Visco-frictional trapping of particles in a submerged porous medium

Particle filtration through a porous medium is a much-studied field due to its numerous industrial or natural applications, particularly for deep bed and cake filtration regimes. The percolation regime has received relatively less attention and corresponds to the continuous motion of particles in a porous environment under the principal effect of gravity, and generally without any interaction other than contact with the porous matrix. In the present case, both the moving particles and the porous medium are composed of spherical particles of diameters d and D, respectively. Dry percolation in such conditions has been analysed and exhibits lateral diffusion as well as a constant mean transit velocity which scales with [1]. The behavior is significantly different in the case studied here, where the porous stack is fully saturated. A previous study experimentally investigated the transition from deep filtration to percolation regime under laminar flow conditions by matching the refractive index of the liquid to that of the particles of the porous system [2]. Here, based on the same experimental technique, we focus on the particular case where there is no fluid flow, for which we observe a new type of particle trapping, by contrast with either the dry condition or the flow situation. We show that this capture comes from a combination of solid friction and viscous effects, which strongly reduce the velocity of the moving particles by lubrication and can also suppress any rebound. This visco-frictionnal trapping is highly metastable and the captured particles can easily be moved again. We will also present a statistical analysis of the trajectories followed in the porous material, including transit velocity, penetration depth and radial dispersion.

[1] Ippolito, I, Samson, L., Bourlès, S., Hulin, J.-P., Diffusion of a single particle in a 3D random packing of spheres, Eur. Phys. J. E 3, 227–236 (2000).

[2] Ghidaglia, C., de Arcangelis, L., Hinch, J., Guazzelli, E., Transition in particle capture in deep bed filtration, Phys. Rev. E 53, R3028(R) (1996).