Tunable Superhydrophobic Surfaces Fabricated by Nanosphere Lithography
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Vicast microresonators are exceptionally smooth, and the scattering component of optical loss is therefore very low (such microresonators are said to be material-loss-limited). Because of these properties, Vahala and co-workers believe that their method “has a secondary application for rapid evaluation of optical loss in previously untested polymers.” The researchers reported that with the use of other polymers such as poly(methyl methacrylate), which is known to exhibit even lower material losses than PDMS or Vicast, “replicated devices with Q factors in excess of 100 million, that is, comparable with their masters, could be molded and used to probe nonlinear optical and thermo-optic tuning effects.” STEVEN TROHALAKI
Tunable Superhydrophobic Surfaces Fabricated by Nanosphere Lithography Superhydrophobic materials received much attention after the discovery of water-repellent behavior in micro- and nanostructured plant surfaces. In examining the influence of nanostructure on surface water-repellent behavior, J.-Y. Shiu, C.-W. Kuo, and P. Chen of Academia Sinica and C.-Y. Mou of National Taiwan University have fabricated well-ordered, tunable superhydrophobic surfaces with a variable water-contact angle (tuned from 132° to 170°) using nanosphere lithography and oxygen plasma. They accomplished this by creating rough surfaces covered with low-surface-energy molecules and by roughening the surface of hydrophobic materials. As reported in the February 24 issue of Chemistry of Materials, the scientists obtained single- and double-layer close-packed polystyrene (PS) arrays by spin-coating monodisperse PS bead solutions onto substrates. This approach to nanosphere lithography achieves arrays of self-ordered, close-packed nanostructures. In a first approach, they used 440-nm-diameter PS beads to form single-layer arrays and later reduced the beads to 360 nm, 330 nm, and finally 190 nm by oxygen plasma treatment. The nanosphere separation remained constant during the oxygen plasma treatment. After the oxygen plasma treatment, the arrays were coated with a 20-nm-thick gold film and modified with octadecanethiol (ODT). The apparent water-contact angle of the single-layer surfaces changed monotonically from 132° (for 440-nmdiameter arrays) to 168° (for 190-nm-diameter size-reduced arrays). The water-contact angles on the nanostructured surfaces were much larger than that of an ODT-modified gold surface on a flat substrate (114°). The researchers said that this is an indication that the surface nanostructure governs the superhydrophobic behavior of surfaces. Two well-established models, one developed by R.N. Wenzel and one by A.B.D. Cassie and S. Baxter, are typically used to describe the water dewetting behavior on a rough surface. The researchers believe that their results are more consistent with the second model, which predicts a decrease in contact angle with bead diameter for the sizereduced beads in agreement with their results, whereas the first model predicts an increase in contact angle with bead diameter. The resear
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