Superhydrophobic surfaces efficiently repel water in a phenomenon that is also known as the lotus effect. Indeed, the lotus leaf is a symbol of purity in many cultures because of its ability to remain clean: when rain falls onto the leaf, the drops of water that form on the surface roll off, taking any dirt with them.

Now, a team led by Guihua Yu and Yi Shi has made a new type of superhydrophobic surface comprising a 3D silica nanostructure replicated from a hydrogel template. The resulting hybrid coating consists of 3D interconnected nanofibres with uniform diameters of about 100 nm. Its morphology is very much like that of the lower surface of a lotus leaf, which contains micron-sized bumps that, in turn, are covered with nanoscale hair-like tubes. The nanofibres trap air under any water drops falling on them, creating a surface that repels water.

Stretched to their limit

The films produced by these inherently 3D nanotextured hydrogel templates remain superhydrophobic even when stretched to their limit – and after more than 5000 stretching cycles at 100% strain. This is a first, since most superhydrophobic surfaces made to date lose their hydrophobic properties when exposed to a strain of more than 30%.

The films can be coated onto virtually any substrate, including metals, cement, wood, fabrics and plastics, thanks to their good wettability. They are also optically transparent (letting through 98% of light falling on them). They might come in handy as screen filters and sponges for separating oil from water, says Yu, since they can absorb up to 40 times their weight in oil.

The researchers made their superhydrophobic films using a polyaniline (PAni) hydrogel template. First, they mixed three precursor solutions together: an aqueous solution of oxidative initiator; an aqueous solution of aniline monomer and phytic acid; and tetraethoxysilane in isopropanol. The polyaniline hydrogel polymerizes and gels out fairly fast, and forms into a 3D structure within three minutes.

Thanks to the high acidic, high-water-content hydrogel matrix, the silica layer preferentially coats onto the PAni nanostructured template. Next, the silica layer is chemically modified, or “silanized” by depositing trichloro(octadecyl)silane onto the template to produce a superhydrophobic surface. The overall process is simple and can be scaled up to produce large amounts of superhydrophobic film, team member Lijia Pan tells nanotechweb.org.

The Texas-Nanjing researchers say that they are now busy looking at making super-oleophobic (oil-repelling) surfaces using the same hydrogel matrix template but a different version of their process.

The current work is detailed in Nano Letters.