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Granular materials: Sand and soil

When a fire occurs in a sandy forest the soil can become naturally extremely water-repellent (see Stefan Doerr's website here). This leads to erosion when it rains and difficulty in re-establishing vegetation and, in the extreme case, impressively destructive debris torrents (click here to see a video). 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 publication below.


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