This document summarizes research on liquid-like thin films conducted by Alokmay Datta and collaborators. Key points include: 1. Langmuir monolayers of stearic acid with cadmium ions exhibit irreversible fracture like a solid, while with cobalt ions they spread out and reconnect like a liquid upon decompression and recompression. 2. Monolayers of stearic acid on a solid surface with cadmium ions exhibit self-affine height correlations characteristic of a solid, while with cobalt ions they show logarithmic correlations characteristic of a liquid. 3. Neutral polymer films like polystyrene below a thickness of about the polymer radius of gyration arrange in

1. Alokmay Datta
Applied Material Science Division
Saha Institute of Nuclear Physics
Kolkata
India
‘Liquid-like’ Films

2. What I would like to talk about
 Liquids
 When liquids do not behave as ‘liquids’
 How to ‘know’ a liquid
 Langmuir monolayers and multilayers
‘Solid-like’ and ‘liquid-like’
 Polymer films
‘long-rod’ and ‘short-rod’ liquids

3. My c0-workers
My Students (Past and Present)
 Sarathi Kundu, Institute of Advanced Study in Science & Technology, India
 Sudeshna Chattopadhyay, Northwestern University, USA
 Smita Mukherjee, Indian Institute of Technology, India
 Nupur Biswas
From Saha Institute
 Milan K. Sanyal
 Munna Sarkar
 Mrinmay Mukhopadhyay
From Kyoto University
 Masatoshi Ichikawa
 Kenichi Yoshikawa

4. Liquids
Scattering Set-up for Liquid Surfaces – Beamline 18B (India-Japan Beamline), Photon Factory

5. Simple & Complex Liquids
 Water
 Liquid metals

 Polymers
 Liquid crystals
 Colloids
 Surfactants
 Lipids
Intermolecular Potential
Spherically symmetric
Short range
Isotropic and Viscous
Intermolecular Potential
Absence of symmetry
Long/Short range
Anisotropic and Visco-elastic

6. Bulk Liquids
Solid Liquid Gas
Energy per particle increases
Order increases
Bulk liquids, either complex or simple, can be identified as ‘liquids’
from their mechanical properties
For confined liquids, however, it is not easy to do that
We need to take recourse to structural correlations

7. The Three Phases
Things are different at liquid surfaces and under confinement

8. Liquid Surface and Films
 For liquid surface molecules exhibit capillary fluctuations
 Motions due to gravity and thermal energy.
 This gives rise to a height-height correlation of the liquid surface,
whether the liquid is bulk or confined
 This correlation has a continuous spectrum of periodicities, the lowest being the
molecular size and the highest being the size of the liquid or film body
 This correlation depends logarithmically on r, the separation between the heights
Unbalanced force pulls the
surface molecules inward.
 Surface Tension

9. Self-affine Surfaces
]})/(exp[1{2)( 22
0
H
Lrrg  
These surfaces are created by fractional Brownian motion.
On them N steps taken with step-size r to cover the length of a curve,
implies that the curve at that scale has length l = Nr, with N = C/rD, 0<D<1.
They have the typical height-height correlation given by

10. Layering in Simple Fluids: TEHOS
Phys. Rev. Lett. 82 , 2326 (1999)

11. Confined Liquids
Confined simple liquids
Ordered state of matter
What happens to the complex liquids, such as Langmuir films and polymers ?

12. Monolayers on Water Surface – ‘Solid’-like
π = 5mN/m
CdSt – irreversible fracture behaviour
π = 30mN/m
With cadmium ions
in subphase, stearic
acid monolayer, when
compressed (up
to π = 30 mN/m),
shows the formation
of “crystallites” that
remain unaffected
when decompressed
from π = 30 mN/m
to π = 5 mN/m.
The CdSt monolayer at 5 mN/m
distinctly shows the presence of a
monolayer on which crystallites are
formed.

13. Monolayers on Water Surface – ‘Liquid’-like
π = 30mN/m π = 5mN/m
CoSt – soap bubble-like behaviour
In the presence of
cobalt ions, on
the other hand, the
monolayer “spreads
out” gradually as π
changes from 30
mN/m to 5 mN/m
and forms completely
interconnected “soap-
bubble-like” features
on decompression.
These aggregate
continuously to form
the monolayer on
recompression.
These decompression and recompression behaviours
of Langmuir monolayers with cadmium and cobalt
ions in subphase suggest, respectively, the essentially
irreversible fracture of a solid and the reversible,
interconnected-bubble features of a soapy liquid.

14. Monolayers on Solid Surface – ‘Solid’-like
3ML1ML 3ML
CdSt – self-affine behaviour Height-height correlation
g (r) = <[ h(r0+r) – h(r0) ]2>
]})/(exp[1{2)( 22
0
H
Lrrg  
Conversion factor.
Scan size (μm) m r(nm)
0.2 1 0.78
0.5 1 1.95
1.0 1 3.91
2.0 1 7.81
5.0 1 19.53
10.0 1 39.06
r << L, g(r) ∝ R2H
r >> L, g(r) = 2σ2,
(I) 500 × 500 nm2;
(II) 1 × 1 μm2;
(III) 2 × 2 μm2;
(IV) 5 × 5 μm2;
(V) 10 × 10 μm2 scan size.

15. Monolayers on Solid Surface – ‘Liquid’-like
CoSt – liquid-like behaviour
1ML 3ML3ML
Logarithmic function
f (r) = d ln(ar2 + br + c)

16. The Parameters
CdSt Film
Scan size (μm) σ (A° ) L (nm) H
1 ML 0.5 34.0 4.4 0.83
1.0 35.0 2.8 0.76
2.0 39.3 1.3 0.74 3
ML 0.5 39.0 15.9 0.75
1.0 49.0 6.4 0.72
2.0 47.3 1.4 0.78
5.0 35.0 0.3 0.78
10.0 48.0 0.1 0.85
CoSt Film
Phys. Rev. E 84, 021606 (2011)

17. What kind of ‘Liquid’ is this?
Phase Images
CdSt
CoSt
‘domains’
‘waves’
CdSt CoSt

18. Why Does this Happen – Role of Molecular Dipoles
CoSt – No Molecular Dipoles
No Long-range Forces
CdSt –Molecular Dipoles
Long-range Forces
Phys. Rev. E 83, 041604 (2011).

19. Polymers – Neutral and Charged
Polystyrene – neutral polymer DNA – polyelectrolyte
- -  CH  CH2  CH  CH2  - -

20. Confined Neutral Polymer
0.1 0.2 0.3
10
-6
10
-2
10
2
302Å
500Å
600Å
840Å
1139Å
R/RF
qz
(Å
-1
)
0 100 200 300 400 500 600
0.24
0.32
ElectronDensity(eÅ
-3
)
0 200 400 600 800
0.16
0.24
0 400 800 1200
0.0
0.5
substrate
0 100 200 300 400 500
0.24
0.32
0 50 100 150 200 250 300
0.24
0.32
Depth from the surface (Å)
PS Mw = 560900, Rg = 203.71Å
Europhys. Lett., 36(4), 265, (1996), Phy. Rev. B, 72, 155418 (2005), Macromolecules, 40, 9190 (2007)
0.26 0.28 0.30 0.32 0.34 0.36
500
400
300
200
100
0
Depthfromsurface(Å)
Electron Density (eÅ
-3
)
~212 Å ~Rg
Below a certain film thickness
polymer molecules are arranged
in layer quite like a simple liquid
PS5C
MPT potential
Entangled
Disentangled & layered
Confinement

21. 400 600 800 1000 1200
0.00
0.04
0.08
0.12 PS2C
PS5C
(eÅ
-3
)
Film Thickness, d (Å)
Confinement versus Entanglement – Role of
Molecular Weight
400 600 800 1000 1200
0
50
100
150
200
250
PS2C
PS5C
AH
(meV)
Film thickness, d (Å)
Phy. Rev. B, 72, 155418 (2005), Macromolecules, 40, 9190 (2007)
δ = order parameter= ρmax-ρmin AH = Hamaker constant  cohesive energy
 With increasing molecular weights i.e. chain lengths layering vanishes.
 There exists a competition between ‘entanglement’ and ‘confinement’.
AH= PS (max
2 - min
2)

22. -50 0 50 100 150
0.0
0.2
0.4
0.6
Electrondensity(eÅ
-3
)
Depth from the surface (Å)
pristine film
buffered film
Confined Charged Polymer
0.0 0.2 0.4 0.6 0.8
10
-9
10
-7
10
-5
10
-3
10
-1
10
1
10
3
buffered film
Reflectivity
qz
(Å
-1
)
pristine film
Pristine film  no counterion, Buffered film  10mM buffer added. 1 buffer molecule  1 Na+ ion
5.06 nm
0.00 nm
5.06 nm
0.00 nm
0.00 Pi
-0.00 Pi
-
 16.81 nm
0.00 nm
0.00 nm
16.81 nm
-
0.00 Pi
-0.00 Pi
Pristine film Buffer film
 Pristine film has three layers
- simple liquid like
 Counterions in buffered film
destroys layering – polymer like
Communicated

23. 10
-5
10
-4
10
-3
10
-2
10
-1
10
0
DSC(Normalized)
qy
(Å
-1
)
21.8
o
21.5
o
21.2
o
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
2 = 1.8
o
2 = 1.5
o
2 = 1.2
o
DSC(Normalized)
qy
(Å
-1
)
Confinement versus Entanglement – Role of
Counter-ions
Communicated
Pristine film Buffer film
g (r) = <[ h(r0+r) – h(r0) ]2>
Height-difference correlation function,
Self-affine liquid like correlation function,
]})/(exp[1)]}{
2
ln([2{)( 22
0
H
E Lr
r
Brg 


0= roughness, L =correlation length, H= Hurst exponent,
B= kBT/γ, γ = surface tension, κ = lower cut-off wavevector
Film 2θ κ (E-7)
(Å-1)
Pristine 1.2 5.01
1.5 6.0
1.8 6.527
Buffer 1.2 2.0
1.5 2.10
1.8 2.0
 /2
 g ∆ρ = density fluctuation
 In buffered film along its depth density
fluctuation decreases - layering is absent.
 Films have liquid like correlations.
 DNA molecules behave as liquid of rods.

24. What next?
1. How do two 2D liquids mix – competition between entropy and interaction?
2. What is a ‘dried-up’ film: ‘Solid-like’ or ‘liquid-like’? What decides that?
3. Can thinning be treated at par with other ‘fields’ (temperature, pressure, etc.)
that cause phase transitions? How does it enter the energy term?