Rare earth oxides make lasting water-repellent surfaces

Water-shedding surfaces that are robust in harsh environments could have broad applications in many industries including energy, water, transportation, construction and medicine. For example, condensation of water is a crucial part of many industrial processes, and condensers are found in most electric power plants and in desalination plants.

Hydrophobic materials - ones that prevent water from spreading over a surface, instead causing it to form droplets that easily fall away - can greatly enhance the efficiency of this process. But these materials have one major problem - most employ thin polymer coatings that degrade when heated, and can easily be destroyed by wear.

MIT researchers have now come up with a new class of hydrophobic ceramics that can overcome these problems. These ceramic materials are highly hydrophobic, but are also durable in the face of extreme temperatures and rough treatment.

The work, by mechanical engineering post-doc Gisele Azimi and associate professor Kripa Varanasi, along with two graduate students and another post-doc, is described this week in the journal Nature Materials. Durability has always been a challenge for hydrophobic materials, Varanasi says - a challenge he says his team has now solved.

Ceramics are highly resistant to extreme temperatures, but they tend to be hydrophilic (water-attracting) rather than hydrophobic. The MIT team decided to try making ceramics out of a series of elements whose unique electronic structure might render the materials hydrophobic: the so-called rare earth metals, which are also known as the lanthanide series on the periodic table.

Since all of the rare earth metals have similar physico-chemical properties, the team expected that their oxides would behave uniformly in their interactions with water. ''We thought they should all have similar properties for wetting, so we said, 'Let's do a systematic study of the whole series,''' says Varanasi, who is the Doherty Associate Professor of Ocean Utilization.