A two-tier nanostructure comprising a 20-nm-diameter "nanograss" on 100-nm-diameter nanopillars exhibits robust and near-perfect superhydrophobicity, that is, water contact angles close to 180 degrees and sliding angles close to 0 degrees. Although the solid fraction was very low (similar to 0.02%), this surface could support a drop of water under a pressure of 234 Pa.
