Listening to Imbibition and Drainage - Bursts and Jumps at the Front.

The macroscopically steady and regular motion of fluid displacement fronts through porous media often conceals considerable activity and bursts occurring at the pore scale involving rapid pressure jumps and interfacial reconfiguration of the front line.  In contrast to well understood sequential and slow dynamics associated with pore filling and emptying described by percolation theory, the characteristics of rapid interfacial jumps underlying such displacement processes and associated acoustic emissions (AE) from these events remain unclear. This study attempts to quantify characteristics of bursts resulting from rapid air-water interfacial reconfiguration via their AE signals and detailed pressure measurements. We plan to relate number of AE events, amplitudes and other acoustic characteristics with porous medium properties and imposed boundary conditions. AE measurement resulting from front displacement in Hele-Shaw cells packed with glass beads of different sizes show marked differences between acoustically-rich drainage process relative to low activity during imbibition under similar conditions. We link AE activity and pressure jumps in the liquid phase with amplitudes of air entry pressures and width of pore throats. We also show model calculations for relating measured acoustic signal with interfacial energy release during meniscus reconfiguration in simple geometries. Acoustic emission information may provide novel means for interrogation of pore and throat sizes, better understanding of displacement processes, and may also assist with quantifying the role of dissipation occurring at a macroscopic displacement front due to rapid interfacial bursts.