Understanding the onset of mudflows and rainwater droplet impact dynamics on hydrophobic tilted surfaces

Abstract:

Post-wildfire mudflows occur in hillslopes where rain is unable to infiltrate soil due to grain-surface hydrophobicity, which is induced by organic matter burn. Instead, rainwater blankets downhill and erodes soil, forming mudflows. The rate of the soil erosion increases more than one order of magnitude in post-wildfire areas due to delayed wetting and increased surface runoff. Onset of mudflows depends, among other factors, on the mobility of rainwater over the hydrophobic granular surface. While a raindrop tends to remain in place and subsequently infiltrate wettable soil, hydrophobic surface has negligible drag on a water drop, which speeds up downhill due to gravity. To better understand the onset of mudflows in hydrophobic area, we start with looking at the micromechanics of droplet dynamics and motion over a hydrophobic sand surface. A series of droplet tests are conducted on hydrophobic surfaces tilted 30°, 40° and 45° from the horizontal. The droplet movement is tracked with high speed cameras at 900 fps and compared to a regular sand surface. Droplet velocity at the impact point, the rolling velocities, spread times and spread factors are obtained from measurements. Advancing and receding water contact angles are also measured for each slope and surface. Results quantify a two-phase raindrop behavior upon hitting an inclined surface. By comparing the droplet energy before and after the impact, it was found that the energy dissipation during the impact is low for hydrophobic surface compared to the regular surface. Fig. 1a shows the impact moment of the droplet on a 45° inclined hydrophobic surface. When the droplet hits a hydrophobic surface, more energy is conserved and the droplet rebounds immediately. Consequently, the higher energy leads to a second jump, which can be observed in tracked patterns of the droplet after impact point in Fig. 1c. It can be concluded that the higher rate of erosion and mudflows in post-fire areas can be related to the higher energy conservation of the rain droplet during the impact on the hydrophobic surface, which leads to a more destructive potential of each rain droplet after the initial impact. Contrarily, a regular wettable soil surface, behaves more as a damper and dissipates a larger portion of a droplet impact energy during the rainfall compared to a hydrophobic surface.