Chaotic Transport of Solutes in Unsaturated Porous Media.

Unsaturated porous media, characterized by the combined presence of several immiscible fluid phases in the pore space, are highly relevant systems in nature, because they control the fate of contaminants and the availability of nutrients in the subsoil. However, a full understanding of the mechanisms controlling solute mixing in such systems is still missing. In particular, the role of saturation in the development of chaotic solute mixing has remained unexplored. Using three-dimensional numerical simulations of flow and transport at the pore scale, built upon X-ray tomograms of a porous medium at different degrees of liquid (wetting)-phase saturation, we show the occurrence of chaotic dynamics in both the deformation of the solute plume, as characterized by computed chaos metrics (Lyapunov exponents), and the mixing of the injected solute. Our results show an enhancement of these chaotic dynamics at lower saturation and their occurrence even under diffusion-relevant conditions over the medium's length, also being strengthened by larger flow velocities. These findings highlight the dominant role of the pore-scale spatial heterogeneity of the system, enhanced by the presence of an immiscible phase (e.g., air), on the mixing efficiency. This represents a stepping stone for the assessment of mixing and reactions in unsaturated porous media.

Hierarchical Porous Wood Cellulose Scaffold with Atomically Dispersed Pt Catalysts for Low-Temperature Ethylene Decomposition.

Despite the excellent catalytic properties of individual nanoparticles and atomic clusters, the current capabilities to assemble them into a complex system are insufficient for many practical applications. An objective of this work is to develop a fabrication technology that allows for the simultaneous control of the nanoparticle surface chemistry, elemental distribution, microscale geometry, and large-scale assembly. Using a cellulose structure derived from wood, we fabricate hierarchical porous cellulose scaffolds combining with cerium-doped TiO2. This hybrid material serves as the support for atomically dispersed Pt catalysts and is used to successfully decompose ethylene at 0 °C. The fabrication concept developed in this work would allow mitigating the conflict between the required large active surfaces and the difficulties in handling nanopowders in environmental catalysis, including food preservation and indoor air purification. We thus discover a promising route to manufacture multifunctional materials with complex structures by combining a controllable chemical synthesis with the nature-designed wood scaffold.

Bioinspired Struvite Mineralization for Fire-Resistant Wood.

High-performance wood materials have attracted significant attention in recent years because of excellent property profiles achieved by relatively easy top-down processing of a renewable resource. A crucial flaw of the renewable wood scaffolds is the low flame retardancy, which we tackled by bioinspired mineralization in an eco-friendly processing step. The formation of the biomineral struvite, commonly found in urinary tract stones, was used for the infiltration of hierarchical wood structures with the necessary ions followed by an in situ synthesis of struvite by ammonium steam fumigation. Struvite decomposes prior to wood, which absorbs heat and releases nonflammable gas and amorphous MgHPO4 resulting from the degradation, which promotes insulating char formation. As a result, the mineralized wood can hardly be ignited and the treatment strongly suppresses the heat release rate and smoke production.

Bond Performance of Sand Coated UHM CFRP Tendons in High Performance Concrete.

The bond behaviour of novel, sand-coated ultra-high modulus (UHM) carbon fibre reinforced polymers (CFRP) tendons to high performance concrete (HPC) was studied by a combined numerical and experimental approach. A series of pull-out tests revealed that the failure type can vary between sudden and continuous pull-out depending on the chosen sand coating grain size. Measuring the same shear stress vs. tendon draw-in (τ-δ) curves in the same test set-up, for sand coated CFRP tendons with a longitudinal stiffness of 137 and 509 GPa, respectively, indicated that the absolute bond strength in both cases was not influenced by the tendon's stiffness. However, the τ-δ curves significantly differed in terms of the draw-in rate, showing higher draw-in rate for the UHM CFRP tendon. With the aid of X-ray computed tomography (CT), scanning electron microscopy (SEM) and visual analysis methods, the bond failure interface was located between the CFRP tendon and the surrounding sand-epoxy layer. For further investigation, a simplified finite element analysis (FEA) of the tendon pull-out was performed using a cohesive surface interaction model and the software Abaqus 6.14. A parametric study, varying the tendon-related material properties, revealed the tendon's longitudinal stiffness to be the only contributor to the difference in the τ-δ curves found in the experiments, thus to the shear stress transfer behaviour between the CFRP tendon and the concrete. In conclusion, the excellent bond of the sand-coated UHM CFRP tendons to HPC as well as the deeper insight in the bond failure mechanism encourages the application of UHM CFRP tendons for prestressing applications.

Three-dimensional discrete element modeling of triggered slip in sheared granular media.

This paper reports results of a three-dimensional discrete element method modeling investigation of the role of boundary vibration in perturbing stick-slip dynamics in a sheared granular layer. The focus is on the influence of vibration within a range of amplitudes and on the fact that above a threshold early slip will be induced. We study the effects of triggering beyond the vibration interval and their origins. A series of perturbed simulations are performed for 30 large slip events selected from different reference runs, in the absence of vibration. For each of the perturbed simulations, vibration is applied either about the middle of the stick phase or slightly before the onset of a large expected slip event. For both cases, a suppression of energy release is on average observed in the perturbed simulations, within the short term following the vibration application. For cases where vibration is applied in the middle of the stick phase, a significant clock advance of the large slip event occurs. In the long term after vibration, there is a recovery period with higher-energy release and increased activity in the perturbed simulations, which compensates for the temporary suppression observed within the short term.

Hysteresis in swelling and in sorption of wood tissue.

The swelling and shrinkage of four Picea abies (L. Karst) wood tissue homogeneous samples, of porosity varying between 45% and 78%, is documented with high-resolution synchrotron radiation phase-contrast X-ray tomographic microscopy. We report measurements of the reversible moisture-induced orthotropic swelling/shrinkage strains. Hysteresis is observed when the swelling/shrinkage strain is considered as a function of relative humidity, except for the very high porosity sample. Hysteresis is no longer present when swelling/shrinkage strains are considered versus moisture content, indicating that wood deforms to the same extent whether an amount of moisture is desorbed or adsorbed. Furthermore, swelling anisotropy, in the tangential and radial directions, is found to increase with increasing porosity. The most homogeneous behaviour for a group of cells is found for 30-50 cells, smaller/larger groups having higher orders of variations.

Time reversal reconstruction of finite sized sources in elastic media.

The ability of the time reversal process to reconstruct sources of finite size relative to a wavelength is investigated. Specifically the quality of the spatial reconstruction of a finite sized source will be presented through the use of time reversal experiments conducted on an aluminum plate. The data presented in the paper show that time reversal can reconstruct a source equally well regarding less of its size, when the source is a half wavelength or less in size. The quality of spatial reconstruction when the source is larger than a half wavelength progressively decreases with the size of the source.

Experimental implementation of reverse time migration for nondestructive evaluation applications.

Reverse time migration (RTM) is a commonly employed imaging technique in seismic applications (e.g., to image reservoirs of oil). Its standard implementation cannot account for multiple scattering/reverberation. For this reason it has not yet found application in nondestructive evaluation (NDE). This paper applies RTM imaging to NDE applications in bounded samples, where reverberation is always present. This paper presents a fully experimental implementation of RTM, whereas in seismic applications, only part of the procedure is done experimentally. A modified RTM imaging condition is able to localize scatterers and locations of disbonding. Experiments are conducted on aluminum samples with controlled scatterers.

Hysteretic swelling of wood at cellular scale probed by phase-contrast X-ray tomography.

We investigated the three-dimensional, microscopic, dimensional changes of Picea abies (L. Karst) wood samples due to controlled steps of the ambient relative humidity. The study was performed at the wood cellular scale by high-resolution synchroton radiation phase-contrast X-ray tomographic microscopy (srPCXTM). Tomographic images were taken after the samples achieved moisture equilibrium at five adsorption and four desorption steps. For spruce latewood, swelling and shrinkage are found to be larger, more hysteretic and more homomorphic than for earlywood. Furthermore, while latewood undergoes similar strains in the transverse directions, earlywood radial strains are less than a third of the tangential strains. The less homomorphic and smaller swelling/shrinkage of earlywood in radial direction is found to be caused by the presence of rays.