Nanostructures lift the fog

To state the obvious, water makes things wet. What may not be so obvious, however, is that even superhydrophobic surfaces can succumb to water’s propensity to moisten. Researchers often fashion tiny hydrophobic bumps, pillars, and protrusions on these surfaces to help keep water away. But they’ve found that fine fog droplets can still slip into the spaces between nanostructures and accumulate into larger drops to wet surfaces. So researchers led by David Quéré of ESPCI Paris launched a systematic investigation into the design of nanotextured hydrophobic surfaces that get around the fogging problem (Nat. Mater. 2017, DOI: 10.1038/nmat4868). By plasma etching self-assembled thin films of a polystyrene-poly(methyl methacrylate) block copolymer, the team created arrays of rods or cones, depending on the etching conditions. The researchers then coated the arrays with hydrophobic fluorinated chlorosilane groups. Water droplets initially grew on all the textured surfaces, but substrates with tightly packed cones eventually kicked the droplets off. The shape and proximity of the cones caused growing droplets to contort dramatically, which generated enough pressure to expel water from the surface with unprecedented efficiency, Quéré explains. He adds that such droplet departure has been observed previously, but “what seems unique with nanocones is the rate of departure is much, much larger than previously reported.” Understanding this behavior could help scientists design better antifogging windshields, mirrors, and solar cells, the researchers say.