Armored Water Droplets: How They Form and Their Role in Water Redistribution and Soil Erosion.

A new regime of soil hydrophobicity is explored where the gas-liquid-solid contact angle is in the range of 60 to 90 degrees. For contact angles less than approximately 60 degrees, a water droplet easily imbibes into the soil surface, while for contact angles greater than 90 degrees, a droplet would sit on the soil surface as a bead. However, for intermediate situations, as discussed herein where the contact angle is approximately between 60-90 degrees, when a water droplet contacts the soil surface, the droplet forms a bead that is quickly coated (< 1/10 sec) with dry, soil particles. This coating does not penetrate the droplet, but rather forms an armor riding on the air-water interface. Even though such particle coatings can be produced by electrostatic attraction of hydrophobic powders (very small particles), this paper poses a different mechanism that is relevant to larger (soil) particles, and focuses on the armoring of water droplets as a natural soil phenomena occurring in soils exhibiting intermediate hydrophobicity. Droplet armoring has significant environmental consequences because the particle coating frees the droplet to roll down slope, eroding the soil surface until it comes to rest at a topographic low. In hill planting, this redistribution of moisture can lead not only to hill erosion, but plant drought stress. The range of contact angles pertinent to this phenomena were derived mathematically and tested using Quincy soil as a medium. Understanding this phenomenon will provide the agricultural community a target for remediation techniques.