Mol. Cryst. Liq. Cryst., 1983, Vol. 98, pp. 183-191
0026-8941/83/9804-0183/S18.50/0
© 1983 Gordon and Breach, Science Publishers, Inc.
Printed in the United States of America
Electrohydrodynamic Behavior in
Twisted-Wedge Nernatic Structures
1
"
J.A. MARTIN-PEREDA, M.A. MURIEL and F.J. LOPEZ
Departamento de Electrónica
Cuántica,
E.
T.
S.
Ing.
Telecomunicación,
Universidad Politécnica de
Madrid,
Ciudad
Universitaria,
Madrid-3,
Spain
(Received January 31, 1983)
A new type of domain for nematic liquid crystals with a twisted-wedge structure is presented.
This new type of domain appears from the low frequency range to 10 kHz. This behavior was
observed for square and pulsed excitations.
The liquid crystal was W-(p-methoxybenzylidene)-p'-butylaniline) (MBBA) used at room
temperature. These domains offer a higher degree of complexity than conventional Williams
domains. The corresponding stability chart is presented.
I. INTRODUCTION
In nematics, electrohydrodynamic instabilities have been extensively stud-
ied by numerous workers and seem presently to be well understood, at least
at low frequencies. Different types of domain appear depending on the
voltage and frequency applied to the nematic liquid crystal cell. Two
regions clearly appear, separated by a cut-off frequency, f
c
. For dc or ac
fields where/ < f
c
, there are two different modes, Williams domains (stria-
tions or cellular patterns) and dynamic scattering, both associated with
hydrodynamic flow. For ac fields with/ > /., a fast turn-off mode appears
with striations or chevrons with a few jam-periodicity, associated with bend
oscillations. For instance, in the case of MBBA, the plot of voltage thresh-
old, V
c
, of instabilities vs frequency/, of the applied field, has an 5-shape,
much more pronounced for square wave excitation than for sine wave
Presented at the Ninth International Liquid Crystal Conference, Bangalore, December
6-10, 1982.
183
184 J. A. MARTIN-PEREDA, M. A. MURIEL and F. J. LOPEZ
excitation; for a 50 /xm-thick sample,/, is between 75 and 100 Hz. Volt-
ages lie between 10 volts for the conduction regime (low frequencies) and
around 100 volts for the dielectric regime (high frequencies). The sample
was in almost every case contained in a sandwich cell having Sn0
2
-coated
glass plates, and the nematic liquid crystal had a homogeneous planar
texture.
1
'
2,3
'
4
Williams domains are at angles to the direction of rubbing.
Recently, a new type of domain, with a static aspect, was observed at
frequencies above the disappearance of the chevron pattern.
5
This structure
appears in the dielectric region (till to 8 kHz) by raising the frequency at
constant voltage until disappearance of the chevron structure. This new
type of domain was observed both for homogeneous cells and for twisted
geometries.
In this paper we extend some results previously reported by us
6
showing
a new stability chart with some features not presented before for nematic
liquid crystal cells with twisted wedge geometries.
II.
EXPERIMENTAL
The liquid crystal device employed in this work is similar to the one
reported by us as a light deflector, both analogue
7
and digital,
8
and as an
optical modulator.
9
We have used the same wedge structure, with a pris-
matic angle around 2° and a Mylar spacer in just one side of the cell.
Internal transparent electrodes were deposited by the conventional spraying
method. We have used Sn0
2
and ITO electrodes. However a different
nematic liquid crystal and molecular configuration have been used in this
case,
namely MBBA in a twisted configuration in the wedge cell. The
working temperature was room temperature and the MBBA had a large
number of charge carriers. As is known, MBBA has a refractive index An
of about 0.24 and a negative dielectric anisotropy of -0.53e
0
. The cell is
shown in Figure 1.
With this configuration we have observed the behavior of the liquid
crystal for frequencies between 0 Hz and 10 kHz, for different conditions
of the applied electric field. We used square and pulsed signals with peak
values up to 60 volts.
The diffraction pattern was observed by illuminating the sample with a
parallel nonpolarized 0.5 mW He-Ne laser beam, normal to the front
surface. With a lens, it was possible to see the domains inside the cell
projected on a screen. With this method, by applying a dc voltage to the
cell, one observes domain formation at 10 volts, as has been reported
previously for non-twisted structures.
10
However, the main features appear
for ac voltages. The experimental set-up is shown in Figure 2.
ELECTROHYDRODYNAMICS IN NEMATICS
185
100jjm ,
5n0
2
k—•]
/
L
FIGURE 1 Liquid crystal wedge cell.
111.
RESULTS
We have applied a square voltage, with zero volts as mean value, with
frequencies from
1
Hz to 10 kHz and found four different regions for every
observed frequency. From 0 volts to a certain V
x
, no diffraction pattern is
obtained. From this V¡ to another V
2
, a structure similar to Williams do-
mains can be observed.
1
'
4
The size of these domains was the same for the
entire range of frequencies. From V
2
to a certain value V
3
we got a very
different diffraction pattern shown in Figure 3. As can be seen, this main-
tains the main features of the Williams domain diffraction pattern, but with
two sets of points parallel to it. The corresponding domains are shown in
Figure 4. These domains are very different from the conventional Williams
domains. The pattern is stationary at fixed voltage without any turbulence
up to V
3
, where a structure like the one shown in Figure 5 appears. This
structure is very difficult to keep static and for still higher voltages tur-
bulence dominates. The diffraction pattern is shown in Figure 6. Samples
of high quality can show as many as 11 parallel lines comprising more than
31 points in each one.
186
J. A. MARTIN-PEREDA, M. A. MURIEL and F. J. LOPEZ
SCREEN
WEDGED
LIQUID CRYSTAL
CELL
HE-NE
LASER
BEAM
0.5 mW
FIGURE 2 Experimental set-up.
The corresponding diagram for the above situation is shown in Figure 7
where the four regions indicated above are depicted. The main feature of
this representation is the peak around 10 Hz. The other interesting point is
the strong increase in voltage from 1000 Hz. The region below 10 Hz
corresponds to electrohydrodynamic instabilities modulated at the same
frequency as the voltage. This modulation can be seen clearly on the
diffraction pattern. From 10 Hz the modulation disappears and the domains
are static in
nature.
From 1000 Hz, the voltage difference between different
regions becomes smaller and becomes very close around 10 kHz. For higher
frequencies, we were not able to obtain any diffraction pattern.
The results were very similar with a pulsed voltage. The corresponding
stability chart is shown in Figure 8. The same peak at 10 Hz appears as
does the strong increase in voltage from 1000 Hz. It is interesting to point
ELECTROHYDRODYNAMICS IN NEMATICS 187
FIGURE 3 Diffraction pattern corresponding to voltages V
2
< V < V, where V
2
and
V
3
are
defined in the text.
FIGURE 4 Domains corresponding to the diffraction pattern of Fig. 3.
