Singin’ in the Rain

Photo courtesy of Brookhaven National Laboratory

When it comes to designing extremely water-repellent surfaces, shape and size matter. That's the finding of a group of scientists at the U.S. Department of Energy's Brookhaven National Laboratory, who investigated the effects of differently shaped, nanoscale textures on a material's ability to force water droplets to roll off without wetting its surface.

"Here comes the rain again" – the Eurythmics

November often brings hard rains and damp, drizzly days. That's what makes recently-published work by researchers at the Office of Science's Brookhaven National Lab (Brookhaven Lab) so intriguing. The researchers, led by Antonio Checco of the Condensed Matter Physics and Materials Science Department, discovered how to emphatically repel the rain – as well as water droplets of all sizes.

To make that happen, you have to tailor materials to tiny (nanometer) scales, and that's just what the researchers did. As described in the journal Advanced Materials, Checco and his team used a new technique (more on that below) to create and test new materials with textures that were different at the nanoscale – billionths of a meter, or really, really small.

Scientists festooned one type of material with tiny pillars and another with tiny cones. They also varied the spacing between the features, since that has an effect on how the surface captures tiny bubbles of air, which repel water droplets. The researchers then (intentionally) spilled water on the materials to see which would repel water best and captured the event using a high-speed camera capable of shooting some 30,000 frames per second.

The effects were striking.

Checco and his team found that materials topped with tiny cones repelled water better than those topped with tiny pillars. Even diminutive droplets more easily rolled off the surface without wetting. What's more, the cones held their water repellent abilities even when the surface was sprayed with a pressurized syringe at speeds faster than that of a falling raindrop.

Just as importantly, this new technique can be scaled up to far larger surfaces. That's often a challenge when making materials with tiny features – they're tough to duplicate with precision and consistency at larger scales. (An intricate sculpture might be made once with difficulty, but filling a museum or hall with copies of same sculpture is an effort on an entirely different scale, as shown by Kyoto's Sanjusangendo Hall, home of 1001 statues.)

Enlarge Photo

Image courtesy of Brookhaven National Laboratory

Side view scanning electron microscope image of a silicon surface textured with (a) cylindrical pillars and (b) nanocones.

The technique crafted by Checco and his collaborators, Drs. Rahman and Black at Brookhaven Lab's Center for Functional Nanomaterials, solves that problem. It couples the tendency of some materials (block copolymers) to spontaneously self-organize with thin-film processing methods commonly used in fabricating electronic devices to create intricate features – such as tiny, water-repelling cones – that can be duplicated on larger scales.

Materials coated with such water-repellent surfaces could prove handy for a wide range of applications such as steam turbine power generators and automotive and aircraft windshields. Other uses may follow as well. Admittedly, it'll likely be a while before such products can be readily purchased, and there are likely to be more than a few soakings along the way. But as Axl Rose reminded, "Nothin' lasts forever/Even cold November rain."

The Center for Functional Nanomaterials is one of five DOE Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE's Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit http://science.energy.gov.

The Department's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit http://science.energy.gov/about. For more information about Brookhaven Lab, please go to: http://www.bnl.gov/world/.

Charles Rousseaux is a Senior Writer in the Office of Science, Charles.rousseaux@doe.science.gov.