1,437 research outputs found
Accurate freezing and melting equations for the Lennard-Jones system
Analyzing three approximate methods to locate liquid-solid coexistence in
simple systems, an observation is made that all of them predict the same
functional dependence of the temperature on density at freezing and melting of
the conventional Lennard-Jones system. The emerging equations can be written as
in normalized units. We suggest to
determine the values of the coefficients at freezing and melting
from the high-temperature limit, governed by the inverse twelfth power
repulsive potential. The coefficients can be determined from the
triple point parameters of the LJ fluid. This produces freezing and melting
equations which are exact in the high-temperature limit and at the triple
point, and show remarkably good agreement with numerical simulation data in the
intermediate region.Comment: 6 pages, 1 figur
Ionization enhanced ion collection by a small floating grain in plasmas
It is demonstrated that the ionization events in the vicinity of a small
floating grain can increase the ion flux to its surface. In this respect the
effect of electron impact ionization is fully analogous to that of the
ion-neutral resonant charge exchange collisions. Both processes create slow ion
which cannot overcome grain' electrical attraction and eventually fall onto its
surface. The relative importance of ionization and ion-neutral collisions is
roughly given by the ratio of the corresponding frequencies. We have evaluated
this ratio for neon and argon plasmas to demonstrate that ionization enhanced
ion collection can indeed be an important factor affecting grain charging in
realistic experimental conditions.Comment: 7 pages, 1 figure, submitted to Physics of Plasma
Supersonic dislocations observed in a plasma crystal
Experimental results on the dislocation dynamics in a two-dimensional plasma
crystal are presented. Edge dislocations were created in pairs in lattice
locations where the internal shear stress exceeded a threshold and then moved
apart in the glide plane at a speed higher than the sound speed of shear waves,
. The experimental system, a plasma crystal, allowed observation of this
process at an atomistic (kinetic) level. The early stage of this process is
identified as a stacking fault. At a later stage, supersonically moving
dislocations generated shear-wave Mach cones
Anisotropic shear melting and recrystallization of a two-dimensional complex (dusty) plasma
A two-dimensional plasma crystal was melted by suddenly applying localized
shear stress. A stripe of particles in the crystal was pushed by the radiation
pressure force of a laser beam. We found that the response of the plasma
crystal to stress and the eventual shear melting depended strongly on the
crystal's angular orientation relative to the laser beam. Shear stress and
strain rate were measured, from which the spatially resolved shear viscosity
was calculated. The latter was shown to have minima in the regions with high
velocity shear, thus demonstrating shear thinning. Shear-induced reordering was
observed in the steady-state flow, where particles formed strings aligned in
the flow direction.Comment: 7 pages, 8 figures, submitted to Physical Review
Cosmic ray secondary nuclei and the structure of the galaxy
The consequencies of diffusive acceleration of cosmic rays in supernova shocks propagation through an inhomogeneous interstellar medium are explored. The acceleration takes place in the hot, tenuous, intercloud gas, while nuclear collisions, leading to the production of cosmic ray secondaries, predominantly occur in those regions where the supernova shocks collide with interstellar clouds. A simple model is used to calculate the interaction of a (cosmic ray + gas) shock with a cloud, and thus determine the gross topology. Extending this to the whole system, using mean cloud sizes and space densities, allows us to calculate the secondary/primary cosmic ray abundance ratios for light and heavy nuclei
Effect of strong wakes on waves in two-dimensional plasma crystals
We study effects of the particle-wake interactions on the dispersion and
polarization of dust lattice wave modes in two-dimensional plasma crystals.
Most notably, the wake-induced coupling between the modes causes the branches
to "attract" each other, and their polarizations become elliptical. Upon the
mode hybridization the major axes of the ellipses (remaining mutually
orthogonal) rotate by . To demonstrate importance of the obtained
results for experiments, we plot spectral densities of the longitudinal and
transverse waves revealing distinct fingerprints of the mixed polarization.
Furthermore, we show that at strong coupling the hybrid mode is significantly
shifted towards smaller wave numbers, away from the border of the first
Brillouin zone (where the hybrid mode is localized for a weak coupling).Comment: 10 pages, 6 figure
The Bohm sheath criterion in strongly coupled complex plasmas
A modification of the classical Bohm sheath criterion is investigated in complex plasmas containing Boltzmann electrons, cold fluid ions and strongly coupled microparticles. Equilibrium is provided by an effective 'temperature' associated with electrostatic interactions between charged grains. Using the small-potential expansion approach of the Sagdeev potential, a significant reduction of the ion Bohm velocity is obtained for complex plasma parameters relevant for experiments. The result is of consequence for all problems involving ion drag on microparticles, including parametric instability, structure formation, wave propagation, etc
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