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

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
Intermolecular Potential
Spherically symmetric
Short range
Isotropic and Viscous
Polymers
Liquid crystals
Colloids
Surfactants
Lipids
Intermolecular Potential
Absence of symmetry
Long/Short range
Anisotropic and Visco-elastic

6. Energy per particle increases
Bulk Liquids
Order increases
Solid Liquid Gas
Energy per particle 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
Unbalanced force pulls the
surface molecules inward.
 Surface Tension
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

9. Self-affine Surfaces
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
CdSt – irreversible fracture behaviour
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.
π = 30mN/m
π = 5mN/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
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.
π = 30mN/m
π = 5mN/m
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
CdSt – self-affine behaviour
Height-height correlation
g (r) = <[ h(r0+r) – h(r0) ]2>
3ML
1ML
r << L, g(r) ∝ R2H
r >> L, g(r) = 2σ2,
Conversion factor.
Scan size (μm) m r(nm)
500 × 500 nm2;
1 × 1 μm2;
2 × 2 μm2;
5 × 5 μm2;
10 × 10 μm2 scan size.

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

16. The Parameters CdSt Film Scan size (μm) σ (A° ) L (nm) H
1 ML ML
CoSt Film
Phys. Rev. E 84, (2011)

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

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

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

20. Confined Neutral Polymer
PS Mw = , Rg = Å
PS5C
MPT potential
Entangled
Disentangled & layered
Confinement
~212 Å ~Rg
Below a certain film thickness
polymer molecules are arranged
in layer quite like a simple liquid
Europhys. Lett., 36(4), 265, (1996), Phy. Rev. B, 72, (2005), Macromolecules, 40, 9190 (2007)

21. Confinement versus Entanglement – Role of Molecular Weight
δ = order parameter= ρmax-ρmin
AH = Hamaker constant  cohesive energy
AH= PS (max2 - min2)
With increasing molecular weights i.e. chain lengths layering vanishes.
There exists a competition between ‘entanglement’ and ‘confinement’.
Phy. Rev. B, 72, (2005), Macromolecules, 40, 9190 (2007)

22. Confined Charged Polymer
Pristine film  no counterion, Buffered film  10mM buffer added. 1 buffer molecule  1 Na+ ion
Pristine film has three layers
- simple liquid like
Counterions in buffered film
destroys layering – polymer like
5.06 nm
0.00 nm - 
0.00 nm nm - 
Pristine film
Buffer film
Communicated

23. Confinement versus Entanglement – Role of Counter-ions
Pristine film
Buffer film
Film 2θ
κ (E-7)
(Å-1)
Pristine
1.2
5.01
1.5
6.0
1.8
6.527
Buffer
2.0
2.10
Height-difference correlation function,
g (r) = <[ h(r0+r) – h(r0) ]2>
In buffered film along its depth density fluctuation decreases - layering is absent.
Films have liquid like correlations.
DNA molecules behave as liquid of rods.
Self-affine liquid like correlation function,
0= roughness, L =correlation length, H= Hurst exponent,
B= kBT/γ, γ = surface tension, κ = lower cut-off wavevector
∆ρ = density fluctuation
Communicated

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

25. Thank you