ILCC 2002 - Edinburgh, July 2002

VAN DER WAALS INTERACTION AND STABILITY OF MULTILAYERED LIQUID-CRYSTALLINE SYSTEMS

dr. Andreja [arlah

Univerza v LjubljaniFakulteta za matematiko in fiziko, Oddelek za fiziko

THERMODYNAMIC ORIGIN OF INTERACTION BETWEEN MACROSCOPIC BODIES

modified free energy of the system

interaction

disjoining pressure

structural force

fluctuation-induced force

van der Waals force

DISJOINING PRESSURE

DISJOINING PRESSURE

structural force

fluctuation-induced force

van der Waals force

Orientational wetting - heterophase systems

surface wetting layer nonhomogeneous degree of order logarithmic divergence of the wetting layer thickness

high ordering power high disordering power

0 20 40 60 80 100-1000

-800

-600

-400

-200

0

P [P

a]

d [nm]

0 20 40 60 80 100-1000

-800

-600

-400

-200

0

P [P

a]

d [nm]

due to modified average order

structural force due to modified average order

DISJOINING PRESSURE

Hybrid nematic system

0 10 20 30 40 50-3

-2

-1

0

1

2

3

P [P

a]

d [nm]

10 1000.01

0.1

1

10

100

P [k

Pa]

d [nm]

structural force

fluctuation-induced force

van der Waals force

DISJOINING PRESSURE

structural force

fluctuation-induced force

van der Waals force

due to modified spectrum of fluctuations of the order parameter

surface-induced modification of the average order

changed potential for fluctuations

boundary conditions

0.90 0.92 0.94 0.96 0.98 1.000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

rela

xati

on r

ate

q0.0 0.1 0.2 0.3 0.4 0.5

-1.0

-0.5

0.0

0.5

1.0

fluc

tuat

ion

mod

e

z/d

fluctuation-induced force due to modified spectrum of fluctuations

DISJOINING PRESSURE

0.0 0.1 0.2 0.3 0.4 0.5-1.0

-0.5

0.0

0.5

1.0

fluc

tuat

ion

mod

e

z/d

0.90 0.92 0.94 0.96 0.98 1.000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

rela

xati

on r

ate

q

Orientational wetting - fluctuations of the degree of order

character

rangecorrelation length

boundary conditions

fluctuation-induced force due to modified spectrum of fluctuations

DISJOINING PRESSURE

finite short range exponential

decay

infinite long range power-law decay

strong-weak repulsive

strong-strongweak-weak attractive

structural force

fluctuation-induced force

van der Waals force

DISJOINING PRESSURE

due to interaction of fluctuating dipoles (permanent or instantaneous) in molecules

VAN DER WAALS INTERACTION BETWEEN ANISOTROPIC BODIES

vdW interaction between macroscopic bodies

due to changed spectrum of electromagnetic field modes

due to changed occupancy of states

anisotropic vs. isotropic bodies

different boundary conditions

different penetration depth

analytical calculation possible only for uniaxial symmetry

thickness dependent

VAN DER WAALS FORCE IN ANISOTROPIC MEDIA

Hamaker constant for uniaxial media

the relevant parameters are instead of averages

0.0 0.5 1.0 1.5 2.0-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

(b)(a)

An>

0 /A0

b0 50 100

-40

-20

0

20

40 (a)

P [

Pa]

d [nm]0 50 100

(b)

d [nm]

(b) change of the character of the van der Waals interaction at the ordering transition

ordered stateisotropic state

repulsive interaction

attractive interaction

5CB at room temperature

!

VAN DER WAALS FORCEat the ordering transition

20 30 40 50 60 70 80-3

-2

-1

0

1

2

3

van der Waals force

structural force

P [P

a]

d [nm]

RENORMALIZATION OF AVERAGE ORDER DUE TO THE VAN DER WAALS INTERACTION

different symmetry of isotropic and nematic phase nonzero van der Waals force acting on the wetting layer

change of the critical exponent

1E-5 1E-4 1E-3 0.01 0.1

1

DdW

[nm

]

q - qNI

(IN)STABILITY OF THIN LIQUID DEPOSITIONS

wrinkling of the free liquid surface due to thermal fluctuations

enhanced fluctuations

decreased fluctuations

stable film decomposition of film into drops

I. Dreven{ek (2000)F. Vandenbrouck, et al., PRL 82, 2693 (1999)

substrate: PVCN on BK7 glass

LC evaporationabout 40 minutes

LC evaporationabout 3 hours

[I. Dreven{ek (2000)]

(IN)STABILITY OF THIN LIQUID DEPOSITIONS

the same result also for quartz and sapphire substrates

U N S T A B L E

solid substrate-LC-airadditional layer of

water on the solid substrate

unstability of thin LC films on different solid substrates

due to thin layer (~0.3 nm) of water

VAN DER WAALS FORCEstability of thin depositions

0 2 4 6 8 10

-100

0

100sapphire

d [nm]

P [P

a]

-100

0

100PVCN

P [P

a]

-100

0

100quartz

P [P

a]

0 2 4 6 8 10

-100

0

100sapphire

d [nm]

P [P

a]

-100

0

100PVCN

P [P

a]

-100

0

100quartz

P [P

a]

usually, in experimental set-up the solid substrate is covered with a natural oxide layer, adsorbed hydrocarbons, water, etc.

unstable

usually, in experimental set-up the solid substrate is covered with a natural oxide layer, adsorbed hydrocarbons, water, etc.unstable film

stable films ?

SUMMARY & CONCLUSIONS

the effect of anisotropy of dielectric permittivity of media on the magnitude and character of the van der Waals interaction

determination of the Hamaker constant for the van der Waals interaction between uniaxial media

neglecting dielectric and optical anisotropy can yield wrong character of the interaction

renormalization of phenomenological description of the heterophase system

important for the interpretation of stability of thin films characterized by highly anisotropic arrangement

importance of additional layers