22 research outputs found
Vortices in vibrated granular rods
We report the experimental observation of novel vortex patterns in vertically
vibrated granular rods. Above a critical packing fraction, moving ordered
domains of nearly vertical rods spontaneously form and coexist with horizontal
rods. The domains of vertical rods coarsen in time to form large vortices. We
investigate the conditions under which the vortices occur by varying the number
of rods, vibration amplitude and frequency. The size of the vortices increases
with the number of rods. We characterize the growth of the ordered domains by
measuring the area fraction of the ordered regions as a function of time. A
{\em void filling} model is presented to describe the nucleation and growth of
the vertical domains. We track the ends of the vertical rods and obtain the
velocity fields of the vortices. The rotation speed of the rods is observed to
depend on the vibration velocity of the container and on the packing. To
investigate the impact of the direction of driving on the observed phenomena,
we performed experiments with the container vibrated horizontally. Although
vertical domains form, vortices are not observed. We therefore argue that the
motion is generated due to the interaction of the inclination of the rods with
the bottom of a vertically vibrated container. We also perform simple
experiments with a single row of rods in an annulus. These experiments directly
demonstrate that the rod motion is generated when the rods are inclined from
the vertical, and is always in the direction of the inclination.Comment: 6 pages, 10 figure, 2 movies at http://physics.clarku.edu/vortex uses
revtex
Cracking Piles of Brittle Grains
A model which accounts for cracking avalanches in piles of grains subject to
external load is introduced and numerically simulated. The stress is
stochastically transferred from higher layers to lower ones. Cracked areas
exhibit various morphologies, depending on the degree of randomness in the
packing and on the ductility of the grains. The external force necessary to
continue the cracking process is constant in wide range of values of the
fraction of already cracked grains. If the grains are very brittle, the force
fluctuations become periodic in early stages of cracking. Distribution of
cracking avalanches obeys a power law with exponent .Comment: RevTeX, 6 pages, 7 postscript figures, submitted to Phys. Rev.
Properties of layer-by-layer vector stochastic models of force fluctuations in granular materials
We attempt to describe the stress distributions of granular packings using
lattice-based layer-by-layer stochastic models that satisfy the constraints of
force and torque balance and non-tensile forces at each site. The inherent
asymmetry in the layer-by-layer approach appears to lead to an asymmetric force
distribution, in disagreement with both experiments and general symmetry
considerations. The vertical force component probability distribution is robust
and in agreement with predictions of the scalar q model while the distribution
of horizontal force components is qualitatively different and depends on the
details of implementation.Comment: 18 pages, 12 figures (with subfigures), 1 table. Uses revtex,
epsfig,subfigure, and cite. Submitted to PRE. Plots have been bitmapped.
High-resolution version is available. Email [email protected] or
download from http://rainbow.uchicago.edu/~mbnguyen/research/vm.htm
Pharmacological Properties and Physiological Function of a P2X-Like Current in Single Proximal Tubule Cells Isolated from Frog Kidney
Although previous studies have provided evidence for the expression of P2X receptors in renal proximal tubule, only one cell line study has provided functional evidence. The current study investigated the pharmacological properties and physiological role of native P2X-like currents in single frog proximal tubule cells using the whole-cell patch-clamp technique. Extracellular ATP activated a cation conductance (P2Xf) that was also Ca2+-permeable. The agonist sequence for activation was ATP = αÎČ-MeATP > BzATP = 2-MeSATP, and P2Xf was inhibited by suramin, PPADS and TNP-ATP. Activation of P2Xf attenuated the rundown of a quinidine-sensitive K+ conductance, suggesting that P2Xf plays a role in K+ channel regulation. In addition, ATP/ADP apyrase and inhibitors of P2Xf inhibited regulatory volume decrease (RVD). These data are consistent with the presence of a P2X receptor that plays a role in the regulation of cell volume and K+ channels in frog renal proximal tubule cells
The statistical mechanics of granular systems composed of irregularly shaped grains
Abstract In this paper we will suggest a model for the packing properties and phase behaviour of a granular material whose constituents are elongated in nature, using the concepts of configurational statistical mechanics. We will show that, depending upon the shape of the grains, the system need not necessarily undergo a discontinuous first-order phase transition (even at minimum close packing). We will also briefly discuss the relationship between this model and more conventional models, such as Onsager's hard rod model
Eigen analysis of the stability and degree of information content in correlation matrices constructed from property time series data
Property is an asset which forms part of the portfolios of many investors, particularly
institutional ones, along with equities and bonds. Techniques from physics, particularly
that of random matrix theory, have provided powerful insights into the behaviour of
financial assets. A large database providing time series data for over 10,000 individual properties is available for the UK. Some of the data is available at an annual and some at a monthly
frequency. However, even at the monthly frequency, only a relatively small number of
observations is available, certainly in comparison with that available with financial
assets. A key issue in translating methods of analysis in financial markets to property data is
whether they are applicable given the small number of data points available. This paper
addresses this issue. Using the tools of random matrix theory, we find that a great deal of information is contained within property data. The correlations between different types and
geographical locations of property tend to have far more true information and be more
stable over time than is the case with financial data, despite the large number of
observations available with the latter