771 research outputs found
Multi-particle structures in non-sequentially reorganized hard sphere deposits
We have examined extended structures, bridges and arches, in computer
generated, non-sequentially stabilized, hard sphere deposits. The bridges and
arches have well defined distributions of sizes and shapes. The distribution
functions reflect the contraints associated with hard particle packing and the
details of the restructuring process. A subpopulation of string-like bridges
has been identified. Bridges are fundamental microstructural elements in real
granular systems and their sizes and shapes dominate considerations of
structural properties and flow instabilities such as jamming.Comment: 9 pages, 7 figure
Smoothing of sandpile surfaces after intermittent and continuous avalanches: three models in search of an experiment
We present and analyse in this paper three models of coupled continuum
equations all united by a common theme: the intuitive notion that sandpile
surfaces are left smoother by the propagation of avalanches across them. Two of
these concern smoothing at the `bare' interface, appropriate to intermittent
avalanche flow, while one of them models smoothing at the effective surface
defined by a cloud of flowing grains across the `bare' interface, which is
appropriate to the regime where avalanches flow continuously across the
sandpile.Comment: 17 pages and 26 figures. Submitted to Physical Review
Glassy dynamics in granular compaction
Two models are presented to study the influence of slow dynamics on granular
compaction. It is found in both cases that high values of packing fraction are
achieved only by the slow relaxation of cooperative structures. Ongoing work to
study the full implications of these results is discussed.Comment: 12 pages, 9 figures; accepted in J. Phys: Condensed Matter,
proceedings of the Trieste workshop on 'Unifying concepts in glass physics
A two-species model of a two-dimensional sandpile surface: a case of asymptotic roughening
We present and analyze a model of an evolving sandpile surface in (2 + 1)
dimensions where the dynamics of mobile grains ({\rho}(x, t)) and immobile
clusters (h(x, t)) are coupled. Our coupling models the situation where the
sandpile is flat on average, so that there is no bias due to gravity. We find
anomalous scaling: the expected logarithmic smoothing at short length and time
scales gives way to roughening in the asymptotic limit, where novel and
non-trivial exponents are found.Comment: 7 Pages, 6 Figures; Granular Matter, 2012 (Online
ROLE OF NITRIC OXIDE (NO) IN CAPSAICIN MEDIATED ANTI-PLATELET ACTIVITY IN IN VITRO, IN VIVO, EX-VIVO MODEL OF PLATELET AGGREGATION ASSAY AND ARTERIAL THROMBOSIS IN RAT: POTENTIAL THERAPEUTIC TARGET?
Objective: Although recent advances in the treatment of congestive heart disease, mortality among patients' remains a questionable remark. Therefore, we evaluated the role of capsaicin on in vitro and ex vivo platelet aggregation induced by Adenosine Di-Phosphate (ADP) as well as in in vivo thrombosis models and role of NO, KATP was also identified in the capsaicin-induced anti-platelet animal model as well as in vivo model of arterial thrombosis.Methods: According to body weight wistar rats were divided into five groups. Group I and Group II was treated with saline and capsaicin (3 mg/kg, i. v), while animals from Group III were treated with N(ω)-nitro-L-arginine methyl ester (L-NAME) (30 mg/kg, i. v) 30 min before administration of capsaicin (3 mg/kg, i. v). Group IV animals were treated with glibenclamide (10 mg/kg,i. v) 30 min before administration of capsaicin (3 mg/kg, i. v). Group V was considered as a positive control and administered clopidogrel (30 mg/kg, p. o). Animals were subjected for in vitro, ex-vivo platelet aggregation assay. ADP (30µM) was utilized as an aggregating agent in these experiments. After these assays; animals of each group were subjected for subaqueous tail bleeding time in a rat model and FeCl3-induced arterial thrombosis model in rats.Results: In ADP-induced in vitro platelet aggregation, a significant reduction in % platelet aggregation was observed at 50µM (64.35±4.641) and 100µM (52.72±4.192) concentration of capsaicin as compared to vehicle control (85.82±3.716). Capsaicin (3 mg/kg, i. v) also showed a significant reduction (49.53±4.075) in ex-vivo ADP-induced platelet aggregation as compared to vehicle control (89.38±2.057). In FeCl3 induced arterial thrombosis model, Capsaicin (3 mg/kg, i. v) exhibited an increase in time to occlusion in this rodent model and presence of the L-NAME and glibenclamide had inhibited the activity of capsaicin.Conclusion: In our study, capsaicin (50 µM, 100µM) exhibited potent anti-platelet activity in ADP-induced platelet aggregation, similarly capsaicin exhibited significant anti-platelet action in the ex-vivo study. Moreover, the presence of L-NAME and glibenclamide inhibited the anti-thrombotic and anti-platelet action of capsaicin. Therefore, it was concluded that NO and KATP may be involved in the anti-thrombotic action of capsaicin
Competition and cooperation:aspects of dynamics in sandpiles
In this article, we review some of our approaches to granular dynamics, now
well known to consist of both fast and slow relaxational processes. In the
first case, grains typically compete with each other, while in the second, they
cooperate. A typical result of {\it cooperation} is the formation of stable
bridges, signatures of spatiotemporal inhomogeneities; we review their
geometrical characteristics and compare theoretical results with those of
independent simulations. {\it Cooperative} excitations due to local density
fluctuations are also responsible for relaxation at the angle of repose; the
{\it competition} between these fluctuations and external driving forces, can,
on the other hand, result in a (rare) collapse of the sandpile to the
horizontal. Both these features are present in a theory reviewed here. An arena
where the effects of cooperation versus competition are felt most keenly is
granular compaction; we review here a random graph model, where three-spin
interactions are used to model compaction under tapping. The compaction curve
shows distinct regions where 'fast' and 'slow' dynamics apply, separated by
what we have called the {\it single-particle relaxation threshold}. In the
final section of this paper, we explore the effect of shape -- jagged vs.
regular -- on the compaction of packings near their jamming limit. One of our
major results is an entropic landscape that, while microscopically rough,
manifests {\it Edwards' flatness} at a macroscopic level. Another major result
is that of surface intermittency under low-intensity shaking.Comment: 36 pages, 23 figures, minor correction
Searching and fixating: scale-invariance vs. characteristic timescales in attentional processes
In an experiment involving semantic search, the visual movements of sample
populations subjected to visual and aural input were tracked in a taskless
paradigm. The probability distributions of saccades and fixations were obtained
and analyzed. Scale-invariance was observed in the saccadic distributions,
while the fixation distributions revealed the presence of a characteristic
(attentional) time scale for literate subjects. A detailed analysis of our
results suggests that saccadic eye motions are an example of Levy, rather than
Brownian, dynamics.Comment: Accepted to Europhysics Letters (2011
A two-species continuum model for aeolian sand ripples
We formulate a continuum model for aeolian sand ripples consisting of two
species of grains: a lower layer of relatively immobile clusters, with an upper
layer of highly mobile grains moving on top. We predict analytically the ripple
wavelength, initial ripple growth rate and threshold saltation flux for ripple
formation. Numerical simulations show the evolution of realistic ripple
profiles from initial surface roughness via ripple growth and merger.Comment: 9 pages, 3 figure
- …