Fine particle percolation dynamics in porous media

This is the authors' abstract. We don't add key points for this paper.

The influences of restitution coefficient, e n , interparticle friction, μ , and size ratio, R , on gravity-driven percolation of fine particles through static beds of larger particles in the free-sifting regime ( R ≳ 6.5 ) remain largely unexplored. Here, we use discrete element method simulations to study the fine particle percolation velocity, v p , and velocity fluctuations, v rms , for 7 ≤ R ≤ 50 and a range of e n and μ . Varying e n modulates the degree of particle excitation and thereby alters the nature of particle trajectories: at low e n , percolation is dominated by gravity, whereas increasing e n amplifies velocity fluctuations thereby reducing the mean percolation velocity. Increasing μ decreases v rms but its influence on v p varies with v rms , decreasing v p for low v rms and increasing v p for high v rms . Although the influence of size ratio is weaker, larger values of R increase both v p and v rms . We also assess the influence of different excitation mechanisms, specifically using static, randomly excited, and sheared beds, finding that an inverse correlation between v p and v rms persists across all cases and is well-described by the Drude model, where increased scattering reduces mobility, when v rms is large. However, for weakly excited particles with low v rms , the Drude analogy breaks down. In this regime, we introduce a staircase-inspired model that accounts for the gravitationally dominated percolation behavior. These findings provide fundamental insight into the mechanisms governing percolation dynamics in porous media and granular systems.