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University of MaineOrono, July 16, 2008

Faculty of Engineering Institute of Materials Science Chair of Materials Science and Biomaterials

Superhydrophobic Aluminum Surfaces:

Preparation Routes, Properties and Artificial Weathering Impact

M. Thieme 1, C. Blank 1, A. Pereira de Oliveira 2, H. Worch 1,R. Frenzel 3, S. Höhne 3, F. Simon 3, H. Pryce Lewis 4, A. J. White 4

1 Technische Universität Dresden (TUD), Institute of Materials Science, Dresden, Germany

2 TUD, now at: Unicamp, Campinas, Brazil3 Leibniz Institute of Polymer Research Dresden, Dresden, Germany4 GVD Corporation, Cambridge, MA, USA

Contact angle 6 – Orono, July 14-16, 2008 Thieme et al., #2

Superhydrophobicity

Introduction

Theory:

•Type of interaction

•Nature of forces

•Triple line motion

•Design, bionics

Model Systems:

•Model arrays (pillars, hoodoos, …)

•Fractal surfaces

•Prediction - experiment

Analytical Issues:

•Contact angle measurement

•Surface characterization

Preparation Issues:

•Novel pathways

•Switchable systems

•Stability (tensides, irradiation, abrasion, …)

•Application


Introduction

Outline

Introduction

Our Experimental Attempts

Selected Results

- Two-step Preparation Routes

- Stability Under Artificial Weathering

Summary & Outlook


Introduction

Background

Superhydrophobicity for self-cleaning purposes

Aluminum as desired base material

Idea: Superhydrophobically furnished Al facades (transparent coating), looking like...

Source: Luxalon, 2000

former general store Centrum Dresden

Source: Kantschew, 2005


Experimental Measures

Preparation of Coated Inorg.-org. Hybrid

Systems (large area capability)

Micro-roughening+

Chem. Modification

Model Exposure

Artificial Weathering

(cyclic moisturing/ drying & irradiation, total duration 15 d)

‘WTH’

In-situ Characterization

EIS

CA(DCA)

XPS

Cross-sect.

FT-IRRAS

SEM

Cryo-fractur-

ing

Fluor-esc.

micro-scopy

EIS

AFM

EQCM

Ex-situ Characterization

NMR

Micro-hard-ness

LSM

Qual. criteria


Preparation Routes Micro-roughening

Al substrates:

Alloys

99.5 (1050),

AlMg1 (5005),

AlMgSi0.5 (6060)

Sheet, rod

Sulfuric Acid Anodization

Intensified Conditions

‘SAAi’

HT Micro-Embossing

‘ME’

+ Micro-blasting

‘MB’

Phosphoric-Chromic Acid Anodization[C. Blank et al., Prakt. Metallogr. Sonderband36, 491-496 (2004)]

Laser Ablation

[M. Thieme et al., Adv. Eng. Mat. 3, 691-695

(2001)]

1 2

SAAi:2.3 M H2SO440°C 30 mA cm-2

1200 s

ME:SiC tool (350 °C)285 °C 120 MPa

MB:corundum600 - 1200 grit

SAAu:20°C 15 mA cm-2

1200 s


Alkylsilanes

‘HTMS’

Fluoroalkylsilane

‘PFATES’

-Si-O-Si-O-O O

Preparation Routes Chemical Modification

Dissolved Compounds

Wet-born Coatings

Aminosilane+ Teflon® AF

‘AS/TAF’

Chitosan ac. (cathodically) + Alkyl maleicanhydride copolymer

‘Chs/POMA’

Tetradecanephosphonic

acid

‘TDPA’

SAMs

1

oxide

-Si-O-Si-O-O O

HO-P=OO

oxide oxide

Hot-filament CVD

(GVD Corp.)

Hexafluoro-propylene

oxide

PTFE

Var. in substrate

temperature, pressure,

post temper

(SC, AC, LP, A1, A2)

FF

NN

NN

oxide

OH

HOCH 2

OO

OHO

CH 2OHNH

NHO

2

2

AO

O

FO

2


SAAi

1 µm

1 µm

T = 40 °C, j varied, t = 1200 s

5 µm

28 ...42 mA cm-2

35 ...21 ...15 ...SAAu

T varied, j = 28 mA cm-2, t = 1200 s

5 µm

40 °C35 °C30 °C45 °C 50 °C

145°//103° 149°//54°155°//148°

148°//149°150°//150°

114°//36° 116°//32° 154°//150° 158°//156° 155°//91°

Parameter variations

ResultsMicro-roughening by SAAi

• optimal ranges of T, j exist for achieving SH

10 µm

Contact angles for modification using HTMS


ResultsMicro-roughening by SAAi

SAAi

10 µm

1 µm 1 µm

5 µm

Metal deposited+ cross-sect.

• SAAi: specific morphology & modified oxide compo-sition in comparison to usual anodic oxide SAAu

O-H strAl-Ox

S-Ox?

2 µm

Cryo-fractured


20 µm

20 µm

20 µm

20 µm

2 µm

20 µm

10 µm

ResultsMicro-roughening by ME/ MB

Tool

ME ME + MB-600

ME + MB-1200 + SAAi

‚Cold‘ ME

HT-ME HT-ME + MB

2 µm

• Purely mechanical & mixed approaches:

specific morphologies very similar to Lotus leaf


Results Chemical Modification by Solutes

SAAi + wet-born coatings (Chs/POMA, PFATES, AS/TAF)

• CA: superhydrophobicity achievable

• SEM: coatings generally not detectable

θa ≈ 152°

θr ≈ 150°


Li and Neumann:

θa = 133°, θr = 110°γl = 72.8 mJ m-2 (water),β = 0.0001247 (m2 mJ-1)2;

γs = 5.5 mJ m-2

PTFE, for comparison: γs = 18 mJ m-2

θ advancing angleγs surface energyγl surface tensionβ constant

[ ]2sl

l

s )((exp21cos γγγγθ −−+−= ß

Further approaches:

Zisman: critical surface tension γcrit = 11.6 mJ m-2,

Girifalco-Good-Fowkes-Young: disperse portion γs

d = 10.8 mJ m-2


Surface energies of pickled state (nearly flat) + AS/TAF

• free surface energies of modified surface extremely low



SAAi + wet-born coatings Chs Chs/POMA

AS/TAF

PFATES

• FT-IRRAS: POMA, PFATES, TAFcan be ±clearly identified viaC-H and C-F vibration bands

• XPS: different carbon bindings incl. >CF2, -CF3

Chs+POMA

PFATESAS/TAF



2 µm

20 µm

SAAi + Chs

Higher pH, j, t: craters, spots

• SEM, EIS: cathodicallydeposited Chs may be harmful for oxide integrity

NH3+ + HO- ↔ NH2 ↓ + H2O


ME + MB-600+ SC-1000

SAAi+ SC-1000

Results Chemical Modification by PTFE

• ‘thick PTFE coatings’ (50 nm to 1000 nm) can be deposited on different micro-rough substrates without leveling effects

• SEM: different, controllable morphology(budded protrusions / filmy flakes)

• CA: superhydrophobicity achievable, depending on substrate roughness

flat surface+ SC-1000

SAAi+ A1-500

10 µm

157±1, 155±1

1 µm

152±1, 151±1

2 µm

151±1, 149±1

2 µm

144±3, 94±6

SAAu+ SC-50

2 µm

151±2, 140±2


SAAi + PTFE

PTFE-LP

PTFE-SCPTFE-SC

PTFE-A1

PTFE-SC on MB

PTFE-LP


• IRRAS: different PTFE deposition conditions with minor deviations in C-F bands

• XPS: generally mostly >CF2 bindings; with PTFE-LP deviations detectable (more C-H and O)


SAAu + PTFE-SC


• EIS: growth of impedance at medium frequencies with increasing PTFE thickness; low frequency limit ±uniform;

EIS curve shapes/ positions similar to earlier results with SAAi + PFATES and SAAi + AS/TAF[M. Thieme & H. Worch, J. Solid State Electrochemistry 9, 737-745 (2006)]


50

75

100

125

150

175

SAAi + C

hs +

POMA

SAAi + P

FATES (2

x)

SAAi + A

S/TAF (2

x)

SAAi + S

C (250

-1000

nm, 6

x)

SAAi + C

hs +

SC (250

-1000

nm, 4

x)

SAAi + A

1-500

(2x)

SAAi + A

2-500

SAAi + A

C-500

SAAi + LP

-500

SAAu + S

C (50-1

000 n

m, 3x)

Pickled

+ SC-50

0 SC-10

00

MB-1200

+ SC-10

00

MB-600 +

SC-10

00ME +

SC-1000

ME + MB-12

00 + S

C-1000

ME + MB-60

0 + S

C-1000

Con

tact

ang

le /

°before WTH, adv. CAbefore WTH, rec. CAafter WTH, adv. CAafter WTH, rec. CA

Results Impacts from Artificial Weathering

• Practically no CA changes ( ): SAAi + PFATES, + AS/TAF,+ PTFE-A1, -AC; ME + SC, + MB-1200 + SC, + MB-600 + SC

• Moderate changes ( ): SAAi + PTFE-SC, -A2, -LP; MB-600 + SC

• Considerable changes ( ): SAAi + Chs + PTFE-SC

• Dramatic worsening ( ): SAAi + Chs/POMA, SAAu + SC,Pickled + SC, Flat + SC, MB-1200 + SC

CA Survey



SAAi + wet-born coatings before/ after WTH

AS/TAFPFATES

AS/TAF+ WTH

PFATES+ WTH

• PFATES: practically unchanged; portion of F-bound carbon remained on a high level

• AS/TAF: decrease of low-energy bound carbon; agrees with increased F/C concentration ratio aminosilane component undergoes degradation in the course of the exposure!



SAAi + PTFE variants before/ after WTH

SAAi + SC

SAAi + SC+ WTH

• PTFE-SC: low, but marked increase of H-bound carbon, along with increase of O content

• PTFE-AC: relatively lowest WTH-related changes acc. to XPS – finding corresponds to little changes in CA!

SAAi + AC+ WTH

SAAi + AC


Conclusions

Superhydrophobic hybrid systems in multifold variants

Micro-roughening feasible i) by specific anodizationor ii) by micro-embossing/ -blasting

SAAi large-area capable; ME to be developed as a continuous rolling technique

Chemical modification feasible i) by different nanoscalic wet-born coatings or ii) by HFCVD-produced sub-microscalic PTFE layers

Weathering stability of F-containing systems superior to POMA; PTFE-AC better than SC procedure

Resistivity to abrasive effects must be enhanced, in spite of higher ‘buffer’ in case of thicker coatings –subject of further work


Acknowledgments

Saxon State Ministry of Science and the Fine Art (SMWK)

German Academic Exchange Service (DAAD)

Mr. T. Burkhardt and J. Engelmann (FhG-IWUChemnitz)

Mrs. K. Galle (TUD) and Mrs. B. Schneider (IPF)

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