Eur. Phys. J. E 25, 277–289 (2008)
DOI: 10.1140/epje/i2007-10290-4
Lehmann effect in a compensated cholesteric
liquid crystal: Experimental evidence with fixed
and gliding boundary conditions
A. Dequidt, A. Żywociński and P. Oswald
Eur. Phys. J. E 25, 277–289 (2008)
DOI 10.1140/epje/i2007-10290-4
THE EUROPEAN
PHYSICAL JOURNAL E
Lehmann effect in a compensated cholesteric liquid crystal:
Experimental evidence with fixed and gliding boundary conditions
A. Dequidt1,a , A. Żywociński1,2 , and P. Oswald1,b
1
2
Université de Lyon, Laboratoire de Physique, École Normale Supérieure de Lyon, CNRS, 46 Allée d’Italie, 69364 Lyon, France
Department III, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Received 24 September 2007 and Received in final form 23 November 2007
c EDP Sciences / Società Italiana di Fisica / Springer-Verlag 2008
Published online: 3 April 2008 –
Abstract. In a recent letter (Europhys. Lett. 80, 26001 (2007)), we have shown that a compensated
cholesteric liquid crystal (in which the macroscopic helix completely unwinds) may be subjected to a
thermomechanical torque (the so-called Lehmann effect), in agreement with previous findings of Éber
and Jánossy (Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982)). These results prove that one must take into
account the chirality of the molecules and the absence of inversion symmetry at the macroscopic scale
when deriving the constitutive equations of the phase at the compensation temperature. In this paper,
we present the details of our experimental work and a new experiment performed in a sample treated
for planar gliding anchoring. The latter experiment, coupled with a numerical simulation, supports the
existence of a thermomechanical coupling in a compensated cholesteric.
PACS. 61.30.-v Liquid crystals – 05.70.Ln Nonequilibrium and irreversible thermodynamics – 65.40.De
Thermal expansion; thermomechanical effects
1 Introduction
When a cholesteric liquid crystal is placed in a temperature gradient parallel to its helical axis, a torque acts on
its director n, of expression [1, 2]
ΓLehmann = −νG⊥ ,
(1)
where G⊥ = (n × G) × n is the component of the temperature gradient G = ∇T perpendicular to n. On condition
that the director be free to rotate at the boundaries (which
is a real problem in experiments), this equation predicts
that the helix of a cholesteric liquid crystal must rotate
at a constant angular velocity Ω when it is submitted to
a temperature gradient parallel to its axis
Ω = −νG/γ1 ,
(2)
where γ1 is the rotational viscosity. Historically, this phenomenon was discovered experimentally by Lehmann at
the beginning of the 20th century [3, 4] by observing the rotation of the internal texture of cholesteric droplets heated
from below and was explained more recently, in 1968, by
Leslie [5] from symmetry arguments.
In 1982, Éber and Jánossy proposed to measure the
Lehmann coefficient ν in a compensated cholesteric liquid crystal, i.e. at the special temperature Tc at which
a
b
e-mail: alain.dequidt@ens-lyon.fr
e-mail: Patrick.Oswald@ens-lyon.fr
the equilibrium twist q = 2π/p (where p is the cholesteric
pitch) vanishes and changes sign [6, 7]. They found that
ν is different from 0 at Tc in spite of the fact that the
phase has a nematic-like structure. To prove this result,
they performed an ingenious experiment with a cholesteric
mixture possessing a compensation temperature, consisting of measuring the birefringence of a homeotropic sample placed in a temperature gradient perpendicular to the
director n. Éber and Jánossy found that the director experiences a Lehmann torque (ν = 0) at Tc and concluded
that it was from microscopic origin and due to the chirality
of the molecules. Their experiment was immediately criticized by Pleiner and Brand who claimed that the Lehmann
coefficient must vanish at the compensation point [8, 9].
Their argument (which we will comment in the conclusion) was as follows: “since it is the symmetry of the phase
which determines the structure of the macroscopic equations, it is clear that the thermomechanical coupling constant has to vanish at the compensation point, since there
the symmetry is exactly that of the nematic phase” [9].
They thus concluded that the result of Éber and Jánossy
(ν = 0) was wrong and due to some experimental artifact. This affirmation led to a polemic between theorists
and experimentalists, the latter claiming that their results
were reliable and not forbidden theoretically because of
the chirality of the molecules [10].
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The European Physical Journal E
In view of this controversial situation, we redid the experiment and extended it to another geometry in order
to know who was right. Our results were published in a
letter [11] and led to the conclusion that molecular chirality was important, in agreement with Éber and Jánossy
findings.
On the other hand, our results were reported without
details about the experiments and the method for calculating the experimental errors. In this paper we fill this gap
and we present a new experiment performed with a sample
treated for gliding planar anchoring. This new experiment
together with a numerical simulation have confirmed our
previous conclusions.
The plan of the article is the following. In Section 2,
we recall the principle of the Éber and Jánossy experiment
in homeotropic anchoring and we show that it can be extended to the planar geometry. We then calculate for each
geometry the director distortions in the temperature gradient and the expression for the phase shift between the
ordinary and extraordinary components of a laser beam
crossing the sample at the compensation temperature. We
show that this shift depends on the optical indices, on
the elastic constants and on an effective Lehmann coefficient of expression νef f = ν + K2 dq/dT , where K2 is
the twist Frank constant. In Section 3, we describe the
experiment and we deduce from the measurement of the
optical phase shifts two different combinations of the effective Lehmann coefficient with the optical indices and
the two other Frank constants K1 and K3 . In Section 4,
we explain how we measured dq/dT as well as the elastic
constants Ki (i = 1, 2, 3) and the optical indices. In Section 5, we give the value of the Lehmann coefficient ν and
we recall the principle of the maximum-likelihood method
used to calculate the error bars. In Section 6 a new experiment performed with a sample treated for gliding planar
anchoring is described. After characterizing the anchoring,
we show the existence of a continuous rotation of the director at the compensation point when the sample is placed
in a vertical temperature gradient. A simple model and a
numerical simulation allow us to reproduce the observations qualitatively. Finally, we draw conclusions about the
role of the molecular chirality in Section 7.
2 The Éber and Jánossy experiment: theory
In the original experiment of Éber and Jánossy, the
cholesteric liquid crystal is introduced between two parallel glass plates treated for strong homeotropic anchoring. The sample is then placed inside a temperature gradient parallel to the glass plates. In this geometry, the
cholesteric helix unwinds when its pitch is typically larger
than the sample thickness. As a consequence a band of
homeotropic nematic phase, centered on the compensation temperature, forms in the sample. This region is bordered by cholesteric fingers which are well visible under
the microscope (for a review about the helix unwinding,
see [2, 12]) (Fig. 1). In practice, the nematic phase is a
little distorted because of the presence of the temperature
Fig. 1. Homeotropic sample in a large temperature gradient
(d = 40 µm and G = 51 ◦ C/cm). The unwound zone is 465 µm
wide.
gradient. The director field distortions are obtained by
solving the torque equilibrium equations. We recall that
in our notations, n · ∇ × n = −q at equilibrium. To first
order in temperature gradient G, they read
∂ 2 nx (x, z)
∂ 2 nx (x, z)
∂ny (x, z)
+K3
+K1
= 0, (3)
2
∂z
∂z
∂x2
∂ 2 ny (x, z)
Gνef f + K3
2
2∂z
∂ ny (x, z)
∂nx (x, z)
+K2
−
2q
= 0,
(4)
∂x2
∂z
2K2 q
2q
with νef f = ν + dK
dT . The x-axis is chosen parallel to the
temperature gradient and the z-axis perpendicular to the
glass plates. The system is invariant along the y-direction.
In practice, second derivatives with respect to x can be
neglected, which can be justified a posteriori (the sample
thickness d is always much smaller than the width of the
nematic band) so that the previous equations become:
∂ 2 nx (x, z)
∂ny (x, z)
+ K3
= 0,
∂z
∂z 2
∂ 2 ny (x, z)
∂nx (x, z)
Gνef f + K3
− 2qK2
= 0.
∂z 2
∂z
2K2 q
Solving these equations gives
⎞
⎛
K2
1 1 sin q(d − 2z) K3 ⎠
Gνef f ⎝ z
nx =
,
d
− +
K2
2K2 q
d 2 2
sin qd K
3
K2
K2
sin
q(d
−
z)
sin
qz
K3
K3
Gνef f
ny =
d
.
K
2K2 q
sin qd 2
(5)
(6)
(7)
(8)
K3
These equations generalize the solution given by Éber and
Jánossy [6] since they are still valid out of the compensation point Tc . In particular, they give back the spinodal
limit for the nematic phase as nx and ny diverge when
qd = π(K3 /K2 ) or d/p = K3 /(2K2 ).
The solution can be linearized in q in the vicinity of
the compensation temperature (at which q = 0), which
A. Dequidt et al.: Lehmann effect in a compensated cholesteric liquid crystal
279
gives
Gνef f K2
d
qz
z
−
(z − d),
3K32
2
Gνef f
z(d − z).
2K3
(9)
This distortion of the director field can be detected
optically by measuring the phase shift ΦH between the ordinary and the extraordinary components of a laser beam
crossing the sample. In practice, the beam is never strictly
perpendicular to the sample, which may be a source of error. For this reason, we calculated the phase shift at the
compensation temperature by taking into account a small
misalignment of the laser beam. Let θ be the angle (assumed to be small) between the beam and the normal to
the sample and ϕ the azimuthal angle of the beam with
respect to the temperature gradient. A straightforward
calculation yields:
nx =
ny =
2 2
Gνef f 2
ne − n2o
kno d
d
K3
240n2e
Gνef f 2 n2e − n2o
θ(sin ϕ)kd,
d
+
K3
12n2e
ΦH = −
The solution satisfying boundary conditions reads simply
(10)
where no and ne are the ordinary and extraordinary indices, respectively, k = 2π/λ the angular wave number of
dq
the laser, while νef f = ν +K2 dT
at Tc as q = 0. This equation shows that, at normal incidence (θ = 0), ΦH is proportional to G2 and d5 , a result already given by Éber and
Jánossy in reference [6]. On the other hand, an additional
term linear in G appears when the laser beam is slightly
misaligned, but the term in G2 remains unchanged.
This experiment can be also performed in planar anchoring, provided that the molecular alignment direction
be perpendicular to the temperature gradient. In this geometry, a nematic band forms which is centered on the inversion temperature. At equilibrium, this band is bordered
by two χ-disclination lines behind which regions twisted
by 2π form (Fig. 2).
As in the homeotropic case, the nematic is distorted
by the temperature gradient. The governing equations for
the director field become in this case
2
2
∂ nx
∂ nx
∂ 2 nz
∂ 2 nz
K2
−
+
+
K
= 0, (11)
1
∂z 2
∂z∂x
∂x2
∂z∂x
2
∂ nz
∂ 2 nx
+
−Gνef f + K1
∂z 2
∂x∂z
2
∂ nz
∂ 2 nx
= 0,
(12)
−
+K2
∂x2
∂x∂z
2q
with, as before, νef f = ν + dK
dT . In practice ∂/∂x ≪
∂/∂z, which allows us to simplify the previous equations
as follows:
K2
∂ 2 nx
= 0,
∂z 2
−Gνef f + K1
(13)
∂ 2 nz
= 0.
∂z 2
Fig. 2. Planar sample in a moderate temperature gradient
(d = 58 µm and G = 7.2 ◦ C/cm). The anchoring direction is
vertical and the temperature gradient horizontal. The unwound
zone between the two χ-disclination lines is 1100 µm wide. In
the center, one can see the image of the laser beam (highly
attenuated to not saturate the camera).
(14)
nx = 0
and nz = −
Gνef f
z(d − z).
2K1
(15)
These distortions can be detected by measuring the
phase shift ΦP between the ordinary and extraordinary
components of a laser beam crossing the sample. A
straightforward calculation gives in this case at the compensation temperature:
2 2
Gνef f
ne − n2o
Gνef f
kne d5 −
θ
ΦP = −ψ +
K1
240n2o
K1
n2 − n2
× e 2 o kd3 sin ϕ
12no
+
d cos2 ψ2
sin ψ
− 2 2
2
kno
k no (ne − no )
cos ϕ , (16)
with ψ = kd(ne − no ). As in the homeotropic case, the
phase shift contains (in addition to the constant term
−ψ) a linear term in G which vanishes when θ = 0 and a
quadratic term in G proportional to d5 and independent
of angles θ and ϕ.
From these calculations, we see that measuring ΦH and
ΦP on the one hand, and no , ne , Ki (i = 1, 2, 3) and dq/dT
on the other hand, allows us to determine the Lehmann
coefficient ν. In the next section, we present our experiment and recall how to measure ΦH and ΦP .
3 Experiment
Our liquid crystal is a mixture of 4-n-octyloxy-4′ cyanobiphenyl (8OCB from Synthon Chemicals GmbH & Co) and
of cholesteryl chloride (CC from Aldrich) in proportion
1:1 in weight with a compensation point at 59 ◦ C and a
clearing point at 67 ◦ C. The 8OCB was purified by one
of us (AZ) and the CC was used without further purification. For practical reasons, we did not use the mixture
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The European Physical Journal E
of Éber and Jánossy, namely 8CB + CC in equal weight
proportions, because the CC crystallizes very rapidly in
this mixture at the compensation temperature (close to
40 ◦ C). This problem was considerably reduced by replacing 8CB by 8OCB, certainly because the compensation
temperature was higher by about 20 ◦ C. It was nonetheless crucial (to avoid the crystallization of the CC) to
melt and systematically homogenize the mixture at high
temperature (typically during 5 min at 100 ◦ C) before filling each sample by capillarity at about 60 ◦ C. We also
took care to prepare the mixture in small quantities to
limit its degradation, pretty fast when it is heated above
100 ◦ C. The mixture was also filtered in the cholesteric
phase through 0.2 µm nylon filter to remove the maximum
number of dust particles. The glass plates were treated either for homeotropic anchoring with DMOAP (according
to the Kahn procedure [13]) or for planar anchoring with
a rubbed polyimide layer baked at 300 ◦ C during 2 hours
(ZLI 2650 from Merck). Nickel wires of calibrated diameters were used as spacers to fix the sample thickness. The
temperature gradient was imposed by placing the sample in a directional solidification cell (for its description,
see Ref. [14]), itself mounted on the stage of a polarizing microscope. A semi-reflecting plate was placed under
the condensor of the microscope to illuminate the sample
with a He-Ne laser (2 mW, λ = 633 nm). The laser beam
was first expanded to 6 mm with a commercial beam expander and then it was focused inside the sample with
the condensor. Its diameter at the waist was of the order
of 5 µm and did not change significantly over the sample thickness (ranging between 50 and 110 µm). An x-y
translation stage allowed manual positioning of the cell
in order that the laser spot lied exactly in the middle of
the nematic band, at the compensation temperature Tc .
The phase shift between the ordinary and extraordinary
components of the laser beam crossing the sample was
measured using a rotating analyzer, a quarter-wave plate,
a photodiode and a lock-in amplifier following the method
of Lim and Ho [15]. A synoptic of experimental setup is
shown in Figure 3.
3.1 Results in homeotropic anchoring
An example of curve obtained with a homeotropic 100 µmthick sample is shown in Figure 4. The solid line represents
the best fit of the data to a parabola ΦH = aH G − bH G2 .
νef f n2e −n2o
3
According to equation (10), aH = K
12n2 θ sin ϕkd
3
ν
n2 −n2
e
ef f 2 e
) 240n2o kno d5 . In Figure 5, we plotted the
and bH = ( K
3
e
fit parameter bH as a function of the sample thickness d.
Within experimental errors, bH is proportional to d5 as
predicted by the theory [16]. Fitting bH with a linear law
in d5 leads to
2 2
νef f
ne − n2o
kno = (9, 3 ± 0.8) × 1013 rad K−2 m−3 .
K3
240n2e
As for values of aH obtained from the fits, they give expected typical angles θ ≈ 1 − 3◦ .
motor
photodiode
reference
signal
sensor
rotating
analyzer A
M
I
C
R
O
S
C
O
P
E
quarter-wave
plate L
LOCK-IN
y
anchoring direction
(planar samples)
L
cold oven
hot oven
(fast
axis)
A ωt
(slow
P and L axis)
π/4
x
gradient
sample
condensor
o
v
e
n
polarizer P
semi-reflecting
plate
o
v
e
n
gradient
LASER
Fig. 3. Schematic diagram of the experimental setup used to
measure the phase shift between the ordinary and extraordinary components of the laser beam crossing the sample. In
practice, we chose ω/2π ≈ 100 s−1 .
We also studied the influence of a shift in temperature
with respect to Tc on the value of ΦH . We checked that
the signal does not change in a measurable way when the
laser spot is slightly decentred with respect to Tc (by no
more than ±0.05 ◦ C). On the other hand, ΦH starts to
decrease for larger shift in a significant way, in particular
when approaching the fingers. In the same time the signal
becomes noisy because it starts to depend on the position
of the spot along the y-direction (perpendicular to the
temperature gradient). We nevertheless checked that in a
systematic way, ΦH passes through a maximum at middistance of the fingers (i.e., at temperature Tc ). A typical
curve is shown in Figure 6. It is well fitted by a parabola
of type ΦH = ΦH (Tc ) + a(T − Tc )2 .
In order to exploit this experimental result, we looked
theoretically for the temperature dependence of ΦH by
taking into account the local tilt of the director given by
equations (7) and (8). In principle, the phase shift may be
expanded in a power series of q around the compensation
point as follows:
ΦH (q, νef f ) = a(νef f ) + b(νef f )q + c(νef f )q 2 + . . . , (17)
where a(νef f ) = ΦH (0, νef f ) is given by equation (10).
This term, as well as b and c, are functions of d, G and
the material constants K1 , K2 , K3 , no and ne which we
assume to be independent of the temperature in the vicinity of Tc . A straightforward calculation at first order in q
showed that b is strictly null. As a consequence, ΦH must
pass through a maximum at a temperature Tmax solution
A. Dequidt et al.: Lehmann effect in a compensated cholesteric liquid crystal
0
58.8
T (°C)
59.0
281
59.2
−20
−40
−60
ΦH ( °)
ΦH (°)
−35
−40
−80
−45
−50
−100
−55
−10
0
−5
5
10
−60
G (°C / cm)
Fig. 4. Phase shift as a function of the temperature gradient
(d = 100 µm, homeotropic anchoring).
bH (K −2cm 2)
slope 5
2
0.1
−20
0
20
q (µm−1 )
40x10
−3
Fig. 6. Phase shift ΦH as a function of temperature T and equilibrium twist q. The data fit reasonably with a parabola with
the maximum at Tc within experimental errors. The dashed
vertical lines mark the limits between the nematic band and
the fingers. The sample thickness is d = 87 µm and the temperature gradient G = 11 ◦ C/cm.
8
6
4
−40
8
6
4
2
5
6
7
8
9
d(μm)
100
Fig. 5. Fit parameter bH as a function of the sample thickness
d. The d5 -dependence is well satisfied.
of equation
∂ΦH
∂a ∂νef f
∂c ∂νef f 2
∂q
=
+
q + 2cq
= 0. (18)
∂T
∂νef f ∂T
∂νef f ∂T
∂T
Experimentally, Tmax = Tc . As q = 0 at this temperature, we deduce from the previous equation that
∂νef f
= 0.
∂T
(19)
Because dq/dT = 0 at Tc , this implies that
∂νef f
= 0.
∂q
(20)
Consequently, we find that νef f is independent of q
within our experimental errors. On the other hand, we
cannot conclude at this stage that the Lehmann coefficient
ν is null at Tc . This point will be addressed later, once the
dq
has been measured.
corrective term K2 dT
3.2 Results in planar anchoring
Experiments in planar geometry were much more delicate
to perform than the previous ones for several reasons.
The first one came from the difficulty to prepare thick
planar samples in the “nematic” phase at the compensation temperature. For this purpose, we used a Mettler
oven in which the sample can be moved easily in order to
visualize its texture under the microscope. In practice, we
did not succeed to prepare samples thicker than 70 µm.
The reason was mainly due to the difficulty for the χdisclination lines to move in thick samples because they
trap dust particles, which finally stop their motion. Another difficulty was to be sure that the sample, once prepared, was well in the “nematic” phase. Indeed, a thick
sample twisted by ±π within the thickness has the same
optical contrast between crossed polarizer and analyzer as
a “nematic” sample because of the adiabatic rotation of
the plane of polarization of the light. To know the director orientation, we used the following method. The sample
was rapidly heated until destabilization occurs: during this
process, bands develop, which are perpendicular to the director orientation in the middle of the sample. It was then
enough to look at the band orientation as bands perpendicular to the anchoring direction indicates that the sample is well in the nematic phase, whereas bands parallel
to the anchoring direction reveals a ±π-twist of the director field. Once the “nematic” sample was prepared, it was
transfered as fast as possible into the directional solidification cell. Because of the temperature gradient, the sample
immediately destabilized on both sides of the “nematic”
central zone. This led first to periodic bands parallel to the
temperature gradient which progressively disappeared by
leaving two zones twisted respectively by 2π and −2π. After a transient of a few hours, all the stripes disappeared,
leaving a “nematic” zone separated from two twisted zones
by two non singular double χ-disclination lines [2]. These
two lines are well visible in Figure 2.
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The European Physical Journal E
66
140
64
62
I (a.u.)
ΦP (°)
130
120
110
58
56
54
100
52
−10
0
G (°C / cm)
10
0
Fig. 7. Phase shift as a function of the temperature gradient
(d = 58 µm, planar anchoring).
2
bP (K −2cm 2)
60
0.1
9
8
7
6
20
25
Fig. 9. Transmitted intensity between crossed polarizers as a
function of the local thickness in a wedge planar sample.
5
4
νef f
K1
2
n2e − n2o
kne = (25 ± 6) × 1013 rad K−2 m−3 .
240n2o
The next step was to measure the material constants
no , ne , K1 , K2 , K3 and dq/dT at the compensation temperature. Our results are given in the next section.
3
6
10
15
d (μm)
In Figure 8, we plotted the fit parameter bP as a function of the sample thickness d. Within experimental errors,
bP is proportional to d5 as predicted by the theory [17].
Fitting bP with a linear law in d5 leads to
slope 5
5x101
5
7
d(μm)
Fig. 8. Fit parameter bP as a function of the sample thickness
d. The d5 -dependence is still well satisfied.
The second problem was due to the presence of a constant term in the phase shift ΦP measured experimentally.
Indeed, the term −ψ = −kd(ne − no ) appears in the expression of ΦP as we can see in equation (16). This term
is very large (of the order of 20π) in comparison with the
variations we want to measure, less than 1 rad. In order to
avoid that this term changes, we took care to perform
all the measurements at the same place in the sample
(so that d is constant). It was also crucial before each
measurement to check that the laser spot was exactly at
equal distances of the two disclination lines bordering the
“nematic” zone. This condition was achieved by finely adjusting the two oven temperatures and was essential to
avoid that the birefringence changes. Finally, it was important to perform measurements as fast as possible to
avoid any variation in the birefringence due to a degradation of the sample. With these precautions, it was possible
to consider that the term −ψ in ΦP was constant.
An example of curve obtained with a planar 58 µmthick sample is shown in Figure 7. The solid line represents the best fit of the data to a parabola ΦP =
aP G −bP G2 + c. According
to equation (16), aP =
−
d cos2 ψ
n2e −n2o
ψ
3
2
− k2 n2sin
12n2o kd sin ϕ +
kn2o
o (ne −no )
νef f 2 n2e −n2o
(K
) 240n2 kne d5 and c = −ψ.
1
o
νef f
K1
bP =
θ
cos ϕ ,
4 Material constants
Great care was taken to measure these coefficients. Details
of our experiments are given below.
The birefringence ∆n = ne − no was obtained by measuring with a CCD camera the transmitted intensity between crossed polarizers of a He-Ne laser as a function of
the local thickness in a wedge planar sample (0 to 30 µm).
The thickness profile of the empty cell was previously measured in green light (546 nm) by determining the positions of the equal-thickness fringes with a reflecting Leica
macroscope. Our measurements are reported in Figure 9.
The best fit to a law of type I = a + b cos(cd + f ) (with
c = 2π∆n/λ and λ = 632.8 nm) gives
∆n = 0, 0917 ± 0.0003
The indices were then deduced separately by measuring the phase shift ψ between the ordinary and extraordinary components of a He-Ne laser beam crossing
homeotropic samples of different thicknesses at an incidence angle
i = 15◦ 10′ . We
ψ has for expression
recall that
2i
sin2 i
ψ = kno d
−
1 − sin
1
−
. Our data “ψ as a
n2e
n2o
function of d” are shown in Figure 10. From their fit to a
linear law and the previous value of the birefringence, we
obtain
no = 1.55 ± 0.01
and ne = 1.64 ± 0.01.
A. Dequidt et al.: Lehmann effect in a compensated cholesteric liquid crystal
50
283
T = 59.5°C
ψ (°)
40
30
20
10
0
0
10
20
d (μm)
30
40
T = 59.6°C
Fig. 10. Phase shift between the ordinary and extraordinary
rays as a function of the sample thickness at oblique incidence
(homeotropic anchoring).
The pitch was measured as a function of temperature
by using commercial cells (from Instec, Inc) treated for
planar anchoring. The cells contain silica balls which allow
to fix their thickness accurately (5, 6.8, 9 or 20 ± 0.2 µm).
After filling, the cells were placed in a precision homemade oven both regulated and homogenous in temperature within 0.01 ◦ C. The absolute value of the pitch was
measured by looking for the particular temperatures at
which two zones with two different values of the twist
coexist. Let n and n′ be the numbers of half pitches in
two adjacent zones. It can be easily checked that the two
zones have exactly the same elastic energy at tempera′
)π
. In general, |n − n′ | = 1
ture T such that q(T ) = (n+n
2d
(sometimes 2). Figure 11 shows an example of a 20 µmthick sample in which an unwound zone (nematic phase,
black between crossed polarizers) is in equilibrium with
a half-pitch twisted zone. In this example, the temperature is adjusted in order that the χ-line separating the
two zones form a broken line joining the silica balls. By
contrast, the line segments bend visibly in the direction of
the zone of lower energy when the temperature is changed
by typically ±0.02 ◦ C. This method allowed us to measure
q(T ) (Fig. 12). The best fit of the experimental data to a
polynomial of degree 2: q = a(T − Tc ) + b(T − Tc )2 led to
dq
a = dT
(T = Tc ) = 0.1365 ± 0.001 µm−1 K−1 . The sign of
q was determined using a cell with a strong planar anchoring at the bottom plate and a gliding planar anchoring at
the top plate.
In order to measure the elastic constants, we used three
different methods.
First, we measured the Fréedériks transition in planar
samples of different thicknesses (6.8, 10, and 20 µm) using
a capacitive method (see Fig. 13). From these experiments
(and capacitance measurements of homeotropic samples)
we deduced successively the dielectric constants
ε = 9.4 ± 0.5
and ε⊥ = 4.5 ± 0.2,
the splay constant, or equivalently
K1
= 0.076 ± 0.006 V2 ,
ε0 εa
T = 59.65°C
Fig. 11. Three photographs taken at slightly different temperatures of the broken line separating an unwound zone (nematic
phase, black) from a half-pitch twisted zone (grey). The line
is pinned on the balls used to fix the sample thickness. At
T = 59.6 ◦ C, the line segments are straight, indicating that
the two zones have exactly the same energy (d = 20 µm).
with εa = ε − ε⊥ , as well as the ratio of the bend and
splay constants
K3
= 1.7 ± 0.1.
K1
from the direct
Note that K1 was obtained
measurement
1
of the critical voltage given by Vc = π εK
, while
0 εa
the ratio K3 /K1 was deduced from the fit of the whole
capacitance-vs.-voltage curve according to the procedure
given in [18]. We emphasize that we neglected the flexoelectric effects in the fitting procedure, which is justified
as long as the flexoelectric coefficient difference e3 − e1 is
smaller than 10−11 Cm−1 [19]. This is the case in our mixture, in which we measured e3 − e1 = 3× 10−12 Cm−1 [20].
284
The European Physical Journal E
57.3°C
57.6°C
57.9°C
58.2°C
58.5°C
58.8°C
220
capacitance (pF)
q (μm−1)
0.5
0.0
−0.5
200
180
160
52
54
56
58
60
62
64
0.4
T (°C)
Fig. 14. Cell capacitance as a function of the applied voltage
(π/2-twisted sample, d = 6.8 µm, f = 10 kHz). The solid lines
are just guides for the eyes.
1800
1.10
1600
1.05
1400
1.00
V 2c (V 2 )
capacitance (pF)
Fig. 12. Equilibrium twist as a function of temperature. The
different symbols correspond to samples of different thicknesses
(5, 6.8, 9 and 20 µm).
0.8
1.2
voltage (Vrms)
1200
1000
0.95
0.90
0.85
800
0.80
600
0.1
2
4
6 8
2
1
voltage (Vrms)
4
6 8
10
2
−1.2
−0.8
qd
−0.4
Fig. 13. Cell capacitance as a function of the applied voltage
(d = 10 µm, f = 10 kHz). The solid line is the best fit to the
theory.
Fig. 15. Square of the critical voltage measured in a π/2twisted sample as a function of the equilibrium twist of the
cholesteric. The solid line is the best fit to a linear law.
Second, we measured the Fréedériks transition in a
π/2-twisted planar cell (thickness 6.8 µm, from Instec, Inc)
at different temperatures close to the compensation point.
Curves are shown in Figure 14. In this geometry, the critical voltage is given by [21]
1 K2
K1
1 K3
qd
−
Vc2 = π 2
1+
1+
.
(21)
ε0 εa
4 K1
2 K1
π/2
Below V0 , the nematic phase destabilizes and quickly gives
a translationally invariant configuration (TIC) which then
slowly modulates to form cholesteric fingers (CF). Because V0 is very sensitive to the sample thickness, a special care was taken during the preparation of the samples
to limit their thickness variations (in practice, constant
within ±1 µm). We also worked in our home-made oven
to control at best the temperature (within 0.01 ◦ C). The
experiment was performed with samples of different thicknesses, ranging between 10 and 75 µm. All our data are
collected in Figure 16, in which we plotted V02 as a function of qd. The best fit to equation (22) led to
In Figure 15, Vc2 is plotted as a function of qd. From
the slope of the linear fit, we deduced
K2
= 0.068 ± 0.002 V2 .
ε0 εa
Finally, we measured the spinodal voltage of the nematic phase in homeotropic samples of various thicknesses
as a function of the temperature (or q). We recall that the
nematic-to-cholesteric transition is usually first order (for
a review, see [2, 12]). The spinodal limit of the nematic
phase can be calculated exactly and reads
V02 =
K3
K2 K2
(qd)2 − π 2
.
ε0 ε a K3
ε 0 εa
(22)
K3
= 0.15 ± 0.02 V2
ε0 εa
and
K22
= 0.0312 ± 0.0006 V2 .
K 3 ε0 ε a
It is worth noting that there is no data near the origin. This is normal inasmuch as the unwound (or “nematic”) state is stable when the equilibrium pitch is typically larger than the sample thickness. Consequently, the
A. Dequidt et al.: Lehmann effect in a compensated cholesteric liquid crystal
6 Direct evidence of the Lehmann rotation:
preliminary results
25
20
V 02 (V 2)
285
15
10
5
0
−20
−10
0
qd
10
20
30
Fig. 16. Square of the spinodal voltage as a function of the
product of the thickness and the equilibrium twist of the
cholesteric. The sample thickness was chosen between 10 and
75 µm. The solid line is the best fit to equation (22).
previous value of K3 given by the Y value in the fit curve
at X = 0 is extrapolated.
Finally, note that we neglected in all this section the
variations of the material parameters with respect to the
temperature, which is justified within our experimental
errors near the compensation temperature.
5 Lehmann coefficient
From the measurements described in the two previous
sections, we can calculate all the materials constants,
including the Lehmann coefficient ν. Some of the measured quantities being coupled (for instance, we measured K1 , K3 but also their ratio K3 /K1 ), the error estimation is not simple and was determined by using the
maximum-likelihood method [22]. This global method allowed us to calculate the most probable value and the
68% confidence interval for the elastic constants: K1 =
(3.4 ± 0.4) × 10−12 N, K2 = (2.8 ± 0.2) × 10−12 N and
K3 = (5.9 ± 0.6) × 10−12 N, and for the Lehmann coefficient [23]:
ν = (2.8 ± 0.6) × 10−7 kg K−1 s−2 .
This last result is important as it clearly shows that the
Lehmann coefficient does not vanish at the compensation
temperature, in agreement with the previous results of
Éber and Jánossy [6, 7].
Note, to end this section, that the only measurements
in homeotropic anchoring would lead to ν = (2.9 ± 0.6) ×
10−7 kg K−1 s−2 while those in planar anchoring (less precise) give ν = (2.2 ± 0.9) × 10−7 kg K−1 s−2 . These two
values are compatible within experimental errors, which
reinforces our results.
To prove more convincingly that the Lehmann coefficient is different from 0 at the compensation temperature,
we performed a new experiment using a planar gliding anchoring. This preliminary experiment is presented in the
next section and reveals a continuous rotation of the director in a temperature gradient.
As we mention in the introduction, the main problem to
observe the continuous Lehmann rotation is to make samples in which the director is allowed to freely rotate at the
boundaries. This requires to prepare gliding anchoring on
the glass plates, which is a real experimental challenge. In
this section, we describe our protocol to prepare and optically characterize such an anchoring. We then describe a
preliminary experiment which clearly shows the Lehmann
rotation at the compensation point. Finally, a model is
presented, which reproduces quite well the experimental
observations.
6.1 Planar gliding anchoring: preparation and optical
characterization
Two surface treatments have already been described in
the literature to obtain a planar gliding anchoring. “Gliding” means that the director is “free” to rotate in the azimuthal plane, without “solid friction”. On the other hand,
this does not exclude a viscous friction at the surface. The
first gliding anchoring was found by Dozov et al. [24] and
consists of treating the glass plates with a thin layer of (3glycidoxypropyl) trimethoxysilane (3-GPS). The second
one, better according to Blanc et al. [25], consists of covering the glass with the commercial photopolymer NOA
60 (provided by Norland Optics) which is then polymerized by exposure to UV light. We thus bought this glue
and tried to reproduce this surface treatment. For some
unknown reasons, we did not succeed in preparing a good
gliding anchoring in this way. We thus tried a new surface
treatment which turned out to give good results.
Our protocol was the following. It consisted of spreading by spin-coating a thin layer of the hardener of an epoxy
glue (structalit7 from Eleco). The latter was first dissolved
in a ketone, the 2-butanone (5% in mass of hardener). The
solution was then filtered through 0.2 µm PTFE membrane to eliminate dust particles and then spread by spincoating on the glass plate at 500 rpm for 1 min. At this
concentration, the resin formed a homogeneous thin layer
after evaporation of the ketone.
To characterize this surface treatment, we prepared
a sample with the top glass plate treated in this way,
whereas the bottom plate was treated in strong planar
anchoring (rubbed polyimide). The sample thickness was
10 µm and the mixture 8OCB + CC was introduced by
capillarity in the cholesteric phase. All measurements were
performed in a Mettler oven. To rapidly eliminate the oily
streaks which form after filling, the sample was rapidly
heated into the isotropic phase and then cooled down into
the cholesteric phase. Observations in white light under
the microscope showed that the sample became rapidly
homogeneous (in less than 1 min), without any visible texture. On the other hand, its color between crossed polarizers was function of the temperature, indicating that the
director was rotating on the top glass plate when the temperature was changed.
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The European Physical Journal E
7x10
−3
I⊥ (a.u.)
6
5
4
3
2
1
0
45
Tc
55
50
60
T(°C)
Fig. 17. Transmitted intensity between crossed polarizer and
analyzer as a function of the temperature (d = 10 µm). Dots
are the experimental points. The solid line is the best fit to
equation (23).
To characterize this new surface treatment, we performed two experiments under the microscope.
First, we increased by successive increments of 0.2 ◦ C
the temperature from the compensation temperature. At
each temperature, we visually searched for the analyzer
position for which the transmitted intensity was minimum
when the polarizer was parallel to the anchoring direction
on the bottom plate. Observations were made in green
light (λ = 546 nm). At the compensation temperature, we
checked that the extinction was obtained between crossed
polarizer and analyzer, which was characteristic of a nematic phase. By increasing the temperature, we observed
that we needed to rotate the analyzer anticlockwise to get
the minimum intensity, showing that the director was rotating in the same direction on the top glass plate (to a
first approximation, the plane of polarization of the light
follows the director rotation when the cholesteric pitch p
is larger than d∆n). In this way, its was possible to determine the sign of q: positive above Tc and negative below it.
The second experiment was more quantitative as we
measured with a photodiode the transmitted intensity
across the sample between crossed polarizer and analyzer
as a function of the temperature. The polarizer was parallel to the anchoring direction at the bottom plate. A
temperature ramp was imposed by a Mettler oven. The
temperature was measured with a thermocouple placed
beside the sample. The temperature and the light intensity were measured sequentially every 1 s with a Keithley 2001 multimeter equipped with a scanner card. Apart
from systematic small temperature shifts (smaller than
0.5 ◦ C) mainly due (in our opinion) to the fact that the
thermocouple was not in contact with the liquid crystal,
all curves we obtained were similar whatever the heating (or cooling) speed (chosen between 0.5 ◦ C/min and
5 ◦ C/min). One typical curve is shown in Figure 17. It
has an oscillating shape with an absolute minimum (corresponding to a complete extinction) at Tc . We shall note
that in this example, the values of Tc and of the clearing
temperature were smaller than expected by about 3–4 ◦ C.
This was certainly due to a problem of pollution of the
liquid crystal when it comes in contact with the hardener
of the epoxy glue. On the other hand, we observed that
Tc did not change significantly during many hours once
the sample was prepared. In order to analyze this curve,
we calculated the expression of the transmitted intensity
by assuming a planar gliding anchoring at the top glass
plate. We found
√
κ2 + cos 2qd 1 + κ2
1
I
= − cos(2qd)
I0
2
2(1 + κ2 )
√
sin 2qd 1 + κ2
√
,
(23)
− sin(2qd)
2 1 + κ2
ne −no
with κ = 2π
= k∆n
λ
2q
2q . To fit our data, we chose as
fit parameters the compensation temperature Tc (assuming that the equilibrium twist is still given by q(µm−1 ) =
0.1365(T − Tc ) + 0.0019(T − Tc )2 , with the temperatures
in ◦ C, see Sect. 4), the sample thickness d, the intensity I0
and the birefringence we took in the form ∆n = a − b(T −
Tc ). The fit of the experimental curve to this law is good as
seen in Figure 17. It led to Tc = 55, 9 ◦ C, I0 = 7, 3 × 10−3 ,
d = 9.1 µm, and ∆n = 0.083 − 0.0021(T − Tc ). Note that
the values of the thickness and of the birefringence at the
compensation point found from the fit are close to the expected values: 9.1 µm instead of 10 µm for the thickness
and 0.083 instead of 0.09 for the birefringence. These results made us confident in our initial assumption that the
new anchoring is planar and gliding. On the other hand,
we observed that it was degraded after one day (hysteresis and memory effects occurred). This is not surprising as
the hardener of the epoxy glue (which is not polymerized)
certainly slowly dissolves in the liquid crystal. We will not
describe these aging effects in this paper because they are
out of the scope of our study.
6.2 Experimental evidence of a continuous Lehmann
rotation at the compensation point
In order to observe the Lehmann effect, we prepared a
10 µm-thick sample with the two glass plates treated for
gliding planar anchoring. The sample was filled with the
cholesteric mixture and annealed at the compensation
temperature during one hour in a Mettler oven without
the cover. Immediately after filling, the sample was full of
disclination lines perpendicular to the glass plates, forming a typical structure à noyaux or “Schlieren texture”
characteristic of the nematic phase. Due to their mutual
attraction, the disclination lines of opposite signs progressively annihilated. After one hour, the sample was almost
free of disclination lines with “large” zones (of a few tenths
of mm in size) in which the director orientation was everywhere approximately the same. The next step was to
impose a vertical temperature gradient while maintaining
the sample at the compensation temperature. To do this,
we started to blow air onto the top of the sample while
increasing the temperature of the oven. To maintain the
liquid crystal at the compensation temperature, we simultaneously observed its optical texture under the microscope. This method, although very simple, was efficient
A. Dequidt et al.: Lehmann effect in a compensated cholesteric liquid crystal
287
Transmitted light (a. u.)
140
120
100
80
60
40
20
0
500
1000
Time (s)
1500
Fig. 19. Intensity measured with a camera video inside the
square shown in Figure 18. Crosses are experimental points and
the solid line is the best fit to a sinusoidal law of period T =
100 s. Only one-third of the recording is represented. During
the full recording the director rotated locally by about 25 π.
Fig. 18. Zone inside the sample where the director rotates
continuously by forming concentric rings encircling a central
defect. The rings propagate inward and collapse on the defect.
The square in the first photograph marks the place where we
measured the intensity (see Fig. 19). Close to the right side of
the photos, one can see a disclination line, the two extinction
branches of which rotate at constant angular velocity. Photos
are taken between crossed polarizer and analyzer. Each photo
is 450 µm wide.
because the grey level of the extinction branches starting
from the cores of the disclination lines (which are almost
black at the compensation temperature) is very sensitive
to the temperature. In this way, it was possible to increase
the temperature of the oven by about 5 ◦ C above the compensation temperature, thus imposing a vertical temperature gradient to the liquid crystal layer. We then started
to record with a video camera the time evolution of the
texture. We immediately observed that the branches of
the disclination lines were rotating, generating black and
white fringes. This observation revealed that the director
was continuously rotating under the action of the temperature gradient. At some places, the fringes formed rings
encircling small defects of the surface treatment (certainly
due to a dewetting of the hardener of the epoxy resin). At
these places, the rings were moving continuously inward
and collapsed in the centre as shown in Figure 18. To determine the periodicity of the process we measured the
transmitted intensity at one point of the sample (more
exactly inside a small square shown in Fig. 18) as a function of time (Fig. 19). This graph shows that the period,
which corresponds to the passage of one fringe—and thus,
to a rotation of the director by π/2—is close to T ≈ 100 s.
We also checked qualitatively that the rotation was slowing down when the temperature gradient was decreased.
On the other hand, it was impossible with our setup to
reverse the temperature gradient to check that it was possible to unwind the director field. In the following subsection a very simple model is developed to explain this
observation.
6.3 Comparison with the theoretical model
One supposes that the director lies in the (x, y)-plane and
is invariant by translation along the z-axis. To simplify,
we assume that K1 = K3 ≡ K and that backflow effects
are negligible. In this case, the torque equation becomes:
γ1
∂ϕ
= −νG + K
∂t
∂ 2 ϕ 1 ∂ϕ
1 ∂2ϕ
+ 2 2
+
2
∂r
r ∂r
r ∂θ
,
(24)
where γ1 is the local rotational viscosity, ϕ the angle between the director and the x-axis, and (r, θ) the polar
coordinates.
In order to model the central anchoring defect, we assume that its main effect is to locally increase the liquid
crystal viscosity. Under this assumption, viscosity γ1 in
equation (24) becomes a function of r of the type:
r
2
γ1 + δγ1 e−( r0 ) ,
(25)
where r0 represents the typical radius of the defect. Solving with Mathematica equation (24) with γ1 given by
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The European Physical Journal E
equation (24) yields
νG
t
b
2
δγ1 r02
r
r2
−νG
+
c
log(r)
C +Γ 0, 2 − log
b 4K
r0
r02
γ1 − b r2
.
(26)
−νG
b 4K
ϕ = a−
In this expression, C is the Euler constant, Γ the incomplete Euler function, and a, b, and c are integration constants. In order that ϕ does not diverge at the center of
the defect, one must take c = 0. Constant a corresponds
to a phase shift. It may be vanished by changing the time
origin. In the following, we shall take a = 0. Finally, one
must take b = γ1 to cancel the torque ∂ϕ/∂r at infinity.
With this choice of the constants, the solution reads
2
r
νG
δγ1 r02
r2
t − νG
ϕ=−
.
C + Γ 0, 2 + log
γ1
γ1 4K
r0
r02
(27)
This solution contains two terms. The first one is just
a function of time: it gives the period T of the process
which is the time interval between the passage of two black
fringes at the same point of the sample. As there is extinction when the director is either parallel or perpendicular
to the polarizer, we obtain
π γ1
T =
,
(28)
2 ν|G|
where we introduced |G| as G may be positive or negative
(in our previous experiment, G was negative as we heated
from below). The second term depends on the spatial coordinate r only. It gives the spatial organization of the
director and the number of rings (between crossed polarizers) around the defects. We shall note that the larger
δγ1 , the larger is the number of rings. On the other hand,
period T is independent of the “strength” of the central
defect, which makes this quantity very interesting to measure experimentally as it only depends on the Lehmann
coefficient ν and on the rotational viscosity γ1 .
This solution may be generalized in order to take into
account the presence of a disclination line on the right
of the central defect as observed experimentally in Figure 18. In this case, the solution is obtained by adding to
solution (27) the quantity mθd where m is the topological
rank of the defect and θd the polar angle measured from
the core of the disclination line. This solution is represented in Figure 20 over one period by taking m = −1/2
(with ν > 0 and G < 0). We note that the simulation is
very close to the experiment and that the sign of m was
chosen in order that the two extinction branches starting from the core of the line rotate in the same sense as
in the experiment. Unfortunately, we cannot deduce from
this preliminary experiment the value of the Lehmann coefficient because the temperature gradient in the liquid
crystal layer is largely unknown. In addition, although we
measured viscosity γ1 in a previous article [20], its value
may be different in the present sample because of the pollution by the glue. Thus, the next step will be to check
1
6
2
7
3
8
4
9
5
10
Fig. 20. Representation (between crossed polarizers) of the
solution calculated numerically. The parameter δγ1 has been
chosen in order that the number of rings be approximately the
same as in experiment.
the viscosity value and to make a special setup to impose
a controlled temperature gradient.
7 Conclusion
In conclusion, we have found, in agreement with the pioneer work of Éber and Jánossy, that the Lehmann coefficient does not vanish at the compensation temperature
in a cholesteric liquid crystal. We additionally found that
the Lehmann coefficient is independent of the equilibrium
twist q within our experimental resolution. On the other
hand, we cannot say anything about a possible dependence of this coefficient on the actual twist n · ∇ × n as
in our experiments with fixed boundary conditions the
cholesteric phase is almost completely unwound because
of the strong (homeotropic or planar) anchoring on the
glass plates. These results experimentally prove the existence of a Lehmann effect of molecular origin which is
certainly strongly reinforced by the collective behavior of
the molecules responsible for the quadrupolar order of the
phase. This conclusion is in disagreement with the theory defended by Pleiner and Brand. Indeed, these authors
A. Dequidt et al.: Lehmann effect in a compensated cholesteric liquid crystal
would be right if, as they claim in reference [9], the symmetries of a compensated cholesteric were exactly the same
as those of a usual nematic phase. This is incorrect because a compensated cholesteric made of chiral molecules
does not have symmetry planes and inversion centres. The
same reasoning applies to an isotropic solution of chiral
molecules and this is why such a solution has a rotatory power contrary to isotropic liquids made of achiral
molecules.
We would also like to emphasize that we determined
this coefficient in two different geometries (homeotropic,
but also planar, which is new) and that our measurements
of the material constants were often redundant, but always compatible with one another. In addition, the two
experiments yielded similar results as we have emphasized
before.
A well-informed reader could also object that our results contradict those of Madhusudana et al. [26, 27] about
the electric Lehmann effect at the compensation point.
For this reason, we redid their experiment with our mixture. Although our observations were essentially the same
as theirs, we showed that they could not result from an
electric Lehmann effect, but more simply from flexoelectricity [20].
Finally, we observed a continuous Lehmann rotation of
the director at the compensation temperature in a sample treated on both sides for planar gliding anchoring.
The next step is now to make a new experimental setup
to study the Lehmann rotation in samples treated with
this new surface treatment. One crucial point will be to
measure accurately the vertical temperature gradient, for
instance by using contact thermocouples. The setup will
also have to allow the possibility of reversing the temperature gradient. Measurements of the thermal conductivity
of the liquid crystal will also be necessary to calculate the
exact temperature gradient inside the liquid crystal. Finally, we plan to improve and to better characterize the
surface treatment which strongly pollutes the liquid crystal for the moment.
We thank Yves Pomeau, Sriram Ramaswamy and Pawel Pieranski for helpful discussions. This work has been supported by
the Polonium Program No. 11622QC.
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