Cavitation-Associated Water Evaporation From Porous Media Through In Situ NMR Characterization

Abstract

Evaporation of water from porous media is essential for a large variety of applications, whereas the opacity of porous matrix imposes considerable challenges in unveiling complicated phase-change phenomena. Air invasion was previously reported as the major desaturation mechanism, while cavitation in porous media is not well studied. Herein we characterize the transient distribution and evaporation of water in homogeneous tight porous media with nondestructive nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). By monitoring the amplitude change of NMR transverse relaxation time T2, we investigate the dynamical pore filling status and water content during evaporation. Interestingly, we find that the T2 spectrum shifts immediately after the evaporation starts, indicating the emptying of big pores from the entire medium. Disconnected void clusters at different depths in the porous medium are also observed from MRI scanning and optical images. These observations indicate the emergence of cavitation across the entire porous media along with the evaporation from open surface. Cavitation occurs when the water is stretched to metastable state by large capillary pressure from the evaporating meniscus. By studying the evaporation from hydrophilic membrane-separated porous media, we further demonstrate the existence of cavitation-associated evaporation. The preferential water vaporization from the bottom part can still be found from T2 spectrum analysis and optical imaging even when the water-permeable membrane cuts off possible air invasion. Our findings confirm cavitation-associated evaporation is one of the primary mechanisms for tight porous media, which provides valuable guidance for evaporation and moisture control.