Finer Measurement Scales for Induced Hydrophobicity Using the Water Droplet Penetration Test

The Water Droplet Penetration Test (WDPT) is commonly used in most soil water repellency (SWR) research and is particularly prominent in field studies after wildfire events. Suppose a water droplet does not infiltrate the soil within the first five seconds. This soil is considered to contain some degree of water repellency, classified by the overall penetration time. Our results show an inflection point in the plot of the height of a droplet vs. droplet penetration time during a WDPT trial. This inflection point is indicative of a combination of two possible flow patterns influencing droplet penetration, one governing and the other—caused by particle lift—drastically impeding the infiltration rate. The reorganization of the intrinsic particle lift at the air–water interface leads to contact angles hindering the expected penetration, delaying the expected infiltration rate to degrees larger than a continuously flat porous hydrophobic surface would. The particle lift creates an instability that can create two competing regimes, leading to two sets of water droplet penetration times. The similarity among sorptivity values for coarse grains at higher hydrophobicity levels, medium grains at intermediate hydrophobicity levels, and fine grains at lower hydrophobicity levels suggests that interpretation of the WDPT needs to be adjusted based on grain size.