2,004 research outputs found
Shocks near Jamming
Non-linear sound is an extreme phenomenon typically observed in solids after
violent explosions. But granular media are different. Right when they jam,
these fragile and disordered solids exhibit a vanishing rigidity and sound
speed, so that even tiny mechanical perturbations form supersonic shocks. Here,
we perform simulations in which two-dimensional jammed granular packings are
dynamically compressed, and demonstrate that the elementary excitations are
strongly non-linear shocks, rather than ordinary phonons. We capture the full
dependence of the shock speed on pressure and impact intensity by a
surprisingly simple analytical model.Comment: Revised version. Accepted for publication in Phys. Rev. Let
Jaming and Geometry of Two-Dimensional Foams
We experimentally probe the vicinity of the jamming point J, located at a
density corresponding to random close packing (), in
two dimensional, bidisperse packings of foam bubbles. We vary the density of
the foam layer and extract geometrical measures by image analysis. We confirm
the predicted scaling of the average contact number Z with and compare
the distribution of local contact numbers to a simple model. We further
establish that the distribution of areas strongly depends on .
Finally, we find that the distribution of contact forces systematically
varies with density.Comment: 6 pages, 5 figures, submitte
Sheared force-networks: anisotropies, yielding and geometry
A scenario for yielding of granular matter is presented by considering the
ensemble of force networks for a given contact network and applied shear stress
. As is increased, the probability distribution of contact forces
becomes highly anisotropic, the difference between average contact forces along
minor and major axis grows, and the allowed networks span a shrinking subspace
of all force-networks. Eventually, contacts start to break, and at the yielding
shear stress, the packing becomes effectively isostatic. The size of the
allowed subspace exhibits simple scaling properties, which lead to a prediction
of the yield stress for packings of arbitrary contact number.Comment: 4 pages, 4 figure
In-vitro activation of complement system by lactic acidosis in newborn and adults.
INTRODUCTION: Complement activation occurs secondary to a variety of external stimuli. Lactic acidosis has been previously shown to activate the complement factors C3a and C5a. In the present investigation we examined the differential effect of lactic acidosis on anaphylatoxin levels in cord and adult blood. Furthermore we aimed to determine if the entire complement cascade could be activated by lactic acidosis. METHODS: Cord and adult blood samples (n = 20 each) were collected and incubated for one hour in either untreated condition or with the addition of lactate in two concentrations (5.5 mmol/l vs. 22 mmol/l). Following incubation, levels of C3a, C5a and sC5b-9, and blood gas parameters were determined. RESULTS: Anaphylatoxin (C3a and C5a) and sC5b-9 levels increased with the addition of lactate in a dose-dependent manner in cord and adult blood (C3a: 1 h, 5.5 mmo/l, 22 mmol/l: 418/498/622 microg/l in cord blood; 1010/1056/1381 microg/l in adult blood, p<0,05; similar results were found for C5a and sC5b-9). CONCLUSION: Lactic acidosis leads to an activation of the entire complement system in neonates and in adults. This activation is dose-dependent and more pronounced in adults as compared to neonates
Spatiotemporal Chaos, Localized Structures and Synchronization in the Vector Complex Ginzburg-Landau Equation
We study the spatiotemporal dynamics, in one and two spatial dimensions, of
two complex fields which are the two components of a vector field satisfying a
vector form of the complex Ginzburg-Landau equation. We find synchronization
and generalized synchronization of the spatiotemporally chaotic dynamics. The
two kinds of synchronization can coexist simultaneously in different regions of
the space, and they are mediated by localized structures. A quantitative
characterization of the degree of synchronization is given in terms of mutual
information measures.Comment: 6 pages, using bifchaos.sty (included). 7 figures. Related material,
including higher quality figures, could be found at
http://www.imedea.uib.es/PhysDept/publicationsDB/date.html . To appear in
International Journal of Bifurcation and Chaos (1999
COMBINED GEOMETRIC AND THERMAL ANALYSIS FROM UAV PLATFORMS FOR ARCHAEOLOGICAL HERITAGE DOCUMENTATION
The aim of this work is to study the value and potential of UAV technology as an instrument for documenting and analyzing a heritage site on both the detailed scale and the wider territorial scale. In particular, this paper will focus on the application of an UAV platform on the archeological site of Isola Comacina (Comacina Island), in the Lago di Como (Lake Como, Lombardy, Northern Italy). The work considers the advantages of different metric scales and the use of both RGB and thermal imagery, along with other terrestrial data (total station measurements and laser scans), in order to arrive at a working heritage information model. In particular, the archaeological remains on Isola Comacina have been intensively studied before by standard techniques but unfortunately no wider context is provided. A part of the research is the investigation of new methodologies offered by accurate geometric reconstructions combined with thermal imagery acquired by means of UAV platforms, e.g. the support of this type of imagery to discover rock formations partially buried
Bounds on the shear load of cohesionless granular matter
We characterize the force state of shear-loaded granular matter by relating
the macroscopic stress to statistical properties of the force network. The
purely repulsive nature of the interaction between grains naturally provides an
upper bound for the sustainable shear stress, which we analyze using an
optimization procedure inspired by the so-called force network ensemble. We
establish a relation between the maximum possible shear resistance and the
friction coefficient between individual grains, and find that anisotropies of
the contact network (or the fabric tensor) only have a subdominant effect.
These results can be considered the hyperstatic limit of the force network
ensemble and we discuss possible implications for real systems. Finally, we
argue how force anisotropies can be related quantitatively to experimental
measurements of the effective elastic constants.Comment: 17 pages, 6 figures. v2: slightly rearranged, introduction and
discussion rewritte
An Integration of Chemistry, Biology, and Physics: The Interdisciplinary Laboratory
As a new venture to integrate research and education, a pilot section of a first-year laboratory sequence known as the Interdisciplinary Laboratory (ID Lab) was introduced on the Harvey Mudd campus during the 1999–2000 academic year and continues to be offered. The ID Lab attempts to bridge laboratory experiences from biology, chemistry, and physics for the first-year student. Taught by a team of faculty from these disciplines, the course seeks both to illustrate commonality of investigative methods and laboratory techniques in these sciences and to introduce discipline-specific principles. Experiments with a chemistry component include the Molecular Weight of Macromolecules, the Synthesis and Characterization of Liquid Crystals, A Structure–Activity Investigation of Photosynthetic Electron Transport, a Genetic Map of a Bacterial Plasmid, and The Carbonate Content of Biological Hard Tissue. This article provides some details of the experiments conducted, information on the philosophy and mechanics of the course, and a discussion of student reaction to this innovative and novel course
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