8,090 research outputs found
Volume fluctuations and geometrical constraints in granular packs
Structural organization and correlations are studied in very large packings
of equally sized acrylic spheres, reconstructed in three-dimensions by means of
X-ray computed tomography. A novel technique, devised to analyze correlations
among more than two spheres, shows that the structural organization can be
conveniently studied in terms of a space-filling packing of irregular
tetrahedra. The study of the volume distribution of such tetrahedra reveals an
exponential decay in the region of large volumes; a behavior that is in very
good quantitative agreement with theoretical prediction. I argue that the
system's structure can be described as constituted of two phases: 1) an
`unconstrained' phase which freely shares the volume; 2) a `constrained' phase
which assumes configurations accordingly with the geometrical constraints
imposed by the condition of non-overlapping between spheres and mechanical
stability. The granular system exploits heterogeneity maximizing freedom and
entropy while constraining mechanical stability.Comment: 5 pages, 4 figure
Large phenotype jumps in biomolecular evolution
By defining the phenotype of a biopolymer by its active three-dimensional
shape, and its genotype by its primary sequence, we propose a model that
predicts and characterizes the statistical distribution of a population of
biopolymers with a specific phenotype, that originated from a given genotypic
sequence by a single mutational event. Depending on the ratio g0 that
characterizes the spread of potential energies of the mutated population with
respect to temperature, three different statistical regimes have been
identified. We suggest that biopolymers found in nature are in a critical
regime with g0 in the range 1-6, corresponding to a broad, but not too broad,
phenotypic distribution resembling a truncated Levy flight. Thus the biopolymer
phenotype can be considerably modified in just a few mutations. The proposed
model is in good agreement with the experimental distribution of activities
determined for a population of single mutants of a group I ribozyme.Comment: to appear in Phys. Rev. E; 7 pages, 6 figures; longer discussion in
VII, new fig.
The packing of granular polymer chains
Rigid particles pack into structures, such as sand dunes on the beach, whose
overall stability is determined by the average number of contacts between
particles. However, when packing spatially extended objects with flexible
shapes, additional concepts must be invoked to understand the stability of the
resulting structure. Here we study the disordered packing of chains constructed
out of flexibly-connected hard spheres. Using X-ray tomography, we find long
chains pack into a low-density structure whose mechanical rigidity is mainly
provided by the backbone. On compaction, randomly-oriented, semi-rigid loops
form along the chain, and the packing of chains can be understood as the
jamming of these elements. Finally we uncover close similarities between the
packing of chains and the glass transition in polymers.Comment: 11 pages, 4 figure
Velocity correlations in dense granular gases
We report the statistical properties of spherical steel particles rolling on
an inclined surface being driven by an oscillating wall. Strong dissipation
occurs due to collisions between the particles and rolling and can be tuned by
changing the number density. The velocities of the particles are observed to be
correlated over large distances comparable to the system size. The distribution
of velocities deviates strongly from a Gaussian. The degree of the deviation,
as measured by the kurtosis of the distribution, is observed to be as much as
four times the value corresponding to a Gaussian, signaling a significant
breakdown of the assumption of negligible velocity correlations in a granular
system.Comment: 4 pages, 4 Figure
Escaping from nonhyperbolic chaotic attractors
We study the noise-induced escape process from chaotic attractors in
nonhyperbolic systems. We provide a general mechanism of escape in the low
noise limit, employing the theory of large fluctuations. Specifically, this is
achieved by solving the variational equations of the auxiliary Hamiltonian
system and by incorporating the initial conditions on the chaotic attractor
unambiguously. Our results are exemplified with the H{\'e}non and the Ikeda map
and can be implemented straightforwardly to experimental data.Comment: replaced with published versio
Presynchronizing PGF2α and GnRH injections before timed artificial insemination CO-Synch + CIDR program
Fixed-time artificial insemination is an effective management tool that reduces the
labor associated with more conventional artificial insemination programs requiring
detection of estrus. The 7-day CO-Synch + controlled internal drug release (CIDR)
insert protocol has been shown to effectively initiate estrus and ovulation in cycling
and non-cycling suckled beef cows, producing pregnancy rates at or greater than 50%
in beef cows. The gonadotropin-releasing hormone (GnRH) injection that begins the
CO-Synch + CIDR program initiates ovulation in a large proportion of cows, particularly
anestrous cows. The CIDR, which releases progesterone intravaginally, prevents
short estrous cycles that usually follow the first postpartum ovulation in beef cows. Our
hypothesis was that inducing estrus with a prostaglandin injection followed 3 days later
with a GnRH injection, 7 days before applying the 7-day CO-Synch + CIDR protocol,
might increase the percentage of cycling cows that would exhibit synchronous follicular
waves after the onset of the CO-Synch + CIDR protocol. We also hypothesized that
the additional GnRH injection would increase the percentage of anestrous cows that
would ovulate, thereby increasing pregnancy outcomes
Dynamics of axial separation in long rotating drums
We propose a continuum description for the axial separation of granular
materials in a long rotating drum. The model, operating with two local
variables, concentration difference and the dynamic angle of repose, describes
both initial transient traveling wave dynamics and long-term segregation of the
binary mixture. Segregation proceeds through ultra-slow logarithmic coarsening.Comment: 4 pages, 3 Postscript figures; submitted to PR
Expansions for the Bollobas-Riordan polynomial of separable ribbon graphs
We define 2-decompositions of ribbon graphs, which generalise 2-sums and
tensor products of graphs. We give formulae for the Bollobas-Riordan polynomial
of such a 2-decomposition, and derive the classical Brylawski formula for the
Tutte polynomial of a tensor product as a (very) special case. This study was
initially motivated from knot theory, and we include an application of our
formulae to mutation in knot diagrams.Comment: Version 2 has minor changes. To appear in Annals of Combinatoric
Propagating front in an excited granular layer
A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated
vertically on a flat plate, shows remarkable ordering transitions and
cooperative behavior just below 1g maximum acceleration. We study the stability
of a quiescent disordered or ``amorphous'' state formed when the acceleration
is switched off in the excited ``gaseous'' state. The transition from the
amorphous state back to the gaseous state upon increasing the plate's
acceleration is generally subcritical: An external perturbation applied to one
bead initiates a propagating front that produces a rapid transition. We measure
the front velocity as a function of the applied acceleration. This phenomenon
is explained by a model based on a single vibrated particle with multiple
attractors that is perturbed by collisions. A simulation shows that a
sufficiently high rate of interparticle collisions can prevent trapping in the
attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May
199
Velocity Distributions of Granular Gases with Drag and with Long-Range Interactions
We study velocity statistics of electrostatically driven granular gases. For
two different experiments: (i) non-magnetic particles in a viscous fluid and
(ii) magnetic particles in air, the velocity distribution is non-Maxwellian,
and its high-energy tail is exponential, P(v) ~ exp(-|v|). This behavior is
consistent with kinetic theory of driven dissipative particles. For particles
immersed in a fluid, viscous damping is responsible for the exponential tail,
while for magnetic particles, long-range interactions cause the exponential
tail. We conclude that velocity statistics of dissipative gases are sensitive
to the fluid environment and to the form of the particle interaction.Comment: 4 pages, 3 figure
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