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Wetting of surfaces
Mika LindénDepartment of Physical Chemistry
Åbo Akademi UniversityTurku, Finland
Outline
• Wetting theory– Cassie equation
– Wenzel equation
– Combined Cassie-Wenzel equation
• Superhydrophobic surfaces
• Superhydrophilic surfaces
• Evaluation of wetting theories for surfaceswith nanometer-scale heterogeneities
• Applied wetting, examples
Wettability background
• Wettability is an importantphenomenon in many industrialprocesses, like adhesion, self-cleaningsurfaces etc.
• Wetting is dependent on bothchemical composition and morphologyof the surface
A classical example is the self-cleaning properties of the lotus leaf where the water repellency is due to a combination of surface roughness and hydrophobicity. (ref. Barthloot, W., Neinhuis, C. Planta 1997, 202, 1-8)
Contact angle
• the balance of forces at the line of contact between the liquid and the solid determines the contact angle
gsg = gsl + glg cos qc
• the angle qc is called the Young contactangle if the surface is ideally flat …
• Water contact angle > 90o = hydrophobic surface• Water contact angle < 90o = hydrophilic surface
There are different contact angles…
• Three different contactangles can be defined, the static, receding, and the advancing contact angle
• For a real surface
qrec < qstat < qadv
qstat
qrec
qadv
Wetting of rough surfaces: the Wenzel equation
The wetting of a rough, hydrophilic, one-component surface is oftendescribed by the Wenzel equation, where the apparent contactangle is given by
where r is the ratio between the real (A3D) and projected surfacearea (A2D) of the sample, and qY is the Young contact angle.
YA r qq coscos
A3D
A2D
r = A3D/A2D
Consequences of the Wenzel equation
• For a hydrophilic surface, i.e. when the Young contactangle < 90oC, rougness makes the surface becomemore hydrophilic
• For a hydrophobic surface, i.e. when the Young contact angle > 90oC, rougness makes the surfacebecome more hydrophobic
Wetting of chemically heterogeneous surfaces, the Cassie equation
In the case of a flat, chemically heterogeneous surface, the wetting can be described by the Cassie approach. For a two component system, the wetting is described by
where f1 and f2 are the area fractions of material 1 and 2 (f1 + f2 =1), and q1 and q2 the contact angles of pure materials 1 and 2, respectively.
2211 coscoscos qqq ffA
Wetting of rough surfaces –the Cassie-Baxter equation
If air is assumed to be entrapped in the voids of a rough, hydrophobic surface (q1 > 90º, q2 = 180º), the Cassie equation is written
This formalism is generally referred to as the Cassie-Baxter equation.
For hydrophilic films, capillary effects has also been shown to be important for the creation of superhydrophilic surfaces.
211 coscos ffA qq
Which type of contact angles should be used in the modeling when trying to assess the chemical
composition of the surface?
• Many researchers support the idea of using the advancing contact angle as the best measure, whileothers use the static contact angle
• The receding contact angle is less frequently used
Practical examples
1. Superhydrophobic films
Superhydrophobic surfaces through
biomineralization
The open porous structure of nucleated and grown calcium phosphate, here
predominantly octacalciumphosphate, is an ideal candidate for a
superhydrophobic surface, as it has a lot of open pores that can entrap air
Surface functionalization using organophosphates is possible
25 30 35
0
1000
2000
3000
4000
5000
6000
7000
[30
0]
[00
2]
[11
2]
Inte
nsity/a
rb.u
nit
2q/°
[21
1]
Confirmation of successful zonyl surface functionalization by XPS
• XPS shows clearly that zonyl is attached to the surface
• From the CF3 to CF2 ratios we can estimate that the mean number of carbon atoms in the chain being either CF3 groups or CF2 groups is 6.5
CF2
CF3
C=O
C-O
C-C
Järn et al., Langmuir, 2008
Superhydrophobic calcium phosphate
Wetting of nanopatterned films
Nanopatterned inorganic thin films
Dip-coating
Drying
Thermal decomposition
Selective functionalization
The pore diameter can be tuned between 10 and 50 nm depending on the size of the surfactant used as the template and the solvent composition.
Pores are aligned in the direction perpendicular to the substrate, which make these films ideal for applications related to microelectronics and sensing, to name a few.
Ref. A. Fisher, M. Kuemmel, M. Järn, M. Linden, C. Boissière, L. Nicole, C. Sanchez, D. Grosso, Small, 2 (2006) 569
Film characterization(SEM, AFM, ellipsometry)
TiO2 on Au, 1µm*1µm AFM image
TiO2 on SiO2, 1µm*1µm AFM image
Mean pore diameter 11 nm Film Thickness 5.5 nm
Mean pore diameter 31 nm Film thickness 11 nm
Mean pore diameter 29 nm Film thickness 11 nm
Combination of Wenzel and Cassie equations
2211 coscoscos qqq FFA
21
11
ffr
frF
21
22
ffr
fF
Cassie-Wenzel
Wenzel Cassie
Structural data
Electrochemically measured f-values agree well with the calculated for small pore films
Sample r fTiO2 (Cassie-Baxter)structural cycl. volt.
FTiO2 (Cassie-Wenzel)structural cycl. volt
TiO2/Au small pores
1.66 0.75 0.78 0.83 0.85
TiO2/Au large pores
1.79 0.65 0.44 0.76 0.58
TiO2/SiO2
small pores1.66 0.75 - 0.83 -
TiO2/SiO2
large pores1.96 0.63 - 0.76 -
All necessary parameters for the modelling of Cassie-Baxter and Cassie-Wenzel is obtained from AFM, SEM and ellipsometry
Superhydrophilic surfaces
Järn et al. Chem. Mater. 2008
TiO2/Au large pores
0
10
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30
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50
60
70
80
90
stat
TiO2
TiO2/Au
Au
Water contact angles- TiO2/Au
Full spreading as the droplet finds itself on a composite surface of solid and liquid, with a liquid film ahead of the droplet. The thickness of the film (about 10 nm) is enough to cause water penetration into the grooves.
(See J. Bico, C. Tordeux, D. Quéré, Colloids Surf. A 2002, 206, 41)
Cassie-Wenzel
Functionalized composite films (droplet volume 2 µL)
Cassie-Baxter
Cassie-Wenzel
TO-TiO2/Au small pores
50
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120
stat adv
TO-TiO2
TO-TiO2/Au
TO-Au
TiO2 is hydrophobized, and constitutes the continuous network
Functionalized composite films (droplet volume 2 µL)
The wetting behavior of small droplets can be modelled with the Cassie-Baxter equation, i.e. air seems to be entrapped in the pores
TO-TiO2/Au large pores
50
60
70
80
90
100
110
120
130
140
stat adv
TO-TiO2
TO-TiO2/Au
TO-Au
Cassie-Baxter
Cassie-Wenzel
Functionalized composite films (droplet volume 10 µL)
The static contact angles of larger droplets agrees well with Cassie-Wenzel predicted contact angles, while the advancing contact angles shows Cassie-Baxter behavior.
TO-TiO2/Au large pores
50
60
70
80
90
100
110
120
130
stat adv
TO-TiO2
TO-TiO2/Au
TO-Au
Cassie-Baxter
Cassie-Wenzel
What determines the wetting?
Addition of liquid
•Are the area based Wenzel and Cassie equations correct?
•Several authors (Extrand, Langmuir 2003, 19, 3793., Gao et al. Langmuir 2007, 23, 3762.) have suggested that the three-phase contact line determines the contact angle.
Intermediate Conclusions
• Regioselective functionalization can readily be achived for hybrid films
• Contact angle measurements are sensitive means for determining the success of the regiofunctionalization
• Contact angle measurements are NOT sensitive to the wettingproperties of the surface which is covered by the droplet
• For nanopatterned films contact angle data can still be describedreasonably well by determination of the relative area fractions, as the heterogeneity length scale is much smaller than the size of the droplet
• In the described cases, both the static and the advanced contact angleworks reasonably well
Variation of f-values for 4 nm thin TiO2
nanopatterns on SiO2 wafers
CF3(CF2)5(CH2)2Si(OC2H5)3
SiO2
FAS functionalization
FAS
SiO2
UV light
Siloxane headgroup
SiO2
TiO2TiO2
TiO2TiO2
TiO2TiO2
TiO2 is made hydrophilic, and constitutes the continuous network
Verification of f-values
694 692 690 688 686 684 682
0 min
5 min
15 min
30 min
60 min
120 min
Inte
nsity [
a.u
.]
Binding Energy [eV]In
cre
asin
g U
V tim
e
UV time [min]
0 5 15 30 60 120
F/Ti 2.34 2.03 0.91 0.72 0.42 0.32
fFAS 1 0.87 0.39 0.31 0.18 0.14
F 1s
Contact angle results
0 20 40 60 80 100 1200
20
40
60
80
100
120
TiO2@SiO2_FAS
TiO2_FAS
SiO2_FAS
Sta
tic C
A [
de
gre
es]
UV time [min]
SiO2
TiO2TiO2
SiO2
TiO2TiO2
M. Järn, Q. Xu, M. Lindén, Langmuir, 2010
Static contact angle gives the best agreement!
0 20 40 60 80 100 120
0.0
0.2
0.4
0.6
0.8
1.0 stat CA
adv CA
rec CA
XPS
f FA
S
UV time (min)
• fFAS values calculated from the static CA based on the Cassie-Wenzel
equation are in good agreement with the XPS data
• NOTE! No superhydrophilicity although the TiO2 is ”free”. This is due to
the very thin films that will not facilitate capillary spreading!
Very useful for determination of the chemical composition of films
• Multicomponent films, separate
phases – how much of each
component is exposed on the
surface?
• Multicomponent films, possible
presence of mixed phases – how
much of a certain component is
present in a mixed phase form on
the surface?
• Requires a) elemental analysis of the film, b) possibility for selective
surface functionalization of at least some components