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Superrepellent Surface

Silica substrate studded with specialized microposts rejects even the ‘stickiest’ of fluorinated solvents

by Lauren K. Wolf
December 1, 2014 | A version of this story appeared in Volume 92, Issue 48

Credit: Science
A micropost-covered silica surface gets its superrepellent properties from an overhanging lip on each post’s head.
An SEM image of a surface with regularly spaced protrusions that look like nails that have not been completely hammered in. Inset: a close up of the nailhead.
Credit: Science
A micropost-covered silica surface gets its superrepellent properties from an overhanging lip on each post’s head.
Credit: T. Liu & C.-J. Kim/UCLA/Department of Mechanical & Aerospace Engineering
A microstructured surface is so repellent that, as shown in this video, droplets of water, methanol, and even perfluorohexane (FC-72) bounce when they come in contact with it. The footage has been slowed down by a factor of 20 for viewing purposes.

Organic solvents—especially fluorinated ones—are notoriously “sticky” when drizzled onto water-repellent surfaces. Rather than beading up and rolling off as water does, these low-surface-energy fluids spread out and hang around. Tingyi (Leo) Liu and Chang-Jin (CJ) Kim of the University of California, Los Angeles, have now succeeded in fabricating a surface that repels even the most stick-to-itive of solvents: perfluorohexane (Science 2014, DOI: 10.1126/science.1254787). The researchers hope their strategy will one day improve the efficiency of solvent-cooled electronics and the antifouling ability of biomedical implants. The researchers created their superrepellent silica surface by etching an array of microscale posts into a silicon substrate that eventually gets oxidized. The final surface architecture resembles regularly spaced nails hammered halfway into a board, where each nail head has an overhanging lip. It’s this lip that makes the surface impervious to low-surface-energy fluids such as perfluorohexane, Kim says. The overhang exerts a surface tension that compels the liquid to pull up rather than slide downward and wet the surface, he adds.


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