2,425 research outputs found
How collective asperity detachments nucleate slip at frictional interfaces
Sliding at a quasi-statically loaded frictional interface can occur via
macroscopic slip events, which nucleate locally before propagating as rupture
fronts very similar to fracture. We introduce a novel microscopic model of a
frictional interface that includes asperity-level disorder, elastic interaction
between local slip events, and inertia. For a perfectly flat and homogeneously
loaded interface, we find that slip is nucleated by avalanches of asperity
detachments of extension larger than a critical radius governed by a
Griffith criterion. We find that after slip, the density of asperities at a
local distance to yielding presents a pseudo-gap , where is a non-universal exponent that depends on
the statistics of the disorder. This result makes a link between friction and
the plasticity of amorphous materials where a pseudo-gap is also present. For
friction, we find that a consequence is that stick-slip is an extremely slowly
decaying finite size effect, while the slip nucleation radius diverges as
a -dependent power law of the system size. We discuss how these
predictions can be tested experimentally
Theory for the density of interacting quasi-localised modes in amorphous solids
Quasi-localised modes appear in the vibrational spectrum of amorphous solids
at low-frequency. Though never formalised, these modes are believed to have a
close relationship with other important local excitations, including shear
transformations and two-level systems. We provide a theory for their frequency
density, , that establishes this link for
systems at zero temperature under quasi-static loading. It predicts two regimes
depending on the density of shear transformations (with
the additional stress needed to trigger a shear transformation). If
, and a finite fraction of quasi-localised modes form
shear transformations, whose amplitudes vanish at low frequencies. If
, and all quasi-localised modes form shear
transformations with a finite amplitude at vanishing frequencies. We confirm
our predictions numerically
Management of hyperkalemia in the acutely ill patient.
PURPOSE:To review the mechanisms of action, expected efficacy and side effects of strategies to control hyperkalemia in acutely ill patients. METHODS:We searched MEDLINE and EMBASE for relevant papers published in English between Jan 1, 1938, and July 1, 2018, in accordance with the PRISMA Statement using the following terms: "hyperkalemia," "intensive care," "acute kidney injury," "acute kidney failure," "hyperkalemia treatment," "renal replacement therapy," "dialysis," "sodium bicarbonate," "emergency," "acute." Reports from within the past 10 years were selected preferentially, together with highly relevant older publications. RESULTS:Hyperkalemia is a potentially life-threatening electrolyte abnormality and may cause cardiac electrophysiological disturbances in the acutely ill patient. Frequently used therapies for hyperkalemia may, however, also be associated with morbidity. Therapeutics may include the simultaneous administration of insulin and glucose (associated with frequent dysglycemic complications), β-2 agonists (associated with potential cardiac ischemia and arrhythmias), hypertonic sodium bicarbonate infusion in the acidotic patient (representing a large hypertonic sodium load) and renal replacement therapy (effective but invasive). Potassium-lowering drugs can cause rapid decrease in serum potassium level leading to cardiac hyperexcitability and rhythm disorders. CONCLUSIONS:Treatment of hyperkalemia should not only focus on the ability of specific therapies to lower serum potassium level but also on their potential side effects. Tailoring treatment to the patient condition and situation may limit the risks
Excess Vibrational Modes and the Boson Peak in Model Glasses
The excess low-frequency normal modes for two widely-used models of glasses
were studied at zero temperature. The onset frequencies for the anomalous modes
for both systems agree well with predictions of a variational argument, which
is based on analyzing the vibrational energy originating from the excess
contacts per particle over the minimum number needed for mechanical stability.
Even though both glasses studied have a high coordination number, most of the
additional contacts can be considered to be weak.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Direct measurement of superluminal group velocity and of signal velocity in an optical fiber
We present an easy way of observing superluminal group velocities using a
birefringent optical fiber and other standard devices. In the theoretical
analysis, we show that the optical properties of the setup can be described
using the notion of "weak value". The experiment shows that the group velocity
can indeed exceed c in the fiber; and we report the first direct observation of
the so-called "signal velocity", the speed at which information propagates and
that cannot exceed c.Comment: 5 pages, 5 figure
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