Erodibility of synthetic water repellent granular materials: Adapting the ground to weather extremes.

Granular materials with synthetic water repellent coatings have great potential to be used in ground interfaces (ground-atmosphere-vegetation and ground-structure) as infiltration barriers, due to their altered hydrological properties (suppressed infiltration and decreased sorptivity). However, very few studies have evaluated the impact of synthetic soil water repellency on soil erosion. This paper investigates the effect of water repellency on soil erosional behavior, including splash erosion and rill processes. Twenty-four flume tests were carried out on model slopes under artificial rainfall; soils with three wettability levels were tested, including wettable (contact angle, CA < 90°), subcritical water repellent (CA ~ 90°) and water repellent (CA > 90°). Various rainfall intensities (230 mm/h, 170 mm/h, 100 mm/h and 40 mm/h) and grain sizes (Fujian sand and sand/silt mixture) were adopted. Erosional variables, including splash erosion rate, average sediment concentration, peak sediment concentration and time to peak sediment were measured to quantitatively analyze the behavior. This study confirms the impact of water repellency on soil erosion and unveils the possibility to reduce infiltration at ground-atmosphere interface with controlled soil erosion. The results revealed that: (1) synthetic water repellency does not necessarily lead to increased soil erosion yield; its impact is dependent on grain size with the soil erosion loss increasing for Fujian sand, but decreasing for sand/silt mixtures; (2) splash erosion is positively correlated to soil water repellency and high rainfall intensity, regardless of grain size; (3) the erosion processes for sand/silt mixtures are particle size selective and not affected by soil water repellency, whereas this phenomenon is not observed with Fujian sand.

Lattice Boltzmann simulation of droplet dynamics on granular surfaces with variable wettability.

Soil-composing particles undergo wettability changes, impacting hydraulic and mechanical processes such as erosion and landslides. Such processes evolve at very small scales, typically at the particle level. Here we capture the evolution of liquid interfaces in a single particle and several particles with the lattice Boltzmann (LB) method. The paper presents a three-dimensional LB study on the droplet dynamics on a layer of uniformly packed spherical particles with varying size and intrinsic contact angle (CA) aimed at mimicking conditions comparable to those of real soils. The numerical droplet is initialized close to the granular surface and deposited by gravity. Three spreading and infiltration behaviors were identified: a droplet with a stable apparent CA, a droplet with a metastable apparent CA before infiltration, and immediate infiltration. The results showed that the formation of a droplet with a stable or metastable spherical-cap shape depends on the particle size and the intrinsic CA. Furthermore, the initial wetted zone expansion was found to be governed by inertial effects with its behavior characterized by a power law. Finally, the apparent CA, which is closely related to the intrinsic CA, was found to be influenced by the particle size due to a significant portion of the droplet being embedded into the granular surface for the larger particles and reducing the apparent CA. This paper provides a basis for future research targeting the behavior of droplet interaction with granular surfaces with variable intrinsic CAs (from wettable to superhydrophobic) such as soils and other granular materials for industrial applications. The numerical approach implemented can also be extended to model other dynamic processes for a droplet, such as evaporation, high-velocity impacting, and lateral sliding.