Microscale Heterogeneity in Fluid Distribution Patterns During Drainage of a Homogeneous Sand: Neutron Imaging Visualization and Model Limitations

We use neutron radiography to provide high-resolution images of water distribution patterns inside the pore space of a uniformly packed sand in a flow cell, during drainage, induced by a sequence of suction tensions. The experiments reveal the emergence of heterogeneous water distribution patterns within the pore space of the sand, although its corresponding retention function suggests homogeneity. This demonstrates that truly homogeneous water movement does not occur even under controlled laboratory conditions in an, as much as possible, homogeneous porous medium, and over dimensions of only a few millimeters. Furthermore, we conduct simulations with a Darcy-Richards model aimed at (a) capturing the overall 1D drainage process of the flow cell, to obtain a macroscopic perspective on the system using spatially averaged saturation levels, and (b) gaining a more detailed microscopic understanding of the internal 2D pore space processes and spatial distribution patterns during water drainage. Simulation results show that the model can reproduce the macroscopic 1D drainage process of the flow cell with high accuracy. However, the microscopic, heterogeneous 2D distribution of water observed inside the flow cell can be reproduced only qualitatively, after manually introducing local heterogeneities into the pore space. This highlights that the successful reproduction of macroscopic dynamics with a Darcy-Richards model does not imply that the model can accordingly capture truly heterogeneous process patterns on the microscale.