218 research outputs found
Transforming mesoscale granular plasticity through particle shape
When an amorphous material is strained beyond the point of yielding it enters
a state of continual reconfiguration via dissipative, avalanche-like slip
events that relieve built-up local stress. However, how the statistics of such
events depend on local interactions among the constituent units remains
debated. To address this we perform experiments on granular material in which
we use particle shape to vary the interactions systematically. Granular
material, confined under constant pressure boundary conditions, is uniaxially
compressed while stress is measured and internal rearrangements are imaged with
x-rays. We introduce volatility, a quantity from economic theory, as a powerful
new tool to quantify the magnitude of stress fluctuations, finding systematic,
shape-dependent trends. For all 22 investigated shapes the magnitude of
relaxation events is well-fit by a truncated power law distribution , as has been proposed within the context of plasticity
models. The power law exponent for all shapes tested clusters around
1.5, within experimental uncertainty covering the range 1.3 - 1.7. The
shape independence of and its compatibility with mean field models
indicate that the granularity of the system, but not particle shape, modifies
the stress redistribution after a slip event away from that of continuum
elasticity. Meanwhile, the characteristic maximum event size changes by
two orders of magnitude and tracks the shape dependence of volatility. Particle
shape in granular materials is therefore a powerful new factor influencing the
distance at which an amorphous system operates from scale-free criticality.
These experimental results are not captured by current models and suggest a
need to reexamine the mechanisms driving mesoscale plastic deformation in
amorphous systems.Comment: 11 pages, 8 figures. v3 adds a new appendix and figure about event
rates and changes several parts the tex
Immediate postoperative thrombolytic therapy: An aggressive strategy for neurologic salvage when cerebral thromboembolism complicates carotid endarterectomy
AbstractA 42-year-old man with a high-grade left internal carotid artery (ICA) stenosis demonstrated on a duplex scan was referred to us. A cerebral arteriogram confirmed a greater than 90% left internal carotid stenosis, but with the unexpected finding of a moderate amount of thrombus in the proximal ICA. He underwent emergent left carotid endarterectomy, but during the operation, only a small amount of thrombus was identified as adherent to the atherosclerotic plaque. he awakened in the operating room with a dense right hemiplegia and aphasia. Immediate reexploration demonstrated a patent endarterectomy site, a distal thromboembolectomy was performed without extraction of thrombus, and urokinase (250,000 Units) was infused into the distal ICA. He reawakened with an unchanged right hemiplegia and aphasia. The patient then underwent an urgent postoperative carotid and cerebral arteriogram that demonstrated an embolus to the middle cerebral artery. he was treated with the superselective infusion of urokinase (500,000 Units), with almost complete resolution of the clot. Over the course of the next 48 hours, the patient made a nearly complete neurologic recovery, and he was discharged from the hospital with only a slight facial droop. At 2 months' follow-up he was completely neurologically healthy. To our knowledge this is the first reported case of urokinase administered in the immediate postoperative period in the angiography suite to treat a thromboembolus complicating a carotid endarterectomy. (J Vasc Surg 2000;31:1033-7.
Mechanical prions: Self-assembling microstructures
Prions are misfolded proteins that transmit their structural arrangement to
neighboring proteins. In biological systems, prion dynamics can produce a
variety of complex functional outcomes. Yet, an understanding of prionic causes
has been hampered by the fact that few computational models exist that allow
for experimental design, hypothesis testing, and control. Here, we identify
essential prionic properties and present a biologically inspired model of
prions using simple mechanical structures capable of undergoing complex
conformational change. We demonstrate the utility of our approach by designing
a prototypical mechanical prion and validating its properties experimentally.
Our work provides a design framework for harnessing and manipulating prionic
properties in natural and artificial systems.Comment: Added supplements, 25 pages, 11 figure
Intrinsically motivated graph exploration using network theories of human curiosity
Intrinsically motivated exploration has proven useful for reinforcement
learning, even without additional extrinsic rewards. When the environment is
naturally represented as a graph, how to guide exploration best remains an open
question. In this work, we propose a novel approach for exploring
graph-structured data motivated by two theories of human curiosity: the
information gap theory and the compression progress theory. The theories view
curiosity as an intrinsic motivation to optimize for topological features of
subgraphs induced by the visited nodes in the environment. We use these
proposed features as rewards for graph neural-network-based reinforcement
learning. On multiple classes of synthetically generated graphs, we find that
trained agents generalize to larger environments and to longer exploratory
walks than are seen during training. Our method computes more efficiently than
the greedy evaluation of the relevant topological properties. The proposed
intrinsic motivations bear particular relevance for recommender systems. We
demonstrate that curiosity-based recommendations are more predictive of human
behavior than PageRank centrality for several real-world graph datasets,
including MovieLens, Amazon Books, and Wikispeedia.Comment: 14 pages, 5 figures in main text, and 15 pages, 8 figures in
supplemen
Anisotropic Transport of Quantum Hall Meron-Pair Excitations
Double-layer quantum Hall systems at total filling factor can
exhibit a commensurate-incommensurate phase transition driven by a magnetic
field oriented parallel to the layers. Within the commensurate
phase, the lowest charge excitations are believed to be linearly-confined Meron
pairs, which are energetically favored to align with . In order
to investigate this interesting object, we propose a gated double-layer Hall
bar experiment in which can be rotated with respect to the
direction of a constriction. We demonstrate the strong angle-dependent
transport due to the anisotropic nature of linearly-confined Meron pairs and
discuss how it would be manifested in experiment.Comment: 4 pages, RevTex, 3 postscript figure
Quiescience as a mechanism for cyclical hypoxia and acidosis
Tumour tissue characteristically experiences fluctuations in substrate supply. This unstable microenvironment drives constitutive metabolic changes within cellular populations and, ultimately, leads to a more aggressive phenotype. Previously, variations in substrate levels were assumed to occur through oscillations in the hæmodynamics of nearby and distant blood vessels. In this paper we examine an alternative hypothesis, that cycles of metabolite concentrations are also driven by cycles of cellular quiescence and proliferation. Using a mathematical modelling approach, we show that the interdependence between cell cycle and the microenvironment will induce typical cycles with the period of order hours in tumour acidity and oxygenation. As a corollary, this means that the standard assumption of metabolites entering diffusive equilibrium around the tumour is not valid; instead temporal dynamics must be considered
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