Jamming in frictionless packings of spheres: determination of the critical volume fraction

The jamming transition in granular packings is characterized by a sudden change in the coordination number. In this work we investigate the evolution of coordination number as function of volume fraction for frictionless packings of spheres undergoing isotropic deformation. Using the results obtained from Discrete Element Method simulations, we confirm that the coordination number depends on volume fraction by a power law with exponent α≈0.5 above the critical volume fraction and up to rather high densities. We find that the system size and loading rate do not have an important effect on the evolution of the coordination number. Polydispersity of the packing seems to cause a shift in the critical volume fraction, i.e., more heterogeneous packings jam at higher volume fractions. Finally, we propose and evaluate alternative methods to determine the critical volume fraction based on the number of rattlers, the pressure and the ratio of kinetic and potential energies. The results are all consistent with the critical volume fractions obtained from the fits of the power law to the simulation data

From particle simulations to macroscopic constitutive relations

The goal is to determine the constitutive behavior of granular packings under various deformations (isotropic and anisotropic) from particle simulations. For this we consider deformations, stress, structure and the contact forces as the basis. In a previous study [6,7] we investigated using DEM, the evolution of the coordination number (and the packing structure) and pressure as functions of the volume fraction for a polydisperse granular packing of spheres under isotropic compression. Here we focus on anisotropic deformation by implementing the triaxial test setup in a similar way. We study the effect of polydispersity changing the width of the particle size distribution. We find that an increase in polydispersity leads to a decrease in pressure at constant volume fraction whereas the macroscopic friction angle seems to increase with polydispersity. Furthermore, we performed triaxial test simulations with soft friction which is characterized by a small tangential contact stiffness. Our main observation is that using the same initial packing configuration with different friction coefficients does not lead to an obvious trend in simulation results

Long-term outcomes of omniflow II biosynthetic vascular graft in lower extremity arterial revascularization

Background: This study aims to evaluate the patency rates and long-term outcomes of femoro-popliteal bypass procedures with Omniflow II biosynthetic vascular grafts in patients with occlusive vascular disease. Methods: This retrospective, observational, clinical study included a total of 93 patients (61 males, 32 females; mean age 56.9±7.4 years; range, 43 to 83 years) who underwent femoro-popliteal bypass in which Omniflow II biosynthetic vascular grafts were used due to peripheral arterial disease. The patients were divided into two groups: 62 patients undergoing femoro-popliteal above-knee bypass and 31 patients undergoing the femoro-popliteal belowknee bypass. We evaluated preoperative clinical characteristics, postoperative graft patency rates, and other clinical results. Results: The mean follow-up was 44.9±18.8 months in the femoropopliteal above-knee bypass group and 47.3±22.3 months in the femoro-popliteal below-knee bypass group (p=0.302). The cumulative primary graft patency rates of the femoro-popliteal above-knee bypass and femoro-popliteal below-knee bypass groups at three, four, and five years were 98%, 95% and 78% and 86%, 75% and 45%, respectively (log-rank; p=0.312). The cumulative assisted graft patency rates of the femoro-popliteal above-knee bypass and femoro-popliteal below-knee bypass groups at five years were 87.9% and 65.3%, respectively (log-rank; p=0.530). Conclusion: The Omniflow II biosynthetic vascular graft is suitable for above-and below-knee femoro-popliteal bypass procedures. These grafts may be prefered due to high patency rates, low incidence of aneursym formations, and infections. ©2018 All right reserved by the Turkish Society of Cardiovascular Surgery

Exploiting pattern transformation to tune phononic band gaps in a two-dimensional granular crystal

The band structure of a two-dimensional granular crystal composed of silicone rubber and polytetrafluoroethylene (PTFE) cylinders is investigated numerically. This system was previously shown to undergo a pattern transformation with uniaxial compression by Göncü et al. [Soft Matter 7, 2321 (2011)]. The dispersion relations of the crystal are computed at different levels of deformation to demonstrate the tunability of the band structure, which is strongly affected by the pattern transformation that induces new band gaps. Replacement of PTFE particles with rubber ones reveals that the change of the band structure is essentially governed by pattern transformation rather than particles¿ mechanical properties

Psychological well-being in Europe after the outbreak of war in Ukraine

The Russian invasion of Ukraine on February 24, 2022, has had devastating effects on the Ukrainian population and the global economy, environment, and political order. However, little is known about the psychological states surrounding the outbreak of war, particularly the mental well-being of individuals outside Ukraine. Here, we present a longitudinal experience-sampling study of a convenience sample from 17 European countries (total participants = 1,341, total assessments = 44,894, countries with >100 participants = 5) that allows us to track well-being levels across countries during the weeks surrounding the outbreak of war. Our data show a significant decline in well-being on the day of the Russian invasion. Recovery over the following weeks was associated with an individual’s personality but was not statistically significantly associated with their age, gender, subjective social status, and political orientation. In general, well-being was lower on days when the war was more salient on social media. Our results demonstrate the need to consider the psychological implications of the Russo-Ukrainian war next to its humanitarian, economic, and ecological consequences

A global experience‐sampling method study of well‐being during times of crisis: The CoCo project

We present a global experience-sampling method (ESM) study aimed at describing, predicting, and understanding individual differences in well-being during times of crisis such as the COVID-19 pandemic. This international ESM study is a collaborative effort of over 60 interdisciplinary researchers from around the world in the “Coping with Corona” (CoCo) project. The study comprises trait-, state-, and daily-level data of 7490 participants from over 20 countries (total ESM measurements = 207,263; total daily measurements = 73,295) collected between October 2021 and August 2022. We provide a brief overview of the theoretical background and aims of the study, present the applied methods (including a description of the study design, data collection procedures, data cleaning, and final sample), and discuss exemplary research questions to which these data can be applied. We end by inviting collaborations on the CoCo dataset

Mechanics of Granular Materials: Constitutive Behavior and Pattern Transformation

From pharmaceutical to mining or traveling desert dunes to earthquakes, granular materials are at the heart of many industries and natural phenomena. Improving the efficiency of the machines handling them or, constructing safer buildings requires a critical understanding of their behavior. However, this is not a straightforward task as opposed to what one might think due to the abundance of particulate matter. From a fundamental point of view, it has been only recently realized that they cannot be easily classified as a solid or liquid or even a gas as they are able to mimic all of these states under slightly different conditions. The challenge of the scientific research today, is to establish the link between the collective behavior and properties of individual particles composing granular materials.Such a relation would enable to characterize them with only a few parameters in contrast to billions of particles typically found in practice. In the first part of this thesis, we study the mechanical behavior of idealized sphere packings with discrete element simulations. The polydispersity and coefficient of friction of the particles are varied systematically to characterize their influence on the macroscopic stress strain response. In isotropically deformed packings, the critical volume fraction marking the transition from a solid to fluid like state increases with polydispersity, whereas it decreases with the coefficient of friction. The coordination number, i.e. average number of contact per particle, is discontinuous at this density. During decompression it drops from its isostatic value to zero and obeys a power law at higher volume fractions. The effect of polydispersity on the pressure is determined by the ratio of critical volume fraction and the contact density which is equal to the trace of the fabric times a correction factor that depends only on the moments of the particle size distribution. Using the micromechanical definition of the stress tensor, we derive an incremental constitutive model for the pressure which includes changes of fabric. With one fit parameter the linear regime of lower pressure is described, while with two parameters, the model captures well the non-linear pressure evolution in isotropically deformed polydisperse, frictionless and frictional packings. Anisotropic deformations are studied with triaxial test simulations. The shear strength of the packings is measured by the deviatoric stress ratio which first increases then saturates with increasing particle coefficient of friction. Volumetric strain also depends on the particle friction albeit in a non monotonic way. The maximum compaction after which packings start to dilate, is achieved at a relatively small coefficient of friction. The stress strain response depends indirectly on the polydispersity which determines initial packing conditions. When initially the volume fraction is fixed, the pressure as well as the shear strength decrease with polydispersity. The opposite is observed when the initial pressure is imposed, although the effect of polydispersity on the stress-strain behavior is less significant in this case. Finally, a hypoplastic constitutive model is calibrated with simulation results and the resulting material coefficients are related to particle properties. Most granular materials are amorphous and disordered as realized up to now. However,crystal structures can be built by placing uniform particles on a regular lattice. The second part of the thesis is about pattern transformation in two-dimensional granular crystals composed of bi-disperse soft and hard cylindrical particles. We show with experiments and simulations that upon uniaxial compression the particles undergo structural rearrangements from an initial square to hexagon-like lattice. It is found that the characteristics of the transformation strongly depend on the size ratio of the particles rather than their material properties. If the ratio is small enough the transformation is homogeneous and practically reversible. The band structure of the granular crystal changes due to the pattern transformation. Using a linearized contact force model, we compute the dispersion relation at different levels of deformation and show that band gaps open and close as the structure of the crystal changes.This could find applications in tunable acoustic devices such as filters or vibration isolators. In short, this thesis concerns the mechanics of granular materials subject to different modes of deformation. The constitutive behavior of disordered sphere packings and pattern transformation in regular arrays of cylinders have been studied.Chemical EngineeringApplied Science

Effect of particle friction and polydispersity on the macroscopic stress–strain relations of granular materials

The macroscopic mechanical behavior of granular materials inherently depends on the properties of particles that compose them. Using the discrete element method, the effect of particle contact friction and polydispersity on the macroscopic stress response of 3D sphere packings is studied. The analytical expressions for the pressure, coordination number and fraction of rattlers proposed for isotropically deformed frictionless systems also hold when the interparticle coefficient of friction is finite; however, the numerical values of the parameters such as the jamming volume fraction change with varying microscopic contact and particle properties. The macroscopic response under deviatoric loading is studied with triaxial test simulations. Concerning the shear strength, our results agree with previous studies showing that the deviatoric stress ratio increases with particle coefficient of friction μ starting from a nonzero value for μ = 0 and saturating for large μ. On the other hand, the volumetric strain does not have a monotonic dependence on the particle contact friction. Most notably, maximum compaction is reached at an intermediate value of the coefficient of friction μ ≈ 0.3. The effect of polydispersity on the macroscopic stress–strain relationship cannot be studied independent of initial packing conditions. The shear strength increases with polydispersity when the initial volume fraction is fixed, but the effect of polydispersity is much less pronounced when the initial pressure of the packings is fixed. Finally, a simple hypoplastic constitutive model is calibrated with numerical test results following an established procedure to ascertain the relation between particle properties and material coefficients of the macroscopic model. The calibrated model is in good qualitative agreement with simulation results