The floating water fern, Salvinia, is a unique plant in that it retains pockets of dry air when fully submerged in water. This capability, which provides the plant with buoyancy, is owed to the surface structure of its leaves.
The water fern’s leaves are covered in tiny hairs grouped to look like miniature wire cooking whisks. Each hair is coated in hydrophobic (water-repelling) wax crystals from its base not quite to its tip. The very tip of each hair lacks hydrophobic wax and is hydrophilic, which means it attracts water molecules. It is these hydrophilic tips that help retain air pockets when the plant is submerged. They enable the trapping of a thin layer of air between the leaf surface and the water that they attract.
The whisk shape maximizes the surface area between individual hairs, providing more space for water molecules to “sit” upon. These water molecules are pinned on the tips of the hairs, thus reducing the impact of instabilities in the surrounding water. By having a large surface of hydrophilic hairs, water forms a boundary that aids in reducing drag as the plant moves in the fluid environment by creating less water-plant interaction between hairs.
This combination of hydrophilic patches on hydrophobic surfaces is known as the “Salvinia Effect.” It is responsible for retaining an air layer underwater on the fern’s leaves for up to several weeks.
This summary was contributed by Ashley Meyers.
“A novel mechanism for long-term air retention under water is found in the sophisticated surface design of the water fern Salvinia. Its floating leaves are evenly covered with complex hydrophobic hairs retaining a layer of air when submerged under water. Surprisingly the terminal cells of the hairs are hydrophilic. These hydrophilic patches stabilize the air layer by pinning the air-water interface. This ‘Salvinia Effect’ provides an innovative concept to develop biomimetic surfaces with long-term air-retention capabilities for under water applications.” (Barthlott et al. 2010: 2325)“To demonstrate the pinning effect, individual eggbeater hairs were dipped into water. From a lateral view the shape and size of the water meniscus created, when pulling the hairs out of the water was recorded…As a measure of the hydrophilicity we took the distance between the tip of the hairs and the water surface at the exact instant when the meniscus snapped off. This experiment was done with untreated hairs and hairs that were rendered hydrophobic by dipping them into a Teflon (polytetra-?uoroethylene, PTFE) solution. The water meniscus could be pulled roughly twice as high by the untreated hair…than by the Teflon coated hair…The function of the eggbeater hairs is obvious – they allow the trapping of a thin air layer reaching from the surface of the plant leaf to the top of the hairs. Penetration of water into this well-defined region requires energy for creating an increased contact area between the water and the hydrophobic hairs.This also explains the special eggbeater shape of the structures. In order to stabilize the air–water interface most efficiently, it is desirable that the energy required for the water to penetrate into the region between the hairs is maximized. Therefore it makes sense that the hairs are split into four arms to create as much surface per height difference as possible.To enhance this effect even more, these four arms are bent together at the terminal ends, approaching an almost horizontal angle. This leads to a maximization of the additional water-hair contact area per height difference required for the water to penetrate into the air retention area, i.e., to penetrate deeper than the topmost level of the hairs.” (Barthlott et al. 2010: 2327).