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

60

70

80

90

100

110

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