Tiny Hydrophobic Water Ferns Could Help Ships Economize on Fuel

The hairs on the surface of water ferns could allow ships to have a 10 percent decrease in fuel consumption. The plant has the rare ability to put on a gauzy skirt of air under water. Researchers at the University of Bonn, Rostock and Karlsruhe now show in the journal Advanced Materials how the fern does this. Their results can possibly be used for the construction of new kinds of hulls with reduced friction.

The water fern salvinia molesta is extremely hydrophobic. If it is submerged and subsequently pulled out the liquid immediately drips off it. After that it is completely dry again. Or to be more precise: it was never really wet. For the fern surrounds itself by a flimsy skirt of air. This layer prevents the plant from coming into contact with liquid. And that even with a dive lasting weeks.

Materials researchers call this behaviour 'superhydrophobic'. This property is of interest for many applications such as rapidly drying swimsuits or simply for fuel-efficient ships. Meanwhile, it is possible to construct superhydrophobic surfaces modelled on nature. However, these 'replicas' have a disadvantage: the layer that forms on them is too unstable. In moving water it disappears after several hours at the latest.

The researchers from Bonn, Rostock und Karlsruhe have now deciphered the trick the water fern uses to pin down its airy skirt. It has been known for some years now that on the surface of its leaves there are tiny whisk-like hairs. These are hydrophobic. They keep water in the surroundings at a distance.

Water is 'stapled in place'

But this is only one side of the coin: "We were able to show that the outermost tips of these whisks are hydrophilic, i.e. they love water," Professor Wilhelm Barthlott from the University of Bonn explains. "They plunge into the surrounding liquid and basically staple the water to the plant at regular intervals. The air layer situated beneath it can therefore not escape so easily."

Professor Barthlott is head of the Nees Institute of Biodiversity of Plants in Bonn. There the experiments began which are continued today in conjunction with the Chair of Fluid Dynamics at the University of Rostock and the Institute of Applied Physics at the University of Karlsruhe. "After the solving of the self-cleansing of the lotus leaf twenty years ago, the discovery of the salvinia effect is one of the most important new discoveries in bionics," Professor Thomas Schimmel from the University of Karlsruhe says.

Fuel saved world wide: one percent

And it is one with huge technical potential to boot. Up to now with container ships more than half of the propulsion energy is lost through friction of the water at the hull. With an air layer this loss could be reduced by ten percent according to the researchers' estimate. Since ships are huge fuel guzzlers, the total effect would be enormous. "Probably one percent of the fuel consumption worldwide could be saved this way," is Professor Barthlott's prognosis. "Surfaces modelled on the water fern could revolutionise shipbuilding," Professor Dr. Alfred Leder from the University of Rostock concurs. Wilhelm Barthlott, Thomas Schimmel, Sabine Wiersch, Kerstin Koch, Martin Brede, Matthias Barczewski, Stefan Walheim, Aaron Weis, Anke Kaltenmaier, Alfred Leder, Holger F. Bohn. The Salvinia Paradox: Superhydrophobic Surfaces with Hydrophilic Pins for Air Retention Under Water. Advanced Materials, 2010; DOI: 10.1002/adma.200904411

Shipworm Threatens Archaeological Treasures

The dreaded shipworm is moving into the Baltic Sea, threatening artefacts of the area's cultural heritage. Researchers at the University of Gothenburg, Sweden, suspect that the unfortunate spread is due to climate change, and are currently involved in an EU project to determine which archaeological remains are at risk.

The shipworm is capable of completely destroying large maritime archaeological finds in only 10 years, and while it has avoided the Baltic Sea in the past, since it does not do well in low salinity water, it can now be spotted along both the Danish and German Baltic Sea coasts.

Malmö landmark infested

'The shipworm has for example attacked shipwrecks from the 1300s off the coast of Germany, and we are also starting to see its presence along the Swedish coast, for example at the Ribersborg cold bath house in Malmö,' says Christin Appelqvist, doctoral student at the Department of Marine Ecology, University of Gothenburg.

Effect of climate change

Appelqvist and her colleagues believe that the development may be due to climate change. In short, the increased water temperature may help the shipworms to become adapted to lower salinity. The group is part of the EU project WreckProtect, a cooperative effort to assess which archaeological treasures are at risk. The project includes researchers from Sweden, Denmark and the Netherlands, as well as experts from France and Germany.

Covering the shipwrecks

One of the objectives is to develop methods to protect the shipwrecks, for example by covering them with geotextile and bottom sediment, and another is to try to predict to which areas the shipworm is likely to spread in the future. The researchers say there are some 100 000 well-preserved shipwrecks in the Baltic Sea.

'Around 100 wrecks are already infested in the Southern Baltic, but yet it hasn't even spread past Falsterbo. We know it can survive the salinity of the Stockholm archipelago, although it needs water with higher salinity than that to be able to reproduce,' says Appelqvist