Rain and condensation can make it difficult to see through spectacles or vehicle windscreens. One solution to this everyday problem lies in coating the optical surface with an ultra-water-repellent, transparent film. But for manufacturing purposes, the coatings on plastic surfaces (for example, lightweight spectacle lenses) need to be produced at temperatures below 100 °C to avoid damaging the surface.

Writing in Chemical Vapor Deposition (8, 47–50; 2002), Yunying Wu et al. report a strategy for producing ultra-water-repellent coatings that can be carried out at room temperature. They used a technique called microwave plasma-enhanced chemical-vapour deposition to deposit films of an organic monomer, trimethylmethoxysilane (TMMOS), at a partial pressure of 50 Pa, on a glass or plastic surface. The resulting coatings are highly transparent and give the required high contact angle (above 150°) between the water and the coating (upper figure), and so a less wettable surface.

What is the cause of this ultra-water-repellence? The wettability of surfaces is governed by surface energy, chemistry and texture. The surface of the TMMOS film is believed to be terminated with methyl groups. So Wu et al. compared their coating with a chemically similar film, methyl-terminated self-assembled monolayers. These turned out to be much more wettable, with a water contact angle of about 100° (lower figure). A film of TMMOS prepared at a lower partial pressure (18 Pa) also resulted in a lower water contact angle (110°).

The answer to the ultra-water-repellent behaviour of TMMOS therefore appears to lie instead in its surface texture. Micrographs of the coatings deposited at 50 Pa revealed non-wettable columns of the deposited film a few hundred nanometres in diameter, with pores in between. The authors suggest that the film created at 18 Pa was less water-repellent because its molecules are more densely packed and there are no pores. The methyl-terminated self-assembled monolayer appeared to be perfectly smooth.

The porous nature of the films deposited at 50 Pa means that the surface consists of both air and TMMOS. Wu et al. propose that the apparent surface energy of the film is therefore reduced, increasing its non-wettable properties. The authors don't expand on their theory to consider the events at the molecular scale. But the performance of the coatings shows that they could be of real use to spectacle-wearers out in the rain, or coming in from the cold.