Role of hydrophobic sand particle granularity on water droplet post-impact dynamics

Abstract

This paper investigates, experimentally and theoretically, the effects of sand particle size and hydrophobicity of a water drop impacting horizontal and inclined surfaces within a context of post-wildfire mudflow precursors. Although previous research established that wildfires often turn hillslope surficial soil hydrophobic, little research shows how wildfire-altered soil surfaces affect raindrops' impact behavior. Specific findings point to a significant effect of sand type on the drop impact dynamics. After spreading, the drop gathers back more on finer than coarser hydrophobic sands due to the substrate air-entrapped grooves, as in the Cassie-Baxter model, leading to drop retraction, rebound, splash, and fragmentation into droplets. The results indicate that if the hillslope dominant median grain size is fine, alteration to hydrophobicity would multifold enhance drop downhill mobility. In contrast, coarse sand surfaces exhibit minimal effects on drop mobility after hydrophobization. In addition, the paper proposes a modified Tang et al. model for the maximum spread factor as a function of the median grain size and hydrophobicity. This research offers essential findings from the perspective of the previous limiting understanding that only drop infiltration prevents erosion in hydrophilic slopes.