Seapower - March 2011

“We want to understand how these organisms settle and stick, and send out cues that encourage settlement. … It’s a hard problem. If it was just one organism, we could do that. But we need to find a solution that works for every organism in every part of the ocean, all over the world.”

— JULIE CHRISTODOULOU, DIRECTOR OF THE MATERIALS DIVISION AT THE OFFICE OF NAVAL RESEARCH

Engineering of the University of Washington. “Our
surface is perfect, but the protein is imperfect. It’s not
going to find any place to stick.”
Furthermore, Jiang said, Zwitterionic materials are
super-hydrophillic — which is the opposite of super-
hydrophobic — binding the water molecules so tightly
to the surface that the barnacle cannot push the water
aside to deliver a dab of protein to stake a claim.

Together, these two factors develop a self-polishing coating that discourages organisms from becoming attached.

According to Jiang, Zwitterionic coatings look the same as other hull coatings, but under the microscope the chemical makeup is dramatically different.

By itself, Jiang said the Zwitterionic material would dissolve quickly in water. But Jiang and his research team have found a way to create a coating that is essentially a hydrophobic Zwitterionic precursor, which has the durable properties of other coatings, but becomes hydrophilic upon contact with water. That means as the outer nano-layer wears away, the next layer of molecules that come in contact with the water undergoes a chemical reaction to give it those nonfouling characteristics of a surface with perfectly distributed positive and negative particles and a tightly bound layer of water.

With the right thickness, there are plenty of nano-layers of Zwitterionic surface that will last for many years.

While Zwitterionic materials use chemistry to repel borders, sharklet technology uses topography to create a nano-pattern that resembles shark skin.

Though it seems obvious that the surface of a warship hull should be as smooth as possible so as to provide the least resistance, Anthony Brennan, a professor at the University of Florida’s Department of Material Science and Engineering, is looking at sharks and how their rough skin deters the settling and adhesion of marine life. “We’re investigating what kind of pattern would be effective, and how could it be applied,” Brennan said.

Sharklet surfaces are micro-patterned materials inspired by the structure of shark denticles, which some people erroneously think of as scales.

“The patterns of the Sharklet were an attempt to mimic the shark’s structure. In our first attempt to stop

the ulva — a green algae that fouls via a swimming
zoospore — our patterns shut it down by 85 percent
over the best known anti-fouling surface at the time,”
Brennan said. “Since then, we have continued to study
the behavior of ulva zoospores, cobetia marine bacte-
ria, navicula diatoms, and both barnacles and tube-
worms. We have developed a ‘model’ that enables us to
design surface microtopographies and at least predict
the settlement densities of cobetia marina and
zoospores of ulva with a high level of precision and
accuracy. We have tested in the marine environment in
addition to lab assays using natural marine systems.”
The Navy has made available a 3-D holographic
microscope to researchers to watch how marine organ-
isms approach a surface, adhere to that surface and sig-
nal to others.

“We’re trying to capture the real-time imaging and
kinetics of that process,” Christodoulou said. “This
knowledge will suggest different approaches to solving
the problem.”
The Navy also has a partnership with the U.S. Coast
Guard (USCG) wherein candidates can be evaluated
on smaller vessels at a reduced cost. According to
Webb, many Coast Guard small craft have served as
preliminary screening platforms for products that have
passed the field site performance tests.

“If a candidate system proves successful on a USCG vessel, it can then be scheduled for a U.S. Navy ship. This keeps the actual test and evaluation portion costs low because a typical USCG small craft can be hoisted from the water and painted, whereas a Navy ship requires a very expensive dry docking,” Webb said.

“In general, the Navy does not dry-dock its ships for hull coating alone, so the hull coating demonstrations must be coordinated with previously scheduled maintenance. Therefore, only the ‘best and final’ candidates typically make it to a Navy ship for trials,” he said.