Back in November, I wrote about the work of Di Gao, an assistant professor in the University of Pittsburgh’s Swanson School of Engineering, regarding mimicking the action of lotus leaves to create self-cleaning coatings that could be applied to anything from windows to warehouses. One of Gao’s applications is a superhydrophobic silica nanoparticle–polymer coating that could be used to prevent ice build up on critical surfaces such as roofs, wings, etc. (FYI, the anti-icing material is an acrylic polymer with organosilane-modified silica particles of diameters ranging from 20 nm to 50 nm.)
Yesterday, we received an intriguing release from Pitt reporting that, in response to the oil spill in the Gulf of Mexico, Gao and his researchers have demonstrated a reusable superhydrophilic filter system for separating oil from water and that the filter has already been successfully tested in the Gulf near Louisiana.
Gao hasn’t published directly on this topic yet, so most of the details are still unknown. However, it appears that the filter has a simple cotton substrate that is coated in a hydrophilic–oleophobic polymer that blocks oil while allowing water to pass through. Gao has yet to say exactly what’s in the polymer, but the preparation only requires that the cotton material be dipped in the polymer and allowed to dry.
Some hints to Gao’s approach might be found in a 2007 Langmuir paper that discusses creating a superoleophilic layer on a Si surface:
“We demonstrate that porous Si films fabricated by a convenient gold-assisted electroless etching process, which possess a hierarchical porous structure consisting of micrometer-sized asperities superimposed onto a network of nanometer-sized pores, are able to induce a superhydrophobic phenomenon on an intrinsically hydrophilic hydrogen-terminated Si surface and a superoleophobic phenomenon on an intrinsically oleophilic self-assembled monolayer-coated Si surface. Through comparison with porous Si films consisting of vertically aligned straight pores, which are hydrophilic and oleophilic, we show that an overhang structure resulting from the hierarchical porous structure is essential to preventing water and oil from penetrating the texture of the films and inducing the observed macroscopic superhydrophobic and superoleophobic phenomena.”
Gao’s system works well enough that the oil can still be preserved, and one idea he has for an approach to large oil spills is to use what he describes as trough-shaped versions of his filter, which would be dragged presumably across the surface of the water.
Because of the stresses that are involved with dragging anything through water, I am fairly certain that this means that the cotton substrate would have to be heavily reinforced, for example, by laminating it to strong netting.