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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.

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Computer-generated illustration of Lotus effect. (Credit: William Thielicke.)

A group of researchers from the University of Pittsburgh, University of California Riverside and the Ross Technology Corporation joined a growing list of researchers studying the superhydrophobic property of lotus leaves and now say the insights they gained showed them a way to develop a particle–polymer coating that prevents ice formation in both lab and real–world testing.

Lotus leaves have fascinated researchers for several years because moisture that hits the leaves rolls off and takes with it accumulated dirt and debris. Once this superhydrophobic “Lotus effect” was revealed, many research groups launched efforts to develop nanomaterials with the goal of developing self-cleaning coatings for glass, mirrors, hospital equipment and even whole buildings.

This new group, however, took a slightly different tack and used the lotus leaves to understand the relationship between water-repellency and snow–ice accumulation on superhydrophobic surfaces. In a paper recently published in Langmuir, they report on the successful use of silica nanoparticle–polymer composites to deter icing, especially in situations involving supercooled water, such as freezing rain and “impact ice,” that fouls highways, creates havoc with airplane lift surfaces and drags down electric power lines.

In their paper, the group reports on making a superhydrophobic surface material by combining an acrylic polymer with organosilane-modified silica particles of diameters ranging from 20 nm to 20 μm. They then coated parts of an aluminum plate with the particle-polymer composites and exposed the plate under laboratory conditions to supercooled water. They repeated the experiment 20 times for each particle size. The pay off is that they found excellent anti-icing capabilities when the silica particles were in the 20-50 nm range, but the anti-icing strength decreased significantly when the particles were larger than 50 nm.

An aluminum plate glazed with superhydrophobic coating (left) repelling the supercooled water. For the uncoated plate (right), the water freezes on contact and ice accumulates. (Credit: University of Pittsburgh.)

The group coated half of another aluminum plate and half of a satellite-TV dish antenna with a layer of the 50 nm composite, and placed both of these outdoors near Pittsburgh where they were exposed to a freezing rain last January. The uncoated sides of the plate and dish were covered with ice, but the treated halves were ice-free.

Because of the particle-size dependency, they group cautions against assuming that all superhydrophobic surfaces have anti-icing properties. Likewise, they say further research is needed on understanding ice adhesion, hydrodynamic conditions and the structure of water film on superhydrophobic surfaces where icing still occurs.

The lead author of the paper is grad student Liangliang Cao, and much of the work was done by in Pitt professor Di Gao’s lab.

Update: Di Gao has kindly provided us with additional images of the coating at work:

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