Lattice Boltzmann simulation of fluid-particle interaction in hydrophobic granular materials

Abstract

Soil water repellency or soil hydrophobicity is defined as a reduction of the affinity between soils and water, so that water cannot spontaneously wet soils. In geotechnical engineering, such waterproof property can be utilized to reduce water infiltration into the ground, since water reduces the soil effective stress and may cause geohazards (e.g., landslides and debris flows). This thesis aims to reveal the performance of water repellent soils from a microscopic perspective, providing a pore-scale insight into the droplet dynamics on granular surfaces and the liquid distribution inside granular packings. The particle surface contact angle (CA), also defined as the intrinsic CA, is used as a measure of soil water repellency. As either the droplet interaction or porous flows develop at the particle-level, multiphase Lattice Boltzmann (LB) methods serve as the appropriate tools for addressing this important aim. The aim will be implemented through three objectives: (1) to track the evolution of droplet spreading and its apparent CA on granular surfaces with varying wettability, (2) to investigate the dynamics of droplets on inclined hydrophobic granular surfaces, and (3) to quantify the liquid distribution, soil water retention and capillary stress in granular packings.

Droplet dynamics is investigated using a phase-field LB method. As for the first objective, various modes of the droplet spreading dynamics were identified and the resulting apparent CAs were measured. The research revealed that apart from sitting above the surface with a stable apparent CA, a droplet may display a transient metastable apparent CA in a spherical cap shape but still infiltrate, of which the formation mechanism is explained by the droplet lateral spreading outpacing the downward infiltration. The relationship between apparent CA and intrinsic CA is found to be dependent on the particle size, which is attributed to a significant portion of the droplet volume embedded into the pores between particles.

To address the second objective, a parametric study was conducted to show how the intrinsic CA, particle size, slope angle and impacting velocity affect the droplet sliding dynamics. A rotational or translational type of droplet movement was also quantitatively characterized. A novel finding is that a droplet can slide in a constant terminal velocity and has multiple discrete choices of its terminal velocity under different impacting velocities. To understand this, a mechanism is proposed: a higher terminal velocity indicates an equally higher gravitational energy input and viscous dissipation rate, which always cancel each other out.

For the last objective, liquid distribution inside a granular packing with hydrophobic particles is investigated using a pseudopotential LB method. The study has demonstrated a significant difference in the liquid morphology from a hydrophilic packing: hydrophobic particles divide a granular packing into several hydrophilic subregions, in which liquid evolves into several segmented groups of liquid clusters. Even at a low degree of saturation, the liquid clusters are in funicular state with lower suctions, but not in pendular state as bridge liquid bridges. Such a microscopic observation explains the significant reduction in the capillary stress caused by hydrophobic particles.