Critical Cleaning For Contamination Control: What is Superhydrophobicity?

By: John B. Durkee, PH.D., P.E.
January 2007

What is Superhydrophobicity?

This word means "water hating" (hydrophobic) at a high level (super). It represents a surface condition that has been attracting a significant level of interest from researchers and business leaders over the past half-decade. In one sense, superhydrophobic (SH) surfaces are the ultimate in no-clean technology. In another sense, they represent early payoff from some of the expensive research done in nanotechnology. In still another sense, these surfaces represent the outcome of following the "right" methodology for design of products — that of "Mother Nature." While superhydrophobic products aren't yet available at Wal-Mart, the technology is interesting and will be significant. In this column, I'll cover two embodiments of super-hydrophobicity — surfaces and what are called "liquid marbles." Imagine:

• Aluminum aircraft structures (aerospace vehicle coatings) that don’t form ice at altitude and don’t need to be de-iced,
• Tubing through which water flows without significant pressure loss,
• Metal building panels, truck and rail car bodies, ornamental metal figures, and drain piping that don’t need to be cleaned because water drains from them carrying particles with it,1
• Fasteners that don’t rust because water doesn’t adhere to their surfaces,
• Marine structures on which barnacles don’t grow, and that move through water with much less frictional drag,
• Waterproof coatings,
• Metal surfaces that reflect incident light in a diffuse 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,
• Lens which don’t fog, and
• Ketchup bottles and honey jars that empty easily.

All of these applications or products represent super-hydrophobic surfaces — ones from which water molecules are strongly repelled.

A NON-WETTING SURFACE
SH occurs in nature. About a generation ago it was noticed that the leaf of the Lotus plant sheds water because the water doesn’t touch (wet) the leaf’s structural surface.² This is shown in both a full-size and highly magnified view in Figures 1 and 2.

INDUSTRIAL RESEARCH
Today, R&D budgets, especially in Europe, are flush as researchers try to learn: what is the best selection of height, diameter, and spacing distance for asperities to produce the most-water repellent surface, and how should that surface be produced.

GREAT BALLS OF WATER
" Liquid Marbles" (LM) are small volumes of water which are superhydrophobic! The water is coated under intense shear, but at low mixing energy, with either fumed silica powder or a natural spore called Lycopodium. A stable LM can contain 95% water by volume.

LMs "bead up" on surfaces as does the water droplet in Figure 2. An LM, shown in Figure 3, can bounce and roll around surfaces without leaking. LMs can roll down a plane with a velocity that is determined by a competition between gravity and friction.

CHANGE IN CRITICAL CLEANING
Superhydrophobic surfaces will be implemented into structures, like dental implants or semiconductors, where the character of the surface is central to the value of the product. Instead of managing work to produce surfaces without chemical features (contamination), we will soon be managing work to produce surfaces which have a certain type of chemical contamination.

References

1. This approach has already been commercialized with glass (by PPG) and polycarbonate (by GE) materials.
2. Wilhelm Barthlott and Christoph Neinhuis made this observation and published it in Nature. Also see http://iri.ibl.fr/bn/pubs/ralf/blossey03.pdf

John Durkee, PhD, PE is the author of the book Management of Industrial Cleaning Technology and Processes,published by Elsevier (ISBN 0-0804-48887).He is an independent consultant specializing in critical cleaning for contamination control.Contact him at PO Box 847,Hunt,TX 78024 or 122 Ridge Road West,Hunt,TX 78024;830-238-7610;Fax 612-677-3170;or jdurkee@precisioncleaning.com.