Video of REO hydrophobic surfaces in action. Credit: MIT News Office; YouTube.
I meant to write about this topic several months ago when the editors of Nature Materials published a remarkable paper about a relatively simple method to make a hydrophobic, which, in this case, are made from ceramic materials. The thrust of the paper was that rugged ceramic materials could be made to be intrinsically hydrophobic through the use of rare-earth oxides. The paper was written by a research group at MIT led by Kripa K. Varasani and I was again reminded of the topic last week when MIT Technology Review published a related story.
The pursuit of hydrophobicity has entailed everything from the studying of lotus leaves to the creation of fairly exotic coatings and films. In this case, however, the thrust of the investigators work was relatively straightforward: They wondered if the tendency of ceramics to be hydrophilic (e.g., with certain metal oxides, such as alumina, water gets bound when its oxygen atoms share electrons with the aluminum atoms, and, in turn, the oxygens in the ceramic share their electrons with hydrogen in the water) could be reversed by interfering with the hydrogen bonding and therefore prevent the ceramic from accepting electrons from water.
The investigators insight, according to the paper, is that rare-earth oxides, which have unfilled 4f orbitals, but these orbitals “are shielded from interactions with the surrounding environment by the full octet of electrons in the 5s2p6 outer shell.” The introduction of rare earths into the ceramic composition, then, might prevent the bonding and render the material hydrophobic.
Led by Varanasi, a materials scientist, the group tested their idea by making simple ceramic disks from powders composed of pure rare earth oxides (REOs) of 13 elements in the lanthanides series—from cerium oxide to lutetium oxide. The fourteenth, promethium oxide, was purposely skipped over because it is radioactive.
They gave a mirror finish to the disks by polishing in order to minimize roughness and texture effects, and then put them to several tests. Sure enough, the worked! The authors write, “As hypothesized, all the REOs are hydrophobic: water contact angels range between 100° and 115°. Also, the polar component of the surface free energy for all REOs was found to be negligible. Moreover, there is minimal variance in the wetting properties over the entire series.”
Moving closer to application-related properties, the group tested the disks with steam condensation, water droplet impingement, high-temperatures, and abrasion wear. These tests “demonstrated dropwise condensation, complete water droplet bounce-off [see video, above], and sustained hydrophobicity after high-temperature exposure and abrasion.”
Of course, demonstrating that some ceramics could be intrinsically hydrophobic was quite an accomplishment*, but Varanasi’s groups goal also had a practical side. They say, for example, that hydrophobic ceramics could play an important role by improving the efficiency of steam-based energy generation. In a separate Nature article about this research, Varanasi tells the publication that a problem with current generators is that steam condenses into water on the rotating blades of the turbine and causes the loss of energy. He says the efficiency loss from this effect could be as great as 30 percent. Likewise, with wind turbines, accumulated water can freeze on turbine blades, again creating efficiency losses and, perhaps, catastrophic failure. He says that in both examples, a superhydrophobic surface composed of the REOs could make an enormous difference.
As ceramists and other materials scientists and engineers know, there are many reliable methods for applying ceramics surfaces that could be employed to add hydrophobic surface to substrates, although the researchers caution that the effects of materials geometries and mismatches of coefficients of thermal expansion would have to be considered.
But, overall, using the REOs to achieve hydrophobic surfaces isn’t all that difficult, and, not surprisingly, Nature reports, “Varanasi is now working with energy and technology companies partnered with the MIT Energy Initiative, which co-funded his work, to test the ceramics in real-world applications.”
* Nature reports that credit for noticing the hydrophobicity of ceria may go to a research group led by Chin Li Cheung, working at the University of Nebraska-Lincoln’s Department of Chemistry and Nebraska Center for Materials and Nanosciences, that was focused on other topics and didn’t pursue the hydrophobic properties further.