A strong electroacoustic effect of a ferroelectric liquid crystal cell was observed with a 1 MV/m electric square field in the frequency range from 3 kHz to 30 kHz. The sound emitted can be heared with the naked ear and easily detected with a simple microphone. The electroacoustic effect vanishes in the chiral SA-phase

Dierking, Ingo

Gießelmann, Frank

Zugenmaier, Peter

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STRONG ELECTROACOUSTIC EFFECT IN 
FERROELECTRIC LIQUID CRYSTAL CELLS 
F. GIEBELMANN. I. DIERKING and P. ZUGENMAIER 
Institut fur PhysikaHsche Chemie 
der Technischen Universitat Clausthal 
Arnold-Sommerfeld-Str. 4 
D-3392 Clausthal-Zellerfeld 
Federal Republic of Germany 
A strong electroacoustic effect of a ferroelectric liquid crystal cell was 
observed with a 1 MV 1m electric square field in the frequency range from 
3 kHz to 30 kHz. The sound emitted can be heated with the naked ear and 
easily detected with a simple microphone. The electroacoustic effect 
vanishes in the chiral SA-phase. 
INTRODUCTION 
In surface stabilized geometries liquid crystals with chiral smectic C" 
phases exhibit a fast electrooptic effect l ) caused by their ferroelectric 
properties2.3) . Due to their ferroelectric behaviour, electromechanic effects 
like piezoelectricity, flexoelectricity or electrostriction are expected"'). 
Because the liquid-like mechanical behaviour of low molecular mass liquid 
crystals seems to be unsuitable for a study of electromechanical effects at 
equilibrium conditions, recent investigations of electromechanical effects 
focus on FLC-polymers and FLC-elastomers'·7}. As will be presented in 
this letter, low molecular mass FLC's in thin surface stabilized cell also 
show a remarkable electromechanic effect, if the frequency of the 
activating field is in the range of high sonic or ultrasonic frequencies. 
EXPERIMENTAL 
The compound 4-(S)-( + )-2-[chloro-3-methyl-butyryloxyJphenyl-4-(de-
cyloxy)-benzoate8) ill is used in this study and exhibits a monotropic s~ 
phase within the phase sequence: cryst. . (Sd ·45 ' C . SA . 64 ' C • isotr. 
IO.'i 
106 F GI EBElM ANN . I DI ERKING and P ZUGENM AIER 
At 40·C the director tilt angle is 14 degrees, the spontaneous polarization 
- 40 nC cm-', the internal rotational viscosity - 50 mPa S-I and the splay 
elastic constant 1.5· 10-11 N m-' ')_ 
1 
The compound is filled in a commercially available 41'm cell (E. H. C. 
Co_ Ltd_, Tokyo) with transparent Ito electrodes and parallel rubbed 
polyimide layers which leads to a planar orientation of the LC phases. The 
exact thickness of the cell used was determined to 3.85 I'm by the 
interference spectrum of the empty cell with application of the corrections 
given by YanglO) considering the optical influence of the polyimide and Ito 
layers_ 
The cell is placed on a home made hot stage in a polarizing microscope 
and connected to the voltage supply (HP 8116A pulse/function generator 
and Krohn Hite KH-7500 broadband amplifier). The hot stage consists of 
a single hot·plate which can be rotated around two axes, parallel and 
perpendicular to the optical axis of the microscope. 
A well oriented S~·phase is obtained after slowly cooling down from 
the isotropic phase. At 40 ·C the original chevron structure of the cell is 
transformed into a bookshelf structure by applying a high amplitude (~ 15 
MY m·l) and low frequency (~ 200 Hz) electric square fieldOl. Sub· 
sequently the amplitude of the applied field is decreased to I MV m-I and 
the frequency increased from 3 to 30 kHz. The sound emitted from the cell 
can be heared with the naked ear, if the frequency of the applied field is 
below the cut·off frequency of the human ear. 
The acoustic response is recorded after amplification (fektronix AM 
502) with a digitzing memory oscilloscope (Hewlett-Packard 54 200A) by 
a simple microphone placed next to the cell and perpendicular to the 
substrate normal . For eliminating low frequency statistical noise, 64 frames 
of the signal are averaged. 
ELECfROACOUSTIC EFFECf IN FLC CELLS 107 
RESULTS AND DISCUSSION 
At 40 'C and application of an electric square field of I MV m" 
amplitude the macroscopically observable electrooptic effect of the cell 
almost vanishes between a frequency of 1 to 2 kHz. In this frequency range 
fiuctuations within the texture can be observed between crossed polars. At 
higher frequencies these visible fluctuations decrease and a sound can be 
b: 40 'c 
o. 1\ 
• 
, . 
d: 46 'c 
,. 
, 
, 
• • : 
. ,. 
o. 
a: 
,. 
! 
, 
= • 
" 
-I" I!-r---,o:;.'---;:, •• -~-;";;---~=-~--;i,,. 
,. I. ,. ,. 
I, . ,~ .. 
... I • ,. ,. 
I, . , ft " 
T,. ,. ,. 
c: 44 °C 
,. 
, 
:- . 
, 
.,. 
• 
e: 48 'C 
,. 
, 
:- I 
• 
'0 . 
• 
o • ,. I . '. 
,. 
I . .. . . ,. 
L I I I L I l 
o • ,. 
'" '. I . 
FIGURE 1 Applied electric field (a) and acoustic responses, plotted as 
amplified microphone voltage vs . time, at different temperatures; b: 40 °C, c: 44 
°C, d: 46 DC and e: 48°C. The spikes on the response curves are due to the 
electromagnetic induction from the electrode connectors and serve as markers for 
the field reversal . 
IOI! F GIEBE\.\1A:-":'" [ DI ERK ING .m.:! P Z UGES\1AIER 
heard. The acoustic effect disappears, if the temperature is increased above 
the S~ - SA phase transition temperature or if the monotropic S~ phase 
crystallizes, and it also vanishes, if an empty cell or load resistance is used. 
These observations lead to the conclusion, that the electroacoustic effect is 
correlated with the S~ phase. 
Fig. I depicts the acoustic response of the cell plotted as amplified 
microphone voltage vs. time for a 7 kHz square field (Fig. la) with 
I MV m' ! amplitude. At 40 ' C (Fig. Ib) the acoustic response essentially 
consists of vibrations in the fundamental and second harmonic frequency. 
The amplitude decreases slightly, if the temperature is raised to 44 'C 
(Fig. Ie), very close to the S~ - SA phase transition. The amplitude of the 
electroacoustic effect breaks down rapidly after passing the phase transition 
at 46 ' C (Fig. Id) and is scarcely detectable at 48 'C (Fig. Ie). The spikes 
on the response curves are due to the electromagnetic induction from the 
electrode connectors to the unshielded microphone wires. 
a: 4 k.Hz b: 10 kHz 
,. ,. 
· ,. ': 1111 
V 1\ ~ • VI· , · • • • • r 
,: - !IIi ': -1 • 
. ,. .,. , ,. ,. 
• 
, . ,. ,. •• ,. f, .. , ~ • T •• • ~ .. 
c: 18 kHz d: 26 kHz 
,. ; : l ': 1111 , 
• • • 
· ~ ' · V 
-;111 ,1. t . 
• 
,. ,. 
• 
,. , . 
T, . ,~ .. ,,- , .. 
FIGURE 2 Acoustic response to an electric square field with 1 MY m'l 
amplitude ploned as amplified microphone voltage vs. time at several electric field 
frequencies and 40 ' C; a: 4 kHz, b: iO kHz, c: 18 kHz and d: 26 kHz. 
ELECTROACOUSTIC EFH:.CT I~ FLC CELLS 109 
The frequency behaviour of the acoustic response is rather complex. 
Some representative response curves to a I MV m·1 electric square field are 
depicted in Fig. 2. At an electric field frequency of 4 kHz (Fig. 2a) the 
response shows a frequency characteristic consisting essentially of the 
fundamental, second and third harmonic frequency (the electric field period 
is marked by the distance of the induction spikes, cf. Fig. I). The con-
tribution of the third harmonic frequency is lost between 5 and 6 kHz and 
a response curve, as shown in Fig . 2b at 10 kHz electric field frequency, 
is observed. The second harmonic response vanishes at field frequencies 
above 12 kHz and the response to the electric square field is almost a sine 
wave with the fundamental frequency, depicted at 18 kHz frequency of the 
electric field in Fig. 2c. An electroacoustic effect can still be observed in 
the ultrasonic frequency range as shown at 26 kHz in Fig. 2d, but the 
acoustic effect can not be detected anymore above a field frequency of 
30 kHz. 
The amplitude changes as detected at different frequencies should be 
regarded as preliminary results, since neither the frequency characteristic 
of the microphone nor the influence of standing waves and interference due 
to the surrounding geometry are known. Amplitude maxima are detected 
at electric field frequencies of 4, 6, 13, 18 and 26 kHz. 
It is worth to note, that the acoustic effect can be supressed by applying 
a small dc-offset field of about 0.2 - 0.3 MV m· l . The response can be 
damped, if some mechanical pressure is exerted on the upper glass plate of 
the cell. 
An explanation of the electroacoustic effect cannot be provided at the 
time. The following observations are found : 
The electroacoustic effect arises after the macroscopically 
observable electrooptic effect breaks down. 
The electroacoustic effect is correlated with the S~ phase 
temperature range. In the chiral SA phase only a poor or no 
response is observed. 
The electroacoustic effect leads to a strong nonlinear response 
with regard to the applied electric square field . The relaxation 
of contributions with higher harmonic frequencies can be 
observed at certain field frequencies . 
110 F GrEB EL~1A N N. I DIERKI NG and P ZUGEN MAIER 
The electroacoustic effect can be suppressed by small electric 
dc-offset fields. 
An explanation based on the electroclinic effect, which causes a change 
in the smectic layer thickness, can almost be excluded because of the 
nonlinearity and the frequency characteristics and because the response 
almost vanishes in the chiral SA phase. The correlation with the S~ phase, 
the cut-off frequency at 30 kHz and the suppression of the effect by dc-
offset fields") point towards a description in relation with the Goldstone 
mode. But pure Goldstone mode fluctuations occur at constant tilt angle 
and, therefore, constant smectic layer thickness, and seem not suitable for 
an explanation of a mechanical effect. Explanations for the effect may be 
found in reversible changes of the orientationallayer structure or reversible 
domain wall motions. 
ACKNOWLEDGEMENT 
This work was supported by the Deutsche Forschungsgemeinschaft . 
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Strong electroacoustic effect in ferroelectric liquid crystal cells

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Strong electroacoustic effect in ferroelectric liquid crystal cells

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