(76c) Super-Hydrophobic Porous Silicon Surfaces

It is commonly known that the water contact angle of hydrogen-terminated Si surfaces is around 70º, and increasing the surface roughness typically decreases the contact angle which is already less than 90º. However, we here report the super-hydrophobicity observed on hydrogen-terminated porous silicon surfaces. The porous silicon samples are prepared by a metal-assisted electroless etching process, which produces a labyrinthine structure with pores ranging from 0.1 to 1 µm in diameter. These as-prepared porous silicon surfaces are super-hydrophobic with a water contact angle of greater than 150º, and a sliding angle of less than 2º. Water droplets are observed to bounce back when falling onto the surfaces from air. Still water droplets drip off rapidly when the substrate is slightly tilted. We have also found that these porous Si surfaces, although super-hydrophobic to water droplets, can be completely wet when immersed in a pool of water. All of these observations resemble the phenomena that have been observed on some plant leaves in nature, which however can not be explained by the common understandings for the ?lotus effect?: a hydrophobic wax-induced hydrophobicity enhanced by the surface roughness. A possible explanation will be discussed in this talk based on our experimental results, which may also help us in understanding the previous observations that some leaves with absence of hydrophobic waxes on the surfaces also exhibit super-hydrophobicity, as well as recent findings that the waxes on the super-hydrophobic surfaces of some lotus leaves do not possess an intrinsic water contact angle of greater than 90º.