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:
