Nano-coated mesh captures oil but lets water through

“Our goal is to reach a transparency in the 90-percent range,” Bhushan said. “In all our coatings, different combinations of ingredients in the layers yield different properties. The trick is to select the right layers.”

He explained that certain combinations of layers yield nanoparticles that bind to oil instead of repelling it. Such particles could be used to detect oil underground or aid removal in the case of oil spills.

The shape of the nanostructures plays a role, as well. In another project, research assistant Dave Maharaj is investigating what happens when a surface is made of nanotubes. Rather than silica, he experiments with molybdenum disulfide nanotubes, which mix well with oil. The nanotubes are approximately a thousand times smaller than a human hair.

Maharaj measured the friction on the surface of the nanotubes, and compressed them to test how they would hold up under pressure.

“There are natural defects in the structure of the nanotubes,” he said. “And under high loads, the defects cause the layers of the tubes to peel apart and create a slippery surface, which greatly reduces friction.”

Bhushan envisions that the molybdenum compound’s compatibility with oil, coupled with its ability to reduce friction, would make it a good additive for liquid lubricants. In addition, for micro- and nanoscale devices, commercial oils may be too sticky to allow for their efficient operation. Here, he suspects that the molybdenum nanotubes alone could be used to reduce friction.

The release notes that this work began more than ten years ago, when Bhushan began building and patenting nano-structured coatings that mimic the texture of the lotus leaf. From there, he and his team have worked to amplify the effect and tailor it for different situations.

“We’ve studied so many natural surfaces, from leaves to butterfly wings and shark skin, to understand how nature solves certain problems,” Bhushan said. “Now we want to go beyond what nature does, in order to solve new problems.”

“Nature reaches a limit of what it can do,” agreed Brown. “To repel synthetic materials like oils, we need to bring in another level of chemistry that nature doesn’t have access to.”

— Read more in Dave Maharaj and Bharat Bhushan, “Nanomechanical behavior of MoS2 and WS2 multi-walled nanotubes and Carbon nanohorns,” Scientific Reports 5, Article number: 8539 (23 February 2015) (doi:10.1038/srep08539); and Philip S. Brown and Bharat Bhushan, “Mechanically durable, superoleophobic coatings prepared by layer-by-layer technique for anti-smudge and oil-water separation,” Scientific Reports 5, Article number: 8701 (3 March 2015) (doi:10.1038/srep08701)