Experimental Study of the Moisture Distribution on the Wetting Front during Drainage and Imbibition in a 2D Sand Chamber

During drainage and imbibition, different kinds of flow mechanisms can be seen at the pore scale (e.g., "Haines jumps" during drainage and ganglion dynamics during imbibition). However, it is argued in the literature that pore-scale flow mechanisms can be average-out when upscaling. It is further doubted whether the effect of flow mechanisms at the pore scale on moisture distribution can be observed at the Darcy scale. In the present study, the difference in moisture distribution during drainage and imbibition in a 2-D sand chamber is investigated. To accomplish this objective, light transmission method is used to record the moisture contents during drainage and imbibition. Results show that during drainage and imbibition, different moisture distributions are observed: (a) during drainage, moisture contents fluctuate in a larger range and "fingers" can be seen on the wetting front; (b) while during imbibition, moisture contents fluctuate in a smaller range and the wetting front is more regular. These "fingers" observed on the wetting front during drainage is attributed to the unstable pore-scale drainage events (or "Haines jumps"). Furthermore, the Hurst coefficient is used to capture the moisture distribution during the experiments and the Hurst coefficient during drainage is significantly larger than the Hurst coefficient during imbibition. The larger Hurst coefficients during drainage periods indicate that the moisture distribution fluctuates in a larger range during drainage. While the smaller Hurst coefficients during imbibition periods indicate that the moisture distribution is constrained to around its mean value and fluctuates in a relatively smaller range. The difference in moisture distribution during drainage and imbibition clearly show that pore-scale mechanisms have a distinct impact on the moisture distribution at the Darcy scale and would not average-out when up-scaling.