Wind-dispersed plants have evolved ingenious ways to lift their seeds 1,2. The common dandelion uses a bundle of drag-enhancing bristles (pappus) to help keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances 3,4 ; however, the engineering underpinning pappus-mediated flight remains unresolved. Here, we have visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached due to the flow passing through the pappus. We hypothesized that the circular disk-like geometry and the porosity of the pappus are the key design features that enable the formation of the separated vortex ring. The porosity gradient was surveyed using microfabricated disks, and a disk with a similar porosity was found able to recapitulate the flow behaviour of the real pappus. The porosity of the dandelion's pappus appears to be tuned precisely to stabilize the vortex, while maximizing the aerodynamic loading and minimizing the material requirement. The discovery of the separated vortex ring signals the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction, and particle retention of biological and manmade structures. Dandelions (Taraxacum officinale agg.) are highly successful perennial herbs, which can be found in temperate zones all over the world 5. Dandelions, like many other members of the Asteraceae family, disperse their bristly seeds using the wind and convective updrafts 6,7. Most dandelion seeds likely land within 2 m 8,9 ; however, in warmer, drier and windier conditions, some may fly further (up to 20,000 seeds per hectare travelling more than 1 km by one estimate) 6,10. Asteraceae seeds routinely disperse over 30 km and occasionally even 150 km 3,4. Plumed seeds comprise a major class of dispersal strategies used by numerous and diverse groups of flowering plants, of which the common dandelion is a representative example. Plumed seeds contain a bundle of bristly filaments, called a pappus, which are presumed to function in drag enhancement (Fig. 1a-c). The pappus prolongs the descent of the seed, so that it may be carried farther by horizontal winds 11 , and it may also serve to orientate the seed as it falls 7,12. Dandelion seeds fall stably at a constant speed in quiescent conditions 2,13-15. For wind-dispersed seeds, maintaining stability while maximizing descent time in turbulent winds may be useful for long-distance dispersal 16,17. It is not clear, however, why plumed seeds have opted for a bristly pappus rather than a wing-like membrane, which is known to enhance lift in some other species (e.g., maples 1). Here, we uncover the flight mechanism of the dandelion, characterizing the fluid dynamics of the pappus and identifying the key structural features enabling its stable flight. To examine the flow behaviour around the pappus, we built a vertical wind tunnel (Fig. 1d, and M1), designed so that the seed ca...
Pressure-solution creep is one of the most common crustal deformation mechanisms, inducing changes in the porosity and permeability of rocks. For a variety of rock types undergoing pressure solution, it has been shown that the presence of phyllosilicates may significantly enhance the rate of the pressure-solution process. In this experimental investigation, we present 4dimensional (three dimensions + time) X-ray microtomographic data that contrast deformation by pressure-solution of a pure NaCl aggregate with that of a mixture of NaCl and biotite. The results show that for mixed samples (NaCl+biotite), phyllosilicates induce a marked reduction in porosity and pore connectivity and contribute to an increase in the local strain rates by an order of magnitude over pure NaCl samples. At the same time, phyllosilicates do not induce strain localization in the sample. We discuss various possible explanations for these observations including a possible positive feedback between the porosity distribution and pressure solution. Our study yields novel insights into the local effects of phyllosilicates during pressure-solution creep and provides full 4-dimensional imaging and characterization of the coupled evolution of porosity and pore connectivity over previously unprecedented experimental time scales.
Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.
Weathering of silicate-rich industrial wastes such as slag can reduce emissions from the steelmaking industry. During slag weathering, different minerals spontaneously react with atmospheric CO2 to produce calcite. Here, we evaluate the CO2 uptake during slag weathering using image-based analysis. The analysis was applied to an X-ray computed tomography (XCT) dataset of a slag sample associated with the former Ravenscraig steelworks in Lanarkshire, Scotland. The element distribution of the sample was studied using scanning electron microscopy (SEM), coupled with energy-dispersive spectroscopy (EDS). Two advanced image segmentation methods, namely trainable WEKA segmentation in the Fiji distribution of ImageJ and watershed segmentation in Avizo ® 9.3.0, were used to segment the XCT images into matrix, pore space, calcite, and other precipitates. Both methods yielded similar volume fractions of the segmented classes. However, WEKA segmentation performed better in segmenting smaller pores, while watershed segmentation was superior in overcoming the partial volume effect presented in the XCT data. We estimate that CO2 has been captured in the studied sample with an uptake between 20 and 17 kg CO2/1,000 kg slag for TWS and WS, respectively, through calcite precipitation.
Often carrying a high-volume fraction of vesicles, basaltic rocks can be an important reservoir horizon in petroleum systems, and are considered an excellent candidate for CO2 storage by in situ mineral trapping. The frequency of amygdaloidal basalts in many sequences highlights the prevalence of mineralisation, but when the vesicle network has been filled, the basalts can act as impermeable seals and traps. Characterising the spatial and temporal evolution of the porosity and permeability is critical to understanding the petro-physical properties and CO2 storage potential of basalts. We exploit X-ray computed tomography (XCT) to investigate the precipitation history of an amygdaloidal basalt containing a pore-connecting micro fracture network now partially filled by calcite as an analogue for CO2 mineral trapping in a vesicular basalt. The fracture network likely represents a preferential pathway for CO2-rich fluids during mineralisation. We investigate and quantify the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We provide a semi-quantitative technique for defining reservoir potential and quality through time and understanding sub-surface flow and storage. We found that permeability evolution is dependent on the precipitation mechanism and rates, as well as on the presence of micro fracture networks, and that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro fracture network. These results prompt further studies to determine CO2 mineral trapping mechanisms in amygdaloidal basalts as analogues for CO2 injections in basalt formations.
Pressure-solution creep is an important fluid-mediated deformation mechanism, causing chemo-mechanical transformations and porosity and permeability changes in rocks. The presence of phyllosilicates, in particular, has previously been hypothesized to further reduce porosity and pore connectivity. Nevertheless, a full characterization of the spatiotemporal evolution of permeability during this process has yet to be reported. A pure NaCl aggregate and a mixture of NaCl and biotite were deformed through pressure-solution creep while monitoring their microstructural evolution through computed X-ray microtomography. The evolution of permeability and fluid velocity of the samples were computed by using the pore geometries from the X-ray microtomography as input for the Lattice-Boltzmann modeling.The results indicate that, as deformation proceeds, porosity and permeability decrease in both samples. In the salt-biotite sample pressure solution creep causes the formation of a compaction band perpendicular to the direction of loading, forming a barrier for permeability. Along the other two directions, pore connectivity and permeability are retained in the marginal salt layers, making the sample strongly anisotropic. The presence of biotite controls the way pore coordination number evolves and hence the connectivity of the pathways. Biotite flakes create an enhanced porosity decrease leading to compaction and reduction of pore connectivity. This reduction in porosity affects local stresses and local contact areas, reducing over time the driving force. According to a texture-porosity process, the reduction in porosity causes salt ions to dissolve in the marginal salt and precipitate within the biotite-bearing layer, where the bulk volume of salt grains increases over time.
<p>CO<sub>2</sub> mineralization is a natural process that occurs during weathering of silicate materials that are calcium/magnesium-rich and aluminum-poor (Kelemen et al., 2020). During this process, silicates convert to carbonates, making silicate-rich materials such as ultramafic rocks and alkaline wastes suitable for CO<sub>2</sub> removal from air. &#160;Using slag to sequester CO<sub>2</sub> is particularly attractive as it is a by-product of a key industry, and it can utilize CO<sub>2</sub> from the emission source, therefore reducing the need for CO<sub>2</sub> and slag transportation, or draw down of CO<sub>2</sub> already in the atmosphere. It is estimated that steel slag has the potential to capture ~150-250 Mt CO<sub>2</sub> yr<sup>-1</sup> now, and ~320-870 Mt CO<sub>2</sub> yr<sup>-1</sup> by 2100 (Renforth, 2019).</p><p>Although the chemical composition of alkaline wastes shows that CO<sub>2</sub> capture can significantly offset emissions from corresponding industries, recent observations reveal that the CO<sub>2</sub> uptake in alkaline wastes in underutilized (Pullin et al., 2019). Here, we use image-based analysis to understand the microstructures of CO<sub>2</sub> mineralization in slag. We use X-ray Computed Tomography (XCT) to visualize slag internal structures and to calculate reactive surface area and pore connectivity. We then use scanning electron microscopy (SEM), coupled with energy dispersive spectroscopy (EDS) to study the distribution of elements within the studied sample.</p><p>In our study, we use a slag sample collected from the former Ravenscraig Steelworks in Lanarkshire, Scotland, where steelmaking took place from 1950s until 1992 (Stewart, 2008), leaving behind a slag heap that has been weathering since then. Our analysis demonstrates that calcium carbonate precipitates as pore-lining. Surface passivation and low surface-connected porosity were identified as processes that can cause reduction in CO<sub>2</sub> uptake.</p><p>&#160;</p><p>References</p><p>&#160;</p><p>Kelemen, P.B., McQueen, N., Wilcox, J., Renforth, P., Dipple, G., Vankeuren, A.P., 2020. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights. Chem. Geol. 550, 119628. https://doi.org/10.1016/j.chemgeo.2020.119628</p><p>Pullin, H., Bray, A.W., Burke, I.T., Muir, D.D., Sapsford, D.J., Mayes, W.M., Renforth, P., 2019. Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste. Environ. Sci. Technol. 53, 9502&#8211;9511. https://doi.org/10.1021/acs.est.9b01265</p><p>Renforth, P., 2019. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401. https://doi.org/10.1038/s41467-019-09475-5</p><p>Stewart, D., 2008. Fighting for Survival: The 1980s Campaign to Save Ravenscraig Steelworks. J. Scottish Hist. Stud. 25, 40&#8211;57. https://doi.org/10.3366/JSHS.2005.25.1.40</p>
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