |||| 3
22
3
1
Radial nematic-filled sphereEffect of the flexoelectricity are typically
small in the absence of the external fields (Fflex < 1% Ftotal), but in some geometries like nematic filled sphere can become substantial importance.
Radial nematic-filled sphere with point-like defectexistance of analytical solution of flexoelectric
quantities for isotropic medium only splay deformation of a director field director field can be represented in spherical
coordinate system as n=(1,0,0)
0,0,2
1)0,0,1(
)(
1
2
21
1
r
er
r
re
nnePflex
0,0,2
)(
0
1
0
0
r
eE
CPE
PE
flex
flex
Radial nematic-filled sphere with hedgehog defectElectric potential induced by flexoelectricity
affects the nematic profile, primarily in the core region of the defects.
(a)Electric field induced by flexoelectricity and spatial distribution of elastic (b), dielectric (c) and flexoelectric (d) contribution to the total free energy
ConclusionCoupled numerical method was developed for the
study of flexoelectricity in nematic liquid crystals to show us that flexoelectricity induces substantial electric potential in the regions surrounding the defects
Flexoelectricity affects defect cores and changes their size
Flexoelectricity could change stability of defect configurations in confined geometries
Flexoelectric contribution to the total free energy has quadratic dependence on flexoelectric coefficient and could become important factor for materials with high flexoelectric coefficients