Pine straw in the rain

Pine needles suspended high in the canopy fall to the ground to provide excellent ground cover for pine stands on steep slopes. These slender fibers provide the soil a protective layer against the erosive effects of falling raindrops. When a droplet undergoes one or more impacts with the fibers during its travel downward, it loses kinetic energy and a portion of its ability to loosen soil particles. This curtails the first step of the erosion process, detachment. In this study, we replicate the phenomenon of rainfall atop ground cover and the proceeding droplet breakup using synthetic slender fibers to simulate pine needles. Through high-speed videography and digital tracking, we compare the energy dissipated by slender fibers of differing profiles, elasticities, and wetting properties. Certain morphologies of these slender fibers are found to cause breakup of falling droplets while others allow droplets to reform into a single mass post-impact. Hydrophobic fibers and those with non-circular profiles provide the highest degree of droplet breakup. Prediction of fiber impact characteristics is provided by consideration of droplet energies.