Influence of surface hierarchy of superhydrophobic surfaces on liquid slip.

We investigated how the surface hierarchy of superhydrophobic (SHPo) surfaces influences liquid slip by testing well-defined microposts that have nanoposts only on their top. Contrary to the commonly held belief, our results show that such hierarchical surfaces do not always lead to an increase of slip length despite their reduced solid fraction and enhanced hydrophobicity compared to single-scale surfaces. Adding nanoposts on top of the microposts resulted in an increase of slip length only if the original microposts had a solid fraction above a threshold value. For solid fractions below this threshold, adding nanoposts decreased the slip length. We propose that there were not enough nanoposts on the top surface of very thin microposts to support the liquid pressure, allowing the liquid to intrude down to the top corners of the microposts.