2,008 research outputs found
Impulse distributions in dense granular flows: signatures of large-scale spatial structures
In this paper we report the results of simulations of a 2D gravity driven,
dissipative granular flow through a hopper system. Measurements of impulse
distributions P(I) on the simulated system show flow-velocity-invariant
behavior of the distribution for impulses larger than the average impulse .
For small impulses, however, P(I) decreases significantly with flow velocity, a
phenomenon which can be attributed exclusively to collisions between grains
undergoing frequent collisions. Visualizations of the system also show that
these frequently colliding particles tend to form increasingly large linear
clusters as the flow velocity decreases. A model is proposed for the form of
P(I), given distributions of cluster size and velocity, which accurately
predicts the observed form of the distribution. Thus the impulse distribution
provides some insight into the formation and properties of these ``dynamic''
force chains.Comment: 4 pages, 4 figure
Universal Robotic Gripper based on the Jamming of Granular Material
Gripping and holding of objects are key tasks for robotic manipulators. The
development of universal grippers able to pick up unfamiliar objects of widely
varying shape and surface properties remains, however, challenging. Most
current designs are based on the multi-fingered hand, but this approach
introduces hardware and software complexities. These include large numbers of
controllable joints, the need for force sensing if objects are to be handled
securely without crushing them, and the computational overhead to decide how
much stress each finger should apply and where. Here we demonstrate a
completely different approach to a universal gripper. Individual fingers are
replaced by a single mass of granular material that, when pressed onto a target
object, flows around it and conforms to its shape. Upon application of a vacuum
the granular material contracts and hardens quickly to pinch and hold the
object without requiring sensory feedback. We find that volume changes of less
than 0.5% suffice to grip objects reliably and hold them with forces exceeding
many times their weight. We show that the operating principle is the ability of
granular materials to transition between an unjammed, deformable state and a
jammed state with solid-like rigidity. We delineate three separate mechanisms,
friction, suction and interlocking, that contribute to the gripping force.
Using a simple model we relate each of them to the mechanical strength of the
jammed state. This opens up new possibilities for the design of simple, yet
highly adaptive systems that excel at fast gripping of complex objects.Comment: 10 pages, 7 figure
Spontaneous Chelation-Driven Reduction of the Neptunyl Cation in Aqueous Solution.
Octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) siderophore analogues are known to be efficacious chelators of the actinide cations, and these ligands are also capable of facilitating both activation and reduction of actinyl species. Utilizing X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, as well as cyclic voltammetry measurements, herein, we elucidate chelation-based mechanisms for driving reactivity and initiating redox processes in a family of neptunyl-HOPO and CAM complexes. Based on the selected chelator, the ability to control the oxidation state of neptunium and the speed of reduction and concurrent oxo group activation was demonstrated. Most notably, reduction kinetics for the NpV O2 +/ /NpIV redox couple upon chelation by the ligands 3,4,3-LI(1,2-HOPO) and 3,4,3-LI(CAM)2 (1,2-HOPO)2 was observed to be faster than ever reported, and in fact quicker than we could measure using either X-ray absorption spectroscopy or electrochemical techniques
From interacting particle systems to random matrices
In this contribution we consider stochastic growth models in the
Kardar-Parisi-Zhang universality class in 1+1 dimension. We discuss the large
time distribution and processes and their dependence on the class on initial
condition. This means that the scaling exponents do not uniquely determine the
large time surface statistics, but one has to further divide into subclasses.
Some of the fluctuation laws were first discovered in random matrix models.
Moreover, the limit process for curved limit shape turned out to show up in a
dynamical version of hermitian random matrices, but this analogy does not
extend to the case of symmetric matrices. Therefore the connections between
growth models and random matrices is only partial.Comment: 18 pages, 8 figures; Contribution to StatPhys24 special issue; minor
corrections in scaling of section 2.
Discovery of a Variable Star Population in NGC 2808
We have applied the image subtraction method to images of the peculiar,
bimodal-horizontal branch globular cluster NGC 2808, taken over a total of six
nights over a range of five months. As a result, we have found, for the first
time, a sizeable population of variable stars in the crowded inner regions of
the cluster, thus raising the known RR Lyrae population in the cluster to a
total of 18 stars. In addition, an eclipsing binary and two other variables
with periods longer than 1 day were also found. Periods, positions and
(differential) light curves are provided for all the detected variables. The
Oosterhoff classification of NGC 2808, which has recently been associated with
a previously unknown dwarf galaxy in Canis Major, is briefly discussed.Comment: 8 pages, 4 figures. A&A, in pres
M75, a Globular Cluster with a Trimodal Horizontal Branch. I. Color-Magnitude Diagram
Deep UBVI photometry for a large field covering the distant globular cluster
M75 (NGC 6864) is presented. We confirm a previous suggestion (Catelan et al.
1998a) that M75 possesses a bimodal horizontal branch (HB) bearing striking
resemblance to the well-known case of NGC 1851. In addition, we detect a third,
smaller grouping of stars on the M75 blue tail, separated from the bulk of the
blue HB stars by a gap spanning about 0.5 mag in V. Such a group of stars may
correspond to the upper part of a very extended, though thinly populated, blue
tail. Thus M75 appears to have a trimodal HB. The presence of the "Grundahl
jump" is verified using the broadband U filter. We explore the color-magnitude
diagram of M75 with the purpose of deriving the cluster's fundamental
parameters, and find a metallicity of [Fe/H] = -1.03 +/- 0.17 dex and -1.24 +/-
0.21 dex in the Carretta & Gratton (1997) and Zinn & West (1984) scales,
respectively. We discuss earlier suggestions that the cluster has an
anomalously low ratio of bright red giants to HB stars. A differential age
analysis with respect to NGC 1851 suggests that the two clusters are
essentially coeval.Comment: 19 pages, 15 figures, emulateapj5/apjfonts style. Astronomical
Journal, in press. This version contains some very low-resolution figures,
due to the size constraints of astro-ph. We strongly encourage the interested
reader to download instead the preprint with full-resolution figures, which
can be found at http://www.astro.puc.cl/~mcatelan
Magneto-optical Trapping of Cadmium
We report the laser-cooling and confinement of Cd atoms in a magneto-optical
trap, and characterize the loading process from the background Cd vapor. The
trapping laser drives the 1S0-1P1 transition at 229 nm in this two-electron
atom and also photoionizes atoms directly from the 1P1 state. This
photoionization overwhelms the other loss mechanisms and allows a direct
measurement of the photoionization cross section, which we measure to be
2(1)x10^(-16)cm^(2) from the 1P1 state. When combined with nearby laser-cooled
and trapped Cd^(+) ions, this apparatus could facilitate studies in ultracold
interactions between atoms and ions.Comment: 8 pages, 11 figure
Airy processes and variational problems
We review the Airy processes; their formulation and how they are conjectured
to govern the large time, large distance spatial fluctuations of one
dimensional random growth models. We also describe formulas which express the
probabilities that they lie below a given curve as Fredholm determinants of
certain boundary value operators, and the several applications of these
formulas to variational problems involving Airy processes that arise in
physical problems, as well as to their local behaviour.Comment: Minor corrections. 41 pages, 4 figures. To appear as chapter in "PASI
Proceedings: Topics in percolative and disordered systems
Observation of p-wave Threshold Law Using Evaporatively Cooled Fermionic Atoms
We have measured independently both s-wave and p-wave cross-dimensional
thermalization rates for ultracold potassium-40 atoms held in a magnetic trap.
These measurements reveal that this fermionic isotope has a large positive
s-wave triplet scattering length in addition to a low temperature p-wave shape
resonance. We have observed directly the p-wave threshold law which, combined
with the Fermi statistics, dramatically suppresses elastic collision rates at
low temperatures. In addition, we present initial evaporative cooling results
that make possible these collision measurements and are a precursor to
achieving quantum degeneracy in this neutral, low-density Fermi system.Comment: 5 pages, 3 figures, 1 tabl
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