Issue Date: January 7, 2008
A NEW SURFACE studded with closely spaced "nanonails" repels virtually any liquid. Initially the liquid forms droplets that rest on the nail heads. When electricity is applied, however, the liquid slips down past the nail heads and spreads out between the nail shanks, wetting the surface completely (Langmuir 2008, 24, 9). Such materials could be used as self-cleaning surfaces or in electric batteries.
Physicists Tom N. Krupenkin and J. Ashley Taylor of the University of Wisconsin, Madison, and colleagues from Bell Laboratories designed the "superlyophobic" material. They created the nails by etching a silicon wafer to form conductive silicon shanks and dielectric silicon oxide heads, then coated the structure with a hydrophobic fluoropolymer. The surface's liquid-repellent properties almost entirely rely on its topography rather than its chemical composition, Krupenkin notes. "Essentially any geometry that features 'overhang' analogs to a nail head would produce similar results," he adds.
Krupenkin's team previously constructed electrically tunable surfaces that can be reversibly switched between wettable and superhydrophobic. But this type of surface works only with liquids, such as water, with high surface tension—the attractive force that pulls molecules into droplets. Liquids with low surface tension—such as skin oil, gasoline, or common organic solvents—ruin the surface by wetting it regardless of the voltage.
Such contaminants are ubiquitous and hard to guard against, so the researchers set out to design a surface that could repel any liquid. The nanonail surfaces they created could be used in lab-on-a-chip applications that utilize organic solvents. They could also be used in membranes that extend the life of an electric battery by switching it off when it isn't needed.
Other groups are developing repellent surfaces, too. For example, MIT researchers recently reported ones inspired by lotus leaves (C&EN, Dec. 10, 2007, page 7).
- Chemical & Engineering News
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