Break-up and mobilization of DNAPL by acoustic excitation: Experimental evidence and pore network modeling.

Dense non-aqueous phase liquids (DNAPLs) are long-term groundwater contaminants due to their high toxicity and slight solubility in water. The use of acoustic waves to remobilize trapped ganglia in subsurface porous systems have some advantages over pre-existing solutions including eliminating the bypassing effect and new environmental hazards. Designing an effective acoustically assisted remediation method for such purposes relies on understanding the underlying mechanisms and developing validated models. In this work, pore-scale microfluidic experiments were run to investigate the interplay between break-up and remobilization under sonication at different levels of flow rate and wettability conditions. Based on the experimental observation and pore-scale physical characteristics, a pore network model was developed and verified against the experimental results. Such a model was developed based on a two-dimensional network and scaled up to three-dimensional networks. In the experiments, processing of two-dimensional images showed that acoustic waves can remobilize trapped ganglia. The other observed effect of vibration is to break up blobs and reduce the mean ganglia size. Recovery enhancements were greater in hydrophilic micromodels as compared to hydrophobic system. A strong correlation was found between the remobilization and breakup indicating that the trapped ganglia are breaking up due to acoustic stimulation firstly and then a background viscous force may get them flowing under the new generated fluid distribution. In modeling, the simulation results of residual saturation reasonably matched with experimental observations. The differences between the prediction by the model and the experimental data at verification points is less than 2% for data before and after the acoustic excitation. The transitions from three-dimensional simulations were used to propose a modified capillary number. This study gives a better understanding of the mechanisms behind the effect of acoustic waves in porous media and provides a predictive tool for evaluating enhancement in fluid displacement.