Swatting at pesky insects or air-borne particles its a common, everyday activity for nearly all living creatures. Its a way to keep clean and get rid of anything that might cause future problems.
But what about machines and vehicles that cant take a swing at annoying parasites, particularly ones submerged in water?
Continual biofouling the accumulation of microorganisms, plants, algae, or animals on wet surfaces has been a long-term problem for the global shipping industry. Now researchers at Duke Universitys Pratt School of Engineering have developed a strategy that could enable ships to rid themselves of creatures and substances that hitch a ride on their hulls.
Coating ships in a new material that shakes itself on command can eliminate several problems associated with keeping ships clean, said Xuanhe Zhao, a Duke mechanical engineering researcher.
If youve ever seen a horse or cow shake its skin or tail to get rid of flies, thats analogous to shaking off something thats bugging the ship, he said. Weve introduced a new mechanism that can deform, and that deformation can literally detach the biofouling materials adhering to the surface.
Why slough the ships?
Apart from being unsightly, barnacles and other biofouling substances can inhibit a ships ability to function. Even a small amount can cause difficulties, said Gabriel López, a Duke biomedical and mechanical engineering professor. He is also the director of Research Triangle Materials Research Science and Engineering Center.
Even a small layer of slime can significantly increase drag, and as drag on the ship goes up, the fuel consumption goes up, as well, he said. Pollution goes up, and more greenhouse gases are produced.
Leaving biofouling materials stuck to ships can also have another environmental impact. For ships sailing worldwide, there is a risk invasive species will be transferred from their native habitat to ones where they can be damaging. For example, nutrient-hoarding zebra mussels from Russia were brought via ship hull to the Great Lakes in the late 1990s. Within 10 years, these invaders had wreaked havoc on the regions fishing industry and levied more than $3 billion in damages.
Ridding ships of biofouling materials is also of military import, López said. The more barnacles and bacteria a ship carries, the noisier the vessel is, making it easier to detect.
How it works
Traditionally, the shipping industry used less-than-optimal means to protect vessels. For more than 40 years, global maritime companies coated their roughly 30,000 ships with paint containing tributyltin, an inexpensive, effective and poisonous barnacle- and algae-killer. An international treaty banned its use in 2007.
Another protection method has been a polymer coating that reduces biofouling substances ability to adhere to the boat. Its a temporary fix, though, because bacteria and barnacles eventually adapt to the polymer and attach themselves anyway, Zhao said.
This new solution, however, starts with an environmentally safe silicon rubber polymer coating on the ships hull. Running voltage through the flat polymer turns it into a capacitor a passive structure that stores energy and generates an electric field, he said. This cleaning strategy then relies on electrostriction, a property of electrical nonconductors that allows them to change shape when exposed to electricity, to slough away the offending substances.
There are patterned channels air channels beneath the polymer, and if you blow air into the channels, it will increase the hydrostatic pressure and buckle up the polymer surface, Zhao said. We basically form a wrinkle on the surface of the polymer, and the biofouling substances simply detach.
Although this strategy must be tested on the large scale, it does offer two advantages other cleaning solutions have lacked, Zhao said. The silicon rubber polymer could potentially last for years, and this method eliminates the need to dry dock a ship for cleaning. An electric current can be sent to the polymer anytime, anywhere, and the ship will slough off biofouling material.
Challenges, future uses
To date, Zhao and López have only tested this self-sloughing mechanism with areas only a few centimeters wide. But, according to Jan Genzer, a chemical and biomolecular engineer at North Carolina State, larger experiments are needed to prove its a valid cleaning solution.
Its a very clever, very different way to think about this problem like trying to stand on a trampoline while its being shaken, said Genzer, who created a different cleaning solution. The real question is, though, can it be applied to a ship? How will the research translate to application? Will the discovery need to be modified?
In his discovery, Genzer and his colleagues created a dense layer of molecules capable of repelling biofouling organisms by repeatedly hitting a stretched piece of rubber with reactive oxygen and allowing the rubber to rebound. The result, he said, was a roughness that helped prevent barnacle and algae accumulation.
Even though the self-sloughing mechanism has only been applied to shipping so far, both Zhao and Genzer agree that there are additional uses for this technology. Representatives from the food industry, lubrication companies, and medical device manufacturers have all expressed interest in this development, Zhao said.
Any time you have synthetic material in contact with some sort of water that might have bacteria or other microorganisms in it, you will form this biofouling layer, he said. So, its a very pervasive problem.
Ultimately, Genzer said, concentrating on strategies to remove biofouling materials rather than trying to create ones that prevent accumulation from even beginning will be a productive plan. Organisms adapt to survive when faced with foul-resistant substance, and that can do more harm than good to an ecosystem.
People are now realizing foul-resistant coatings are more realistic. They allow deposits to settle, but they can be cleaned by shaking or running the ship at particular speeds, Genzer said. You cant outsmart Mother Nature. Shes been around for millions of years, and shes developed ways for organisms to survive in different conditions. If we can get surfaces clean, we should be happy, and leave it at that.