Naval engineers have been working on minimizing the amount of shear stress and friction which a ship experiences in water for many years and one of the more promising solutions to the problem is thought to be the use of superhydrophobic coatings which can repel water away from hulls. However, although superhydrophobic films have already been shown to decrease drag substantially, they have a very limited lifetime and are not suitable for ships travelling over long periods of time.
On the other hand, some underwater plants have complex superhydrophobic structures that are capable of retaining a stable film of air at their surface for several months at a time and a closer look at their properties could help with longevity.
A collaboration of German-based scientists has shown that Salvinia molesta uses a complex morphology to trap air between its leaves and water. Each leaf surface contains a series of hairs which have a whisk-like structure. The ‘whisks’ are completely coated with wax crystals which are hydrophobic – with the exception of the endmost cells which are covered with highly hydrophilic patches. The hydrophilic parts act as pins, holding water just at the tips of the hairs with the layer of air trapped beneath the pins on the hydrophobic parts of the leaf: the Salvinia Effect.
The combination of hydrophilic pins and superhydrophobic surfaces helps to stabilise the air-water interface on the fern. If the interface breaks down then energy must be spent to rupture the contact. Energy is also needed to restore the interface once it has been broken down. Salvinia molesta avoids wasting energy in this manner with its elastic hairs which allow for deformation of the air-water interface. When the air layer moves, the hairs bend in response and equilibrium is restored quickly before the interface breaks, keeping the system stable over time.
Considering that this fern is commonly associated with rapid dispersal and ensuing ecological problems, it seems ironic that it may help us find the answer to such a major problem currently faced by the maritime industry.