MEASUREMENT OF BIREFRINGENCE AND
TRANSITION TEMPERATURE OF NEMATIC LIQUID
SUBMITTED IN THE PARTIAL FULFILLMENT OF THE
ACADEMIC REQUIREMENT FOR
THE AWARD OF
MASTER OF SCIENCE
Dr. SURAJIT DHARA
AssociateProfessor, Schoolof Physics
I, SOUVIK ROY, hereby declare that my project work on " MEASUREMENT
OF BIREFRINGENCE AND TRANSITION TEMPERATURE OF NEMATIC LIQUID
CRYSTAL" submitted to School Of Physics, University of Hyderabad,
Hyderabad, for partial fulfillment of the Degree of MASTER OF SCIENCE IN
PHYSICS has been carried out by me under the supervision of Dr. SURAJIT
DHARA, School Of Physics, University Of Hyderabad. To the best of my
knowledge, this work has not been submitted for any other degree in any
UNIVERSITY OF HYDERABAD
This is to certify that the project work on " MEASUREMENT OF
BIREFRINGENCE AND TRANSITION TEMPERATURE OF NEMATIC LIQUID
CRYSTAL " is a bonafide work done by Mr. SOUVIK ROY bearing the
Reg.No:13PHMP09 under my guidance in partial fulfillment of the
requirements for the award of degree of MASTER OF SCIENCE IN PHYSICS and
submitted to School Of Physics, University Of Hyderabad. This work has not
been submitted for any other degree in any University.
DR. SURAJIT DHARA
School Of Physics
School Of Physics
First and foremost I would like to thank the Almighty for his grace
upon me, without which, I would not have been able to complete
this project. I am indebted to my parents for their continuous
I would like to express my sincere gratitude to my project supervisor
Dr. SURAJIT DHARA for his generous nature, infinite patience and
helpful guidance. I will always remember his suggestions and
practice his advices in both my professional and personal life.
I am Highly thankfull to Mr. VENKATA SAI DASARI for constant help
and encouragement during the project.
I take the opportunity to thank all my labmates like M.V RASNA,
RASMITA SAHOO,ZUNAID AHMED for their co-operation and
suggestions for my project work.
UNIVERSITY OF HYDERABAD 6TH MAY, 2015
2. ORDER PARAMETER
3 TYPES OF LIQUID CRYSTAL
b) TWISTED NEMATIC
c) SMECTIC PHASES
i) SMECTIC A
ii) SMECTIC C
iii) SMECTIC C*
4. SOME OPTICALPROPERTIES OF LIQUID CRYSTAL:
a) Refractive index.
5. MAKING OF LIQUID CRYSTAL CELL:
6. MEASUREMENT OF THICKNESS OF THE CELL:
7. LIQUID CRYSTAL SAMPLE FOR EXPERIMENT :
b) ORGANIC NAME
c) TRANSITION TEMPERATURE
8. SOMETEXTURES OF LIQUID CRYSTAL UNDERNIKONPOLARISING
c) TEXTURE IN DIFFERENT PHASE TRANSITIONS
d) SOME TEXTURE WITH DIGITAL CAMERA WITH 10X AND 20X OPTICAL ZOOMAT
TEMPERATURE 62 AND 54.
9. MEASUREMENT OF BIREFRINGENCE
b) CALCULATIONS AND GRAPH
c) RESULTS .
1. INTRODUCTION :
The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich
Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting
points (419K and 552K) .In this experiments, Reinitzer increased the temperature of a solid
sample and observed the crystal changing into a hazy liquid. Further increasing of
temperature that sample again changes to transparent liquid .they are found among organic
compounds with molecular shape anisotropy.they are classified into two types namely
thermotropic and lyotropic.thermotropic liquid crystal exhibit mesophases as a function of
its concentration in solvent.conventional liquid crystals are mostly made of rod and disc
type of molecules.
2. ORDER PARAMETER :
Liquid crystals possess some degree of orientational order To quantify how much order is
present in a material an order parameter (S) is defined.Liquid crystal molecules possess
orientational order. The direction of preferred orientation in a liquid crystal is called the
director .The order parameter is given as follows
S= ½ <3cos²θ – 1>
where, θ is the angle made by each molecule with the director axis denoted by n.
3. TYPES OF LIQUID CRYSTAL :
Depending on the shape of the molecules thermotropic liquid crystal can be classified into
iii) bend core type.
Which basically composed with rod like,disc like and bent core molecules.
3.1 calamitic liquid crystal:-
it consist of rigid cores with flexible side chains.there are many mesophases formed by
a) Nematic phase: This phase ischaracterizedbythe moleculesthathave nopositional
orderbut tendto orientinthe same directioni.e.,alongthe director.
b) Twisted nematic phase: InthisCholesteric ortwistedphase alongwithlongrange
orientational orderspatial variation of directorisalsoexistwhichcauseshelical structure.
c) SMECTICPHASES :
These phases shows orientational order along with positional order.the molecules
preferably pointing in one direction. Just like in the nematic phases.in the layer the centre
of mass of the molecules are random i.e liquid like.there are several different types of
i) Smectic A : In smectic A (SmA) phases, on average, the molecules are parallel
to one another and are arranged in layers, with the long axes perpendicular to
the layer plane. Within the layers, the centers of gravity of the molecules are
ordered at random. Thus, smectics A possess the one-dimensional quasi long-
range positional order and within the layers molecules show a relatively high
mobility. The layer thickness is equal to the molecule length. SmA LCs are
optically positive and uniaxial with the optic axis parallel to the molecular long
ii) Smectic c :. For The structure of the smectic C (SmC) liquid crystals is closely
related to the structure of the SmA. The molecules are arranged in layers, but the
long axes of the molecules are tilted to the layers planes.some materials the tilt
angle is constant but for others it is temperature dependent. The centers of
gravity of the molecules are randomly ordered and the moleculesare free to
rotate around their long axes. SmC phases are optically biaxial.
iii) SMECTIC C* : The SmC* phase is similar to the SmC phase but consists of the
chiral molecules, which rotate the direction of the director projection on the
layer plane from one layer to the next. The twist axis of the SmC* is
perpendicular to the layers. Therefore, these phases appear optically positive
uniaxial, and show optical activity and selective reflection similar to the
iv) ANTICLINIC :Here the long axis of molecules make an angle with the layer
4. SOME OPTICAL PROPERTIES OF LIQUID CRYSTAL :
a) REFRACTIVE INDEX : In liquid crystal the speed of light is parallel to the direction is
different from the perpendicular direction.nematic phase has two principal
refractive index for ordinary and extraordinary rays.
b) BIREFRINGENCE: Liquid crystals are found to be birefringent, due to
their anisotropic nature. That is, they demonstrate double refraction (having two
indices of refraction). In the following diagram, the blue lines represent the director
field and the arrows show the polarization vector.
∆n= ne – no.
Where ne and no are the refractive index for extraordinary and ordinary
5. MAKING OF A LIQUID CRYSTAL CELL :
a) Firstwe cut the slidesaround1.5cm² inarea withspecial diamondcutter.
b) Thenwe washthe slideswithsoapsolutionandstartoursolicationprocess.
Firstwe solicate the slideswithbenzenefor30 mints,andthenwithasetone for30 minutes
and afterthat withdistilledwaterfor30 mints.
c) Aftersolicationwe dryupour slideswithdryerandNitrogengasforbubble removing.
d) Afterbubble removingwe use ouralignmentlayerSC7942 on the slidesandputit inthe spin
coater machine andputin a furnace at 180 ͦc for 45 mints, rubbing followsafter the furnace
e) Afterrubbingstickingwith8µmsample withglue.andhasto keepforsome time underUV
f) Cells are prepare foruse.
6. MEASUREMENT OF CELL THICKNESS :
The liquid crystal cell is kept under spectrometer reflecltion probes, which consist of
optical fibre.it is illuminated through outer six fibres of probes. And reflected light is
collected by central fibre and fed to spectrometer.
Then view the spectrum in the computer using a software called spectrasuite .
FORMULA USED : -
d= (λn-λm)/λn λm*(n - m)/2
where n & m are the peaks and λn, λm are the corresponding wavelength.
We can get the values of λ’s accurately using the software.
7. LIQUID CRYSTAL SAMPLE FOR EXPERIMENT:
a) SAMPLE :- 8OCB
b) ORGANIC NAME :- 4-Octyloxy-4’-cyanobiphenyl
c) TRANSITION TEMPERATURE :-
i) CRYSTAL TO SMECTIC - 54 ͦ C
ii) SMECTIC(A) TO NEMATIC - 67 ͦ C
iii) NEMATIC TO ISOTROPIC - 80 ͦ C
8. SOME TEXTURES OF LIQUID CRYSTAL UNDER NIKON
POLARISING MICROSCOPE :
we place the liquid crystal sample 8OCB under Nikon polarising microscope,and with varing
the temperature and take some pictures of the sample at different transition temperature.
a) APPARATUS REQUIRED :
i) Liquid crystal sample holder
ii) Temperature controller.
iii) Nikon porarising microscope.
iv) Digital camera with 10x and 20x optical zoom.
v) Computer Pc.
i) First take the sample 8OCB in between the liquid crystal slide.
ii) Stick it with temperature resistant cellotape with the sample holder.
iii) Sample holder is connected with the temperature controller.
iv) Increase the tempreture beyond 80 ͦ , and then cool it down .
v) Take the textures at 10x and 20x optical zoom by digital camera at the transition
c) TEXTURES AT DIFFERENTPHASETRANSITION:
Textures at isotropic phase : (at temperature 82 ͦ)
Textures fromisotropic to nematic transition : (at temperature 78 ͦ )
Textures at nematic phase : ( at temperature 72 ͦ)
Textures fromnematic to smectic transition : (at temperature 66 ͦ)
Textures at smectic phase: (at temperature 58 ͦ)
d) HERE IS SOMETEXTURES AT PURE SMECTICPHASE: (AT TEMPERATURE 62 ͦ
WITH 10X OPTICAL ZOOM)
e) SOME TEXTURES AT SMECTIC TO CRYSTAL TRANSITION (AT
TEMPERATURE 54 ͦ WITH 20X OPTICAL ZOOM) :
9. MEASUREMENT OF BIREFRINGENCE : -
The birefringence of a uniaxial liquid crystal can be measured by two techniques.
Intensity measurement technique.
Phase modulation technique.
In my experiment i use only intensity measurement technique.
Using dc intensity measurement technique we can measure the birefringence.the
optical set up is shown in the figure.
i) APPERATUS :
a) He –Ne source.
b) Optical bench on a shelf with a hole for passing the light.
c) Glan-thompson polarizers.
ii) EXPERIMENTAL PROCEDURE:
The plane polarised light after the GT polariser passes through the sample,
in which the rubbing direction is 45 ͦ with respect to the first polariser. The
transmitted light then passes through the second polariser and after that goes to a
detector.the detector output is connected to keithly dc voltmeter for measurement
of output intensity.
the temperature of the sample then controlled by temperature controller called
For measurement of temperature dependent birefringence,temperature is varied
with finite step and the corresponding intensity is measured by the detector.
All the measurement is controlled by the computer and with the help of a software
We place the liquid crystal sample between two crossed polariser.and rubbing direction
makes an angle φ with polariser.Assuming the analyser and polariser is in x and y direction
and light propagates in z direction.
iii) WORKING FORMULA:
If the intensity of light after the first polariser is I0 , then intensity at the
detector is given by,
I= I0 Sin²(2φ)Sin²(πΔn/λ)
Setting φ=45 ͦ , the intensity at the detector is given by,
Is = I0 Sin²(πΔn/λ)
And birefringence is given by,
∆n=(λ/πd)arcSin (√(Is/I0) .
b) Calculation & graph :-
measuring the temperature and corresponding intensity, Using the software
Calculate the birefringence for each intensity.
Plot a graph in origin software with temperature is along x axis.
And birefringence along y axis .
THE GRAPH BETWEEN BIREFRINGENCEAND TEMPERATURE:
THE GRAPH FOR COMPAREING THEEXPERIMENTAL VALUEAND
c) RESULTS :
After draw a plot between birefringence and temperature in origin software i can realise
how the birefringence changes with temperature.with increases the temperature
When cooling from temperature 84, ͦI saw the first transition occurs in the graph at
temperature near about 80.208 ͦ . it denotes some phase is changes.and we know
the theoretical transition temperature for isotropic to nematic transition is near
about 80 ͦ,which is nearer to our experimental value. So the transition of phase is
isotropic to nematic.
The second transition occurs at temperature near about 65.947 ͦ. Which is close to
our theoretical nematic to smectic transition temperature 67 ͦ. So the transition
phase is nematic to smectic.
And also my graph is approximately the same to the graph which is plotted with the
EXPERIMENTAL VALUES OF TRANSITIONTEMPERATURE:
FIRST TRANSITION TEMPERATURE : 80.208 ͦ
(DENOTES ISOTROPIC TO NEMATIC TRANSITION)
CORRESPONDING BIREFRINGENCE : 0.0823
SECOND TRANSITION TEMPERATURE : 65.947 ͦ
(DENOTES NEMATIC TO SMECTIC TRANSITION )
CORRESPONDING BIREFRINGENCE : 0.168
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