Rare-earth oxide ceramics found to be robustly hydrophobic
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Sequential snapshots of a water droplet impinging on a surface coated with a thin layer of ceria (~200 nm thick); the droplet cannot form hydrogen bonding with the REO and surface tension causes the water droplet to bounce off the coating illustrating the superhydrophobic nature of these materials. Scale bar 2.5 mm. Reproduced with permission from Nature Mater. (DOI: 10.1038/NMAT3545). © 2013 Macmillan Publishers Ltd.
generated by texturing the REOs using a range of techniques, such as sputtercoating onto smooth and microstructured silica surfaces. Contact angles as large as 160° were measured and video snapshots of water droplets falling onto the microstructured surface show the droplets bouncing off the surface. The origin of the hydrophobicity of the REOs can be attributed to their electronic structure. While most ceramics and metals are hydrophilic, due to coordinative unsaturation which allows water to form bonds with available valence orbitals, the 4f orbitals of rare-earth atoms are completely shielded by the electrons in the filled 5s and 5p orbitals. They are thus not available to hydrogen bond with water molecules. This causes the water adjacent to the REO surface to exhibit a hydrophobic hydration structure. This work highlights the flexibility and scalability of these intrinsically hydrophobic REOs, where they can be fabricated using standard ceramic processing methods. The research also addresses problems of robustness, and may lead to new hydrophobic applications in harsh environments. Christopher J. Patridge
co-author Ivan Božovíc of BNL said, “They resemble waves rolling across the surface of a lake under a breeze, except that instead of water, here we actually have a sea of mobile electrons.” Once a CDW forms, the electron density loses uniformity as the ripples rise and fall. Detecting CDWs typically requires high-intensity x-rays, but even then, the technique works only if the waves are essentially frozen upon formation. However, if CDWs actually fluctuate rapidly, they may escape detection by x-ray diffraction, which typically requires a long exposure time that blurs fast motion. For their experiment, the researchers grew thin films of La1.9Sr0.1CuO4, a HTS cuprate compound. The metallic cuprates, assembled one atomic layer at a time, are separated by insulating planes of lanthanum and strontium oxides, resulting in a quasi-two-dimensional conductor. When
cooled down to less than 100 K, electron waves began to ripple through the 2D matrix. At even lower temperatures, these films became superconducting. To catch CDWs in action, the research group at MIT, led by Nuh Gedik, used an advanced ultrafast pump-probe spectroscopy technique. Intense laser pulses, “pumps,” cause excitations in the superconducting films, which are then probed by measuring the film reflectance with a second light pulse. The second pulse is delayed by precise time intervals, and the series of measurements allows the lifetime of the excitation to be determined. In a more sophisticated variant of the technique, the researche
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