The Undersides of Boats and Control of the Barnacles and Bacteria Growth
Opportunistic seaweed, barnacles, and bacterial films can quickly befoul almost any underwater surface, but researchers are now using advances in nanotechnology and materials science to design environmentally friendly underwater coatings that repel these biological stowaways. Biological build-up on the undersides of boats can increase drag, adding to fuel costs, and colonies of sea life can also disrupt the operation of ocean sensors and other underwater equipment. Anti-fouling paints, designed to kill the colonizers, often contain heavy metals or other toxic chemicals that might accumulate in the environment and unintentionally harm fish or other marine organisms. To replace toxic paints, scientists and engineers are now looking for ways to manipulate the physical properties of surface coatings to discourage biological colonization.
Biofouling is the growth of bacteria, plants and animals on surfaces immersed in water. It affects ships and underwater infrastructure including pipes and cables, oil platforms, and seismic survey equipment.
In the past, marine biofouling was managed with toxic paints or polymers, but these had undesirable effects on non-target organisms.
"Sea water is a very aggressive biological system," says Gabriel Lopez, whose lab at Duke University studies the interface of marine bacterial films with submerged surfaces.
While the teeming abundance of ocean life makes coral reefs and tide pools attractive tourist destinations, for ships whose hulls become covered with slime, all this life can, quite literally, be a big drag. On just one class of U.S. Navy destroyer, biological build-up is estimated to cost more than $50 million a year, mostly in extra fuel, according to a 2010 study performed by researchers from the U.S. Naval Academy and Naval Surface Warfare Center in Maryland.
The researchers, led by Gabriel Lopez of Duke University, focused on a class of materials called stimuli-responsive surfaces that alter their physical or chemical properties in response to a stimulus, such as a temperature change.
Currently the group experiments on two different types of stimuli-responsive surfaces: one that changes its texture in response to temperature, and the other in response to an applied voltage. When the surface is exposed to the appropriate stimuli, it will wrinkle on the micro- or nano-scale, shaking off slimy colonies of marine bacteria in a manner similar to how a horse might twitch its skin to shoo away flies.
Non-toxic anti-fouling coatings prevent any attachment of microorganisms thus negating the use of biocides. Further, these coatings are usually based on polymers and researchers are able to design self-healing coatings.
There are two classes of currently recognized non-toxic anti-fouling coatings. The most common class relies on low friction and low surface energies. This results in hydrophobic surfaces. These coatings create a smooth surface which can prevent attachment of larger microorganisms. For example, fluoropolymers and silicone coatings are commonly used. These coatings are ecologically inert but have problems with mechanical strength and long term stability. Specifically, after days biofilms (slime) can coat the surfaces which buries the chemical activity and allows microorganisms to attach.
The second class of non-toxic anti-fouling coatings are hydrophilic coatings. They rely on high amounts of hydration in order to increase the energetic penalty of removing water for proteins and microorganisms to attach. This is not yet commercially available.
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