Wrinkle effect

Pesky barnacles are an expensive problem for ship owners. NC State engineers are trying to give them some relief, taking cues from Mother Nature.

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When Drs. Jan Genzer and Kirill Efimenko set out to create a coating for ship hulls that resisted pesky barnacles but didn’t poison the water, they began with a simple premise.

“You cannot outsmart Mother Nature,” Genzer said. “She will find ways to combat you.”

Rather than fighting nature, the researchers tried to mimic it. Their creation, which shares characteristics with certain living materials, was a non-toxic “wrinkled” coating that could potentially save boat owners millions of dollars in cleaning and fuel costs. The research, published in the journal ACS Applied Materials & Interfaces in May 2009, in collaboration with researchers at the University of Birmingham in the United Kingdom, showed for the first time that surface coatings containing nests of different-sized wrinkles are effective in preventing barnacles from firmly latching on to the coatings.

To understand how Genzer, Celanese Professor and Efimenko, research assistant professor, both in the Department of Chemical and Biomolecular Engineering, arrived at their creation, it is helpful to understand the importance of rough and wrinkled surfaces in nature.

In general, rough surfaces tend to stay free of unwanted dirt and other debris, while smooth surfaces tend to accumulate it. This seems counterintuitive, but natural examples abound. Whales, whose massive frames are covered with smooth skin, often carry barnacles as unwanted hitchhikers.

But sharks have sandpaper-like skin that remains barnacle-free. Another example of this phenomenon can be seen in plants. Surfaces of plant leaves are hierarchically-roughened, and this unique surface topography helps channel water across the surface, keeping it free of dirt and other deposits. This helps the plant avoid injury and perform photosynthesis.

These natural phenomena have prompted researchers in many fields to consider roughness and wrinkles as they design surface coatings.

Genzer and Efimenko stumbled onto their wrinkled coating almost by accident. In 2005, they were conducting an experiment on a piece of silicon rubber and inadvertently created a hardened wrinkly surface. Upon examination, the researchers saw five distinct generations of wrinkles of various sizes, with each new generation “carrying” the old one.

“Basically, the wrinkles have their ‘infant’ wrinkles, which have their ‘infant’ wrinkles, and so on,” Genzer said. The researchers began thinking about how to form their wrinkles consistently and how industry and government could take advantage of their discovery. The U.S. Office of Naval Research sponsored their work.

Barnacles latching onto ships might appear to be just an aesthetic problem, but looks can be deceiving. Barnacles increase the ship’s “drag” in water, forcing the engine to burn more fuel to maintain the same speed. After several months, a ship’s fuel consumption increases substantially, costing ship owners—including the military—plenty of extra cash.

“It’s like running your air conditioner with the windows open,” Genzer said. Barnacle buildup also forces owners to remove ships from the water and place them on dry docks for cleaning. This procedure is cost prohibitive, and it also costs ships valuable time at sea when they could be making money.

Barnacles are particularly troublesome to the military, which often stations its giant aircraft carriers and battleships in harbors for long periods of time.

“Military ships, in peacetime, spend most of their time in a harbor somewhere. They don’t move,” Genzer said. “That’s going to cost you, because you’re going to have tons of marine deposits.”

For many years, ship owners fought this problem by coating their hulls with toxic substances that resisted barnacle buildup. But those substances killed fish and other marine life in harbors, causing governments around the world to ban ships from using them. These new regulations created interest in developing ship coatings with wrinkled topographies—essentially transforming the whale-like hulls of today into the shark-like hulls of tomorrow.

To see if their accidental discovery could stand up to barnacles, Genzer and Efimenko needed to test it. They repeated their initial experiment by stretching a piece of rubber mechanically and then exposing it to an ultra-violet ozone (UVO) treatment, which formed a thin skin on the rubber. After the skin formed, they released the tension and allowed the material to readjust, which formed the “wrinkles.”

The shapes and sizes of these wrinkles were particularly important. The researchers had learned that the surface roughness has to be comparable to the size of the organism that’s trying to catch a ride on the ship. If the roughness is too fine, then the organism might think the surface is flat and attach itself, which means a few hundred extra pounds of costly “cargo” for ship owners. If the wrinkles are too large, barnacles will cluster between them.

But by applying just the right combination of the UVO treatment and the degree of strain on the surface, Genzer and Efimenko created five generations of wrinkles that formed concurrently. These tiny wrinkles ranged in length from a fraction of a millimeter all the way down to tens of nanometers (a nanometer is one-billionth of a meter).

With the coatings created, the researchers applied them to several slabs of hull-like materials and dunked them in the ocean off Wilmington, N.C. Then they waited. After one month, the flat coatings were covered with barnacles, and just plain “nasty,” Genzer said. But the wrinkled materials looked to be barnacle-free. The researchers waited some more, and even after 18 months in the water, the wrinkled materials remained free of barnacles, while flat coatings with the same chemical composition showed plenty of barnacle buildup.

“The results are very promising,” Efimenko said. “We are dealing with a very complex phenomenon. Living organisms are very adaptable to the environment, so we need to find their weakness. And this hierarchical wrinkled topography seems to do the trick.”

Perfecting the “wrinkling” process will require much more work. For the coatings to be effective, the researchers explained, they’ll need to work against the array of ocean organisms that live in harbors around the world, so they’re working with marine biologists on multiple testing sites in Hawaii, Florida, the United Kingdom and Singapore.

They’ll also continue work on refining the mechanical, physical and chemical properties of the coatings. Some day, researchers hope, their efforts at copying Mother Nature will allow ships everywhere to sail from harbors free of barnacles. “This is just the beginning of this process,” Efimenko said. end of story

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