266 research outputs found
Three-dimensional shear in granular flow
The evolution of granular shear flow is investigated as a function of height
in a split-bottom Couette cell. Using particle tracking, magnetic-resonance
imaging, and large-scale simulations we find a transition in the nature of the
shear as a characteristic height is exceeded. Below there is a
central stationary core; above we observe the onset of additional axial
shear associated with torsional failure. Radial and axial shear profiles are
qualitatively different: the radial extent is wide and increases with height
while the axial width remains narrow and fixed.Comment: 4 pages, 5 figure
Granular packing simulation protocols: tap, press and relax
Granular matter takes many paths to pack. Gentle compression, compaction or
repetitive tapping can happen in natural and industrial processes. The path
influences the packing microstructure, and thus macroscale properties,
particularly for frictional grains. We perform discrete element modeling
simulations to construct packings of frictional spheres implementing a range of
stress-controlled protocols with 3D periodic boundary conditions. A
volume-controlled over-compression method is compared to four stress-controlled
methods, including over-compression and release, gentle under-compression and
cyclical compression and release. The packing volume fraction of each method
depends on the pressure, initial kinetic energy and protocol parameters. A
non-monotonic pressure dependence in the volume fraction, but not the
coordination number occurs when dilute particles initialized with a non-zero
kinetic energy are compressed, but can be reduced with the inclusion of drag.
The fraction of frictional contacts correlates with the volume fraction
minimum. Packings were cyclically compressed 1000 times. Response to
compression depends on pressure; low pressure packings have a constant volume
fraction regime, while high pressure packings continue to get dense with number
of cycles. The capability of stress-controlled, bulk-like particle simulations
to capture different protocols is showcased, and the ability to pack at low
pressures demonstrates unexpected behavior
Fractal dimensions of jammed packings with power-law particle size distributions in two and three dimensions
Static structure factors are computed for large-scale, mechanically stable,
jammed packings of frictionless spheres (three dimensions) and disks (two
dimensions) with broad, power-law size dispersity characterized by the exponent
. The static structure factor exhibits diverging power-law behavior for
small wavenumbers, allowing us to identify a structural fractal dimension,
. In three dimensions, for ,
such that each of the structure factors can be collapsed onto a universal
curve. In two dimensions, we instead find for
. Furthermore, we show that the fractal behavior
persists when rattler particles are removed, indicating that the long
wavelength structural properties of the packings are controlled by the large
particle backbone conferring mechanical rigidity to the system. A numerical
scheme for computing structure factors for triclinic unit cells is presented
and employed to analyze the jammed packings.Comment: 5 figures, 1 tabl
Functional profiling of single CRISPR/Cas9-edited human long-term hematopoietic stem cells.
In the human hematopoietic system, rare self-renewing multipotent long-term hematopoietic stem cells (LT-HSCs) are responsible for the lifelong production of mature blood cells and are the rational target for clinical regenerative therapies. However, the heterogeneity in the hematopoietic stem cell compartment and variable outcomes of CRISPR/Cas9 editing make functional interrogation of rare LT-HSCs challenging. Here, we report high efficiency LT-HSC editing at single-cell resolution using electroporation of modified synthetic gRNAs and Cas9 protein. Targeted short isoform expression of the GATA1 transcription factor elicit distinct differentiation and proliferation effects in single highly purified LT-HSC when analyzed with functional in vitro differentiation and long-term repopulation xenotransplantation assays. Our method represents a blueprint for systematic genetic analysis of complex tissue hierarchies at single-cell resolution
Decomposing the misery index: A dynamic approach
YesThe misery index (the unweighted sum of unemployment and inflation
rates) was probably the first attempt to develop a single statistic to measure the level
of a population’s economic malaise. In this letter, we develop a dynamic approach to
decompose the misery index using two basic relations of modern macroeconomics:
the expectations-augmented Phillips curve and Okun’s law. Our reformulation of the
misery index is closer in spirit to Okun’s idea. However, we are able to offer an improved
version of the index, mainly based on output and unemployment. Specifically,
this new Okun’s index measures the level of economic discomfort as a function of
three key factors: (1) the misery index in the previous period; (2) the output gap in
growth rate terms; and (3) cyclical unemployment. This dynamic approach differs
substantially from the standard one utilised to develop the misery index, and allow
us to obtain an index with five main interesting features: (1) it focuses on output,
unemployment and inflation; (2) it considers only objective variables; (3) it allows
a distinction
between short-run and long-run phenomena; (4) it places more
importance
on output and unemployment rather than inflation; and (5) it weights
recessions
more than expansions
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