Emergence of periodic chimneys during fluidization at a coarse-fine grains interface

In this work, we focus on the influence of an interface on liquid rise through an immersed granular bed. Based on laboratory experiments, we consider the migration of water injected at constant flow rate at the bottom center of a Hele-Shaw cell filled with two layers of grains immersed in water. The bottom layer is made of coarse grains, large enough to ensure liquid percolation without grain motion. The top layer consists of a bidisperse medium of fine grains and dusts about four times smaller in diameter, which can penetrate the interface between coarse and fine grains. When the liquid invades the cell, above a critical flow rate, the dusts are washed out of the coarse grains, a process called elutriation. The flow pattern self-organizes, generating fluidization chimneys at the interface between coarse and fine grains with regular spatial distribution. A model based on pressure-drop estimations predicts the pattern wavelength, which depends on the dust size and the number of coarse grains in the cell gap.