Clay micromechanics : an analysis of elementary mechanisms of clay particle interactions to gain insight into compression behaviour of clay

The macroscopic response of geomaterials is controlled by the processes occurring at the microscale. Understanding these processes is key to interpret experimental data, understand fundamental modes of stress-strain behaviour, inform ‘continuum’ macroscopic constitutive models, and develop quantitative predictive tools based on Discrete Element Method (DEM) approaches. Unlike granular materials, mechanisms at the particle scale controlling macro-mechanical behaviour of clays are still largely ignored. This paper presents an analysis of elementary mechanisms of clay particle interactions with the aim of gaining an insight into behaviour of clay and advance the process of defining suitable contact laws to be implemented into DEM formulations

Animated molecular dynamics simulations of hydrated caesium-smectite interlayers

Computer animation of center of mass coordinates obtained from 800 ps molecular dynamics simulations of Cs-smectite hydrates (1/3 and 2/3 water monolayers) provided information concerning the structure and dynamics of the interlayer region that could not be obtained through traditional simulation analysis methods. Cs(+ )formed inner sphere complexes with the mineral surface, and could be seen to jump from one attracting location near a layer charge site to the next, while water molecules were observed to migrate from the hydration shell of one ion to that of another. Neighboring ions maintained a partial hydration shell by sharing water molecules, such that a single water molecule hydrated two ions simultaneously for hundreds of picoseconds. Cs-montmorillonite hydrates featured the largest extent of this sharing interaction, because interlayer ions were able to inhabit positions near surface cavities as well as at their edges, close to oxygen triads. The greater positional freedom of Cs(+ )within the montmorillonite interlayer, a result of structural hydroxyl orientation and low tetrahedral charge, promoted the optimization of distances between cations and water molecules required for water sharing. Preference of Cs(+ )for locations near oxygen triads was observed within interlayer beidellite and hectorite. Water molecules also could be seen to interact directly with the mineral surface, entering its surface cavities to approach attracting charge sites and structural hydroxyls. With increasing water content, water molecules exhibited increased frequency and duration of both cavity habitation and water sharing interactions. Competition between Cs(+ )and water molecules for surface sites was evident. These important cooperative and competitive features of interlayer molecular behavior were uniquely revealed by animation of an otherwise highly complex simulation output

Venous hemodynamics in neurological disorders: an analytical review with hydrodynamic analysis.

Venous abnormalities contribute to the pathophysiology of several neurological conditions. This paper reviews the literature regarding venous abnormalities in multiple sclerosis (MS), leukoaraiosis, and normal-pressure hydrocephalus (NPH). The review is supplemented with hydrodynamic analysis to assess the effects on cerebrospinal fluid (CSF) dynamics and cerebral blood flow (CBF) of venous hypertension in general, and chronic cerebrospinal venous insufficiency (CCSVI) in particular.CCSVI-like venous anomalies seem unlikely to account for reduced CBF in patients with MS, thus other mechanisms must be at work, which increase the hydraulic resistance of the cerebral vascular bed in MS. Similarly, hydrodynamic changes appear to be responsible for reduced CBF in leukoaraiosis. The hydrodynamic properties of the periventricular veins make these vessels particularly vulnerable to ischemia and plaque formation.Venous hypertension in the dural sinuses can alter intracranial compliance. Consequently, venous hypertension may change the CSF dynamics, affecting the intracranial windkessel mechanism. MS and NPH appear to share some similar characteristics, with both conditions exhibiting increased CSF pulsatility in the aqueduct of Sylvius.CCSVI appears to be a real phenomenon associated with MS, which causes venous hypertension in the dural sinuses. However, the role of CCSVI in the pathophysiology of MS remains unclear

Cooperative management and its effects on shade tree diversity, soil properties and ecosystem services of coffee plantations in western El Salvador

We compared how management approaches affected shade tree diversity, soil properties, and provisioning and carbon sequestration ecosystem services in three shade coffee cooperatives. Collectively managed cooperatives utilized less diverse shade, and pruned coffee and shade trees more intensively, than individual farms. Soil properties showed significant differences among the cooperatives, with the following properties contributing to differentiation: N, pH, P, K, and Ca. Higher tree richness was associated with higher soil pH, CEC, Ca, and Mg, and lower K. Higher tree densities were associated with lower N, K, and organic matter. Although we found differences in the incidence of provisioning services (e.g., fruit), all plantations generated products other than coffee. No differences were observed between C-stocks. The history and institutional arrangements of cooperatives can influence management approaches, which affect ecosystem properties and services. Our study corroborates that interdisciplinary investigations are essential to understand the socio-ecological context of tropical shade coffee landscapes