1,061 research outputs found
Gas Transport in Porous Media: Simulations and Experiments on Partially Densified Aerogels
The experimental density dependence of gas (argon and nitrogen) permeability
of partially densified silica aerogels in the Knudsen regime is quantitatively
accounted for by a computer model. The model simulates both the structure of
the sintered material and the random ballistic motion of a point particle
inside its voids. The same model is also able to account for the densit y
dependence of the specific pore surface as measured from nitrogen adsorption
experiments.Comment: RevTex, 11 pages + 5 postscript figures appended using "uufiles".
Published in Europhys. Lett. 29, p. 567 (1995
Work-rate of substitutes in elite soccer: A preliminary study
The aim of this study was to investigate the work-rate of substitutes in professional soccer. A computerised player tracking system was used to assess the work-rates of second-half substitutes (11 midfielders and 14 forwards) in a French Ligue 1 club. Total distance, distance covered in five categories of movement intensity and recovery time between high-intensity efforts were evaluated. First- and second-half work-rates of the replaced players were compared. The performance of substitutes was compared to that of the players they replaced, to team-mates in the same position who remained on the pitch after the substitution and in relation to their habitual performances when starting games. No differences in work-rate between first- and second-halves were observed in all players who were substituted. In the second-half, a non-significant trend was observed in midfield substitutes who covered greater distances than the player they replaced whereas no differences were observed in forwards. Midfield substitutes covered a greater overall distance and distance at high-intensities (p<0.01) and had a lower recovery time between high-intensity efforts (p<0.01) compared to other midfield team-mates who remained on the pitch. Forwards covered less distance (p<0.01) in their first 10-minutes as a substitute compared to their habitual work-rate profile in the opening 10-minutes when starting matches while this finding was not observed in midfielders. These findings suggest that compared to midfield substitutes, forward substitutes did not utilise their full physical potential. Further investigation is warranted into the reasons behind this finding in order to optimise the work-rate contributions of forward substitutes
Discontinuous percolation transitions in real physical systems
We study discontinuous percolation transitions (PT) in the diffusion-limited
cluster aggregation model of the sol-gel transition as an example of real
physical systems, in which the number of aggregation events is regarded as the
number of bonds occupied in the system. When particles are Brownian, in which
cluster velocity depends on cluster size as with
, a larger cluster has less probability to collide with other
clusters because of its smaller mobility. Thus, the cluster is effectively more
suppressed in growth of its size. Then the giant cluster size increases
drastically by merging those suppressed clusters near the percolation
threshold, exhibiting a discontinuous PT. We also study the tricritical
behavior by controlling the parameter , and the tricritical point is
determined by introducing an asymmetric Smoluchowski equation.Comment: 5 pages, 5 figure
Influence of surfactants on the structure of titanium oxide gels : experiments and simulations
We report here on experimental and numerical studies of the influence of
surfactants on mineral gel synthesis. The modification of the gel structure
when the ratios water-precursor and water-surfactant vary is brought to the
fore by fractal dimension measures. A property of {\em polydispersity of the
initial hydrolysis} is proposed to explain these results, and is successfuly
tested through numerical experiments of three dimensional chemically limited
aggregation.Comment: 12 pages, 4 Postscript figures, uses RevTe
Stochastic Model for the Motion of a Particle on an Inclined Rough Plane and the Onset of Viscous Friction
Experiments on the motion of a particle on an inclined rough plane have
yielded some surprising results. For example, it was found that the frictional
force acting on the ball is viscous, {\it i.e.} proportional to the velocity
rather than the expected square of the velocity. It was also found that, for a
given inclination of the plane, the velocity of the ball scales as a power of
its radius. We present here a one dimensional stochastic model based on the
microscopic equations of motion of the ball, which exhibits the same behaviour
as the experiments. This model yields a mechanism for the origins of the
viscous friction force and the scaling of the velocity with the radius. It also
reproduces other aspects of the phase diagram of the motion which we will
discuss.Comment: 19 pages, latex, 11 postscript figures in separate uuencoded fil
Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime
Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean and a standard deviation . The wave vector dependent
scattered intensity is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given , the location of the
crossover decreases as increases. The dependence of on
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994
Vector chiral and multipolar orders in the spin-1/2 frustrated ferromagnetic chain in magnetic field
We study the one-dimensional spin-1/2 Heisenberg chain with competing
ferromagnetic nearest-neighbor J_1 and antiferromagnetic next-nearest-neighbor
J_2 exchange couplings in the presence of magnetic field. We use both numerical
approaches (the density matrix renormalization group method and exact
diagonalization) and effective field-theory approach, and obtain the
ground-state phase diagram for wide parameter range of the coupling ratio
J_1/J_2. The phase diagram is rich and has a variety of phases, including the
vector chiral phase, the nematic phase, and other multipolar phases. In the
vector chiral phase, which appears in relatively weak magnetic field, the
ground state exhibits long-range order (LRO) of vector chirality which
spontaneously breaks a parity symmetry. The nematic phase shows a quasi-LRO of
antiferro-nematic spin correlation, and arises as a result of formation of
two-magnon bound states in high magnetic fields. Similarly, the higher
multipolar phases, such as triatic (p=3) and quartic (p=4) phases, are formed
through binding of p magnons near the saturation fields, showing quasi-LRO of
antiferro-multipolar spin correlations. The multipolar phases cross over to
spin density wave phases as the magnetic field is decreased, before
encountering a phase transition to the vector chiral phase at a lower field.
The implications of our results to quasi-one-dimensional frustrated magnets
(e.g., LiCuVO_4) are discussed.Comment: v1. 20 pages, 18 figures: v2: 21 pages, 19 figures, Title modified
slightly. Some references, Fig.16, and a note are added. To appear in Phys.
Rev.
Crystallization in a model glass: influence of the boundary conditions
Using molecular dynamics calculations and the Voronoi tessellation, we study
the evolution of the local structure of a soft-sphere glass versus temperature
starting from the liquid phase at different quenching rates. This study is done
for different sizes and for two different boundary conditions namely the usual
cubic periodic boundary conditions and the isotropic hyperspherical boundary
conditions for which the particles evolve on the surface of a hypersphere in
four dimensions. Our results show that for small system sizes, crystallization
can indeed be induced by the cubic boundary conditions. On the other hand we
show that finite size effects are more pronounced on the hypersphere and that
crystallization is artificially inhibited even for large system sizes.Comment: 11 pages, 2 figure
Fluctuating Bond Aggregation: a Model for Chemical Gel Formation
The Diffusion-Limited Cluster-Cluster Aggregation (DLCA) model is modified by
including cluster deformations using the {\it bond fluctuation} algorithm. From
3 computer simulations, it is shown that, below a given threshold value
of the volumic fraction , the realization of all intra-aggregate
bonding possibilities prevents the formation of a gelling network. For ,
the sol-gel transition occurs at a time which, in contrast to DLCA,
doesnot diverge with the box size. Several results are reported including small
angle scattering curves and possible applications are discussed.Comment: RevTex, 9 pages + 3 postscript figures appended using "uufiles". To
appear in Phys. Rev. Let
Structure and dynamics of a model glass: influence of long-range forces
We vary the amplitude of the long-range Coulomb forces within a classical
potential describing a model silica glass and study the consequences on the
structure and dynamics of the glass, via molecular dynamics simulations. This
model allows us to follow the variation of specific features such as the First
Sharp Diffraction Peak and the Boson Peak in a system going continuously from a
fragile (no Coulomb forces) to a strong (with Coulomb forces) glass. In
particular we show that the characteristic features of a strong glass
(existence of medium range order, bell-shaped ring size distribution, sharp
Boson peak) appear as soon as tetrahedral units are formed.Comment: 5 pages, 4 figures. To be published in J.Phys.: C
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