ELECTRIC AND MAGNETIC FIELD EFFECTS

ELECTRIC AND MAGNETIC FIELD EFFECTS IN LIQUID CRYSTALS

Thesis Submitted to the UNIVERSITY OF MYSORE

for the Degree of

DOCTOR OF PHILOSOPHY

BY

PRAKASH P. KARAT

Liquid Crystals Laboratory Raman Research Institute Bangalore 560006 (INDIA)

May 1977

X hareby dealam that the .Irbeeia was aorngeeed by indepmdently and

t b f i t W e not form4 the beau ^the

award o f m y

Drrgree,

a t p l o n r ~ . h e o a i a t e -

ahip, B@lloweUp

or

_{othes r-r} $ t t l e ,

I aertlf'y tba* thU OSIesla ha@ been

c o ~ p o s e d by ilr.

Pnrk.sh P.

ltorat boead

on imestl-

gationa oslrrierd out by

U m

a t the _Liqufd Cryatal8 Laboratory,

h m a ~

Raeoar~h

mretitute

under my sugemiarion. The aub)act mtttur of' %hi@ thesis here not pra'+ia\;lsl$ Pomed %ha baalia cP the owud

of any Begrere, diplorre, waoabtar&kip, isllowship

ar

ather aiallar: 6 i t l e .

Profolaaor 5. Clmnbrrslllekhar L i q u i d C r y ef;& Uboratory

#aman Jlasrtr~~uroh fna t i t u t s BIIiCALBAE 560006

%he work r q o r l a d in t h i a theeis w a e c ~ m i e d out,

in

_{Rrrlasln tiesear0h f mtitute,} Bauti:rdlore un0sr fbe

@dams of Br(ofeeaor 8. Chmclrraeokbr.

I am d e ~ l y iz%3&tod t o hla f o r

his

alimulating guidtinoe and kern infezeat ^{a80 t o ;Inem} tli.P.Nilabhueudaru for h$8 valuable rsluggeopb,P sns and oomf ant help.

f en: a l a n vary f l a l a f u 2 %o 4111 BIT oollaragucl8 in the laboratory for their help and caosgsratiun at various

stage@ ol the fntrsstigatiarws.

In

~\errtic?uPear may tharnbs a r e

due to

Prof'emsos C.31;. Khetrapd,

Dr.

G a s . b n g w t h , Pr. A.C. Kmwar, Pilr, U.D. Eini,

Nz-.

5.g. femandes,

Mr. B.P. SadaabAva9

Mr.

K.T. BEilakrlLahnun, I\W. J.f'cSXr~~lanerbhan, Mr. A.P. Jqyarajan, H m J, Rmaohinni!xen, !4r*R.k.Gtibr&mny~~1 Mr. 3. Hngbvaobtjr rtiuzd Pa. K. VaeuCLbvan.

I

gmtefhl t o $he arrsmgement

,

^{S t ,} PlraiXomno

Oollege, Puftur ( S e x . ) f o r g~anfitng the study

Iscare,

m d t o at, Rev. 2.3. F u ~ t a 8 0 , B l ~ h ~ p , C h u r ~ h of South I n d i a , Kysora Southern :itweless %or f i n n c c i d help.

P i n a l l y , ¹ t h a k my w i f e Janet Sar her osmtent;

enaousageasn t

.

ZMa fbeafs deasribsr, aoae irntaetigatiam

orr

the eftaote

of

_argaaWo and eleettrlo

i i a U s

oa

?aenatio l i p l i d crya0.l~. ^58.. ima6tlgatlonr lay be olueeffied broa4Xy

in* four

gastet

(b) ^{S h Q of} tho

altastfo oonetrsnts in mlatfaa

to I Qran48 ~ oziant;att~osrl a d e r (Lbrpterrr IXlt iud I?) ^$

(a) Study of *he

ela8ttcr oom%ant@ in falsr*Aen

fo rhort w e ordapr ([lRap%sr O)

r

( d )

Pbtorfio

w d dyrasnria eftaautmr of elestrilo f&elde (Qhaptars VTZ end V I X X )

.

Xn Obaptsr

I

we giver

e

~tesard introbwtlon

&J the xsubjeaf with particmloa:

r~fsareaoe

t o the

imestiga+foM ~ p f # ~ t @ b

fn

W s rstst of Ute the8%er+

between two g h s e p l a t e @ with t h e dlreotor mahored t o the w a l l s a t eithar end, a emill t w i a t deformtion

Ln

the m e d i m oannot be 4eteotsd optforilly when l i g h t is inoidsnt normal t o the _emple g r a m booauee the o l e o t r i c veutor merely f o l l o w s the airgotor (.Mauguin

1 5 1 I ) . This _i~ a oonsequencra of t h e largo b i r e i r i ~ a n o @ of the medlum f a f M s d i r e a t i o n of p??op&%g~tion, ao

t h t the phme retardation p e r unit length i e v0zy Inrge ctompared t o t h e t w S e t p a r unit Icwth. Under such eircumctanees, the 'adiabatio theoretin

'

h o l d s ^good.

I t i a shown t h a t the tramition from ^the undiatorted

t o tho _{d f} e t o r t d state aan be obesrved o p t i a d l l y i f the bir~fringenae is ~ m & 1 . 20 reduce the eifeotive birefringenoe ^ian experimental sef up i s B o s c r i b d in whioh l i & t is allowed t o enter ^the a m p l e oblipuoly,

l.0.

,

^{making a very} s m a l l angle with r e o g o c t t o the sample plane (iastead of b e i q normal t o It). The trmpernture vuriotfon& of kZ2 _of p-asoxyonirrole (PAA) and p-oaoxyphcnc tole (PAP) mea~ured by t h i s n e t h o d

are greaented. Indegendent mt~artruremants on % i f f errent s m p l a e , &ve ooaslstent reeulte a n d moreover the data on PAA agree w e l l w i t h thorscl reporfed e m u o r ,

confirming that t h i s aizaple rasthod iar a reliable one.

The temperature variation o f

Lz2

of F A A an8 PAP shows an

s2

dependence ( S b a i w the order parameter) i n

aoniomo&tp w i t h the

me-

_{N e l d} theory. 2hir rrew mthod w o e w a d

i n

a l l the aeeeurcrcnts o f

whioh are deercribad

in

rrubsequ8nt ohapters.

fa

Ohaptor 111 we prseent erperinental

detcminatiom of the aplay (k,,), t w i s t (kp2) and bend (k3>) elastia constants md the order pprmetcr

( 2 ) a@ finatlono ^of ttrpprratura

for

the 6th,

Tth

^{and 8th} homo1 oguee of 4 ' -n-rlbl-4-oyanobiphenylpi (noo)

5CB, 6CB and 7CB shew only the maatla phase, while 8CB ahowa the ausotia A phaaa as _{w e Z l} at lower fwppe- raturea. T b u e compound@ m a stron(gly p o l a r , room

tmpsrcture li _guid c r y & t d s _with large positive d i e l o a t r i c aniootrapy.

X-ray

d l f traotion studleu

(Laadbatter

a.

^{1575) ehow that theae oompamde}

belonl. t o thts crlorsa of nor& aybotaotlo nomatjoa wherein small p u p a with ~ ~ e u t I a - L i k e order arcs present csvcsn in the i o o tropia pheae. Puthermore, the aompounOa form doublo l a y e r s with lnterdi&ttstod moloouloa

,

^{beoause of the} antigarallel a oxrelati on between the rctrongly p o l a r n i t r i l e group8 attaahad t o on@ end of t h e raoleau2o. The r e a l t s obtilined

by

ue

orhow some intereating

features. In

_the first ploor kJJ and

t o .as

_extent kI1 deviate from the S*

deyendenoe. They inoreaaa sore rapidly a8 the temperature

I .

deorrased. On the other b o d k2*

showa t h e S 2 depandsnoe (axaept f

n

_the oase o f BQB,

whioh w i l l be dleauaaed ^{i n} the f i f t h o b p t e r ) . ^'Phoao reaults oan be interpreted t o maan that the temperature ratlfatione o f the longitudinal Prhd

tranrrverrse

earre- l o t i o n length6 ( ^Fl, and ) u r m.qual.

Howevsr

the ratio6 of t h e elcrertio oonst;antr

An

5=, 6UB and 7- are praoticrel3y the blae (it axif &yen relative temperature

(THI -

^2). Thie suggeeta t h a t the t m p r

rarfurr tariaUon of ahort range order i m

similar

in these ooaptmnda, although the d e o l u t e values of t h e ahort r a g e order a r e not ths e m s .

Rear

TBI,

Ir3) 6howo the odd-even .ifeat whcrelu,

5 ehows

alternetion

only f o r t h e first three member8 a l u d i e d and inareass froa TCl3 t o BGB. The l a t e n t heat of transition ( A

If),

k I 1 and kZ2 .how the 8aae

trend aa

s.

Moreoror near ^I k33 18 lee@

than k I 1 f o r 8CB rrhioh i s a most

unwual

resu2t. !These

obetitxlratione o m be interpreted by aaawning that the

ratio

(

FII/ f,) near tax

_de0rea.e. between

70B

and UOB.

Ilacuucenents

of

^kii and S have a l s o bean made

on

another steriee of

now

ooapouftde prepared reoantly

i n

our chaoistr~r laboratory ( S e d u h i v a 1976).

ris.,

lhees eetera are oolourlsss, ohaaAoally etable an8 ponsess high v i a a o s i t y

in

the neaatio phma. They

also have a

nitrils

group attache4 t o

one

and of t h e moleaule and e x h i b i t etrong p o s i t i v e dieleatrio

microtropy. He have s t u d l e d the reeond, t h i r d ,

fourth lana eighth, ain$h tea* b o m o l o g ~ e of thfo eeriee. (The other members

wra

not invrrstuated

baaauae they have a ahort nsmatic

renge,

and, further- more. most of t h o r a r e monotropic). 02 these 10 WOPD ehowa a moaotropio aaecrtio A pharss. The result6

on

t h i ~ serlso~, dieau~ssed in Chapter SV, u e more BOPPU- ontsd than in the sass of oyanobipbaayla. lPor i a s t m o e , 3 and

A H

incrs@e aoaaiderably between 4 OXCPO an8

8 OHCi'D. k33 and t o o ema11.r extent k,, a l e o e x h i b i t an upward jump between theae two members.

However

and kZ2 do not show ony auo!. j u p . Horeover the

r~wt.Alwl

of a b e t i s

uamtaantas at the sarm

ralatSve

tcciigsrature dlffer cronaAderPbly .olopg t h e different oolpouadr studied. Both k l ond

kZp

_{btoodLy follow}

an

S~ dep.ni~eo~e

An

the aeoo*, t h i r d , f o u r t h o~ _eighth h 0 ~ 1 0 ~ 8 rlth th.

emeption

& kg2 oi 4 OilGPO whioh a s h i b i t 8 r a i m A a a n t l g slower rrariatioa. ^Oa the ofhsr hanil

SJ

~ P r i s a mere rqyldly with tmperature

In

OBL8O8.

Ae yet,

no

X-ra+y

Ba+a

a r e clvdlable on t h e

coapowda. Giws the

rolooul@s

are strooglp polor, i t

is probable that they $ o m 4 0 u b l ~ ~ 1 0 1 e m ~ t laprs, as

in

E l m ~ a a e of 0th01~ polar i o l e ~ u P ~ a ( ~ ~ 6 1 1 0 1 1 157J.

Laadbetter

a a. lsm).

^1;t

is

^{ob~trroed that the mltiag:}

polnt o f fhti aryatsd.

afiaoremae~

adeadily betwe- treoond

ant3 f o u r t h a.rabexe

in

t h e seritng, and &owe B l a r ~ e i n o r e a ~ e

between

%he

four*

read fifth haarolo~uera. There- after kt &#orin de~rgasea et;eailily tll'J, ^the ninth nramber.

If &a poaaiblr t h f the urf & t i i &txuoture ohaWe8 befw@m

fourth an4 f i f t h asmbere ernd t h e riia~t~re s f &bore r a y @ o.der ^may

&LBO

be different between the lower (<1) pod

hlgbar (

>

⁵⁾ horoLo&usu. Thir mny exgiain ^the trend.

in the p h y e i o a prapertfca that are dasuribeb above.

In

particular we have made

meaaurmentrr on

8GB, 10 OHCPO, poyanobenaylidene-p-+oo tyloxyaniIine (OBOOA) and ootylory oyanobiphaayl (8 0033).

To

inorease the nemetio range we prepwed two mixtures of 8 O W and 7CB. de Oemea ha8 ahown that in thees oeetbb the

twiet

and bend eonstoat6 ocm be expresstab at klL

-

^(kii)o

+

^Op(T

- T')7

^{when (k& ie the}

'normal' e l a a t i u oomtant (in the absence of corre- lations).

Tm

i f the transition i s a t r i o t l r

seoond order.

In

a31 the oompounde etub3ed exaept BCD, the values of y 1s around 0.6. For 8 5 in _which the ^A-M I s d i a t l n a t l y f i r e t order y -- 1.0" Rowgrer i t must be emphueieed that a quantitatively meanla&Pul

oatimats of _{t h e} o r i t i c a l ^exponent requires ^the agaeure- asnt t o be oarried o u t within a f u w ~li&fid005r(et88 of the traneition. In t h e graeont ^{sf udy} the nscusurmsnts were not made f o this degree of prcclsion _and the value

of y obtained hers o a n o t be rsgasded ear aoaurate onough t o throw any list ^on the gueetion of whether t h e A-IS traneiCion exhibit8 h e l i w a like or olasalcral

mean f i e l d behaviou~. (Zndacsd t h i e ob3eation i s probably v a l i d in many of the pr@viours a t u d i o s , ^{8 r g .}

,

Ohsung

sf

al.1573, Uelaye

ef &.

$573).

. 1111

In

Ohapter

VI

we preeent o~eraaurerorante of t h e threshold

for

a t wiated nematia @ell

(IN

cell). 2he

of i t o use in d i s ~ l a j deviasae. The theory of the TN oell we8 dav8loped by Lselie ( 1 S7Q)

,

^{who derived on}

expreeslon f o r the threahold magnetio f i e l d

(AL)

beyond uhiah d e f o r ~ o t i a a oaaurr in the a m p l e . 3 2 i s we11 known that wb- viewed through p a r a l l e l polarizers kept parallel

ar

perpendiaulru t o the director st

e i t h e r boundtlhg 6urfao8, the o p t i c a l threahold (Elo) is a l w a y u ~ r e a t a r than

IT&

(Gerri tsma

a.

^{1 971 )}

.

^Thia

i s

wain

due to the a%Aclbatlc theorem referred to e=Iier. Ye show t b t

Bt

oan be dctomlned optloally.

if the set up i a aenaitlve t o 8-11 ohangee fxa the birefringenoe of the sample. Uslni; t h l e method, we have determined the thrcohold tiL aoaurately. Ueing k r

kZ2 a

kj3 value0

(from

^{Chapter 111),wo have} oheokad t h a t teelie's

orprsaoian

f o r the o r i t i a a l f i e l d a g e s 8 well with the oboervea value. Further, we hare shown that BLxo ( x o b e i n g the s m p l e thickmos) 1s Andependent o f asmple thloknoas prorlag that HL is the true Lsarlir threshold. Thi8 i& the firrrt verifl-

o a t i o n of L e a U e V r theow f o r &he t w i s t e d nematto eel&.

The next t w o uhqptcbre dlsoues & o m eblsotria field e f f o e t a .

It

As well known t h a t nb!metic l i q u i d erystala have a low Srequenay d&elcotrLa relaxation ( " lutt~s) whioh is Bus t o the raorientcrtion of the

dipoles about an arxia nor*fporl t o the dirtsotor. Thia r a l a x e t ion oan be understood

In

terna of the nenatiu p o t e n t i a l , which a xaoleoule hac t o overcome in trying t o rotate abou* Ate elliort a x i ~ (PIQ)$~w and 9wpe lS6G).

fn

Chapter V f X we present a study of t h i s

low

irequenay selorxation ^by obeemation ^{of the} 9ree4erfckus threshold both in the kTJ and k,, gaoaetrfee. The grincipla 18 t o measure t h e aritiodjrl c~agnetio f i e l d correerponding t o t h e Fr~ederlokes trsmioiti on

in

the prssenoe of on dternzlting s t a b i l i s i n g e l e o t r i o f i e l d .

The e l e c t s i o f i e l d rir along the undiertorted direlatar suis an8 the _magnetic f i e l d normal t o i t . C& the f r e q ~ s n o y

of t h e e l c a t r i a P l e l d c p p r o a a h a ~ fha Pre?$~tmcy of relaxation the o r i t i o a l aagaatic f i e l d

(Re)

dearsaeos cormiderc~bly and tend. t o approauh B,.

In

_the case of 6CB, even ^for frequellaies h i ~ g h ~ r than the relaxation frequency. lie

>

^{Ho (the} v d u s in eero e l e c t r i c f i e l d ) , i .e., the diePeotraa miaotroyy xembiru weakly g o s i t i v e . fiowever in the caee o f 9 &Gi?C, beyou3 a p a t i c u l c u :

frequency f (depending upon the telp2erature) ^8. beams less than

A,

fndiontfng t h a t

A

c hap ohengeP sign.

.LII

The graph of log fR (relaxation frepuenoy) Vb.

4

^givee

a slope whioh Is proportional t o the t o t a l aotioation enorgy Q. For

9

OHCPC, Q wm eetiarrated t o be 1 eV.

We have also made observations of the Praedarlokaa

thrashold

i n

^{the k l l} g e o u t r y due t o an e l a c t r l c field (with no magnetio f i e l d present). Xn t h e case

of

9

WCPC a

a r i t i w i l

f i e l d Boea not pertain t o a

pure Freederiokss traneitlon

a t

very high frequencies

(

>

^{1 IQ1s)}

same eleotcohgdrodyn~~lo

i n s t a b i l i t y makes

a

@trow

oontribution. Indeed as tha ~ l s c t t r i c PisXd i r inorebeeti bayon4 the t h r s ~ h o l d v d u a s t r i a t i o n s analogous t o 'William@ boadmt ^{o r e} sessn. The domain width hae the &tame order of athlignitutle sre t h e ~ m y l e thickness. Poesible moheuli~m~ for t h i s Wgh frequency instabf l i % g are disouaee8 b r i e f l y .

In

Chapter TI11 we prerant some new typeo of

o p t f a d pattorne clue t o eXeotrohydrodymiaia i m t u b l l i t l s s

i n

^{nematic P313BA.} %?ha wual method of obsleming these domains, ie t o uae the 8smdwlcln' getmetry, the

d i s e a t f on of obeomatron belng along the o l o o t r i c f i e l d . We have w e d a geometry in which fhe Blrectioa of

observeti on l a d o n g the initial undlstorted dlrcsotor axis a n d nonnnl t o the applied eleotric f i e l d . Jurst above the tkre8hol8 voltage, i f appear6 that the vortioee w o in the plane oontainlng the e l c o t r l c

f i e l d and the Blreaotion of observation. With imrexlee of _{t h e} voltage the pattern observed bcoome ooapP1catsd and three dimemional flow p&terns a r e seen. The

domain widthas

&re

opproximate2y half the elactrode

separation.

Uowerer

the threah o l d v o l t ago is muoh larger than that in the u 8 U rsandwioh geometry. An sxplanetion is gSverr of the o r i a n of this _type of

f l o w plaftem.

Another

interasti=

obsorvatl oa greoent ed Sn thls _Chapter $8 the, s i f o a t a f cbleatrio f i e l d s on nematic3 b r o p h t a o f

MBBA

ewpsndebdl

in

i t 8 l s o t ~ o p i c l ghaee. The dropil whioh _appeeu. spheriaal a t t e r o f i e l d , beoome e l l l p a o i d a l wfth i t s

mrsdos:

a x i s

normti1

t o the

field.

For

very h i @ f i e l d # the droplets aoquire highly diatortsd, r q l B l y ohang%ng ehapes. Thsae

defomatlons s r o obeorvr8 both

i n DC

and &ow irequsnay

AO fieIda. $e the irequanay is inoreneed, the

deformtation dsorsaeeo an8 f o r higb fiequenoy ^--/ 5 atts no deiozm~tion Is seen. The o r i a n of t h i ~ type of d i s t o r t i o n 18 agsln slootrohydrodyaaraio, but a d e t a i l e d theory u f t h e observe4 beheviour still remaims t o be tlc9oslopeb.

Some of t h e xeeultb dA8ouseed in t h i s theeSs are reported in the f ollo#in&; publloatioao a

1. Some erls~trohydrodyramlo d i s t o r t i o n patterns in a nematio l i q u i d o r y a t d ( g e l ? . H ~ 3 h u s ~ d - s ~ f.E.ECsrat and S. ~hemdraseWlar)

- g w r ~ n t

$o$anoe

,

147-149 (1573).

2. Experimental detenafnation of t h e t w i s t slatttic

oomtant

^{of aer~ntio} L i q u i d orgetdm (N.V.MPzBhuaudaaa, P.P. $-at on8 S, OhaaBrwekhm)

-

Prerssnted at t h e

l[nternatlonal L i q u i d Orystorls C"oalercbnce, Beruy%~alore Da~WQber 1973

-

^{PTi~sla-. ...r 1. 225-236 (1575).}

3.

^{Some new type8 of}

eloatrohyc¶rodynaisio

flow patterns in

nemotle

l i q u i d Oryetala

(P.P.

_Karat,

L V .

~adhuoubana)

-

^{Preaent.6 at the} l n t o r m t i ^{o m l} L i q u i d C r y a t d u Ooaisrenoe,

Bawalom,

Eeomber 1573

-

Pramsnp SUDD~.

1,

285-2843 (1975).

4. E l s e t l o and o p t i o a l propertlee of some 4'-n-alkyl-4- oyanobighsnyls (P.B. #mat and ^{E r V e} ~ a b h w i ~ d ~ )

-

pioleaular

Owstale ^t LIauiU C r y s t a l s

z,

^{> I (1576).}

5 . Elaatioity and orientstional oroer in 8oae 4'-a-

dkyl-4-cyanobiphenylsir Part I

I

(P.P. K a r s t an8 K.V.

Madhu~udana)

-

& o l . C r u s t , ~ i d Crust.

(In

press).

6 . Vsrificration of Leolie'a ewpreassrion for the threshold f i e l d

for

a t w i e t o d nematic! aell (P.P. Yarat and

tz , V . ~ ~ u r ~ u 8 a n a )

- pol

^.Cryst. t i n u i d Or-. (in yrees)

.

7. k 8tudy of the d i e l e o t r l o relaxation

An

mmatio l i q u i d cryetala w i n g the h e s d o r i c k a s transit _{i o n} technique (P.F. Karat and E.V. ~ladhuawlana)

- &&.

C r ~ I t , & u i d Crw. (in p r s s e ) .

-

Plfc~PUllan, k*Lr 1579 me. Rev.

a,

^1419.