Biomimetic super-hydrophobic coatings can be used as long-term protection for stainless steel surfaces. They provide anti-corrosion, anti-fouling, low-friction, and drag-reduction properties. Reporting in Nanotechnology, researchers have realized a stationary super-hydrophobic film made of carbon nanotubes (CNTs) deposited on stainless steel. The group at the University of Rome Tor Vergata use chemical vapour deposition (CVD) to achieve this. The wetting state of the thin film has exceptional durability as a result of the highly adhesive microscopic structure. This mimics the hierarchical microscopic architecture of Salvinia leaves.

 
Hierarchical super-hydrophobic carbon nanotube film on stainless steel.

Biologically inspired materials are modelled on plants and animals. Over 460 million years, the best and most efficient surfaces known so far evolved during the adaption of plants and animals to different environments.

The Salvinia effect

The super-hydrophobic properties of many natural surfaces result from the presence of at least a two-fold superimposed surface morphology. Salvinia is a water plant whose leaf surface exhibits this two-fold hierarchical morphology. It also alternates super-hydrophobic cells at the larger scale with hydrophilic cells at the smaller scale. This unique microscopic structure leads to a super-hydrophobic surface with highly adhesive properties: stably pinning water droplets and retaining a thin air cushion underneath them. In this way, Salvinia always stays dry. This phenomenon is the Salvinia effect.

Mimicking the Salvinia leaf

CVD is the main technique used to produce CNTs on a large scale from metallic nanoparticle catalysts, which act as seeds for their growth. The researchers have developed a method to directly grow carbon nanotube films on stainless steel without a catalyst. This exploits the intimate metallic composition of steel. The researchers have observed with scanning electron microscopy that these films show a two-fold hierarchical morphology. They comprise hydrophobic carbon nanotubes with hydrophilic carbonaceous nanoparticles at the end of their tips. The structures resemble the Salvinia leaf structure but have an adhesion force ten times higher: comparable with that of the gecko foot.

Long-term behaviour

Contact angle measurements on these carbon nanotube coatings show an unprecedented long-term super-hydrophobic behaviour due to the Salvinia effect. This is very promising for the realization of durable super-hydrophobic coatings which improve the properties of stainless steel. Such effects could improve for instance, steel sterilization and its long-lasting quality, water transport speed and indirectly fuel consumption.