Are You Ready for Superhydrophobicity? By John B. Durkee, Consultant in Metal and Critical Cleaning

: :::: :: :: : :: :: : :::::: : : : : : : : ::: :::: ::: ):: :::: ::: :: : : ::::::::::::::::::::::::::::::::::: i n last month's column, I: wroteabout what might be: the:future::of metal finishing.: ~ e : t h e m e W a s thesci~nce i behind understanding and controlling:the character of surfaces(That's: w h a t w e do:in: the :finishL'~g of met~ii:: • I believe superhydrophobicity, t h e subject of:this colum, will: be :a new : t ~ e o f metal:finiShi:~is: ou~comei:: produced in many ways for: a like:: :number of:purposes:vAIl:::become :h dominant characterist ic b f :p rbduc t s manufactured by the next generation of metal finishing op~rators.~: :: : ::i :=:::::: =::::: : : : : - :: :::::: ::: :::: :::: : :::

W H A T IS S U P E R H Y D R O P H O B I C I T Y (SH)? This word means "water hating" (hydrophobic) at a high level (super). For metal f inishing work, the word means unique surfaces tha t shed wate r extremely well without coatings.

N A T U R A L F I N I S H I N G SH occurs in nature. About noticed that the leaf of the Lotus plant sheds wa te r because the water doesn' t touch (wet) the leaf's struc- tura l surface 2. This is shown in both a full-size and highly magnified view in Figures 13 and 2 4.

The native surface struc- ture is shown magnified in Figure 3.

It is the tiny protrusions (called asperities), nanome- ters tall and in diameter, that produce the SH effect. This is the ult imate water- repellent coating for metal finishers.

Scientists, including some from U.S. firms but mostly from European firms, are try- ing to answer two prime questions about this surface.

a generation ago it was

Figure 1.

Figure 2.

Figure 3.

QUESTION 1 What is the best selection of height, diameter, and spacing distance for asperities to produce the most water-repellent surfaceS?

A useful choice is shown ~ - in side view in Figure 4, which should be compared | i | | i | i _ | i l l l i to Figure 2.

Please notice how the sur- Figure 4. face tension forces support the drop on the structur- al "bed of nails6. " Also notice how the area of contact

between the drop and the surface is so much small- er than that of a surface without asperities.

A poor choice is shown in Figure 5. Here, the spac- ing is too broad for the same water drop.

The choice becomes differ- ent if the surface tension of water is changed by addi- tion of a detergent , which reduces the surface tension. This is shown in Figure 6, which should be compared to Figure 4.

Here, there isn't adequate surface tension force to counteract the force of gravity, and the droplet

I . . , l , . . l ....

Figure 5.

=~ ~ _ 1 _ I , .

! 1 I I I i l l l J l l l l I I I

Figure 6.

does wet the surface. Worse, the droplet may become "entangled" in the asperities and cling rather than be repelled from the specially constructed surface.

In other words, the spacing of asperities should be different if the surface is to be repellent of solvents versus water.

QUESTION 2 How should this chosen surface be produced? • One possibil i ty is tha t the asperi t ies would be

"printed," as would an inkjet printer deposit pico- liters of ink on paper.

• Another is via lithography as (controlled etching) would a micro-circuit be "printed" on a silicon semiconductor.

• Another possibil i ty is tha t nano-sized part icles can be bonded to a metal surface 7, The particles could be amine-functionalized silica (silicon diox- ide) and might be applied as a suspension as would a coating be applied by finishers today.

• Yet another possibility would be by chemical vapor deposit ion leading to self-assembly growth of micro-pillars consist ing of well-aligned carbon nanotubes 8. Several ex-amples are shown in Figure 7 9 .

• Still another possibility is that the water-repellent structure might not be assembled as a controlled

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C L E A N I N G T I M E S : :: : ; ~ v?~:::::

mechanical roughness. It

';i j:, !

might be an applied | (grafted) chemical (coat- [ ing) whose molecular properties are such tha t - they repel water m°le- B B cules 1°. These coatings have been spin coated or spray cast on smooth and

Figure 7. flexible a luminum sub- strates. The coatings can be optically t ransparent or colored. Multiple answers to the latter question are likely to

be available in two to five years. Those answers will be how metal finishing of some surfaces will be done early in the next decade. Basically, this would be application of a controlled roughness to a surface 11.

B B

T H E V A L U E O F S U P E R H Y D R O P H O B I C I T Y

Imagine: • Aluminum aircraft structures (aerospace vehicle

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• tubing through which water flows without signifi- cant pressure loss;

• metal building panels, t ruck and railcar bodies, ornamental metal figures, and drain piping that don't need to be cleaned because water drains from them carrying particles with it12;

• fasteners tha t don't rus t because water doesn't adhere to their surfaces;

• marine structures on which barnacles don't grow, and that move through water with much less fric- tional drag;

• waterproof coatings; • metal surfaces that reflect incident light in a dif-

fuse pattern and aren't shiny by design; • and metal surfaces without water spots.

And outside of metal structures: • Paper packages that don't get wet; • ketchup bottles and honey jars that empty easily; • and lenses that don't fog.

DON'T FORGET SUPERHYDROPHILICITY One can also make, though not in the same way, metal surfaces tha t love water. They are called

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46 www.meta i f in ish ing.com

hydrophil ic . I m a g i n e how long a w a t e r - b a s e d pa in t would l a s t w h e n appl ied to a hydrophi l ic surface!

W H A T D O E S T H I S M E A N TO Y O U . = Today, i t 's j u s t a curiosity. But , r e s ea r ch budge t s for new science a b o u t s u p e r h y d r o p h o b i c i t y a re f lush. Too m a n y s m a r t people h a v e too m a n y resources not to produce some ways by which m e t a l sur faces a re f in ished in m u c h di f ferent ways t h a n we do now.

In five to 10 yea r s (or sooner) supe rhydrophob ic i t y t echnology will change the c h a r a c t e r of m e t a l fin- i sh ing work. This technology won ' t be expor ted off- sho re b e c a u s e of cost compe t i t i on , t h o u g h it m a y s t a r t t he re because tha t ' s whe re m u c h of r e s ea r ch is be ing done.

And this technology p robab ly won ' t be d o m i n a t e d by env i ronmen ta l , safety, and h e a l t h concerns as we find today. Skilled, t r a i ned worke r s and owned intel- lec tual p r o p e r t y will be essen t i a l for success, which is not a l w a y s t r u e today. S u r f a c e c l e a n i n g a n d p r e p a r a t i o n won ' t become less i m p o r t a n t .

I t ' s too soon to find a supp l i e r of supe rhydrophob ic p roduc t s or to buy s tock in one, bu t i t 's not too soon to follow its evolut ion on the In t e rne t .

REFERENCES 1. Much about what I learned came from attending two

scientific conferences in Toronto. Both were directed by Dr. K. L. Mittal, a well-known scientist in technolo- gy associated with surfaces. The conferences were the Tenth Internat ional Symposium on Particles on Surfaces--Detection, Adhesion, and Removal, and the Fifth Internat ional Symposium on Contact Angle-- Wettability and Adhesion. Attendees were scientists, not engineers, seeking practical applications.

2. Wilhelm Barthlott and Christoph Neinhuis made this observation and published it in the British Scientific journal Nature. R. McNeill Alexander writes [Nature, Vol. 438 (10 November 2005)] how microscopic obser- vations about the construction and actions of plants and animals have led to discoveries of new products such as self-cleaning glass and "smart textiles" into which bioluminescence has been impregnated, giving the ability to change color. A good summary, published in Nature Materials as Vol. 2 in May 2003 by Ralf Blossey, describes progress in creating self-cleaning structures. You can find it at http://iri.ibl.fr/bn/pubs/ ralf/blossey03.pdf.

3. Figure 1 is a photograph by Zolt Levay from Kenilworth Aquatic Gardens, Washington D.C.

4. Figures 2 and 3 are courtesy of the University of Bonn, Department of Botanik.

5. The earliest work on this problem can be attributed to Wenzel (J. Phys. Colloid Chem., Vol. 53, 1949, p. 1466) and Cassia (Faraday Soc., Vol. 3, 1948, p. 11)

6. This outcome is called the "Fakir Effect" by some.

7. Yue Li, Weiping Cai, Bingqiang Cao, Guotao Duan, Fengqiang Sun, Cuncheng Li, and Lichao Jia, Nanotechnology, Vol 17, 2006, pages 238-243.

8. Wei-De Zhang et al, Nanotechnology, Vol. 16, 2005, pages 2442-244.

9. Figure 7 is courtesy of Lawrence Livermore Laboratory.

10. One material used for this purpose has the same prim- itive structure as does oil--it 's called nhexatriacon- tane (n-C36H74). See Tavana, H. , Amirfazli, A., and Neumann, A. W., Langmuir, Vol. 22, No. 13, 2006, pages 5556-5559.

11. It is odd that metal finishers today spend resources to minimize surface roughness, and in the future they may be trying to apply it in a controlled fashion.

12. This approach has already been commercialized with glass (by PPG) and polycarbonate (by GE) materials.

John B, Durkee, II, PhD, PE,: is a conSU~tant:in ~ a l and critical cleaning. :contact: him in Hunt, =: Texa~ at:: jdurkee@precisionvleaning.com; 830-238-7610;:Fax: 612:677-3t70; :or :830:459-5555 :(celiu:iar)i:::: Dr,: Durkee is: the author of the recently pubiished~k;,:

: "Industrial Cleaning:::Processes::~nd,: Tec:hnbtogy;~ (ISBN :o-o804-4sss7) by Etsev r Ma age e ti:mf

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October 2006 47