ilcc 2002 - edinburgh, july 2002 van der waals interaction and stability of multilayered...
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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
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THERMODYNAMIC ORIGIN OF INTERACTION BETWEEN MACROSCOPIC BODIES
modified free energy of the system
interaction
disjoining pressure
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structural force
fluctuation-induced force
van der Waals force
DISJOINING PRESSURE
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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
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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]
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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
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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
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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
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structural force
fluctuation-induced force
van der Waals force
DISJOINING PRESSURE
due to interaction of fluctuating dipoles (permanent or instantaneous) in molecules
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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
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VAN DER WAALS FORCE IN ANISOTROPIC MEDIA
Hamaker constant for uniaxial media
the relevant parameters are instead of averages
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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
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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
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(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)
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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
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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 ?
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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