275 research outputs found
Role of friction in pattern formation in oscillated granular layers
Particles in granular flows are often modeled as frictionless (smooth)
inelastic spheres; however, there exist no frictionless grains, just as there
are no elastic grains. Our molecular dynamics simulations reveal that friction
is essential for realistic modeling of vertically oscillated granular layers:
simulations of frictionless particles yield patterns with an onset at a
container acceleration about 30% smaller than that observed in experiments and
simulations with friction. More importantly, even though square and hexagonal
patterns form for a wide range of the oscillation parameters in experiments and
in our simulations of frictional inelastic particles, only stripe patterns form
in the simulations without friction, even if the inelasticity is increased to
obtain as much dissipation as in frictional particles. We also consider the
effect of particle friction on the shock wave that forms each time the granular
layer strikes the container. While a shock wave still forms for frictionless
particles, the height and time dependence of the hydrodynamic fields differ for
the cases with and without friction.Comment: final version appeared in Phys. Rev.
Superior pre-osteoblast cell response of etched ultrafine-grained titanium with a controlled crystallographic orientation
Ultrafine-grained (UFG) Ti for improved mechanical performance as well as its surface modification enhancing biofunctions has attracted much attention in medical industries. Most of the studies on the surface etching of metallic biomaterials have focused on surface topography and wettability but not crystallographic orientation, i.e., texture, which influences the chemical as well as the physical properties. In this paper, the influences of texture and grain size on roughness, wettability, and pre-osteoblast cell response were investigated in vitro after HF etching treatment. The surface characteristics and cell behaviors of ultrafine, fine, and coarse-grained Ti were examined after the HF etching. The surface roughness during the etching treatment was significantly increased as the orientation angle from the basal pole was increased. The cell adhesion tendency of the rough surface was promoted. The UFG Ti substrate exhibited a higher texture energy state, rougher surface, enhanced hydrophilic wettability, and better cell adhesion and proliferation behaviors after etching than those of the coarse- and fine-grained Ti substrates. These results provide a new route for enhancing both mechanical and biological performances using etching after grain refinement of Ti. ? The Author(s) 2017.115Ysciescopu
Solar Flares and Coronal Mass Ejections: A Statistically Determined Flare Flux-CME Mass Correlation
In an effort to examine the relationship between flare flux and corresponding
CME mass, we temporally and spatially correlate all X-ray flares and CMEs in
the LASCO and GOES archives from 1996 to 2006. We cross-reference 6,733 CMEs
having well-measured masses against 12,050 X-ray flares having position
information as determined from their optical counterparts. For a given flare,
we search in time for CMEs which occur 10-80 minutes afterward, and we further
require the flare and CME to occur within +/-45 degrees in position angle on
the solar disk. There are 826 CME/flare pairs which fit these criteria.
Comparing the flare fluxes with CME masses of these paired events, we find CME
mass increases with flare flux, following an approximately log-linear, broken
relationship: in the limit of lower flare fluxes, log(CME mass)~0.68*log(flare
flux), and in the limit of higher flare fluxes, log(CME mass)~0.33*log(flare
flux). We show that this broken power-law, and in particular the flatter slope
at higher flare fluxes, may be due to an observational bias against CMEs
associated with the most energetic flares: halo CMEs. Correcting for this bias
yields a single power-law relationship of the form log(CME mass)~0.70*log(flare
flux). This function describes the relationship between CME mass and flare flux
over at least 3 dex in flare flux, from ~10^-7 to 10^-4 W m^-2.Comment: 28 pages, 16 figures, accepted to Solar Physic
Collision statistics of driven granular materials
We present an experimental investigation of the statistical properties of
spherical granular particles on an inclined plane that are excited by an
oscillating side-wall. The data is obtained by high-speed imaging and particle
tracking techniques. We identify all particles in the system and link their
positions to form trajectories over long times. Thus, we identify particle
collisions to measure the effective coefficient of restitution and find a broad
distribution of values for the same impact angles. We find that the energy
inelasticity can take on values greater than one, which implies that the
rotational degrees play an important role in energy transfer. We also measure
the distance and the time between collision events in order to directly
determine the distribution of path lengths and the free times. These
distributions are shown to deviate from expected theoretical forms for elastic
spheres, demonstrating the inherent clustering in this system. We describe the
data with a two-parameter fitting function and use it to calculated the mean
free path and collision time. We find that the ratio of these values is
consistent with the average velocity. The velocity distribution are observed to
be strongly non-Gaussian and do not demonstrate any apparent universal
behavior. We report the scaling of the second moment, which corresponds to the
granular temperature, and higher order moments as a function of distance from
the driving wall. Additionally, we measure long time correlation functions in
both space and in the velocities to probe diffusion in a dissipative gas.Comment: 12 pages, 4 figures, uses revtex
A Statistical Study on Photospheric Magnetic Nonpotentiality of Active Regions and Its Relationship with Flares during Solar Cycles 22-23
A statistical study is carried out on the photospheric magnetic
nonpotentiality in solar active regions and its relationship with associated
flares. We select 2173 photospheric vector magnetograms from 1106 active
regions observed by the Solar Magnetic Field Telescope at Huairou Solar
Observing Station, National Astronomical Observatories of China, in the period
of 1988-2008, which covers most of the 22nd and 23rd solar cycles. We have
computed the mean planar magnetic shear angle (\bar{\Delta\phi}), mean shear
angle of the vector magnetic field (\bar{\Delta\psi}), mean absolute vertical
current density (\bar{|J_{z}|}), mean absolute current helicity density
(\bar{|h_{c}|}), absolute twist parameter (|\alpha_{av}|), mean free magnetic
energy density (\bar{\rho_{free}}), effective distance of the longitudinal
magnetic field (d_{E}), and modified effective distance (d_{Em}) of each
photospheric vector magnetogram. Parameters \bar{|h_{c}|}, \bar{\rho_{free}},
and d_{Em} show higher correlation with the evolution of the solar cycle. The
Pearson linear correlation coefficients between these three parameters and the
yearly mean sunspot number are all larger than 0.59. Parameters
\bar{\Delta\phi}, \bar{\Delta\psi}, \bar{|J_{z}|}, |\alpha_{av}|, and d_{E}
show only weak correlations with the solar cycle, though the nonpotentiality
and the complexity of active regions are greater in the activity maximum
periods than in the minimum periods. All of the eight parameters show positive
correlations with the flare productivity of active regions, and the combination
of different nonpotentiality parameters may be effective in predicting the
flaring probability of active regions.Comment: 20 pages, 5 figures, 4 tables, accepted for publication in Solar
Physic
Non-zero temperature transport near quantum critical points
We describe the nature of charge transport at non-zero temperatures ()
above the two-dimensional () superfluid-insulator quantum critical point. We
argue that the transport is characterized by inelastic collisions among
thermally excited carriers at a rate of order . This implies that
the transport at frequencies is in the hydrodynamic,
collision-dominated (or `incoherent') regime, while is
the collisionless (or `phase-coherent') regime. The conductivity is argued to
be times a non-trivial universal scaling function of , and not independent of , as has been previously
claimed, or implicitly assumed. The experimentally measured d.c. conductivity
is the hydrodynamic limit of this function, and is a
universal number times , even though the transport is incoherent.
Previous work determined the conductivity by incorrectly assuming it was also
equal to the collisionless limit of the scaling
function, which actually describes phase-coherent transport with a conductivity
given by a different universal number times . We provide the first
computation of the universal d.c. conductivity in a disorder-free boson model,
along with explicit crossover functions, using a quantum Boltzmann equation and
an expansion in . The case of spin transport near quantum
critical points in antiferromagnets is also discussed. Similar ideas should
apply to the transitions in quantum Hall systems and to metal-insulator
transitions. We suggest experimental tests of our picture and speculate on a
new route to self-duality at two-dimensional quantum critical points.Comment: Feedback incorporated into numerous clarifying remarks; additional
appendix discusses relationship to transport in dissipative quantum mechanics
and quantum Hall edge state tunnelling problems, stimulated by discussions
with E. Fradki
The Physical Processes of CME/ICME Evolution
As observed in Thomson-scattered white light, coronal mass ejections (CMEs) are manifest as large-scale expulsions of plasma magnetically driven from the corona in the most energetic eruptions from the Sun. It remains a tantalizing mystery as to how these erupting magnetic fields evolve to form the complex structures we observe in the solar wind at Earth. Here, we strive to provide a fresh perspective on the post-eruption and interplanetary evolution of CMEs, focusing on the physical processes that define the many complex interactions of the ejected plasma with its surroundings as it departs the corona and propagates through the heliosphere. We summarize the ways CMEs and their interplanetary CMEs (ICMEs) are rotated, reconfigured, deformed, deflected, decelerated and disguised during their journey through the solar wind. This study then leads to consideration of how structures originating in coronal eruptions can be connected to their far removed interplanetary counterparts. Given that ICMEs are the drivers of most geomagnetic storms (and the sole driver of extreme storms), this work provides a guide to the processes that must be considered in making space weather forecasts from remote observations of the corona.Peer reviewe
Large-scale association analyses identify host factors influencing human gut microbiome composition
To study the effect of host genetics on gut microbiome composition, the MiBioGen consortium curated and analyzed genome-wide genotypes and 16S fecal microbiome data from 18,340 individuals (24 cohorts). Microbial composition showed high variability across cohorts: only 9 of 410 genera were detected in more than 95% of samples. A genome-wide association study of host genetic variation regarding microbial taxa identified 31 loci affecting the microbiome at a genome-wide significant (P < 5 x 10(-8)) threshold. One locus, the lactase (LCT) gene locus, reached study-wide significance (genome-wide association study signal: P = 1.28 x 10(-20)), and it showed an age-dependent association with Bifidobacterium abundance. Other associations were suggestive (1.95 x 10(-10) < P < 5 x 10(-8)) but enriched for taxa showing high heritability and for genes expressed in the intestine and brain. A phenome-wide association study and Mendelian randomization identified enrichment of microbiome trait loci in the metabolic, nutrition and environment domains and suggested the microbiome might have causal effects in ulcerative colitis and rheumatoid arthritis
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