Self-organization of hydrophobic soil and granular surfaces

Soil can become extremely water repellent following forest fires or oil spillages, thus preventing penetration of water and increasing run-off and soil erosion. Here the authors show that evaporation of a droplet from the surface of a hydrophobic granular material can be an active process, lifting, self-coating and selectively concentrating small solid grains. Droplet evaporation leads to the formation of temporary liquid marbles and, as droplet volume reduces, particles of different wettability compete for water-air interfacial surface area. This can result in a sorting effect with self-organisation of a mixed hydrophobic-hydrophilic aggregate into a hydrophobic shell surrounding a hydrophilic core