DEPENDENCE OF ROLL FREQUENCY ON DRIVING
GRC ON LIQUID
CRYSTALS 2005
FREQUENCY IN NEMATIC LIQUID CRYSTAL MIXTURE
G. Acharya, J.T. Gleeson
Department of Physics, Kent State University, Kent, OH 44242 USA
ABSTRACT:
We report on the variation in frequency of
traveling rolls with the variation of driving
frequency on electroconvection
By using Fast Fourier transform(FFT),the frequency of
NT rolls were calculated at different driving frequency
at 60oC. Finally, graphs of NT roll frequencies
were
plotted against the driving frequency(fig.6) The
frequency of rolls were found to be increased with
temperature at fixed onset voltage and variable driving
frequency.
of Nematic liquid crystal mixture driven by ac
voltage at given temperature. The frequency of
rolls are found to be zero at low voltage and
driving frequency and increase with the increase of
voltage and corresponding frequency at given
temperature. However, at higher frequency,instead
of normal traveling rolls,we observed
aperiodic
oblique traveling rolls. The driving frequency at
onset was found to be smaller at lower temperature
than that at higher temperature.Also,we did not
observe the traveling rolls when dc was applied on
the sample cell.
240
Intensity(arbitrary Unit)
230
220
210
200
190
180
170
1
2
3
4
5
6
Time(sec)
INTRODUCTION:
n(O)-Om (CnH2n+1)-(O)-
Since the sample is chemically stable, almost all of
the material properties are known and it is easily
obtained, MV is a suitable material for EHC[5].
-O-CmH2m+1)
22.0%
30.3%
4-heptyloxyphenyl-4'-hexylobenzoate
13.3%
4-butyloxyphenyl-4'-hexylbenzoate
34.4%
4
3
f(Hz)
4-octaphenyl-4'-pentyloxybenzoate
To control the electrical
conductivity, dopant (mixture of equal weight of pheptylbenzoic acid and octyloxyphenol) (3wt%) was
added
to
the
sample.The
Conductivity
varies
slightly for individualal cell[7].Difference in
conductivity between individual cell may cause to
the
variation
of
respective
threshold[4].We
observed temperature dependence of both C┴ and G┴ on
the temperature of filled cell (fig.3)
2
1
0
600
700
800
900
1000
1100
1200
1300
F(Hz)
Figure 6: Frequency of Traveling Rolls vs Driving
Frequency in the 23.39 μm sample cell at 60oC.
60
50
When we increase the ac voltage, beyond certain
critical value Vc, spatiotemporal structures form
which
include
oblique
stationary(fig.1),
normal
stationary(fig.2),
normal
traveling
and
oblique
traveling rolls.
7.2
40
F(Hz)
30
20
7.0
6.8
6.6
V(Vdc)
In case of planar arrangement, if
p and
q be the
components of wave vector parallel and perpendicular
to the director, the states with q = 0 are normal
rolls and the states with q ≠ 0 are oblique rolls.A
point of critical driving frequency where the system
goes from q = 0 to q ≠ 0 is the Lifshitz point.
-COO –
4-hexyloxyphenyl-4'-methoxybenzoate
Also, we studied the variation of DC threshold
with temperature (fig.7)for the cell MV + 2.7%
dopant. No traveling rolls were observed. We had
used 3 cells having roughly area of conducting
cell as 5 by 5 mm2 .
-9
-1
Gperp(10 Ohm )
When ac is applied, in the conduction regime charge
relaxation is faster than the ac frequency. The sign
of the director deflection remains same, but the
charge density alters its sign with the applied ac.
In the dielectric regime, the director deflection
alters its sign with the applied ac field, but the
sign of charge density remains same[4].
Figure 2: Normal Stationary Rolls taken on a 24.22 μm cell at
60oC,9.85V and 150 Hz. The rubbing direction is aligned in the
vertical direction of this picture.
Cperp(10-12F)
Nematic Liquid Crystal (NLC) molecules have common
orientational order but do not have positional order.
As a result, only diffuse scattering occurs in the xray diffraction pattern. The correlation length
ξ║ ≠
ξ┴.As compared to isotropic fluids, a nematic is
described
by
an
additional
director
field
n
=(nx,ny,nz), n2 = 1 and n and -n are indistinguishable.
Nematic phase occurs only to those materials which are
`achiral' or ‘racemic’[1].On heating, some of the
thermodynamic
properties
like
specific
heat,
coefficient of thermal expansion and coefficient of
isothermal compression increase rapidly when the
temperature approaches to Nematic-Isotropic(N-I) point
TNI[2].At TNI there will be complete breakdown of
orientational order and a weak first order phase
transition takes place .To occur EHC,the NLC must be
doped with ionic impurities and also the anisotropy in
the dielectric constant Єa = Є║ - Є┴ must be negative
or only slightly positive[3].
Figure 5: Intensity of single piexl vs
time .
10
0
30
40
50
60
6.0
Figure 3: Variation of C┴ and G┴ with temperature in the 24.22 μm
sample cell.
The sample was kept in hot stage FP 82 on the
microscope. The sample showed a Nematic-Isotropic
phase transition at 63oC. A CCD camera was mounted
on the microscope at about 30 cm from the sample to
cast the image on the TV screen and various image
were captured with the help of snap program.
At different driving frequency,ac voltage at onset
were recorded
at 600C and plotted which followed
the equation V2(f) =V02/(1- f2/fc2) (fig.4)
60
50
6.4
6.2
70
o
T( C)
5.8
5.6
30
40
50
60
T(oC)
Figure 7: Threshold Vdc vs Temperature in the 24.97 μm sample
cell .
CONCLUSION:
The frequency of NT rolls at higher frequencies
increase with increase of driving frequency which
will apply only for the liquid crystal sample
having traveling rolls. For normal stationary
rolls, the frequency
are zero(the width of the
roll roughly equal to the cell gap) for smaller
driving
frequency
and
increases
slowly
with
increase of driving frequency(Wavelength of rolls
decrease). The driving frequency at onset was found
to be smaller at lower temperature than that at
higher temperature.
Vc(Vrms)
40
o
50 C
60oC
With dc supply, we did not report the traveling
rolls. Instead, we fund the decrease of onset
voltage with increase of temperature
30
20
10
Figure 1: Oblique Stationary Rolls taken on a 24.22 μm cell at
60oC,6.22V and 150 Hz. The rubbing direction is aligned in the
vertical direction of this picture.
REFERENCES:
0
0
100
200
300
400
500
F(Hz)
Figure 4: Threshold Voltage vs Driving Frequency in the 24.22 μm
sample cell at different temperature.
EXPERIMENTAL:
The experiment was performed on a mixture of Phenyl
Benzoate derivatives called Mischung V(MV) sandwiched
in the sample cell.The sample had a Nematic range from
below room temperature to 630C.The sample cell consists
of ITO coated Borosilicate. The electrodes have been
treated to induce planar alignment. Electrical contact
between the plates and the wire were made by using
silver epoxy.
The space between the electrodes was measured by using
spectrometer
and
the
capacitance(C┴)
and
conductance(G┴) of the cell by using capbridge.
MV is a four component mixture of following compounds
The cutoff frequency, Fc(below which the regime is
conductive
and
above
which
it
is
dielectric)increases with temperature as well as
with conductivity of the sample
By
increasing
the
driving
frequency
and
simultaneously
the
applied
voltage,
normal
stationary(NS) and normal traveling(NT) rolls were
casted on the screen. Finally central single pixel
was recorded at different time interval.At higher
frequency, instead of NT rolls,we observed oblique
traveling(OT) rolls which were aperoidic.
1
P.G.De Gennes and J.Prost, The
physics of liquid Crystals, Clarendon
Press,Oxford(1993).
2
S.Chandrashakhar,Liquid
Crystals,Cambridge University Press(1992)
3
M.Dannin,Ph.D.thesis,University of
California at Santa Barbara(1995)
4
T.John,U.Behn
and
R.
Stannarius,Phys. Rev. E 65 046229 (2002).
5
J.Shi,C.Wang,V.Surendenatha,K.Kang
,J.T. Gleeson,Liq. Crys. 29,887 (2002).
6
E.H.C. Co Tokyo,Japan
7
T.John and R. Stannarius,Phys. Rev
70,025202(R) (2004).