Photo credit: W. Barthlott/Nees-Institut.The ability of the hairy leaves of some aquatic ferns to quickly absorb large amounts of oil while also being strongly water-repellent make them ideal models for similar materials, according to a new study. Scientists from Karlsruhe Institute of Technology and Bonn University in Germany have been studying these features to help improve a “nanofur” material they have developed that could help clean up oil spills in an environmentally friendly way.
Aquatic ferns, which originate in tropical and subtropical regions but are now found in many parts of Europe, are often considered to be pests as they are so invasive. However, the plants also have significant potential as cheap and quick oil absorbers as their trichomes – hairy extensions of up to 2.5 mm on the surface of their leaves – make them both superhydrophobic and superoleophilic.
In this study, which was published in Bioinspiration & Biomimetics [Zeiger et al., Bioinspir. Biomim. (2016) DOI: 10.1088/1748-3190/11/5/056003], the researchers quantified the oil absorption capacity of four different Salvinia species, comparing their absorption capacity to artificial oil sorbents, with the leaves of the Salvinia molesta and Pistia stratiotes being found to be most effective. They also examined how the presence and morphology of trichomes affect the amount of oil absorbed by their surfaces in terms of the influence of hair length and shape.
“We now know that the shape of the hair ends is important, because they need to support the oil/air interface to ensure maximum oil absorption and retention capability. We will use that result for our artificial surface.”Hendrik Hölscher
Hairy absorbent materials were generally better than non-hairy, and not only the length of the hair but also their shape was found to be significant. As team leader Hendrik Hölscher said “We now know that the shape of the hair ends is important, because they need to support the oil/air interface to ensure maximum oil absorption and retention capability. We will use that result for our artificial surface.”
The usual methods for cleaning up oil spills have their disadvantages – for instance, oil combustion or using chemical substances to accelerate oil decomposition – as they can result in secondary pollution. Although other natural materials can absorb oil, they do not also repel large amounts of water. However, biomimetic nanofur can mimic the properties of superhydrophobic plants, and be fabricated in a comparatively simple way by pressing a hot rough steel plate into a polymer foil. As the surface of the polymer melts, the steel plate is retracted, and micro- and nanoscaled hairs are pulled from the surface. The team now hope to use the findings to further improve on their artificial bioinspired oil absorbent materials.