Acoustic characteristics of fluid interface displacement in drying porous media

Water evaporation from porous media involves many rapid interfacial jumps at the pore-scale as air invades the pore network and displaces the evaporating fluid. We show that this process produces a crackling noise that can be detected using an acoustic emission (AE) instrument. We investigated the acoustic signature of evaporation from porous media using transparent glass cells packed with five types of sand and glass beads differing in particle size distribution and grains shape. Each sample was mounted on a digital balance, saturated with dyed water, left to evaporate under well-controlled atmospheric conditions, and digitally imaged every 20. min to quantify the dynamics of liquid phase distributions. An AE sensor was fixed to each column to record AE events (hits) and their acoustic features. Results indicate that the cumulative number of AE hits is strongly proportional to total evaporative losses. Additionally, the cumulative number of hits shares an inverse relationship with particle size and a direct relationship with grain irregularity. Analysis of the dynamics of liquid phase distributions reveals a strong correlation between the area invaded by air and the cumulative number of AE hits. Our results suggest that AE techniques may hold the potential to non-invasively analyze evaporation from porous media. © 2014 Elsevier Ltd.