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1. Optimizing pink‐beam fast X‐ray microtomography for multiphase flow in 3D porous media

   https://doi.org/10.1111/jmi.12872  

   MEISENHEIMER, D. E. ; RIVERS, M. L. ; WILDENSCHILD, D. ( February 2020 , Journal of Microscopy)

   A fast pink‐beam X‐ray microtomography methodology was developed at the GSECARS 13‐BMD beamline at the Advanced Photon Source to study multiphase flow in porous media. The white beam X‐ray distribution of the Advanced Photon Source is modified using a 1‐mm copper filter and the beam is reflected off a platinum mirror angled at 1.5 mrad, resulting in a pink beam with X‐ray intensities predominately in the range of 40–60 keV. Bubble formation in the wetting phase and wettability alteration of the solid phase from x‐ray exposure can be a problem with high flux and high energy beams, but the suggested pink‐beam configuration mitigates these effects. With a 14‐second acquisition time for capturing a complete dataset, the evolving fluid‐fronts of nonequilibrium three‐dimensional multiphase flow can be studied in real‐time and the images contain adequate image contrast and quality to measure important multiphase quantities such as contact angles and interfacial areas.

   Understanding how fluids are transported through porous materials is pertinent to many important societal processes in the environment (e.g. groundwater flow for drinking water) and industry (e.g. drying of industrial materials such as pulp and paper). To develop accurate models and theories of this fluid transportation, experiments need to track fluids in 3‐dimensions quickly. This is difficult to do as most materials are opaque and therefore cameras cannot capture fluid movement directly. But, with the help of x‐rays, scientists can track fluids in 3D using an imaging technique called x‐ray microtomography (μCT). Standard μCT takes about 15 minutes for one image which can produce blurry images if fluids are flowing quickly through the material. We present a technique, fast μCT, which uses a larger spectrum of x‐rays than the standard technique and acquires a 3D image in 14 seconds. With the large amount of x‐rays utilized in this technique, bubbles can start to form in the fluids from x‐ray exposure. We optimized the utilized x‐ray spectrum to limit bubble formation while still achieving a rapid 3D image acquisition that has adequate image quality and contrast. With this technique, scientists can study fluid transport in 3D porous materials in near real‐time for the improvement of models used to ensure public and environmental health.

    

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2. Modeling Multiphase Flow Within and Around Deformable Porous Materials: A Darcy‐Brinkman‐Biot Approach

   https://doi.org/10.1029/2020WR028734  

   Carrillo, Francisco J. ; Bourg, Ian C. ( February 2021 , Water Resources Research)

   We present a new computational fluid dynamics approach for simulating two‐phase flow in hybrid systems containing solid‐free regions and deformable porous matrices. Our approach is based on the derivation of a unique set of volume‐averaged partial differential equations that asymptotically approach the Navier‐Stokes Volume‐of‐Fluid equations in solid‐free regions and multiphase Biot Theory in porous regions. The resulting equations extend our recently developed Darcy‐Brinkman‐Biot framework to multiphase flow. Through careful consideration of interfacial dynamics (relative permeability and capillary effects) and extensive benchmarking, we show that the resulting model accurately captures the strong two‐way coupling that is often exhibited between multiple fluids and deformable porous media. Thus, it can be used to represent flow‐induced material deformation (swelling, compression) and failure (cracking, fracturing). The model's open‐source numerical implementation,hybridBiotInterFoam, effectively marks the extension of computational fluid mechanics into modeling multiscale multiphase flow in deformable porous systems. The versatility of the solver is illustrated through applications related to material failure in poroelastic coastal barriers and surface deformation due to fluid injection in poro‐visco‐plastic systems.

    

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3. Nonhysteretic Capillary Pressure in Two‐Fluid Porous Medium Systems: Definition, Evaluation, Validation, and Dynamics

   https://doi.org/10.1029/2018WR024586  

   Miller, C. T. ; Bruning, K. ; Talbot, C. L. ; McClure, J. E. ; Gray, W. G. ( August 2019 , Water Resources Research)

   A closure relation for capillary pressure plays an important role in the formulation of both traditional and evolving models of two‐fluid‐phase flow in porous medium systems. We review the traditional approaches to define capillary pressure, to describe it mathematically, to determine parameters for this relation, and to constrain the domain of applicability of this relation. In contrast to the traditional approach, we provide a rigorous, multiscale definition of capillary pressure, define the state domain of interest in practice, summarize computational and experimental approaches to investigate the system state, and apply the methods for two‐fluid states in a model ink bottle system, the classical Finney pack of spheres, and a synthetic sphere pack system. The results of these applications show that a state equation exists that describes capillary pressure without hysteresis. This state equation parameterizes a function that describes the nonwetting phase volume fraction in terms of the capillary pressure, the interfacial area, and the specific Euler characteristic of the nonwetting phase. Furthermore, this state equation applies over the complete range of conditions encountered in practice, and it applies under both equilibrium and dynamic conditions. This state equation involving capillary pressure forms an important foundation for the development of the next generation of macroscale two‐fluid‐phase flow models in porous medium systems.

    

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4. Pore-size dependence and slow relaxation of hydrogel friction on smooth surfaces

   https://doi.org/10.1073/pnas.1922364117  

   Cuccia, Nicholas L. ; Pothineni, Suraj ; Wu, Brady ; Méndez Harper, Joshua ; Burton, Justin C. ( May 2020 , Proceedings of the National Academy of Sciences)

   Hydrogels consist of a cross-linked polymer matrix imbibed with a solvent such as water at volume fractions that can exceed 90%. They are important in many scientific and engineering applications due to their tunable physiochemical properties, biocompatibility, and ultralow friction. Their multiphase structure leads to a complex interfacial rheology, yet a detailed, microscopic understanding of hydrogel friction is still emerging. Using a custom-built tribometer, here we identify three distinct regimes of frictional behavior for polyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres on smooth surfaces. We find that at low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms between the gel and surface that obeys elastohydrodynamic theory. Between these regimes, the frictional force decreases by an order of magnitude and displays slow relaxation over several minutes. Our results can be interpreted as an interfacial shear thinning of the polymers with an increasing relaxation time due to the confinement of entanglements. This transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.

    

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5. A Darcy‐Brinkman‐Biot Approach to Modeling the Hydrology and Mechanics of Porous Media Containing Macropores and Deformable Microporous Regions

   https://doi.org/10.1029/2019WR024712  

   Carrillo, Francisco J. ; Bourg, Ian C. ( October 2019 , Water Resources Research)

   The coupled hydrology and mechanics of soft porous materials (such as clays, hydrogels, membranes, and biofilms) is an important research area in several fields, including water and energy technologies as well as biomedical engineering. Well‐established models based on poromechanics theory exist for describing these coupled properties, but these models are not adapted to describe systems with more than one characteristic length scale, that is, systems that contain both macropores and micropores. In this paper, we expand upon the well‐known Darcy‐Brinkman formulation of fluid flow in two‐scale porous media to develop a “Darcy‐Brinkman‐Biot” formulation: a general coupled system of equations that approximates the Navier‐Stokes equations in fluid‐filled macropores and resembles the equations for poroelasticity in microporous regions. We parameterized and validated our model for systems that contain either plastic (swelling clay) or elastic microporous regions. In particular, we used our model to predict the permeability of an idealized siliciclastic sedimentary rock as a function of pore water salinity and clay content. Predicted permeability values are well described by a single parametric relation between permeability and clay volume fraction that agrees with existing experimental data sets. Our novel formulation captures the coupled hydro‐chemo‐mechanical properties of sedimentary rocks and other deformable porous media in a manner that can be readily implemented within the framework of Digital Rock Physics.

    

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