Ceramics surprise with durable dryness
Coatings that repel water are found in myriad applications — they keep car windscreens clear in storms, for example, and ensure that rainswept hikers stay snug and dry. But they quickly wear away in harsh environments, such as the insides of power-station turbines, where condensation reduces efficiency. As a result, engineers are eager to get their hands on tough materials that are intrinsically hydrophobic.
Now they can, following the discovery that a well-known family of durable ceramics can repel water. That is surprising because most ceramics are hydrophilic. When water meets a ceramic such as aluminium oxide, the water’s oxygen atoms share some of their electrons with vacant electron orbitals on the aluminium atoms, and the oxygens in the ceramic share their electrons with hydrogen in the water. This binds the two together.
But what if a ceramic failed to accept electrons from water? Then the ceramic might actually be hydrophobic, reasoned Kripa Varanasi, a materials scientist at the Massachusetts Institute of Technology (MIT) in Cambridge. He looked to the oxides of the lanthanides — the row of metals nestled almost at the bottom of the periodic table, from cerium to lutetium. The lanthanides’ empty orbitals are buried beneath shells of other electrons, which should make them much less attractive to water’s oxygen, thought Varanasi.
Proving that sometimes the simplest experiments are the best, he and his colleagues tested the idea by making small ceramic discs of the oxides of all the lanthanides except radioactive promethium. The team then dropped water onto the pellets and watched what happened.
Every single one repelled the liquid. When water condensed on to the discs, it did so in neat droplets rather than the films that would form on non-hydrophobic materials such as silicon. And cerium oxide — the cheapest and most widely available of the lanthanide oxides — remained hydrophobic even after a two-hour sauna at 1,000 °C, or a thorough grinding with abrasive silicon carbide.