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In porous geological or environmental systems, many processes such as transport, mixing, chemical reactions, and biological activity are determined by fluid flow, making it essential to understand and characterise flow processes. While continuous-scale reactive transport models generally rely on effective parameters derived from the assumption of homogeneous mixing at the pore scale, natural porous materials have complex pore geometries and predominantly laminar flow conditions, favoring incomplete mixing conditions at the pore scale. Indeed, recent experimental studies (1,2) conducted on transparent bead packs have highlighted the persistence of chemical gradients at the pore scale due to chaotic advection. Through repeated stretching and folding of fluid elements induced by the pore architectures, chaotic advection sustains concentration gradients thus strongly influencing transport and reaction dynamics. Despite its potential importance, direct experimental evidence of chaotic mixing in other natural porous materials, such as sand or rocks, has remained limited due to challenges in imaging pore-scale advective processes.
Here, we present direct three-dimensional observations of chaotic fluid deformation in porous rock samples obtained using fast, high-resolution X-ray tomography at the European Synchrotron Radiation Facility (ESRF). Experiments were conducted on highly permeable sandstone and unconsolidated sand pack samples using a custom-designed core holder enabling the controlled co-injection of two miscible fluids. To ensure that advective transport dominates over molecular diffusion (high Péclet numbers), we used highly viscous fluids (glycerin-water mixture). These conditions favor the observation of pore-scale deformation of fluid fronts to be resolved prior to diffusive smoothing.
X-rays images reveal the complexity of the mixing interface between the two fluids, which obeys stretching and folding in a way similar to what was previously observed in bead packs. In particular, we quantify the growth of the material interface length with respect to advection distance, as well as the strength of concentration heterogeneities persisting at pore scale through a local concentration variance. We relate these measures to recent theoretical prediction of scalar mixing in chaotic flows.
These results highlight the need to incorporate chaotic mixing mechanisms into pore-scale and larger-scale transport models. By providing direct experimental evidence of chaotic advection in natural porous media, this work contributes to improving the predictive capability of reactive transport models.
| References | 1. Heyman, J. et al. (2020). Proceedings of the National Academy of Sciences, 117(24) 2. Heyman, J. et al. (2021). Physical Review Letters, 126(3), 034505. |
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| Country | France |
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