Finite-Size Scaling at the Jamming Transition

We present an analysis of finite-size effects in jammed packings of N soft, frictionless spheres at zero temperature. There is a 1/N correction to the discrete jump in the contact number at the transition so that jammed packings exist only above isostaticity. As a result, the canonical power-law scalings of the contact number and elastic moduli break down at low pressure. These quantities exhibit scaling collapse with a non-trivial scaling function, demonstrating that the jamming transition can be considered a phase transition. Scaling is achieved as a function of N in both 2 and 3 dimensions, indicating an upper critical dimension of 2.Comment: 5 pages, 3 figure

Contact Changes near Jamming

We probe the onset and effect of contact changes in soft harmonic particle packings which are sheared quasistatically. We find that the first contact changes are the creation or breaking of contacts on a single particle. We characterize the critical strain, statistics of breaking versus making a contact, and ratio of shear modulus before and after such events, and explain their finite size scaling relations. For large systems at finite pressure, the critical strain vanishes but the ratio of shear modulus before and after a contact change approaches one: linear response remains relevant in large systems. For finite systems close to jamming the critical strain also vanishes, but here linear response already breaks down after a single contact change.Comment: 5 pages, 4 figure

Characterization and optimization of EBT2 radiochromic films dosimetry system for precise measurements of output factors in small fields used in radiotherapy

International audienceThe accurate determination of absorbed dose in small photon beams, especially for stereotactic radiation therapy, is a difficult task with commercially available detectors. As these small fields are characterized by high dose gradients, a lack of lateral particle equilibrium and a variation of energy spectra with beam sizes, a dosimeter with high resolution, tissue-equivalence and high precision is required. The new radiochromic film EBT2, which meets these criteria, was fully characterized in Institut de Radioprotection et Sûreté Nucléaire (IRSN) for this application. This type of film was tested with the reading system EPSON Dual Lens Perfection V700 flatbed scanner in transmission mode. Warm-up effects of the scanner were studied as well as the influence of the scanner light. Uniformity of unirradiated and irradiated EBT2 films in terms of pixel value was found to be respectively 0.3% (1 SD) and 0.5% (1 SD). An original, accurate and efficient radiochromic film dosimetry protocol was established. The overall uncertainty for dose measurement with EBT2 films using this protocol was estimated at less than 2% (1 SD). Encouraging measurements of output factors were performed on a Novalis system. © 2011 Elsevier Ltd. All rights reserved

JamBashBulk: two-dimensional packing simulation

Authors are listed in alphabetical order. JamBashBulk: The Van Hecke Lab shear-stabilized packing simulation code. JamBashBulk is a C/C++ simulation code that provides three basic operations: Creating shear stabilized packings, Shearing packings with non-square boundary conditions, and Arbitrary deformations via a C/Python interface. See readme.md for more details. For background and the physics behind the simulation, please see: Simon Dagois-Bohy, Brian P. Tighe, Johannes Simon, Silke Henkes, and Martin van Hecke. Soft-Sphere Packings at Finite Pressure but Unstable to Shear. Phys. Rev. Lett. 109, 095703, arXiv:1203.3364. Merlijn S. van Deen, Johannes Simon, Zorana Zeravcic, Simon Dagois-Bohy, Brian P. Tighe, and Martin van Hecke. Contact changes near jamming. Phys. Rev. E 90 020202(R), arXiv:1404.3156. Merlijn S. van Deen, Brian P. Tighe, and Martin van Hecke. Contact Changes of Sheared Systems: Scaling, Correlations, and Mechanisms. arXiv:1606.04799 Merlijn S. van Deen. Mechanical Response of Foams: Elasticity, Plasticity, and Rearrangements. PhD Thesis, Leiden University, 2016. hdl:1887/40902 Several data sets created using the simulation code are available via Zenodo: Shear-stabilized jammed packings. doi:10.5281/zenodo.59216 Contact changes in shear-stabilized jammed packings. doi:10.5281/zenodo.5921

Softening and yielding of soft glassy materials

Solids deform and fluids flow, but soft glassy materials, such as emulsions, foams, suspensions, and pastes, exhibit an intricate mix of solid- and liquid-like behavior. While much progress has been made to understand their elastic (small strain) and flow (infinite strain) properties, such understanding is lacking for the softening and yielding phenomena that connect these asymptotic regimes. Here we present a comprehensive framework for softening and yielding of soft glassy materials, based on extensive numerical simulations of oscillatory rheological tests, and show that two distinct scenarios unfold depending on the material's packing density. For dense systems, there is a single, pressure-independent strain where the elastic modulus drops and the particle motion becomes diffusive. In contrast, for weakly jammed systems, a two-step process arises: at an intermediate softening strain, the elastic and loss moduli both drop down and then reach a new plateau value, whereas the particle motion becomes diffusive at the distinctly larger yield strain. We show that softening is associated with an extensive number of microscopic contact changes leading to a non-analytic rheological signature. Moreover, the scaling of the softening strain with pressure suggest the existence of a novel pressure scale above which softening and yielding coincide, and we verify the existence of this crossover scale numerically. Our findings thus evidence the existence of two distinct classes of soft glassy materials – jamming dominated and dense – and show how these can be distinguished by their rheological fingerprint.Engineering Thermodynamic