Tilted alignment of MBBA induced by short-chain surfactants

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Tilted alignment of MBBA induced by short-chain surfactants

G. Porte

To cite this version:

G. Porte. Tilted alignment of MBBA induced by short-chain surfactants. Journal de Physique, 1976,

37 (10), pp.1245-1252. �10.1051/jphys:0197600370100124500�. �jpa-00208521�

TILTED ALIGNMENT OF MBBA INDUCED

BY

SHORT-CHAIN SURFACTANTS

G. PORTE

Centre d’Etudes Nucléaires de Grenoble 85

X,

38041 Grenoble

Cedex,

^France

(Reçu

^{le 11 mars}

1976,

^{révisé le 3 mai}

1976, accepté

^{le 11 mai}

1976)

Résumé. ^- L’orientation d’un cristal liquide ^{induite par une} série de surfactants à courte chaîne

aliphatique ^a été observée. Le phénomène ^nouveau d’orientation inclinée a été obtenu. L’angle

d’inclinaison est relié à la longueur de la chaîne et à la tension

superficielle

^de la surface solide.

Les énergies d’ancrage caractérisant ce type d’alignement ^{ont été} approximativement ^mesurées.

Quelques interprétations ^{de ce mécanisme assez} complexe ^sont proposées.

Abstract ^- Alignments ^induced

by

short chained surfactants in MBBA are observed : tilted orientations with different tilt angles.

The anchoring energies associated with this type ^of alignment ^are approximately ^measured.

In the last section, interpretations ^of the rather complex mechanisms involved are proposed.

Classification

Physics ^Abstracts

7.130 - 7.840

1. Introduction. ^- Surface active agents ^{such as}

lecithin,

^Versamid

100, hexadecyltrimethyl

^ammonium

bromide are

commonly

^{used to} promote

homeotropic alignment

^{in MBBA}

(p-methoxibenzilidene-n-butyl- aniline)

^{in the}

vicinity

^of

glass

surfaces. These sur-

factants have one common character : when adsorbed

on a

glass

surface their

long aliphatic

chains stand

perpendicularly

^{to the surface.}

Two

explanations

^{have been}

proposed

^{to describe}

the

orientating

effect of such surfaces on nematic

liquid crystals :

- the steric one assumes that the molecules of NLC in contact with the surface are

kept

^{in a}

position

normal to the surface

by

^the

surrounding long

^ali-

phatic

^chains

[1],

- the

thermodynamical

^{one was first}

developed by Creagh

^{and Kmetz}

[2]

^{and Kahn}

[3]

^who

explained

the

homeotropic

orientation

by

the low surface tension

(26 dynes/cm)

of these surfaces.

Rigorous

calculations

supported by

interface free _energy measu- rements

performed by

^{Proust and Ter. Minassian}

[4, 5]

gave much

light

^on this mechanism and showed that the

Creagh

^{and Kmetz}

interpretation

was no more

than an

approximate description

^{of a more}

complex

mechanism.

Anyway

^{both of these}

explanations predict

^{that the}

orientation of the NLC will

critically depend

^{on the}

aliphatic

^chain

length : according

^to the steric effect if the chains are too short

they

^{will not be able to}

keep

the molecules of NLC in the

homeotropic position.

Otherwise the decisive works of Zisman et al.

[6, 7]

^on

surfaces covered

by

adsorbed amines and

acids,

demonstrated that the surface tension increases when the chain

length

decreases. The

thermodynamical explanation

^of

Creagh

^and Kmetz then

predicts

^that

surfaces covered

by

short-chain surfactants would

no

longer

promote

homeotropic alignment.’

In order to make sure of these

predictions

^we

studied the

orienting

effect of

glass

^{surfaces covered}

with monomolecular films of

aliphatic

^monoamines

(CnH2n+ 1-NH2)

^of various chain

lengths (6

ⁿ

16).

We obtained the

following

^{results :}

- n >, 12

homeotropic alignment,

- n , 10 the molecules in contact with the surface

are tilted of an

angle 00

^{from the}

homeotropic position.

The tilt

angle 0o

increases when the chain gets ^shorter.

The

anchoring energies

^{of tilted MBBA on such}

surfaces have been

estimated, according

^{to Kleman}

et

al., by

^the observation of

topological objects :

surface disclination

lines,

Bloch walls. Their

respective

values

give

^some

light

^to the rather

complex

^mecha-

nisms

involved,

^{described in last section.}

2.

Experimental procedure.

^- As surfactants we

used

commercially

^available

aliphatic

^monoamines

(Merck-Shushardt purity grade :

^{at least 98}

%),

without _any further

purification. They

^{were adsorbed}

on common

glass

surface from non-aqueous

solution, using

the method discovered

by

Zisman et al.

[6, 7, 8] :

first the amine is dissolved almost to saturation in

boiling

nitromethane

(Merck purity grade :

^99.5

%).

Then after

cooling,

the solution is

kept

ⁱⁿ

dry

^atmo-

sphere

^to prevent ^the

precipitation

of the amine in contact with accidental traces of water.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197600370100124500

1246

The

glass

^{slides are first}

thoroughly

^{cleaned in hot}

sulfochromic acid

(2 hours,

¹¹⁵

°C),

rinsed for 15 mm

in

boiling

^{distilled water and dried in a}

nitrogen

^flow.

The spontaneous

spreading

^{of a}

drop

^{of water on}

these substrates makes sure that

they

^are

perfectly hydrophilic.

After this

cleaning

treatment, the

glass

^{slide is}

dipped

ⁱⁿ the amine solution and

kept

^{immersed for}

about 10 seconds. When

withdrawn,

^the

glass

^slide

emerges

dry :

^the spontaneous retraction of the amine solution demonstrates the _presence of the adsorbed monomolecular film

[6, 7, 8].

The surfaces obtained are characterized and controlled

by

their surface tension. In order to

perform

these measurements we used an automatic

wetting

balance as described

by

^{J. M. Swaine}

[9].

This method

was derived from the

Wilhelmy (capillary)

^method

for

measuring

surface tension of

liquids.

A

thin,

^{1 cm}

wide, rectangular

^{slide of}

glass,

pre-

viously

^covered

by

the amine

film,

is held under an

electromagnetic

micro-balance

(Prolabo).

^{The lower}

edge

of the slide is then immersed in a reference

liquid

of known surface tension

(we

^used

methylene

^iodide

for its

high

^surface

tension yL

^{= 50.8}

dynes/cm).

The meniscus forms a constant contact

angle

^{a with}

the

plate

and if i is the

pulling

^{force on the}

plate :

The

equilibrium

^{of the}

plate

^can be written :

where

WA

is the free _energy of adhesion of the solid-

liquid

interface. Since both solid and

methylene

^iodide

have no

polar

contribution to their surface

tension,

we can write

according

^{to Fowkes}

[10]

Combining (1), (2)

^and

(3)

^{we obtain :}

(4)

The Fowkes’ formulation for _{Ys has} been

prefered

to the critical surface tension concept ^{of Zisman} because it does not

require

^tedious measurements with numerous

liquids

^{of a}

homologous

^{series to}

extrapolate

^{to a} consistent result.

Anyway,

^as

long

^as

the

liquid

^{and the solid are}

non-polar,

^{the two formu-}

lations lead ^to very similar numerical values.

The surface tension measurements have been _per- formed with

eight aliphatic

^amine adsorbed films :

n ⁼

6,

7,

8, 9, 10, 12, 14,

^{16. The}

pulling

^{force i is}

directly

converted into

dynes/cm by

^the microbalance

processing

system ^{with a 0.1}

dyne/cm

accuracy.

Results are

given

^{on table I and}

figure

^1.

In order to observe the

orienting

effects of such

FIG. 1. ^- Surface tension _ys versus aliphatic ^chain length.

surfaces,

^two

plates

covered with the same amine film are fitted

facing

^each

other, separated by mylar

walls

(15 gm).

^{The NLC}

(MBBA, clearing point 45.6°C)

is introduced

by capillarity

^{between the two}

plates.

^{The cell is then observed}

through

^a

polarizing microscope.

^This

microscope

^is

equiped

^{with a}

Bertrand lens to

permit conoscopical

observations.

TABLE I

3. Results. ^- As

predicted,

^the

long

chain amines induce

homeotropic alignment :

in

parallel light

^{and crossed}

polarizers

^the observation field is

uniformly

^{dark for} any

position

of the cell.

Conoscopical

observation shows the classical Maltese

cross. The

alignment

^is

definitely homeotropic.

With shorter

aliphatic ^chains,

^the

homeotropic

orientation is no

longer

^{obtained. As} the chain

gets shorter,

^{molecules in contact} with the surface are more

and more tilted off the normal

position

As the cell fills _up

(by capillarity)

^{the MBBA flows}

through

^and

conoscopical

observation shows

hyper-

bolic isochromatic bands. This indicates that the

viscosity anisotropy (of NLC) aligns

^{the NLC mole-}

cules

along

^the

flowing

direction in most of the cell thickness. As soon as the cell is

completely

^{full and}

thus the translation movement of MBBA stops, ^this

structure

begin

^{to relax}

by

spontaneous ^formation

of 2

disinclination

loops slowly expanding

^until

they

reach the bounds of the cell. We have then a new

alignment through

^the whole cell area. In

parallel light perfect

extinction occurs when the

flowing

direction of the MBBA is

parallel

^{to one of the crossed}

polarizers.

^A 45° rotation of the stage

gives

^Newton

coloration. These colours indicate an average

optical

pass difference of _{0.5 um} for n ⁼ 9 and about _{0.3 um} for n ⁼ 10.

Since the cell thickness is at least 20 ym with 15 gm

mylar

^walls this leads to small _average tilt

angle through

^{the cell thickness}

(respectively

^{110 et}

60).

In convergent

light

^the

given figure (Fig. 2a, 2b)

is a Maltese cross which is distorted. It remains

unchanged

when the cell is turned

upside

^{down :}

this means that the z ⁼ 0

plane (Fig. 3)

^{is a} symmetry

plane

^{for the NLC} structure. ^{We can} then conclude that the MBBA

alignment

^{is the one}

given

^on

figure

^3.

The tilt

angle

^of the molecules in contact with the surfaces is then

approximately

twice the _average tilt

angle (respectively On = 10 L---

^{100 and}

On = 9 ~ 220).

The _accuracy of these values is rather _poor since the results

really depend

^{on the} estimation of the cell thickness which is hazardous.

FIG. 2. - n ⁼ 10, n ⁼ 9; a) ^The flowing ^{direction of} entering

MBBA is 45° from the crossed polarizers. b) ^{This direction is} parallel ^to the vertical analyser.

Any attempt

^to cancel this distorted structure

(Fig. 3)

^{and to obtain the} undistorted

alignment,

as

represented

ⁱⁿ

figure 6,

remained unsuccessful For these

aliphatic

^chain

lengths

^the isochromatic

hyperbolic

^bands

(Fig. 4)

remain stable after the flow

FIG. 3. - n ⁼ 10, n ⁼ 9, orientation of MBBA.

FIG. 4. - n ⁼ 8, hyperbolic bands observed in convergent light

before heating ^{the cell.}

of MBBA is

stopped.

Since such a pattern

[ 11 ]

^{can be}

obtained for distorted

alignment

^{of the} type repre- sented in

figure 5,

^the orientation of molecules in contact with the surface is not known

by

this observa- tion. In order to obtain a more informative undis- torted

alignment

^the

following

^{treatment was} per- formed. The cell is heated

slightly

^{above the nematic}

isotropic

transition

temperature, cancelling

^{the dis-}

torted

alignment,

^{and then very}

slowly

cooled back to the nematic state

(1 hour)

^{in a} temperature

gradient.

The nematic

isotropic

^interface

gradually

falls down from the cold _upper surface to the hot lower

surface, leading

^to the undistorted

alignment

^of

figure

^6.

If

properly performed ^(slow cooling)

^{this treatment}

leads to

really homogeneous

^{structure all over the cell}

area. This structure is observed in convergent

light.

n ⁼ 8 : the

conoscopical

pattern ^is

given

ⁱⁿ

figure 7a,

7b. It is a Maltese _cross, the centre of which is

largely displaced

^{from the centre} of the field. When

rotating

FIG. 5. - n ⁼ 6, 7, ⁸ distorted tilted structure before heating ^{the cell.}

FIG. 6. - n ⁼ 6, 7, 8, undistorted tilted structure obtained after slow cooling of the cell in a thermal gradient.

1248

FIG. 7. - n ⁼ 8, ^observed pattern ⁱⁿ convergent light after slow cooling : a) the easy direction is 45° from crossed polarisers,

b) ^the easy direction is parallel ^to the horizontal polariser.

the stage, ^{the centre of the cross describes a circular} pass

just

^{outside the}

boundary

^{of the}

field,

^{the black}

branches

(Fig. 7b) preserving

^their

parallelism

^{to the}

cross wires while

sweeping successively

^{across the field.}

This

picture

^denotes

unambiguously [12]

^{a uniaxial}

undistorted

crystal

^{with its}

optical

^axis

making

^an

angle

with the axis of the

microscope according

^{to the}

displacement

^{of the centre of the cross. As the nume-}

rical aperture ^{of the}

microscope

^{is 0.60} the estimation of the

displacement

^{of the cross centre}

gives

^a very

approximate

value of the tilt

angle :

n ⁼ 7 : a similar

figure

^is

given

ⁱⁿ convergent

light (Fig. 8).

^{The tilt}

angle

^{is more}

important

^{and the}

hyperbolic

^bands appear ^on the _upper

right

part ^of the

figure.

^{In such a case the} estimation of the tilt

angle

^is

quite

hazardous but we can say it is in the range :

FIG. 8. - n ⁼ 7, ^observed pattern ⁱⁿ convergent light.

FIG. 9. ^- n ⁼ 6, ^observed pattern ⁱⁿ convergent light.

n ⁼ 6 : now the centre of the

hyperbolic

^isochro-

matic bands

(Fig. 9)

^{is into the} observation field : the tilt

angle

^increased

again.

^{It is estimated to be :}

Table I sums up these results.

Despite

^the very poor accuracy of 0 measurements, the

conoscopical figures clearly

show that 0 increases while n decreases.

One fact must be

thoroughly emphasised :

^{in each}

case, ^even for small tilt

angles (n

⁼

10, n = 9),

^the

NLC molecules lean

homogeneously,

^their

projections

on the surface

plane being parallel

^{to the}

flowing

direction of

entering

^{MBBA. This} easy direction remains stable even after several

heatings

^over

Tc

or after 6 months

ageing.

^The

only

way ^to offset

it,

is to heat the cell over 80 °C. The

resulting alignment (Fig. 10)

^{is then a}

quasi

^two dimensional

degenerated alignment

very similar to the one obtained and studied

by Ryschenkow [13].

^The presence of Bloch walls shows that the tilt

angle

^is

preserved

after this treatment.

FIG. 10. ^- Quasi two-dimensional alignment ^of Ryschenkow type obtained after cancelling ^the easy direction (n ⁼ 7) ( x 100).

4.

Anchoring energies.

^{- Since the} orientation of MBBA in contact with those surfaces is defined

by

^{the tilt}

angle 00

^and

by

the above mentioned easy

direction,

^two

anchoring energies

^{have to be}

FIG.11. ^- Definition of 0, 00 and Q

measured. The first _one, which can be called

W(e - 0o) (Fig. 11)

^{is the} energy per surface unit

required

^{to move}

the molecules

(in

^{contact with the}

surface)

^{off the}

equilibrium 00 position,

^{with their}

projection

^{in the}

surface

plane remaining parallel

^{to the} easy direction.

This _energy can

reasonably

^be

expressed

^as per

Kleman (Fig. 11)

W((p)

^{is then the} energy

required

^{to rotate the mole-}

cules of an

angle

(p off _{the easy}

direction,

the azimuthal

angle 00 remaining

^constant

(Fig. 11).

^As

proposed by

^{Kleman we can write :}

It is now well known from works

published

ⁱⁿ

ref.

[13, 14], [19, 20]

^that

W0o

^and

WQ

^are

^closely

related to certain surface

topological objects :

^Bloch

or Neel walls

and -1

^surface disclination lines. The walls take

place

^{when the}

extrapolation length b,

as defined in ref.

[13, 14]

^{is much} greater ^{than the cell} thickness

Where K is the elastic constant

involved,

^{h the cell}

thickness, Wo

^either

W0o

^or

WQ ^depending

^{on the}

considered

topological object.

With those conditions the

anchoring

energy ^can be calculated :

where e is the wall thickness.

The

relation b >

^h

implies

^that walls will take

place

for low

anchoring

energy. If not, ^we have b

h,

and

± 2

surface disclination lines would then be observed instead of Bloch or Neel walls. These lines

can as well lead to the calculation of

Wo by

^{the follow-}

ing

^{relation :}

e

being

^the line width.

One should not expect ^{an accurate} result from this method since the above

expression

^{assumes that the}

anchoring

energy

Ws

^is

expressed according

^{to the}

relation

(1)

^or

(1’)

which does not

lay

^on

experimental

evidence. Moreover the cell thickness h is still

poorly

related to the

mylar

walls thickness. But

despite

those limitations this method can

give interesting

estimations for the values of

anchoring energies.

W(0 - Bo)

ESTIMATION FOR n ⁼ 7 SURFACES. ^-

It has been mentioned in the

preceding

section that

heating

^{a thin}

(5

ym h 10

ym) n

^{= 7 cell} up to over 80 OC would cancel the _easy direction defined

by

^the

flowing

direction of

entering

^{MBBA and after}

cooling,

^{leads to the}

degenerated alignment

^of

figure

^10. The black branches of Friedel nuclei are the darkest between

perfectly

^crossed

polarizers

^{so that}

one can conclude that :

ocp/oz ~

^{0 all over the} spe- cimen area. The narrow bands

(20 um),

^{which can}

be seen in _many

places,

^{are of a}

higher

^{Newton colour}

than in the

surrounding regions.

^When

they

^are

parallel

^{to one of the}

polarizers, they

^{are crossed}

by

black branches

coming

from nuclei.

Moreover, by rotating

^the

stage,

^{one can reveal}

reversing points,

on those bands. Such characteristics

being

^most

identical to the

Ryshenkov’s

bidimensional ancho- rage, those coloured bands are identified as pure Bloch walls

[11]. Consequently

^{the NLC} deformation is pure twist and the

only

^{elastic constant concerned}

is

K2.

^{Since the}

anchoring

^is

completely degenerated

in the surface

plane

^the

only

^active

anchoring

energy is

W(0 _ 00).

Taking

^{h =} 5 um and e ⁼ 20 _um we have :

This value is

equal

^{to the}

anchoring

energy measured

by Ryschenkov

^on

glass

substrate covered with a film of carbon black from over-heated cellulose

[13].

Weep)

ESTIMATION FOR n ⁼ 7 SURFACES. ^- When

we described

experimental

evidence of tilted

align-

ment, ^we underlined that _very slow

cooling

^{of a}

previously

cleared cell would

spontaneously

^{lead to a}

very

homogeneous

undistorted structute all over the

sample

^{area. If this}

cooling

^is

carelessly performed

the result is much more confused : one can observe many

patches

^{most often too small to}

give

^a

good figure

ⁱⁿ convergent

light,

^but

always

^bounded

by

various

topological objects. Among

^these

objects,

surface lines _appear

bright

between crossed nicols

1250

FIG. 12. - + i surface inversion line forming ^a loop (n = 7) ( x 100).

and black between

parallel

^nicols

(Fig. 12). They

are -i

surface disinclinations lines of Neel type. ^The above mentioned authors determined the variation of the director around such lines and related their width to the

anchoring

energy

WQ

^{of the surface :}

with h - 20 _um and _{5 Jlm} e 10 um.

We have :

The mean value for W is

slightly bigger

^{than the one}

deduced

by

this method for

glass

surfaces rubbed

according

^to Chatelain’s

technique (- ^10-2 dynes/

cm).

In conclusion of this

section,

^{one can} notice the great

discrepancy

between the two

anchoring energies

(1) ^This expression has been established for planar configuration (60 ⁼ 900). ^{For tilted} orientation, ^{it has to be corrected} by ^{a sin} 00

factor (acting ^on the elastic constant Kl ⁺ K3). ^In respect ^{to the}

’

accuracy of the method this correction (0.82 ^{for n =} 7) ^{is of no} importance. ^{Therefore it has not} been taken into account.

characterizing

^this type ^{of surface}

alignment :

respec-

tively

This

discrepancy

suggests ^{the need for different}

interpretations

^{of each} energy.

5. General discussion. ^- One obvious _way of

explaining

^{the easy} direction for NLC

tilting

^{is to}

assume that the viscous

flowing

^{of MBBA}

plays

^the

same role as Chatelain’s

rubbing [15]

^{and thus}

aligns

the amine chains

along

^{its own direction.}

Such an argument ^assumes that surfaces of similar chemical nature

( - CH3)

^{but with no}

aliphatic

^chains

would lead to a

significantly

^smaller

WQ,

^{at least.}

In order to obtain clear evidence for the role of the

chains,

^the

following experiment

^{has been}

performed.

Trimethyl

chlorosilane is adsorbed on

glass plates

from solution in _pure

toluene, according

^{to a method}

very

commonly

^used

by

biochemists to

methylate glass

containers

[16].

^Surfaces

resulting

^{from this}

treatment are

unchained, strongly hydrophobic, methyl

^covered surfaces. Their surface tension measur-

ed

according

^to the above mentioned method is in the range of 36.5 _ys 38.5

dynes/cm.

^{Cells of}

various thickness have been made with those

surfaces,

and observed as usual. Observation in convergent

light

^{indicates a}

completely planar

^structure

aligned along

^{the same} easy direction defined

by

^NLC

flowing.

But in

parallel light,

patterns as seen on

figure

¹³

are observed. These patterns ^studied

by Nehrning

in ref.

[17]

^are Neel inversion walls. The cell

being typically

5 ym

thick,

^{their measured width}

( ~ 10 ym)

leads to the _very low

approximate

^{value of}

W :Q WQ

^{1. 5 x}

^{10 - 3 dynes/cm}

according

^{to Kleman’s method.}

Despite

the poor accuracy of this

estimation,

^this

Fip. 13. ^- Neel inversion walls obtained on trimethyl silane surfaces ( x 150).

calculated value for

WQ

^{is very} much smaller than the 5 x

10 - 2 dynes/cm

obtained for amine monomole- cular films and thus

emphasizes

^{the role}

played by aliphatic

chains when strong

anchoring along

^the

easy direction occurs.

The tilt

angle 00

^{and the}

corresponding anchoring

energy

Woo

^{are not so}

easily

understood. But a recent

experiment performed

^at

College

^de France sug- gests ^a

possible interpretation.

^The

experiment

^is

the

following : organic

^{molecules of}

elongated shape

are

spread

^{on a free surface of water. In the}

equilibrium configuration

^{of the}

resulting film,

^the

organic

^mole-

cules

stand, ordered,

^{normal to the}

surface, leading

to a

given

^value

yL(0)

^{for the}

resulting

^{surface tension.}

When the film is

subjected

^{to a} strong

(>

^{2 000}

G) magnetic

^field

Ho,

^the

long

^{molecules of this ordered}

film are forced to tilt with an

angle 00 (increasing

with

Ho)

^{and a} different surface

tension, yL(0o) > YL(O) increasing

^with

00,

^is then measured. Since the free surface of MBBA is also formed

by

^ordered

[21]

molecules of

elongated shape

^{one could}

easily imagine

that its surface tension is

00 dependant

^{as well. Let us}

then write the surface tension of MBBA as a function of

00 :

The

equilibrium

of the NLC-solid substrate inter- face occurs of course when _TLS

(interfacial

^surface

tension)

is minimum

[5] :

WA(0o) being

^{the free} energy of adhesion of NLC

on the solid substrate and _ys

being

the surface tension of the solid substrate.

Assuming

^{in a first}

approximation

^{that the}

only

interaction forces are

dispersion forces,

^{we can write}

according

^{to Fowkes}

[10] :

and

Thus

writing YLS(OO)

minimum leads to

equation (5) :

Assuming reasonably (this assumption

^is

implicit

in the

Creagh

^{and Kmetz}

interpretation [2])

^{that :}

equation (5)

^{leads to the}

experimentally

^obtained

equilibrium configurations :

No value of

00

^would

satisfy

^{the condition}

so the

equilibrium

^is

given by :

and

YL((0O)

^minimum

implying

The observed

.structure

^{in then}

homeotropic

^as

predicted

^{for low} Ys

by Creagh

^{and Kmetz}

[2].

Then :

is satisfied for one finite value of 0

00 n/2

^thus

leading

^to the observed tilted

configuration, 0o increasing

^with ys.

Then

equation (5) predicts planar

^structure.

These

thermodynamical

considerations about inter- facial surface tension are in

good

agreement ^with

experimental

observations. But as

long

^as the relative

importance

^of permanent

dipoles

^{forces at the inter-}

face is not

measured, quantitative

agreement ^{will not} be _very

significant.

^{Moreover this} argument ^does

not

deny

^the

possible importance

of steric effects from

aliphatic

chains. So further

investigations

^are

required

^{to a clear}

understanding

of these tilt

angles.

6. Conclusion. ^- This work

gives experimental

evidence of tilted orientations induced

by

^{mean of}

short-chain

surfactants.

This particular anchoring

is characterized

by

^two

geometrical

parameters :

- the tilt

angle 00,

- the _easy direction defined

by

the flow of

entering

NLC.

The two

anchoring energies

associated with these parameters have been measured. The

predominant

role of steric effects

appeared

^{in the mechanism}

leading

to the

high anchoring

energy associated with the easy direction.

Despite

^the

good

agreement between the

experi-

mental results and the theoretical

explanation

pro-

posed

^{in last}

section,

^{the tilt}

angle 00

^{is not unambi-}

guously

understood : _very recent

experiments

^showed

that a

phase

transition occurs near

Tc.

This transition is most similar to the one observed

by Ryschenkov [13]

with carbon black covered surfaces : that is to say

1252

that when a thin cell

(5 ym)

^{is heated near}

Tc,

^homeo-

tropic alignment

^takes

place

instead of the tilted

alignment.

^Therefore

long

range Van der Waal’s forces

have,

^most

probably,

^to be invoked to

explain

this transition as

proposed by

de Gennes and Dubois- Violette

[18].

^{The tilt}

angle 00

^{seems then to}

depend

on numerous parameters :

long

range Van der Waal’s

forces,

^short range

dispersion

forces related to super- ficial

tensions,

steric effects... Further

experiments

are

required

^{to reach a better}

understanding

^{of these}

rather

confusing

mechanisms.

7.

Acknowledgments.

^{- This work} in very much indebted to Dr. M. Kleman’s

helpfull

discussions and

pertinent pieces

of advice. The second part of this work would have been

impossible

without his clear

expla-

nations about the relations between

topological objects

^{and surface}

anchoring energies.

References

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[3] KAHN, ^F. J., TAYLOR, B. N., Proc. IEEE 61 (1973) ^823.

[4] PROUST, J. E., ^TER. MINASSIAN-SARAGA, L., C.R. Hebd. Séan.

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[13] RYCHENKOW, G., ^Third cycle thesis, Orsay (1975).

[14] WILLIAMS, C., Third cycle thesis, Orsay (1973).

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