Force Mobilization and Generalized Isostaticity in Jammed Packings of Frictional Grains

We show that in slowly generated 2d packings of frictional spheres, a significant fraction of the friction forces lies at the Coulomb threshold - for small pressure p and friction coefficient mu, about half of the contacts. Interpreting these contacts as constrained leads to a generalized concept of isostaticity, which relates the maximal fraction of fully mobilized contacts and contact number. For p->0, our frictional packings approximately satisfy this relation over the full range of mu. This is in agreement with a previous conjecture that gently built packings should be marginal solids at jamming. In addition, the contact numbers and packing densities scale with both p and mu.Comment: 4 pages, 4 figures, submitte

Convection in rotating annuli: Ginzburg-Landau equations with tunable coefficients

The coefficients of the complex Ginzburg-Landau equations that describe weakly nonlinear convection in a large rotating annulus are calculated for a range of Prandtl numbers $\sigma$. For fluids with $\sigma \approx 0.15$, we show that the rotation rate can tune the coefficients of the corresponding amplitude equations from regimes where coherent patterns prevail to regimes of spatio-temporal chaos.Comment: 4 pages (latex,multicol,epsf) including 3 figure

Jammed frictionless discs: connecting local and global response

By calculating the linear response of packings of soft frictionless discs to quasistatic external perturbations, we investigate the critical scaling behavior of their elastic properties and non-affine deformations as a function of the distance to jamming. Averaged over an ensemble of similar packings, these systems are well described by elasticity, while in single packings we determine a diverging length scale $\ell^*$ up to which the response of the system is dominated by the local packing disorder. This length scale, which we observe directly, diverges as $1/\Delta z$, where $\Delta z$ is the difference between contact number and its isostatic value, and appears to scale identically to the length scale which had been introduced earlier in the interpretation of the spectrum of vibrational modes. It governs the crossover from isostatic behavior at the small scale to continuum behavior at the large scale; indeed we identify this length scale with the coarse graining length needed to obtain a smooth stress field. We characterize the non-affine displacements of the particles using the \emph{displacement angle distribution}, a local measure for the amount of relative sliding, and analyze the connection between local relative displacements and the elastic moduli.Comment: 19 pages, 15 figures, submitted to Phys. Rev.

Local contact numbers in two dimensional packings of frictional disks

We analyze the local structure of two dimensional packings of frictional disks numerically. We focus on the fractions x_i of particles that are in contact with i neighbors, and systematically vary the confining pressure p and friction coefficient \mu. We find that for all \mu, the fractions x_i exhibit powerlaw scaling with p, which allows us to obtain an accurate estimate for x_i at zero pressure. We uncover how these zero pressure fractions x_i vary with \mu, and introduce a simple model that captures most of this variation. We also probe the correlations between the contact numbers of neighboring particles.Comment: 4 pages, 5 figure

Prevention from a community perspective facilitating and constraining factors perceived by nurses in local community health centers in the course of prevention and health promotion

The purpose of this paper is to explore the sti- mulating factors and the obstacles experienced by nurses in community health centres in their work with prevention and health promotion. We conducted the qualitative research design con- sisting of fifteen nurses in five community health centres in Ghent. There was also a focus group and participant observation. All partici- pants experienced stimulating factors as well as obstacles in their work with preventive interven- tions. The most strongly facilitators were the Electronic Medical File and the multidisciplinary approach. The obstacles mentioned by the nurs- es were the communication problems and the financial problems. This study may have an im- portant impact on nursing practice and nursing prevention. Further research is needed on nurs- es working in community health centres and the way they approach their patients in the course of preventive actions

Elastic wave propagation in confined granular systems

We present numerical simulations of acoustic wave propagation in confined granular systems consisting of particles interacting with the three-dimensional Hertz-Mindlin force law. The response to a short mechanical excitation on one side of the system is found to be a propagating coherent wavefront followed by random oscillations made of multiply scattered waves. We find that the coherent wavefront is insensitive to details of the packing: force chains do not play an important role in determining this wavefront. The coherent wave propagates linearly in time, and its amplitude and width depend as a power law on distance, while its velocity is roughly compatible with the predictions of macroscopic elasticity. As there is at present no theory for the broadening and decay of the coherent wave, we numerically and analytically study pulse-propagation in a one-dimensional chain of identical elastic balls. The results for the broadening and decay exponents of this system differ significantly from those of the random packings. In all our simulations, the speed of the coherent wavefront scales with pressure as $p^{1/6}$; we compare this result with experimental data on various granular systems where deviations from the $p^{1/6}$ behavior are seen. We briefly discuss the eigenmodes of the system and effects of damping are investigated as well.Comment: 20 pages, 12 figures; changes throughout text, especially Section V.

Model for the Scaling of Stresses and Fluctuations in Flows near Jamming

We probe flows of soft, viscous spheres near the jamming point, which acts as a critical point for static soft spheres. Starting from energy considerations, we find nontrivial scaling of velocity fluctuations with strain rate. Combining this scaling with insights from jamming, we arrive at an analytical model that predicts four distinct regimes of flow, each characterized by rational-valued scaling exponents. Both the number of regimes and values of the exponents depart from prior results. We validate predictions of the model with simulations.Comment: 4 pages, 5 figures (revised text and one new figure). To appear in Phys. Rev. Let

Critical and non-critical jamming of frictional grains

We probe the nature of the jamming transition of frictional granular media by studying their vibrational properties as a function of the applied pressure p and friction coefficient mu. The density of vibrational states exhibits a crossover from a plateau at frequencies omega \gtrsim omega^*(p,mu) to a linear growth for omega \lesssim omega^*(p,mu). We show that omega^* is proportional to Delta z, the excess number of contacts per grains relative to the minimally allowed, isostatic value. For zero and infinitely large friction, typical packings at the jamming threshold have Delta z -> 0, and then exhibit critical scaling. We study the nature of the soft modes in these two limits, and find that the ratio of elastic moduli is governed by the distance from isostaticity.Comment: 4 pages, 4 figures; discussion update

Is privacy a problem during bedside handovers? : a practice-oriented discussion paper

Bedside handover is the delivery of the nurse-to-nurse handover at the patient's bedside. Although increasingly used in nursing, nurses report many barriers for delivering the bedside handover. Among these barriers is the possibility of breaching the patient's privacy. By referring to this concept, nurses add a legal and ethical dimension to the delivery of the bedside handover, making implementation of the method difficult or even impossible. In this discussion article, the concept of privacy during handovers is being discussed by use of observations, interviews with nurses, and interviews with patients. These findings are combined with international literature from a narrative review on the topic. We provide a practice-oriented answer in which two mutually exclusive possibilities are discussed. If bedside handover does pose problems concerning privacy, this situation is not unique in healthcare and measures can be taken during the bedside handover to safeguard the patient. If bedside handover does not pose problems concerning privacy, privacy is misused by nurses to hide professional uncertainties and/or a reluctance toward patient participation. Therefore, a possible breach of privacy-whether a justified argument or not-is not a reason for not delivering the bedside handover

Critical scaling in linear response of frictionless granular packings near jamming

We study the origin of the scaling behavior in frictionless granular media above the jamming transition by analyzing their linear response. The response to local forcing is non-self-averaging and fluctuates over a length scale that diverges at the jamming transition. The response to global forcing becomes increasingly non-affine near the jamming transition. This is due to the proximity of floppy modes, the influence of which we characterize by the local linear response. We show that the local response also governs the anomalous scaling of elastic constants and contact number.Comment: 4 pages, 3 figures. v2: Added new results; removed part of discussion; changed Fig.