4,038 research outputs found
Quasi-2D dynamic jamming in cornstarch suspensions: visualization and force measurements
We report experiments investigating jamming fronts in a floating layer of
cornstarch suspension. The suspension has a packing fraction close to jamming,
which dynamically turns into a solid when impacted at a high speed. We show
that the front propagates in both axial and transverse direction from the point
of impact, with a constant ratio between the two directions of propagation of
approximately 2. Inside the jammed solid, we observe an additional compression,
which results from the increasing stress as the solid grows. During the initial
growth of the jammed solid, we measure a force response that can be completely
accounted for by added mass. Only once the jamming front reaches a boundary,
the added mass cannot account for the measured force anymore. We do not,
however, immediately see a strong force response as we would expect when
compressing a jammed packing. Instead, we observe a delay in the force response
on the pusher, which corresponds to the time it takes for the system to develop
a close to uniform velocity gradient that spans the complete system.Comment: 7 pages, 7 figure
High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming
A remarkable property of dense suspensions is that they can transform from
liquid-like at rest to solid-like under sudden impact. Previous work showed
that this impact-induced solidification involves rapidly moving jamming fronts;
however, details of this process have remained unresolved. Here we use
high-speed ultrasound imaging to probe non-invasively how the interior of a
dense suspension responds to impact. Measuring the speed of sound we
demonstrate that the solidification proceeds without a detectable increase in
packing fraction, and imaging the evolving flow field we find that the shear
intensity is maximized right at the jamming front. Taken together, this
provides direct experimental evidence for jamming by shear, rather than
densification, as driving the transformation to solid-like behavior. Based on
these findings we propose a new model to explain the anisotropy in the
propagation speed of the fronts and delineate the onset conditions for dynamic
shear jamming in suspensions.Comment: 9 pages, 3 figure
Dynamic shear jamming in dense granular suspensions under extension
Unlike dry granular materials, a dense granular suspension like cornstarch in
water can strongly resist extensional flows. At low extension rates, such a
suspension behaves like a viscous liquid, but rapid extension results in a
response where stresses far exceed the predictions of lubrication hydrodynamics
and capillarity. To understand this remarkable mechanical response, we
experimentally measure the normal force imparted by a large bulk of the
suspension on a plate moving vertically upward at a controlled velocity. We
observe that above a velocity threshold, the peak force increases by orders of
magnitude. Using fast ultrasound imaging we map out the local velocity profiles
inside the suspension which reveal the formation of a growing jammed region
under rapid extension. This region interacts with the rigid boundaries of the
container through strong velocity gradients, suggesting a direct connection to
the recently proposed shear-jamming mechanism.Comment: Accepted for publication in Phys. Rev.
The Effects of Variations in Nuclear Interactions on Nucleosynthesis in Thermonuclear Supernovae
The impact of nuclear physics uncertainties on nucleosynthesis in
thermonuclear supernovae has not been fully explored using comprehensive and
systematic studies with multiple models. To better constrain predictions of
yields from these phenomena, we have performed a sensitivity study by
post-processing thermodynamic histories from two different hydrodynamic,
Chandrasekhar-mass explosion models. We have individually varied all input
reaction and, for the first time, weak interaction rates by a factor of ten and
compared the yields in each case to yields using standard rates. Of the 2305
nuclear reactions in our network, we find that the rates of only 53 reactions
affect the yield of any species with an abundance of at least 10^-8 M_sun by at
least a factor of two, in either model. The rates of the 12C(a,g), 12C+12C,
20Ne(a,p), 20Ne(a,g) and 30Si(p,g) reactions are among those that modify the
most yields when varied by a factor of ten. From the individual variation of
658 weak interaction rates in our network by a factor of ten, only the stellar
28Si(b+)28Al, 32S(b+)32P and 36Ar(b+)36Cl rates significantly affect the yields
of species in a model. Additional tests reveal that reaction rate changes over
temperatures T > 1.5 GK have the greatest impact, and that ratios of
radionuclides that may be used as explosion diagnostics change by a factor of
less than two from the variation of individual rates by a factor of 10.
Nucleosynthesis in the two adopted models is relatively robust to variations in
individual nuclear reaction and weak interaction rates. Laboratory measurements
of a limited number of reactions would help to further constrain predictions.
As well, we confirm the need for a consistent treatment for relevant stellar
weak interaction rates since simultaneous variation of these rates (as opposed
to individual variation) has a significant effect on yields in our models.Comment: accepted by A&A, 14 pages, 5 figures, 2 table
Splash wave and crown breakup after disc impact on a liquid surface
In this paper we analyze the impact of a circular disc on a free surface
using experiments, potential flow numerical simulations and theory. We focus
our attention both on the study of the generation and possible breakup of the
splash wave created after the impact and on the calculation of the force on the
disc. We have experimentally found that drops are only ejected from the rim
located at the top part of the splash --giving rise to what is known as the
crown splash-- if the impact Weber number exceeds a threshold value
\Weber_{crit}\simeq 140. We explain this threshold by defining a local Bond
number based on the rim deceleration and its radius of curvature,
with which we show using both numerical simulations and experiments that a
crown splash only occurs when , revealing that the rim
disrupts due to a Rayleigh-Taylor instability. Neglecting the effect of air, we
show that the flow in the region close to the disc edge possesses a
Weber-number-dependent self-similar structure for every Weber number. From this
we demonstrate that \Bond_{tip}\propto\Weber, explaining both why the
transition to crown splash can be characterized in terms of the impact Weber
number and why this transition occurs for .
Next, including the effect of air, we have developed a theory which predicts
the time-varying thickness of the very thin air cushion that is entrapped
between the impacting solid and the liquid. Our analysis reveals that gas
critically affect the velocity of propagation of the splash wave as well as the
time-varying force on the disc, . The existence of the air layer also
limits the range of times in which the self-similar solution is valid and,
accordingly, the maximum deceleration experienced by the liquid rim, what sets
the length scale of the splash drops ejected when We>\Weber_{crit}
Fermi-surface calculation of the anomalous Hall conductivity
While the intrinsic anomalous Hall conductivity is normally written in terms
of an integral of the electronic Berry curvature over the occupied portions of
the Brillouin zone, Haldane has recently pointed out that this quantity (or
more precisely, its ``non-quantized part'') may alternatively be expressed as a
Fermi-surface property. Here we present an {\it ab-initio} approach for
computing the anomalous Hall conductivity that takes advantage of this
observation by converting the integral over the Fermi sea into a more efficient
integral on the Fermi surface only. First, a conventional electronic-structure
calculation is performed with spin-orbit interaction included.
Maximally-localized Wannier functions are then constructed by a post-processing
step in order to convert the {\it ab-initio} electronic structure around the
Fermi level into a tight-binding-like form. Working in the Wannier
representation, the Brillouin zone is sampled on a large number of equally
spaced parallel slices oriented normal to the total magnetization. On each
slice, we find the intersections of the Fermi-surface sheets with the slice by
standard contour methods, organize these into a set of closed loops, and
compute the Berry phases of the Bloch states as they are transported around
these loops. The anomalous Hall conductivity is proportional to the sum of the
Berry phases of all the loops on all the slices. Illustrative calculations are
performed for Fe, Co and Ni.Comment: 12 pages, 9 figure
Wannier-based calculation of the orbital magnetization in crystals
We present a first-principles scheme that allows the orbital magnetization of
a magnetic crystal to be evaluated accurately and efficiently even in the
presence of complex Fermi surfaces. Starting from an initial
electronic-structure calculation with a coarse ab initio k-point mesh,
maximally localized Wannier functions are constructed and used to interpolate
the necessary k-space quantities on a fine mesh, in parallel to a
previously-developed formalism for the anomalous Hall conductivity [X.Wang, J.
Yates, I. Souza, and D. Vanderbilt, Phys. Rev. B 74, 195118 (2006)]. We
formulate our new approach in a manifestly gauge-invariant manner, expressing
the orbital magnetization in terms of traces over matrices in Wannier space.
Since only a few (e.g., of the order of 20) Wannier functions are typically
needed to describe the occupied and partially occupied bands, these Wannier
matrices are small, which makes the interpolation itself very efficient. The
method has been used to calculate the orbital magnetization of bcc Fe, hcp Co,
and fcc Ni. Unlike an approximate calculation based on integrating orbital
currents inside atomic spheres, our results nicely reproduce the experimentally
measured ordering of the orbital magnetization in these three materials.Comment: 13 pages, 3 figures, 4 table
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