New Wrinkle to Keeping Hulls Shipshape

From sailboats to cruise ships to aircraft carriers, most ships gleam as they sit in harbor or cut through the open water. Below the water line, however, the picture often isn't as pretty. Algae, barnacles, and other organisms can latch onto a ship's hull and create an uneven surface that reduces maneuverability and increases drag. Such biofouling can increase fuel consumption by as much as 30 percent.

Until recently, the Navy coated ship hulls with copper- and tin-based materials, which poisoned the attached organisms to shed the unwanted growth, but environmental concerns brought an end to that practice. The Office of Naval Research has tapped Dr. Jan Genzer and others to find a greener solution to biofouling. The NC State research group led by Genzer, Celanese Professor of Chemical and Biomolecular Engineering, believed it could create a dense coating of molecules that would repel organisms by bombarding a stretched piece of rubber with reactive oxygen and letting the rubber snap back to its normal size. In a bit of serendipity, the process works, but not as originally intended.

When the NC State team examined the treated rubber, they found it was wrinkled. In fact, it had layers of different-sized wrinkles atop each other, Genzer says, because the oxygen created an ultra-thin layer of a silica-like material that crumpled when the stretched rubber was released. Surfaces with periodic roughness naturally tend to be better than smooth ones at remaining free of unwanted debris, and tests in the U.S. and the U.K. have shown that, even after months in the water, surfaces with the wrinkled coating had little organic buildup. "You have so many organisms of different size in the ocean," Genzer says, "so having wrinkles of various sizes works."

Algae, barnacles, and other organisms can latch onto a ship's hull and create an uneven surface that reduces maneuverability and increases drag.

The researchers are now trying to attach various molecules and polymers to the silica-like layer to determine if chemistry can aid topography in keeping ship hulls clean. "The more we work on biofouling, the more we realize that we have more work to do," Genzer says. "There's a sweet spot in there for controlling buildup, and we need to find it."

 

Drs. Jan Genzer, left, and research assistant professor Kirill Efimenko look at samples of a wrinkled surface created by bombarding stretched rubber with reactive oxygen and allowing the rubber to snap back to its normal size. The wrinkles on the surface can be seen clearly in the photo below.