Anisotropic nature of the capillary stress tensor

The paper describes a micromechanical approach that explores the anisotropic nature of the capillary stress tensor and its evolution in pendular granular materials via Discrete Element Modeling (DEM) simulations. Dimensionless parameters are used to address the conditions under which the contribution of capillarity (or cohesive interparticle forces) to the stress transmission within a Representative Elementary Volume (REV) is expected to be considerable. From a series of suction-controlled conventional triaxial tests, numerical results show that the significance of the capillary stress and the relative magnitude of its mean to deviatoric components is directly connected to the characteristic particle size and applied stress. In addition, it is shown that the anisotropic character of the capillary stress tensor intensifies with increasing suction. Furthermore, a simple shear test is conducted at constant mean stress to reveal the development of deviatoric capillary stresses in the absence of any change in mean stress, which cannot be captured by the commonly used Bishop’s stress expression.

Altered Hemodynamics and End-Organ Damage in Heart Failure

Heart failure is characterized by pathologic hemodynamic derangements, including elevated cardiac filling pressures (“backward” failure), which may or may not coexist with reduced cardiac output (“forward” failure). Even when normal during unstressed conditions such as rest, hemodynamics classically become abnormal during stressors such as exercise in patients with heart failure. This has important upstream and downstream effects on multiple organ systems, particularly with respect to the lungs and kidneys. Hemodynamic abnormalities in heart failure are affected by processes that extend well beyond the cardiac myocyte, including important roles for pericardial constraint, ventricular interaction, and altered venous capacity. Hemodynamic perturbations have widespread effects across multiple heart failure phenotypes, ranging from reduced to preserved ejection fraction, acute to chronic disease, and cardiogenic shock to preserved perfusion states. In the lung, hemodynamic derangements lead to the development of abnormalities in ventilatory control and efficiency, pulmonary congestion, capillary stress failure, and eventually pulmonary vascular disease. In the kidney, hemodynamic perturbations lead to sodium and water retention and worsening renal function. Improved understanding of the mechanisms by which altered hemodynamics in heart failure affect the lungs and kidneys is needed in order to design novel strategies to improve clinical outcomes.

Microstructural modelling of autogenous shrinkage in Portland cement paste at early age

Purpose This paper aims to develop a numerical, micromechanical model to predict the evolution of autogenous shrinkage of hydrating cement paste at early age (up to 7 days). Autogeneous shrinkage can be important in high-performance concrete characterized by low water to cement (w/c) ratios. The occurrence of this phenomenon during the first few days of hardening may result in early-age cracking in concrete structures. A good prediction of autogeneous shrinkage is necessary to achieve better understanding of the mechanisms and the deployment of effective measures to prevent early-age cracking. Design/methodology/approach Three-dimensional digital microstructures from the hydration modelling platform μic of cement paste were used to simulate macroscopic autogenous shrinkage based on the mechanism of capillary tension. Elastic and creep properties of the digital microstructures were calculated by means of finite element (FE) method homogenization. Autogenous shrinkage was then estimated as the average hydrostatic strain resulting from the capillary stress that was globally applied on the simulated digital microstructures. For this estimation, two approaches of homogenization technique, i.e. analytical poro-elasticity and numerical creep-superposition were used. Findings The comparisons of between the simulated and experimentally measured deformations indicate that the creep-superposition approach is more reasonable to estimate shrinkage at different water to cement ratios. It was found that better estimations could be obtained at low degrees of hydration, by assuming a loosely packed calcium silicate hydrates (C-S-H) growing in the microstructures. The simulation results show how numerical models can be used to upscale from microscopic characteristics of phases to macroscopic composite properties such as elasticity, creep and shrinkage. Research limitations/implications While the good predictions of some cement paste properties from the microstructure at early age were obtained, the current models have several limitations that are needed to overcome in the future. Firstly, the limitation of pore-structure representation is not only from lack understanding of C-S-H structure but also from the computational complexity. Secondly, the models do not consider early-age expansion that usually happens in practice and appears to be superimposed on an underlying shrinkage as observed in experiments. Thirdly, the simplified assumptions for mechanical simulation do not accurately reflect the solid–liquid interactions in the real partially saturated system, for example, the globally applying capillary stress on the boundary of the microstructure to find the effective deformation, neglecting water flow and the pore pressure. Last but not least, the models, due to the computational complexities, use many simplifications such as FE approximation, mechanical phase properties and creep statistical data. Originality/value This study holistically tackles the phenomenon of autogeneous shrinkage through microstructural modelling. In a first such attempt, the authors have used the same microstructural model to simulate the microstructural development, elastic properties, creep and autogeneous shrinkage. The task of putting these models together was not simple. The authors have successfully handled several problems at each step in an elegant manner. For example, although several earlier studies have pointed out that discrete models are unable to capture the late setting times of cements due to mesh effects, this study offers the most effective solution yet on the problem. It is also the first time that creep and shrinkage have been modelled on a young evolving microstructure that is subjected to a time variable load.

Phase Diagram of Pinch-off Behaviors During Drop-on-Demand Inkjetting of Alginate Solutions

Viscoelastic polymer solutions have been extensively utilized in inkjet printing for a variety of biomedical applications. The pinch-off of viscoelastic jets is a key step toward the generation of droplets in inkjet printing. This complex process is governed by the interplay of four stresses, including inertial stress, capillary stress, viscous stress, and elastic stress. Depending on polymer solution properties and process conditions, four types of pinch-off phenomenon were observed during inkjetting of viscoelastic alginate solutions. In this study, material properties of alginate solutions with different concentrations have been characterized, and three dimensionless numbers (Ohnesorge number Oh, Deborah number De, and Weber number We) have been proposed to analyze different pinch-off behaviors. The phase diagram in terms of these three dimensionless numbers has been constructed to classify the regimes for different pinch-off types during inkjetting of viscoelastic alginate solutions. It is found that (1) at low De and Oh, the capillary stress is mainly balanced by the inertial stress, resulting in front pinching. (2) At medium De and low Oh, with the increase of We, the pinch-off type may change from front pinching to hybrid pinching to exit pinching. (3) At low Oh and high De, the capillary stress is mainly balanced by the elastic stress, resulting in exit pinching. (4) At high Oh and De, the viscoelastic effect is dominant. With the increase of We, middle pinching turns to be exit pinching due to the increase in the initial ligament diameter near the forming droplet.

Endothelial cell Piezo1 mediates pressure-induced lung vascular hyperpermeability via disruption of adherens junctions

Increased pulmonary microvessel pressure experienced in left heart failure, head trauma, or high altitude can lead to endothelial barrier disruption referred to as capillary “stress failure” that causes leakage of protein-rich plasma and pulmonary edema. However, little is known about vascular endothelial sensing and transduction of mechanical stimuli inducing endothelial barrier disruption. Piezo1, a mechanosensing ion channel expressed in endothelial cells (ECs), is activated by elevated pressure and other mechanical stimuli. Here, we demonstrate the involvement of Piezo1 in sensing increased lung microvessel pressure and mediating endothelial barrier disruption. Studies were made in mice in which Piezo1 was deleted conditionally in ECs (Piezo1iΔEC), and lung microvessel pressure was increased either by raising left atrial pressure or by aortic constriction. We observed that lung endothelial barrier leakiness and edema induced by raising pulmonary microvessel pressure were abrogated inPiezo1iΔECmice. Piezo1 signaled lung vascular hyperpermeability by promoting the internalization and degradation of the endothelial adherens junction (AJ) protein VE-cadherin. Breakdown of AJs was the result of activation of the calcium-dependent protease calpain and degradation of the AJ proteins VE-cadherin, β-catenin, and p120-catenin. Deletion of Piezo1 in ECs or inhibition of calpain similarly prevented reduction in the AJ proteins. Thus, Piezo1 activation in ECs induced by elevated lung microvessel pressure mediates capillary stress failure and edema formation secondary to calpain-induced disruption of VE-cadherin adhesion. Inhibiting Piezo1 signaling may be a useful strategy to limit lung capillary stress failure injury in response to elevated vascular pressures.

Phase Diagram of Pinch-Off Behaviors During Drop-on-Demand Inkjetting of Alginate Solutions

Viscoelastic polymer solutions have been extensively utilized in drop-wise manufacturing (such as inkjet printing) for a variety of biomedical applications. The pinch-off of viscoelastic jets is a key step towards generation of droplets in inkjet printing. This complex process is governed by interplay of four stresses including inertial stress, capillary stress, viscous stress, and elastic stress. Depending on polymer solution properties and process conditions, four types of pinch-off phenomenon were observed during inkjetting of viscoelastic alginate solutions. In this study, material properties of alginate solutions with different concentrations have been characterized, and three dimensionless numbers (Ohnesorge number Oh, Deborah number De and Weber number We) have been proposed to analyze different pinch-off behaviors. Phase diagram in terms of these three dimensionless numbers has been constructed to classify the regimes for different pinch-off types during inkjetting of viscoelastic alginate solutions. It is found that: 1) At low De and Oh, the viscoelastic effect is small. The capillary stress is mainly balanced by the inertial stress, resulting in front pinching. 2) At medium De and low Oh, the capillary stress is still mainly balanced by the inertial stress, but the elastic effect starts to show its effect by delaying the ligament thinning near the front-pinching location. With the increase of We, the pinch-off type may change from front pinching to hybrid pinching to exit pinching. 3) At low Oh and high De, the viscous and inertial effects are small. The capillary stress is mainly balanced by the elastic stress, resulting in exit pinching. 4) At high Oh and De, the viscoelastic effect is dominant. The capillary stress is mainly balanced by the viscous and elastic stresses. With the increase of We, middle pinching turns to be exit pinching due to the increase of the initial ligament diameter near the forming droplet.