808 research outputs found
Re-appraisal of the stratigraphy and determination of new U-Pb dates for the Sterkfontein hominin site, South Africa
No full textSterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 ± 0.28 and 2.6 ± 0.30 Ma and Member 4 between 2.65 ± 0.30 and 2.01 ± 0.05 Ma. The age of 2.01 ± 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of ∼2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes
Diffusion across particle-laden interfaces in Pickering droplets
No full textEmulsions stabilized by nanoparticles, known as Pickering emulsions, exhibit remarkable stability, which enables applications ranging from encapsulation, to advanced materials, to chemical conversion. The layer of nanoparticles at the interface of Pickering droplets is a semi-permeable barrier between the two liquid phases, which can affect the rate of release of encapsulates, and the interfacial transfer of reactants and products in biphasic chemical conversion. A gap in our fundamental understanding of diffusion in multiphase systems with particle-laden interfaces currently limits the optimal development of these applications. To address this gap, we developed an experimental approach for in situ, real-time quantification of concentration fields in Pickering droplets in a Hele-Shaw geometry and investigated the effect of the layer of nanoparticles on diffusion of solute across a liquid–liquid interface. The experiments did not reveal a significant hindrance on the diffusion of solute across an interface densely covered by nanoparticles. We interpret this result using an unsteady diffusion model to predict the spatio-temporal evolution of the concentration of solute with a particle-laden interface. We find that the concentration field is only affected in the immediate vicinity of the layer of particles, where the area available for diffusion is affected by the particles. This defines a characteristic time scale for the problem, which is the time for diffusion across the layer of particles. The far-field concentration profile evolves towards that of a bare interface. This localized effect of the particle hindrance is not measurable in our experiments, which take place over a much longer time scale. Our model also predicts that the hindrance by particles can be more pronounced depending on the particle size and physicochemical properties of the liquids and can ultimately affect performance in applications.ChemE/Transport Phenomen
Shifting and persosting in the face of failure: Learning from what did not work
No full textSocial justice activism demands coordinated, concentrated efforts to move the needle in a positive direction. In the author's nine years as a social justice educator, he led multiple large- and small-scale projects for social justice within higher education. In many ways, those efforts failed to create a lasting impact. In the higher education ecosystem, they also took away time from the kind of promotable work which would benefit his case for tenure and promotion. Trying - and failing - to effect institutional changes left him emotionally, psychologically, and physically exhausted. Beyond that, he suffered from feelings of loneliness, exclusion, and lack of direction. For a long time, the author blamed himself for the failure to change the institution to be a place in which he felt comfortable. He also failed to cope with these negative experiences and emotions, often seething in frustration or anger or avoiding similar situations of vulnerability or creativity. It took years for him to remember and internalize lessons of persistence and shifting appraisals in order to maintain motivation for action and survive the stressors of working within an oppressive system.Published versio
Reseñas
No full textObra ressenyada: Robyn S. METCALFE, Meat, Commerce and the City: The London food market, 1800-1850. Londres: Pickering & Chatto Publishers, 2012
Reseñas
No full textObra ressenyada: Robyn S. METCALFE, Meat, Commerce and the City: The London food market, 1800-1850. Londres: Pickering & Chatto Publishers, 2012
Board of County Commissioners v. Umbehr: The Inadequacies of Extending Pickering Analysis to Government Contractors
No full textThis Note challenges the restrictive First Amendment free speech protection that the Supreme Court gave to government contractors in Board of County Commissioners v. Umbehr when it applied the Pickering balancing test, developed nearly thirty years ago in Pickering v. Board of Education in the context of government employees. It does so by first questioning whether the First Amendment free speech protections given to government employees should be similar for government contractors. It then explores whether the Pickering balancing test should be applied to cases involving government contractors as it was in Umbehr.
The author concludes that the Court improperly restricted the First Amendment free speech rights of government contractors by failing to take into account the numerous modifications the Court has made to the Pickering balancing test that have gradually eroded the broader protections of Pickering and by failing to account for the potentially significant differences between public employees and government contractors. The author thus proposes a new test that would both incorporate the Court\u27s views regarding the free speech rights of government employees and account for the potentially significant differences between public employees and government contractors
Fabrication of edible biocompatible carbohydrate particle-based Pickering emulsions
No full textDifferent from conventional emulsions, Pickering emulsions are stabilized by interfacially-adsorbed solid particles. Pickering emulsions have gained great interest in the past decades due to their high stability against coalescence, Ostwald ripening, and the possibility to avoid the deleterious effects linked to emulsifiers used in conventional emulsions. To fulfill the strong market trend of formulating products that are not only edible in theory, but also maintain the consumer perception of being natural, “clean label” and “green”, this study was dedicated to developing particles from biomass-based resources to form Pickering emulsions with potential application in food, cosmetic and pharmaceutic products. As a simple, organic solvent-free process, media-milling was applied to modify two major biomass materials, native starch and cellulose. Three maize starches (normal maize starch, high-amylose maize starch and waxy maize starch) with different amylose/amylopectin ratios were physically modified through media milling process to form milled starch particles. The physiochemical properties of these starches during milling process, including particle size, crystallinity and gelatinization properties were studied. Emulsions stabilized by milled starch particles with different amylose/amylopectin ratios exhibited significant difference in terms of stabilization capability and rheological properties. Milled high-amylose maize starch particles have the best stabilization ability, followed by milled normal maize starch particles. Furthermore, the stabilization capacity of the milled starch particles improves with the increase of milling time. To investigate the feasibility of Pickering emulsions stabilized by milled starch as a novel food-grade formulation for encapsulation and delivery of lipophilic bioactive compounds, curcumin was selected as model delivery target and encapsulated in the oil phase of Pickering emulsion. The digestion profile of curcumin-loaded Pickering emulsion was studied using three in vitro digestion models, simulated static small intestinal digestion model, pH-stat lipolysis model and TNO’s gastrointestinal model (TIM-1). Simulated static small intestinal digestion model and pH-stat model indicated that the bioaccessibility of curcumin encapsulated in Pickering emulsion was enhanced compared with free curcumin suspended in bulk oil phase. A significant improvement of curcumin bioaccessbility was also observed in an emulsion system vs in bulk oil when using TIM-1 model, which simulates the entire human GI tract. Overall, the study's findings showed that curcumin encapsulated in Pickering emulsion stabilized by milled starch possesses benign stability against harsh gastric conditions as well as improved dissolution profiles in small intestinal tract. All are suggested that Pickering emulsion stabilized by milled starch exhibit high potential as encapsulate and delivery system for lipophilic bioactive compounds. Milled cellulose particles of sizes ranging from 38 nm to 671 nm with rod-like shapes have also been successfully fabricated using media milling. Media milling process led to a notable decrease in the particle size and crystallinity of milled cellulose particles with the increase of milling time. The milled cellulose particles were irreversibly adsorbed at the oil/water interface and formed stable emulsions with droplet size around 60~42 µm, which exhibited benign stability over a month storage. Milled cellulose stabilized emulsions also exhibited good stability against a wide range of pH (3, 5, 7, 9) and salt conditions (0.1~100 mM) with slight change in the droplet size. The rheological tests indicated the formation of gel network in the emulsions, which promoted the stability of the emulsions. The in vitro digestion profile and phase behavior of Pickering emulsions stabilized by milled cellulose were evaluated to investigate their feasibility for encapsulation and delivery of lipophilic bioactive compounds. Curcumin encapsulated in Pickering emulsions exhibited benign stability with less than 50% degraded after storage of 30 days. The digestion behavior of emulsions under simulated small intestinal conditions was characterized using a pH-stat lipolysis model. The digestion profiles of emulsions were markedly dependent on the type of lipid and digestion buffer employed in lipid digestion experiments. The rate and extent of lipolysis of emulsions with medium chain triglycerides (MCT) was greater than emulsions with long chain triglycerides (soy bean and canola oil), reaching complete hydrolysis during lipolysis process independent of bile salt and phospholipids concentration. The structure changes of emulsions during digestion were analyzed using optical and fluorescent imaging. Although the initial digestion rate of curcumin encapsulated in Pickering emulsions with soy bean and canola oil was slower than the corresponding conventional emulsions stabilized by Tween/Span, their total extent of lipolysis was higher than that of conventional emulsions under both fasted and fed intestinal digestion conditions. The bioaccessibility of curcumin encapsulated in Pickering emulsions was higher than in corresponding surfactant stabilized conventional emulsions. High-amylose maize starch with different fatty acids (C12:0, C14:0, C16:0, C18:0, C18:1) were complexed using two heat-moisture methods. The structure properties of different starch-fatty acid complexes, including size and shapes of nanoscale supramolecular structures formed, through heat and moisture treatment, were studied. Optical microscope and SEM analysis showed that starch-fatty acid complexes retained the Maltese cross and granular morphology of native starch. X-ray diffraction revealed the crystalline morphology of starch-fatty acid complexes with B- and V-type crystallinity. And the crystallinity of the complexes varied depending on the fatty acids and methods used. USAXS/SAXS experiments demonstrated that fatty acid chain length and level of saturation affected both the lamellar structure as well as the B-type crystalline of the complexes. Moreover, the processing methods also exhibited major influence on the nanostructure of complexes. The resistance of these starch complexes against enzymatic hydrolysis was increased based on the in vitro digestion measurements. And the hydrophobicity of these complexes was enhanced. This was manifested by increased contact angles. The capacity of these starch-fatty acid complexes to form Pickering emulsions was characterized. Starch-saturated fatty acid complexes were able to form stable emulsions that endured heat treatment of 60, 80 and 100℃. However, starch-unsaturated fatty acid complexes could not form stable emulsions. The barrier properties of these emulsions could be adjusted by heat treatment, which led to swelling of starches. Lipolysis profile of PMFs loaded emulsions suggested that certain heat treatment could reduce the accessibility of lipase towards oil droplets and release of PMFs during lipolysis by enhancing the coverage of granules onto the oil-water interface. In conclusion, particles derived from biomass resources starch and cellulose have been successfully fabricated to form Pickering emulsions using simple, environmental-friendly procedures. The resulting formulations were edible, ‘green’, have exhibited exceptional stability and a practical potential to encapsulate and control release of lipophilic ingredients, making them suitable for various applications in cosmetic, food and pharmaceutical industry.Ph.D.Includes bibliographical referencesby Xuanxuan L
Using the fossil charcoal and pollen records from Elands Bay Cave and Boomplaas Cave, South Africa, to reconstruct variability in local hydroclimate and seasonality
No full textGlobal climate change is well documented and recorded by various marine and terrestrial archives, predominantly by benthic ocean sediment and ice cores. These records are characterized by cyclical variation, suggesting changing polar ice volumes from glacial to interglacial conditions, driven by Milankovich cycles and greenhouse gas concentration feedbacks. While these records have informed much of our understanding of past climate variability, they are from either the deep ocean or high latitudes, with a huge bias to the Northern Hemisphere. More terrestrial, mid latitude, Southern Hemisphere (SH) palaeoclimate records are needed to plug this gap in our knowledge. With a rapidly changing and uncertain climate future, these new records should intersect with past records of human activity. South Africa (SA) has a huge geographic advantage, sitting at the confluence of the Southern Atlantic and Indian Ocean systems and hosting an almost unprecedented rich record of human occupancy spanning the last 2.6 Ma, but particularly the Last Glacial Maximum (LGM). An equator-wards shift in the SH Westerlies has long been used to account for increased precipitation in SA during the LGM. While this narrative is challenged by a few records from the Southern Cape, which suggest aridity during the LGM, it remains an overarching idea. In this thesis, I test these two apparently contradictory climate responses by using fossil charcoal and pollen records from two archaeological sites: Boomplaas Cave (BPC) in the Year-round Rainfall Zone (YRZ), and Elands Bay Cave (EBC) in the Winter Rainfall Zone (WRZ). By quantifying changes in Mean Annual Temperature (MAT), Total Annual Precipitation (TAP), and Dry Quarter Precipitation (DQP), I explore the potential of using archaeological charcoal to reconstruct a palaeoclimate throughout the LGM in SA. Specifically, I use a Canonical Correspondence Analysis to interrogate the species response to modern climate and a Weighted Average – Partial Least Squared regression to reconstruct paleoclimate. These results display the promising ability of fossil charcoal records to represent changes in hydroclimate given a few conditions. Both sites record an average decrease in MAT of 5°C at the LGM compared to modern conditions, consistent with existing regional records. Precipitation results from BPC show ~200mm less precipitation around the LGM compared to present day, this is consistent with other proxies from the Southern Cape, as well as global climate model results suggesting decreased precipitation in the region. The EBC charcoal record proved its ability to reconstruct MAT trends and (given a few conditions) the potential to reproduce TAP results. Interestingly, at the LGM, the EBC pollen record suggests a decrease in TAP of ~150mm, which does not agree with existing records (see chapter 2). This decrease in LGM TAP recorded at both sites, together with reconstructions of DQP are used to argue that the WRZ did not expand to reach BPC during the LGM. Overall, the new records present here are more consistent with a pole-wards shift in the SH Westerlies, challenging the long-held notion of a wet LGM in SA
An investigation of the drying process in pickering foams
No full textPickering foams, aqueous foams stabilized by solid particles, can be used as a precursor to fabricate solid porous polymers, ceramics, and composite materials. The fabrication process usually comprises of two subsequent steps of drying and solidification (e.g. sintering) after foam is prepared. Drying (or aging) in aqueous Pickering foams is a complex transport process which involves evaporation and drainage (induced by gravity) of the excess liquid along with deformation of the foam and possible formation of cracks caused by capillary-induced stresses. Crack formation is therefore depended on the drying conditions and the mechanical properties of sample. A comprehensive understanding of the drying process can provide us with predictive tools to select efficient process parameters (e.g. required drying time prior to sintering) as well as providing input prameters for developing numerical models. In this thesis, we first provide experimental data (end of drying time, average moisture content, and effective moisture diffusivity) on drying of Pickering foams stabilized by polymer particles under controlled conditions (i.e. relative humidity and temperature). Drying curves are presented for samples of various initial thicknesses and shapes on substrates of different hydrophobicity and temperatures. Moisture transport is represented via calculating the effective moisture diffusivity coefficients using method of slope. Also, we investigate drying for a bi-component Pickering foam prepared using Multi-walled Carbon Nanotubes (MWCNTs) and polymer particles. We show that the effective moisture diffusivity increases as the average moisture content decreases for all trials and that all data can be collapsed on a master curve. Also, effective moisture diffusivity increased as initial sample thickness as well as substrate temperature increase. On the other hand, effective moisture diffusivity does not depend strongly on the sample shape and MWCNTs concentration. In next part, we explore the effect of initial sample thickness and shape, substrate temperature and wettability, as well as MWCNTs concentration on the crack formations in the samples. We demonstrate that substrate wettability, initial sample thickness, and MWCNTs concentration have a strong influence on the formation and propagation of cracks. We found that decreasing the wettability of the substrate reduces crack formation. Also, increasing the initial sample thickness reduces crack formation. On the contrary, increasing the MWCNTs concentration increases crack formation for all types of substrates. We demonstrate that substrate temperature and sample shape do not seem to influence crack formation but these parameters influence the crack patterns. Such information can be beneficial in using these foams as a precursor to fabricate porous composite porous materials.Ph.D.Includes bibliographical referencesby Omer S. Alabidalkree
Life after Whitehall: The career moves of British special advisers
No full textsponsorship: The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publica-tion of this article: Thanks to the Flanders Research Foundation (FWO) grant number G079619N and KU Leuven, Belgium, for supporting the participation of Heath Pickering in this work. (Flanders Research Foundation (FWO), KU Leuven, Belgium|G079619N)status: Publishe
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