Fast-4D neutron imaging reveals complexity behind water vapour condensation in heterogeneous porous media

fPorous media are fundamental to a wide range of natural and engineering processes ranging from groundwater flow and filtration cleaning, to fuel cells, as well as food processing (e.g. coffee steam extraction). A key and yet poorly understood phenomenon in these systems is condensation, where gas transforms into liquid within the pore network, influencing the efficiency of various processes. In geothermal systems, condensation affects heat transfer efficiency, while in civil structures, it governs the moisture transport impacting material durability and eventually structural integrity. Despite its significance across a variety of fields, condensation in porous media remains underexplored due to complex heat and mass transfer interactions, especially in (micro)fractured structures relevant to geological, civil and process engineering systems. Despite its importance, directly studying condensation and liquid transport in porous media remains a significant challenge due to their opacity as well as the limitation of conventional measurement techniques which often provide only point-wise data, unable to map the entire spatial-signature of the process. X-ray imaging offers a powerful non-invasive means to capture internal structures in 3 spatial dimensions (3D). However, X-rays struggle to detect water vapour due to the low atomic number of its components, limiting their ability to detect condensation and liquid transport. Neutron imaging substantially overcomes this limitation by exploiting its high sensitivity to hydrogen, making it particularly effective for visualizing water distribution in complex porous networks. By combining neutron and X-ray imaging, complementary insights can be achieved—mapping both solid structures and fast-evolving liquid distributions—leading to a deeper understanding of phase change phenomena in porous media.

To investigate this complex phenomenon, researchers from the UGA/G-INP/CNRS Fluid Dynamics Laboratory (LEGI), in collaboration with scientists from ESRF and ILL, joined forces to address the associated challenges. Led by Dr. Arash Nemati, the team designed an experimental setup in which water vapor, with precisely controlled quality and flow rate, was injected into a naturally heterogeneous porous sample containing artificially introduced fractures. The condensation experiments were performed using the NeXT-Grenoble instrument at the Institut Laue-Langevin (ILL), exploiting the instrument’s high cold neutron flux to study the condensation in-situ in 4D. To ensure reliable water quantification, neutron imaging calibration was performed beforehand to account for incoherent scattering effects caused by high hydrogen content, using reference measurements and correction techniques. A multimodal imaging approach was employed (Figure 1): first, synchrotron phase-contrast X-ray imaging was used to obtain the microstructure morphology of the media at high resolutions ; Second, fast neutron tomography was used to track water condensation distribution in 4D (3D + time) with unprecedented detail.

Through correlative analysis of the multimodal datasets, the research team observed the evolution of water condensation within the samples, its interaction with fractures, and its dependence on the porous medium’s microstructure. While fractures acted as primary pathways for vapor migration due to their higher permeability, condensed water preferentially accumulated in the smaller pores of the porous matrix, emphasizing the capillary-driven nature of water migration. Initially, water condensed along fracture surfaces before being absorbed into the matrix through capillarity. Lower-porosity samples exhibited faster propagation of the water condensation front due to their limited absorption capacity. At low vapor flow rates (Figure 2), water accumulated in fractures as patches, whereas higher flow rates kept fractures drier by driving more water into the matrix. Overall, the proposed methodology reliably measured water content, as validated by boundary measurements. Neutron and X-ray imaging further confirmed the critical role of fracture morphology and microstructure in condensation dynamics. These findings are currently used at the base of the development of an advanced multi-physics numerical and analytical tools able to reproduce condensation in porous media.