Granular materials: Sand and soil

Grains of sand, magnified
Grains of sand, magnified

When a fire occurs in a sandy forest the soil can become naturally extremely water-repellent. This leads to erosion when it rains and difficulty in re-establishing vegetation and, in the extreme case, impressively destructive debris torrents. But what does this have to do with superhydrophobicity? The fire volatizes waxes from the litter layer and these then re-condense on the grains of sand.

Since the grains of sand with gaps in-between provide a rough surface and the waxes are hydrophobic this creates a superhydrophobic surface. The same process (on a smaller scale) can be used in the laboratory with grains or beads stuck to a solid substrate. Read more about the similarities between superhydrophobic materials and extreme soil water repellency in the publications below.

Publications

  • Water repellent soil and its relationship to texture and hydrophobicity.
    G. McHale, M.I. Newton and N.J. Shirtcliffe
    Eur. J. Soil Sci. 56 (4) (2005) 445-452.
  • Superhydrophobic surfaces: A model approach to predict contact angle and surface energy of soil particles.
    J. Bachmann and G. McHale.
    Eur. J. Soil Sci. 60 (3) (2009) 420-430.