Retention behaviour of natural clayey materials at different temperatures

The water retention capacity of geomaterials, and especially clayey soils, is sensitive to temperature changes as the physical mechanisms of retention, such as capillarity or adsorption, are affected by it. It is therefore a major issue to be able to define temperature-dependent behaviour of materials, especially for geo-energy and geo-environmental applications involving non-isothermal conditions. This paper presents results of experiments conducted on two representative materials: a hard clay (Opalinus clay) and a plastic clay (Boom clay), both of which have been considered as buffer materials for underground radioactive waste disposal, in Switzerland and Belgium, respectively. Two new devices were developed for this purpose to permit the analysis of water retention behaviour at different temperatures. The behaviour of these two materials at ambient (20°C) and high temperature (80°C) was observed and described through the evolution of the degree of saturation, the water content and the void ratio with respect to suction. It appears that the retention capability of the clays reduces significantly with an increase in temperature; on the other hand, the change in temperature had less of an effect on the total volume variatio

Definition and experimental determination of a soil-water retention surface

This paper deals with the definition and determination methods of the soil-water retention surface (SWRS), which is the tool used to present the hydromechanical behaviour of soils to highlight both the effect of suction on the change in water and total volumes and the effect of deformation with respect to the water retention capability. An experimental method is introduced to determine the SWRS and applied to a clayey silty sand. The determination of this surface is based on the measurement of void ratio, suction, and water content along the main drying paths. These paths are established for five different initial states. The experimental results allow us to define the parametric equations of the main drying paths, expressing both water content and void ratio as functions of suction and initial void ratio. A model of the SWRS for clayey silty sand is established in the space (void ratio - suction - water content). This surface covers all possible states of the soil inside the investigated range for the three variables. Finally, the SWRS is used to study the relations between water content and suction at a constant void ratio and between void ratio and suction at a constant water content

Revealing lithium-silicide phase transformations in nano-structured silicon-based lithium ion batteries via in situ NMR spectroscopy.

Nano-structured silicon anodes are attractive alternatives to graphitic carbons in rechargeable Li-ion batteries, owing to their extremely high capacities. Despite their advantages, numerous issues remain to be addressed, the most basic being to understand the complex kinetics and thermodynamics that control the reactions and structural rearrangements. Elucidating this necessitates real-time in situ metrologies, which are highly challenging, if the whole electrode structure is studied at an atomistic level for multiple cycles under realistic cycling conditions. Here we report that Si nanowires grown on a conducting carbon-fibre support provide a robust model battery system that can be studied by (7)Li in situ NMR spectroscopy. The method allows the (de)alloying reactions of the amorphous silicides to be followed in the 2nd cycle and beyond. In combination with density-functional theory calculations, the results provide insight into the amorphous and amorphous-to-crystalline lithium-silicide transformations, particularly those at low voltages, which are highly relevant to practical cycling strategies.K.O acknowledges a research fellowship from Japanese Society for the Promotion of Science (JSPS). E.S acknowledges support by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme and thanks Churchill College (Cambridge, UK) for a non-stipendiary Raymond and Beverly Sackler Research fellowship. C.J.K and A.E.F acknowledge a research studentship from the Cambridge Nano Science and Technology Doctoral Training Centre (NanoDTC). A.J.M acknowledges the support from the Winton Programme for the Physics of Sustainability. S.H acknowledges funding from ERC grant InsituNANO (project number 279342). C.P.G and C.D thank the Royal Society, and C.P.G thanks European Research Council (ERC). C.P.G. acknowledges support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under Contract DE-AC02-05CH11231, subcontract 6952000.This is the accepted manuscript. The final version is available from Nature Communications at http://www.nature.com/ncomms/2014/140203/ncomms4217/full/ncomms4217.html

A tomographic imagery segmentation methodology for three-phase geomaterials based on simultaneous region growing

peer reviewedX-Ray Computed Tomography (X-Ray CT) is a powerful non-destructive technique used in many domains to obtain the three-dimensional representation of objects, starting from the reconstitution of two-dimensional images of radiographic scanning. This technique is now able to analyze objects within a few microns resolution. Consequently, X-Ray micro-computed tomography (X-Ray μCT) opens perspectivesfor the analysis of the fabric of multi-phase geomaterials such as soils, concretes, rocks or ceramics. To be able to characterize the spatial distribution of the different phases in such complex and disordered materials, automated phase recognition has to be implemented through image segmentation. A crucial difficulty in segmenting images lies in the presence of noise in the obtained tomographic representation, making it difficult to assign a specific phase to each voxel (vx) of the image. In the present study, simultaneous region growing is used to reconstitute the three-dimensional segmented image of granular materials. First, based on a set of expected phases in the image, regions where specific phases are sure to be present are identified, leaving uncertain regions of the image unidentified. Subsequently, the identified regions are grown until growing phases meet each other with vanishing unidentified regions. The methodrequires a limited number of manual parameters that are easily determined. The developed method is illustrated based on three applications on granular materials, comparing the phase volume fractions obtained by segmentation with macroscopic data. It is demonstrated that the algorithm rapidly converges and fills the image after a few iterations

Micro-scale investigation of unsaturated sand in mini-triaxial shearing using X-ray CT

peer reviewedThis paper explores the micro characteristics of unsaturated sand in triaxial shearing by using X-ray computed tomography (X-ray CT). To obtain higher resolution, a mini-triaxial set-up is designed in which the sample is miniaturised to 1 cm in diameter and 2 cm long, allowing scans with a pixel size of 9 μm. Samples are sheared in the mini-triaxial set-up at different constant suction levels (therefore, different degree of saturation). In the meanwhile, the samples are scanned by X-ray CT at various deformation stages, about 0, 2, 5, 10 and 15% axial strain. The three-dimensional (3D) reconstructed image is trinarised based on a region growing technique, which gives access to the microstructure of the solid, liquid and air phases. Then, the 3D image is subdivided into representative volume elements, with length ≈3·8D50, which gives local information of degree of saturation and porosity. It is observed that the sample dilates and water drains out during triaxial test under constant suction condition. The local study shows that the porosity increase and water desaturation are more significant in the middle part of the sample, especially for a higher suction value. This work allows the emphasis of the coupling between dilatancy on shearing (highlighted by the evolution of local porosity) and the evolution of the local degree of saturation in unsaturated granular materials

Nanoemulsion stability: experimental evaluation of the flocculation rate from turbidity measurements

The coalescence of liquid drops induces a higher level of complexity compared to the classical studies about the aggregation of solid spheres. Yet, it is commonly believed that most findings on solid dispersions are directly applicable to liquid mixtures. Here, the state of the art in the evaluation of the flocculation rate of these two systems is reviewed. Special emphasis is made on the differences between suspensions and emulsions. In the case of suspensions, the stability ratio is commonly evaluated from the initial slope of the absorbance as a function of time under diffusive and reactive conditions. Puertas and de las Nieves (1997) developed a theoretical approach that allows the determination of the flocculation rate from the variation of the turbidity of a sample as a function of time. Here, suitable modifications of the experimental procedure and the referred theoretical approach are implemented in order to calculate the values of the stability ratio and the flocculation rate corresponding to a dodecane-in-water nanoemulsion stabilized with sodium dodecyl sulfate. Four analytical expressions of the turbidity are tested, basically differing in the optical cross section of the aggregates formed. The first two models consider the processes of: a) aggregation (as described by Smoluchowski) and b) the instantaneous coalescence upon flocculation. The other two models account for the simultaneous occurrence of flocculation and coalescence. The latter reproduce the temporal variation of the turbidity in all cases studied (380 \leq [NaCl] \leq 600 mM), providing a method of appraisal of the flocculation rate in nanoemulsions

Cationic surfactants for demulsification of produced water from alkaline-surfactant-polymer flooding

In this research, demulsification of produced water (which is an oil-in-water emulsion) from alkaline–surfactant–polymer flooding, containing sodium alkyl sulfate, was evaluated using five different surfactants from the classes of nonionic, amphoteric, and cationic. It was observed that only single-tail cationic surfactants, namely, dodecyltriemthylammonium chloride (DTAC) and alkyltrimethylammonium bromide (ATAB), with a concentration of 1000 ppm, were capable of attaining transparent separated water phases following 3 h separation at room temperature with relative separation efficiencies, determined using fluorescence spectroscopy, of 89.4 and 59%, respectively. However, the cationic surfactant dimethyldioctadecylammonium chloride (DDOAC) could achieve a relative separation efficiency of only 28.4% after 13 days, in contrast to nonionic and amphoteric surfactants that did not reveal any progress in demulsification. Similarly, given the demulsifier concentration of 1000 ppm, only DTAC and ATAB reduced the negative surface charge of oil droplets in the produced water after 3 h separation, and large droplets were formed owing to their coalescence after the addition of the respective demulsifiers as viewed by optical microscopy. The dominant emulsification mechanism is believed to be electrostatic stabilization. Consequently, the proposed demulsification mechanism is the formation/adsorption of cationic–anionic pairs at the oil–water interface. When comparing the demulsification performances of various demulsifying surfactants, although high interfacial activity (low interfacial tension (IFT)) is an essential feature for a demulsifier to be considered effective, it was concluded that lower equilibrium IFT does not necessarily result in superior separation efficiency, and other parameters such as type, tail branch number (i.e., single-tail or double-tail), and purity of surfactant may have profound effects on both separation efficiency and demulsification speed of the emulsion. From the dynamic IFT data, it was realized that DTAC and ATAB caused faster demulsification than DDOAC. The undesirable demulsification performance of DDOAC might have been due to its double-tail structure, which confined its interfacial adsorption. The operational variables, including salinity, pH, and temperature, in the demulsification process by DTAC were optimized with respect to the changes of IFT, and the optimum values were found to be 2 wt %, 7.0, and 35 °C, respectively

Academic writing for publication: Putting the ‘international’ into context

There is a growing body of research on the impact of English-medium publication and associated higher education regimes on knowledge construction. However, not much is known about how academics outside the Global North make decisions about how and where to publish. Through a comparative case study, this article sets out to explore how academics in Ethiopia and Oman engage in writing for publication. Taking an academic literacies lens, the analysis reveals that their decisions were shaped by institutional values at the local level, as well as global hierarchies around knowledge construction. However, issues around identity, languages and disciplinary cultures also influenced how academics chose to position themselves in relation to local and international journals. The findings point to the need for new partnerships between journals in the Global North and South to prevent ‘publication drain’, and for universities to explore ways to address inequalities perpetuated through journal ranking and language hierarchies

Delithiation/lithiation behavior of LiNi<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> studied by in situ and ex situ ^6,7Li NMR spectroscopy

Delithiation and lithiation behaviors of ordered spinel LiNi0.5Mn1.5O4 and disordered spinel LiNi0.4Mn1.6O4 were investigated by using in situ (in operando) 7Li NMR and ex situ 6Li MAS NMR spectroscopy. The in situ 7Li monitoring of the ordered spinel revealed a clear appearance and subsequent disappearance of a new signal from the well-defined phase Li0.5Ni0.5Mn1.5O4, suggesting the two-phase reaction processes among Li1.0Ni0.5Mn1.5O4, Li0.5Ni0.5Mn1.5O4, and Li0.0Ni0.5Mn1.5O4. Also, for the disordered spinel, Li0.5Ni0.4Mn1.6O4 was identified with a broad distribution in Li environment. High-resolution 6Li MAS NMR spectra were also acquired for the delithiated and lithiated samples to understand the detailed local structure around Li ions. We suggested that the nominal Li-free phase Li0.0Ni0.5Mn1.5O4 can accommodate a small amount of Li ions in its structure. The tetragonal phases Li2.0Ni0.5Mn1.5O4 and Li2.0Ni0.4Mn1.6O4, which occurred when the cell was discharged down to 2.0 V, were very different in the Li environment from each other. It is found that 6, 7Li NMR is highly sensitive not only to the Ni/Mn ordering in LiNi0.5Mn1.5O4 but also to the valence changes of Ni and Mn on charge-discharge process