640 research outputs found
Charging and Growth of Fractal Dust Grains
The structure and evolution of aggregate grains formed within a plasma
environment are dependent upon the charge acquired by the micron-sized dust
grains during the coagulation process. The manner in which the charge is
arranged on developing irregular structures can affect the fractal dimension of
aggregates formed during collisions, which in turn influences the coagulation
rate and size evolution of the dust within the plasma cloud. This paper
presents preliminary models for the charge and size evolution of fractal
aggregates immersed in a plasma environment calculated using a modification to
the orbital-motion-limited (OML) theory. Primary electron and ion currents
incident on points on the aggregate surface are determined using a
line-of-sight (LOS) approximation: only those electron or ion trajectories
which are not blocked by another grain within the aggregate contribute to the
charging current. Using a self-consistent iterative approach, the equilibrium
charge and dipole moment are calculated for the dust aggregate. The charges are
then used to develop a heuristic charging scheme which can be implemented in
coagulation models. While most coagulation theories assume that it is difficult
for like-charged grains to coagulate, the OML_LOS approximation indicates that
the electric potentials of aggregate structures are often reduced enough to
allow significant coagulation to occur
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Helical Structures in Vertically Aligned Dust Particle Chains in a Complex Plasma
Self-assembly of structures from vertically aligned, charged dust particle
bundles within a glass box placed on the lower, powered electrode of a RF GEC
cell were produced and examined experimentally. Self-organized formation of
one-dimensional vertical chains, two-dimensional zigzag structures and
three-dimensional helical structures of triangular, quadrangular, pentagonal,
hexagonal, and heptagonal symmetries are shown to occur. System evolution is
shown to progress from a one-dimensional chain structure, through a zigzag
transition to a two-dimensional, spindle-like structure and then to various
three-dimensional, helical structures exhibiting multiple symmetries. Stable
configurations are found to be dependent upon the system confinement, (where
are the horizontal and vertical dust resonance frequencies), the total number
of particles within a bundle and the RF power. For clusters having fixed
numbers of particles, the RF power at which structural transitions occur is
repeatable and exhibits no observable hysteresis. The critical conditions for
these structural transitions as well as the basic symmetry exhibited by the
one-, two- and three-dimensional structures that subsequently develop are in
good agreement with the theoretically predicted configurations of minimum
energy determined employing molecular dynamics simulations for charged dust
particles confined in a prolate, spheroidal potential as presented
theoretically by Kamimura and Ishihara [10]
Cosmic Dust Aggregation with Stochastic Charging
The coagulation of cosmic dust grains is a fundamental process which takes
place in astrophysical environments, such as presolar nebulae and circumstellar
and protoplanetary disks. Cosmic dust grains can become charged through
interaction with their plasma environment or other processes, and the resultant
electrostatic force between dust grains can strongly affect their coagulation
rate. Since ions and electrons are collected on the surface of the dust grain
at random time intervals, the electrical charge of a dust grain experiences
stochastic fluctuations. In this study, a set of stochastic differential
equations is developed to model these fluctuations over the surface of an
irregularly-shaped aggregate. Then, employing the data produced, the influence
of the charge fluctuations on the coagulation process and the physical
characteristics of the aggregates formed is examined. It is shown that dust
with small charges (due to the small size of the dust grains or a tenuous
plasma environment) are affected most strongly
Multipole Expansions of Aggregate Charge: How Far to Go?
Aggregates immersed in a plasma or radiative environment will have charge
distributed over their extended surface. Previous studies have modeled the
aggregate charge using the monopole and dipole terms of a multipole expansion,
with results indicating that the dipole-dipole interactions play an important
role in increasing the aggregation rate and altering the morphology of the
resultant aggregates. This study examines the effect that including the
quadrupole terms has on the dynamics of aggregates interacting with each other
and the confining electric fields in laboratory experiments. Results are
compared to modeling aggregates as a collection of point charges located at the
center of each spherical monomer comprising the aggregate.Comment: 6 page
Formation of Cosmic Dust Bunnies
Planetary formation is an efficient process now thought to take place on a
relatively short astronomical time scale. Recent observations have shown that
the dust surrounding a protostar emits more efficiently at longer wavelengths
as the protoplanetary disk evolves, suggesting that the dust particles are
coagulating into fluffy aggregates, "much as dust bunnies form under a bed."
One poorly understood problem in this coagulation process is the manner in
which micron-sized, charged grains form the fractal aggregate structures now
thought to be the precursors of protoplanetary disk evolution. This study
examines the characteristics of such fractal aggregates formed by the collision
of spherical monomers and aggregates where the charge is distributed over the
aggregate structure. The aggregates are free to rotate due to collisions and
dipole-dipole electrostatic interactions. Comparisons are made for different
precursor size distributions and like-charged, oppositelycharged, and neutral
grains
Modeling Agglomeration of Dust Particles in Plasma
The charge on an aggregate immersed in a plasma environment distributes
itself over the aggregate's surface; this can be approximated theoretically by
assuming a multipole distribution. The dipole-dipole (or higher order) charge
interactions between fractal aggregates lead to rotations of the grains as they
interact. Other properties of the dust grains also influence the agglomeration
process, such as the monomer shape (spherical or ellipsoidal) or the presence
of magnetic material. Finally, the plasma and grain properties also determine
the morphology of the resultant aggregates. Porous and fluffy aggregates are
more strongly coupled to the gas, leading to reduced collisional velocities,
and greater collisional cross sections. These factors in turn can determine the
growth rate of the aggregates and evolution of the dust cloud. This paper gives
an overview of the numerical and experimental methods used to study dust
agglomeration at CASPER and highlights some recent results
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