Superhydrophobic surfaces

Preventing corrosion, bacterial growth and chemical fouling on underwater surfaces.

Aerophilic surfaces – those that rely on or are only active in the presence oxygen – immersed underwater trap films of air known as plastrons. Utilising the protective effects of plastrons could lead to the development of underwater superhydrophobic surfaces capable of preventing corrosion, bacterial growth, the adhesion of marine organisms, chemical fouling and other harmful effects of liquids on surfaces. Material scientists have been attempting to harness the protective effects of plastrons for decades, however they have typically been considered impractical for underwater engineering applications due to their meta-stable performance. Thus far, plastrons have proved highly unstable under water – keeping surfaces dry for only a matter of hours in the lab.

This could all be about to change, however, thanks to a combined team of researchers led by the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS), the Wyss Institute for Biologically Inspired Engineering at Harvard, the Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany and Aalto University in Finland. The team has successfully developed a superhydrophobic surface with a stable plastron that can reportedly last for months underwater, opening up a range of applications in biomedicine and industry.

SUPERHYDROPHOBICITY EXPLAINED

Superhydrophobic materials are highly hydrophobic and extremely hard to ‘wet’. Generally, this type of surface is defined by having the static water contact angle above 150° and contact angle hysteresis less than 5°. Superhydrophobic coatings are often found in nature, appearing on plant leaves and some insect wings, and can be sprayed onto objects to make them waterproof.

“Research into bioinspired materials is an extremely exciting area that continues to bring into the realm of man-made materials elegant solutions evolved in nature, which allow us to introduce new materials with properties never seen before,” says Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science and Professor of Chemistry and Chemical Biology at SEAS. “This research exemplifies how uncovering these principles can lead to developing surfaces that maintain superhydrophobicity underwater.”

NEW CHARACTERISATION METHOD

One of the biggest issues with plastrons is that they need rough surfaces to form, however this roughness makes the surface mechanically unstable and susceptible to any small perturbation in temperature, pressure, or tiny defect. Current techniques to assess artificially-made superhydrophobic surfaces only take into account two parameters, which do not give enough information about the stability of the air plastron underwater.

Alongside a group of researchers from Aalto University and FAU, Aizenberg identified a larger group of parameters including information on surface roughness, the hydrophobicity of the surface molecules, plastron coverage, contact angles and more. When combined with the thermodynamic theory, these additional parameters enabled the group to figure out whether or not the air plastron would be stable. With this new method and a simple manufacturing technique, the team designed an aerophilic surface from a commonly used and inexpensive titanium alloy with a long-lasting plastron that kept the surface dry thousands of hours longer than previous experiments.

“We used a characterisation method that had been suggested by theorists 20 years ago to prove that our surface is stable, which means that not only have we made a novel type of extremely repellent, extremely durable superhydrophobic surface, but we can also have a pathway of doing it again with a different material,” explains Alexander Tesler, former postdoctoral fellow at SEAS and the Wyss Institute, and lead author of the paper.

PROVING STABILITY

To prove the stability of the plastron, the researchers put the surface through its paces via bending, twisting, blasting with hot and cold water, and abrading it with sand and steel to block the surface while remaining aerophilic. The surface managed to survive 208 days submerged in water and hundreds of dunks in a petri dish of blood. It severely reduced the growth of E. coli and barnacles on its surface and stopped the adhesion of mussels altogether.

“The stability, simplicity and scalability of this system make it valuable for real-world applications,” adds Stefan Kolle, a graduate student at SEAS and co-author of the paper. “With the characterisation approach shown here, we demonstrate a simple toolkit that allows you to optimise your superhydrophobic surface to reach stability, which dramatically changes your application space.”

In terms of applications, the superhydrophobic surfaces could be used in biomedicine to reduce infection after surgery or as biodegradable implants such as stents. They could also be leveraged for underwater applications to prevent corrosion in pipelines and sensors. And, in the future, they could be used in combination with the super-slick coating known as SLIPS (Slippery Liquid-Infused Porous Surfaces), developed by Aizenberg and her team more than a decade ago, to protect surfaces even further from contamination.

More information can be found in the paper titled, ‘Long-term stability of aerophilic metallic surfaces underwater’ published the Nature journal.