63 research outputs found
Cooling dynamics of a dilute gas of inelastic rods: a many particle simulation
We present results of simulations for a dilute gas of inelastically colliding
particles. Collisions are modelled as a stochastic process, which on average
decreases the translational energy (cooling), but allows for fluctuations in
the transfer of energy to internal vibrations. We show that these fluctuations
are strong enough to suppress inelastic collapse. This allows us to study large
systems for long times in the truely inelastic regime. During the cooling stage
we observe complex cluster dynamics, as large clusters of particles form,
collide and merge or dissolve. Typical clusters are found to survive long
enough to establish local equilibrium within a cluster, but not among different
clusters. We extend the model to include net dissipation of energy by damping
of the internal vibrations. Inelatic collapse is avoided also in this case but
in contrast to the conservative system the translational energy decays
according to the mean field scaling law, E(t)\propto t^{-2}, for asymptotically
long times.Comment: 10 pages, 12 figures, Latex; extended discussion, accepted for
publication in Phys. Rev.
Collisions of small ice particles under microgravity conditions - II. Does the chemical composition of the ice change the collisional properties?
Context. Understanding the collisional properties of ice is important for understanding both the early stages of planet formation and the evolution of planetary ring systems. Simple chemicals such as methanol and formic acid are known to be present in cold protostellar regions alongside the dominant water ice; they are also likely to be incorporated into planets which form in protoplanetary disks, and planetary ring systems. However, the effect of the chemical composition of the ice on its collisional properties has not yet been studied.Aims. Collisions of 1.5 cm ice spheres composed of pure crystalline water ice, water with 5% methanol, and water with 5% formic acid were investigated to determine the effect of the ice composition on the collisional outcomes.Methods. The collisions were conducted in a dedicated experimental instrument, operated under microgravity conditions, at relative particle impact velocities between 0.01 and 0.19 ms-1, temperatures between 131 and 160 K and a pressure of around 10-5Results. A range of coefficients of restitution were found, with no correlation between this and the chemical composition, relative impact velocity, or temperature.Conclusions. We conclude that the chemical composition of the ice (at the level of 95% water ice and 5% methanol or formic acid) does not affect the collisional properties at these temperatures and pressures due to the inability of surface wetting to take place. At a level of 5% methanol or formic acid, the structure is likely to be dominated by crystalline water ice, leading to no change in collisional properties. The surface roughness of the particles is the dominant factor in explaining the range of coefficients of restitution
A model for collisions in granular gases
We propose a model for collisions between particles of a granular material
and calculate the restitution coefficients for the normal and tangential motion
as functions of the impact velocity from considerations of dissipative
viscoelastic collisions. Existing models of impact with dissipation as well as
the classical Hertz impact theory are included in the present model as special
cases. We find that the type of collision (smooth, reflecting or sticky) is
determined by the impact velocity and by the surface properties of the
colliding grains. We observe a rather nontrivial dependence of the tangential
restitution coefficient on the impact velocity.Comment: 11 pages, 2 figure
A microscopic 2D lattice model of dimer granular compaction with friction
We study by Monte Carlo simulation the compaction dynamics of hard dimers in
2D under the action of gravity, subjected to vertical and horizontal shaking,
considering also the case in which a friction force acts for horizontal
displacements of the dimers. These forces are modeled by introducing effective
probabilities for all kinds of moves of the particles. We analyze the dynamics
for different values of the time during which the shaking is applied to
the system and for different intensities of the forces. It turns out that the
density evolution in time follows a stretched exponential behavior if is
not very large, while a power law tail develops for larger values of .
Moreover, in the absence of friction, a critical value exists which
signals the crossover between two different regimes: for the
asymptotic density scales with a power law of , while for
it reaches logarithmically a maximal saturation value. Such behavior smears out
when a finite friction force is present. In this situation the dynamics is
slower and lower asymptotic densities are attained. In particular, for
significant friction forces, the final density decreases linearly with the
friction coefficient. We also compare the frictionless single tap dynamics to
the sequential tapping dynamics, observing in the latter case an inverse
logarithmic behavior of the density evolution, as found in the experiments.Comment: 10 pages, 15 figures, to be published in Phys. Rev.
Metallo-cryptophanes decorated with Bis-N-heterocyclic carbene ligands: self-assembly and guest uptake into a nonporous crystalline lattice
Pd3L2 metallo-cryptophane cages with cyclotriveratrylene-type L ligands can be stabilized by use of a bis-N-heterocyclic carbene as an auxiliary cis-protecting ligand, while use of more common protecting chelating ligands such as ethylenediamine saw a Pd3L2 to Pd6L8 rearrangement occur in solution. The crystalline Pd3L2 complexes act as sponges, taking up 1,2-dichorobenzene or iodine in a single-crystal-to-single-crystal fashion despite not exhibiting conventional porosity
Cellular model for the compaction of a vertically tapped granular column
A cellular model for the compaction of granular material is described.
It takes into account horizontal redistribution as well
as vertical transfer of particles. Parameters are the width of the
horizontal redistribution and the settling probability. Numerical
simulations of the behaviour
of a granular column in a container are shown as an example, and the
evolution of some characteristic features over time has been followed
for some typical configurations. Experimental results for the time evolution
of the density can be reproduced for a settling probability proportional to
the unoccupied spaces for particles in the lower cells
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