Conductances between confined rough walls

Two- and three-dimensional creeping flows and diffusion transport through constricted and possibly rough surfaces are studied. Asymptotic expansions of conductances are derived as functions of the constriction local geometry. The validity range of the proposed theoretical approximations is explored through a comparison either with available exact results for specific two-dimensional aperture fields or with direct numerical computations for general three-dimensional geometries. The large validity range of the analytical expressions proposed for the hydraulic conductivity (and to a lesser extent for the electrical conductivity) opens up interesting perspectives for the simulation of flows in highly complicated geometries with a large number of constrictions

Quasi-static liquid–air drainage in narrow channels with variations in the gap

This paper studies the shape of an air bubble quasi-statically flowing in the longitudinal direction of narrow channels. Two bottom topographies are treated, i.e., linear and quadratic variations of the gap along the transverse direction. This work analyses the main characteristics of the gas–liquid interface with respect to the wedge aspect ratio. From the convergence of asymptotic, numerical and experimental analyses, we found simple dependences for the finger width and total curvature as a function of channel aspect ratio. These results provide simple and general expressions for the pressure drop needed to overcome capillary forces and push the air finger inside the channel

Les premiers représentants du genre Homo, en quête d’une identité. Apports de l’étude morphologique et de l’analyse cladistique

Il n’existe actuellement aucun consensus concernant la taxinomie et la constitution de l’hypodigme d’Homo habilis Leakey et al. 1964. Quatre hypothèses majeures sont aujourd’hui avancées à partir des restes crâniens et mandibulaires des spécimens attribués classiquement à Homo habilis sensu lato : (1) tous les spécimens appartiennent à un seul et même taxon : Homo habilis ; (2) deux espèces peuvent être identifiées dans ce groupe : Homo habilis sensu stricto et Homo rudolfensis ; (3) ces spécimens n’appartiennent pas au genre Homo mais au genre Australopithecus ; (4) les spécimens de l’espèce rudolfensis devraient être mis dans le genre Kenyanthropus. Le but de cette étude est de réévaluer de façon critique la variabilité au sein de cet ensemble fossile du Plio-Pléistocène et de tester si les spécimens appartiennent bien au genre Homo. Deux types d’étude ont été entrepris : une comparaison morphologique et une analyse cladistique. Dans cette dernière analyse, l’Unité Taxinomique Opérationnelle (OTU) est constituée par le spécimen et non l’espèce, en l’absence de consensus concernant l’attribution taxinomique des spécimens fossiles étudiés. Ces analyses portent sur les pièces originales provenant d’Éthiopie, du Kenya, de Tanzanie, du Malawi et de République d’Afrique du sud. L’étude morphologique permet de mettre en évidence une variabilité considérable au sein des premiers représentants du genre Homo. Deux espèces peuvent être définies au sein de cet ensemble : habilis et rudolfensis. L’analyse cladistique des 122 caractères crâniens montre l’appartenance de ces deux espèces au genre Homo et non aux genres Australopithecus ou Kenyanthropus.No consensus has been achieved concerning the taxonomic significance of the species Homo habilis. Four main hypotheses have been advanced: (1) the specimens from Olduvai, East Turkana and Omo belong to the same palaeospecies:  Homo habilis sensu lato; (2) the hypodigm is heterogenous; two species could be defined : Homo habilis sensu stricto and Homo rudolfensis; (3) these specimens do not belong to the genus Homo but to Australopithecus or (4) Kenyanthropus. The goal of this study is to critically re-evaluate the hypotheses concerning the taxonomy of the specimens attributed to early Homo, and to test whether they belong to the genus Homo or to another genus. A morphological study and numerical cladistic analyses on 122 morphological characteristics were carried out on the original Plio-Pleistocene specimens. The Operational Taxonomic Unit (OTU) is defined by the fossil specimen rather than the species in the absence of consensus concerning the taxonomic attribution of the fossil specimens studied. Indeed, because no consensus concerning the hypodigm of Homo habilis has been achieved, the creation of OTU on the basis of shared anatomy would have introduced circularity into our analysis. The results of these analyses show that: (a) two species could be defined: habilis and rudolfensis; (b) the specimens belonging to these two taxa are included in the clade of Homo ; (c) the conclusions concerning the revision of the genus Homo and the inclusion of the specimens of habilis and rudolfensis to the genus Australopithecus or Kenyanthropus are questioned

The quaternary boundary: 1.8 or 2.6 millions years old? Contributions of early homo

The Quaternary, which is sometimes also termed the Anthropogene Period, often is linked to the beginning of humankind. But what exactly is the basis in the fossil hominin record for this link, in particular what is the early Homo evidence? Since 1999, debate concerning the definition of the genus Homo and the identification of its first members has intensified. At the centre of recent debates, there are four hypotheses: (1) all early Homo specimens belong to the same species: Homo habilis sensu lato; (2) the early Homo hypodigm is heterogeneous, two species could be defined: Homo habilis sensu stricto and Homo rudolfensis; (3) these species do not belong to the genus Homo but to the genus Australopithecus; or (4) it would be more appropriate to put the specimens of Homo rudolfensis into the genus Kenyanthropus. The earliest appearance of the genus Homo will differ depending upon the favoured hypothesis: 2.45 Myr (hypotheses 1 and 2); 1.9 Myr (hypothesis 3); 1.9 or 2.4 Myr (hypothesis 4, depending on the genus attribution for habilis (2.4 Myr if Homo; 1.9 Myr if Australopithecus)). Our study, based on palaeoanthropological data, is focused on the 2.6-1.6 Myr period. In this paper, we propose to review and assess knowledge of early Homo taxonomy in the context of the definition of the Quaternary boundary.Le Quaternaire est souvent considéré comme une période liée à l’apparition du genre Homo et marquée par la diversité de ses représentants, ce qui lui valu parfois d’être défini comme une période « Anthropozoïque ». Mais que nous indique vraiment le registre des homininés fossiles et en particulier des premiers représentants du genre Homo? La définition du genre Homo et l’identification de ces premiers représentants sont l’objet de nombreux débats depuis 1999. Quatre hypothèses sont actuellement débattues : (1) tous les spécimens appartiennent à un seul et même taxon : Homo habilis ; (2) deux espèces peuvent être identifiées dans ce groupe : Homo habilis sensu stricto et Homo rudolfensis ; (3) ces spécimens n’appartiennent pas au genre Homo mais au genre Australopithecus ; (4) les spécimens de l’espèce rudolfensis devraient être mis dans le genre Kenyanthropus. L’émergence du genre Homo serait à 2,45 millions d’années (hypothèses 1 et 2) ; à 1,9 million d’années (hypothèse 3) ; à 1, 9 ou 2,4 millions d’années (hypothèse 4, selon l’attribution de l’espèce habilis, au genre Homo (2,4 Ma), ou au genre Australopithecus (1,9 Ma)). Nous exposons dans ce papier les problématiques actuelles et nos propres résultats (analyse cladistique) concernant l’émergence du genre Homo en Afrique, en les mettant en perspectives avec les différentes propositions concernant la limite du Quaternaire. Notre propos, basé sur les données paléoanthropologiques, est centré sur la période allant de 2,6 à 1,6 millions d’années

Evolutionary perspective of the genus Homo: new approaches

No consensus exists in human evolution research regarding the definition of genus Homo and its mode of evolution. Based on a database that comprises more than 350 characters scored on 22 hominin species from Late Miocene to Holocene, we address methodologically these two questions with a quantitative taxonomic approach that combines Maximum Parsimony (MP) phylogenetic reconstruction with a Phylogenetic Networks method. Based on tree-based MP, we identify three Homo most parsimonious definitio..

Roles of gas in capillary filling of nanoslits

Control and understanding of flows inside fabricated nanochannels is rich in potential applications, but nanoscale physics of fluids remains to be clarified even for the simple case of spontaneous capillary filling. This paper reports an experimental and modelling investigation of the role of gas on the capillary filling kinetics slowdown in nanoslits (depth going from 20 nm to 400 nm) compared to Washburn's prediction. First, the role of gas through the usually observed trapped bubbles during a nanoslits capillary filling is analysed thanks to experiments realized with water, ethanol and silicone oil in siliconglass nanochannels. Bubbles are trapped only when slit depth is below a liquid-dependent threshold. This is interpreted as possible contact line pinning strength varying with wettability. Stagnant trapped bubbles lifetime is investigated for the three liquids used. Experimental results show that bubbles are first compressed because of the increasing local liquid pressure. Once the gas bubble pressure is sufficiently high, gas dissolution induces the final bubble collapse. Influence of the bubbles' presence on the capillary filling kinetics is analysed by estimating viscous resistance induced by the bubbles using an effective medium approach (Brinkman approximation). Surprisingly, the bubbles' presence is found to have a very minor effect on nanoslits capillary filling kinetics. Second, the transient gas pressure profile between the advancing meniscus and the channel exit is computed numerically taking into account gas compressibility. A non-negligible over-pressure ahead of the meniscus is found for nano-scale slit capillary filling. Considering the possible presence of precursor films, reducing cross-section for gas flow, leads to a capillary filling kinetics slowdown comparable to the ones measured experimentally

Le registre d’hominines fossiles du bassin de l’Omo-Turkana

L’objectif de cette présentation est de considérer un registre exhaustif d’hominines fossiles comme objet d’étude en tant que tel et de montrer via quelques exemples l’apport de l’analyse d’un tel registre, à condition qu’il provienne d’un système sédimentaire cohérent. Nous illustrerons ceci en utilisant le registre d’hominines du bassin de l’Omo-Turkana (Kenya/Éthiopie), où les recherches paléoanthropologiques intensives ont commencé en 1967. À ce jour, le registre d’hominines fossiles est..

Nanobubbles and gas dynamics during capillary filling of nanochannels

This paper focuses on capillary filling at the nanoscale where deviations to the Washburn’s classical theory are observed. Imbibition experiments in microfabricated silicon-glass nanochannels with low aspect ratio (width >> depth and depths going from 400 nm down to 20 nm) are performed for several liquids. In all cases, as predicted by the Washburn’s law, liquid invasion front location evolves as the square root of time. However, filling kinetics slowdown compared to the Washburn’s law is measured in nanochannels for depths below ~ 100 nm. Furthermore, below a liquid-dependent depth threshold, we observe spontaneous bubbles formation behind the advancing meniscus. Bubbles dynamics (formation conditions and lifetime) are analyzed thanks to our experimental data involving several liquids and nanochannels depths. Viscous resistance induced by the bubbles presence is estimated using an effective medium approach. Conjointly, gas flow ahead of the advancing meniscus is modeled considering the gas as viscous and compressible. Influence of these effects on the filling kinetics is discussed

Three Periods of Drying of a Single Square Capillary Tube

The drying kinetics of a porous medium is classically described in three main periods, which depend on the interplay between the external and internal mass transfers during evaporation. The first period is described as essentially depending on the external mass transfer, whereas the third period is dominated by the internal mass transfer. The second period is a crossover period. We show experimentally that a similar drying kinetics can be obtained from a much simpler system owing to the effect of corner liquid films: a capillary tube of square cross section

Locus of first crystals on the evaporative surface of a vertically textured porous medium

The evaporation of a saline solution from a heterogeneous porous medium formed by the assembly of a coarse medium column and a fine medium column is studied numerically. We concentrate on the locus of the formation of first crystals on the evaporative surface from the computation of the ion mass fraction distribution at the surface prior to the efflorescence development. Two basic situations considered in previous works, namely the evaporation–wicking situation and the drying situation are considered. The study makes clear that each situation leads to a markedly different locus of the efflorescence formation, except, however, for very high initial salt concentrations. The study emphasizes the key-role of the velocity field induced in the porous domain in the case of the evaporation–wicking situation. In the case of the drying situation, a key aspect lies in the local increase in the ion mass fraction due to the local desaturation, i.e. the local shrinking of the liquid volume containing the ions