58 research outputs found
Submillimetre-sized dust aggregate collision and growth properties
The collisional and sticking properties of sub-mm-sized aggregates composed
of protoplanetary dust analogue material are measured, including the
statistical threshold velocity between sticking and bouncing, their surface
energy and tensile strength within aggregate clusters. We performed an
experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue
materials were micrometre-sized monodisperse and polydisperse SiO2 particles
prepared into aggregates with sizes around 120 m and 330 m,
respectively and volume filling factors around 0.37. During the experimental
run of 150 s under reduced gravity conditions, the sticking of aggregates and
the formation and fragmentation of clusters of up to a few millimetres in size
was observed. The sticking probability of the sub-mm-sized dust aggregates
could be derived for velocities decreasing from 22 to 3 cm/s. The transition
from bouncing to sticking collisions happened at 12.7 cm/s for the smaller
aggregates composed of monodisperse particles and at 11.5 and 11.7 cm/s for the
larger aggregates composed of mono- and polydisperse dust particles,
respectively. Using the pull-off force of sub-mm-sized dust aggregates from the
clusters, the surface energy of the aggregates composed of monodisperse dust
was derived to be 1.6x10-5 J/m2, which can be scaled down to 1.7x10-2 J/m2 for
the micrometre-sized monomer particles and is in good agreement with previous
measurements for silica particles. The tensile strengths of these aggregates
within the clusters were derived to be 1.9 Pa and 1.6 Pa for the small and
large dust aggregates, respectively. These values are in good agreement with
recent tensile strength measurements for mm-sized silica aggregates. Using our
data on the sticking-bouncing threshold, estimates of the maximum aggregate
size can be given. For a minimum mass solar nebula model, aggregates can reach
sizes of 1 cm.Comment: 21 pages (incl. 6 pages of appendix), 23 figure
Low-velocity collision behaviour of clusters composed of sub-mm sized dust aggregates
The experiments presented aim to measure the outcome of collisions between
sub-mm sized protoplanetary dust aggregate analogues. We also observed the
clusters formed from these aggregates and their collision behaviour. The
experiments were performed at the drop tower in Bremen. The protoplanetary dust
analogue materials were micrometre-sized monodisperse and polydisperse SiO
particles prepared into aggregates with sizes between 120~m and
250~m. One of the dust samples contained aggregates that were previously
compacted through repeated bouncing. During three flights of 9~s of
microgravity each, individual collisions between aggregates and the formation
of clusters of up to a few millimetres in size were observed. In addition, the
collisions of clusters with the experiment cell walls leading to compaction or
fragmentation were recorded. We observed collisions amongst dust aggregates and
collisions between dust clusters and the cell aluminium walls at speeds ranging
from about 0.1 cm/s to 20 cm/s. The velocities at which sticking occurred
ranged from 0.18 to 5.0 cm/s for aggregates composed of monodisperse dust, with
an average value of 2.1 cm/s for reduced masses ranging from 1.2x10-6 to
1.8x10-3 g with an average value of 2.2x10-4 g. From the restructuring and
fragmentation of clusters composed of dust aggregates colliding with the
aluminium cell walls, we derived a collision recipe for dust aggregates
(100 m) following the model of Dominik \& Thielens (1997) developed
for microscopic particles. We measured a critical rolling energy of 1.8x10-13 J
and a critical breaking energy of 3.5x10-13 J for 100 m-sized
non-compacted aggregates.Comment: 12 pages, 13 figure
Low-velocity collisions of centimeter-sized dust aggregates
Collisions between centimeter- to decimeter-sized dusty bodies are important
to understand the mechanisms leading to the formation of planetesimals. We thus
performed laboratory experiments to study the collisional behavior of dust
aggregates in this size range at velocities below and around the fragmentation
threshold. We developed two independent experimental setups with the same goal
to study the effects of bouncing, fragmentation, and mass transfer in free
particle-particle collisions. The first setup is an evacuated drop tower with a
free-fall height of 1.5 m, providing us with 0.56 s of microgravity time so
that we observed collisions with velocities between 8 mm/s and 2 m/s. The
second setup is designed to study the effect of partial fragmentation (when
only one of the two aggregates is destroyed) and mass transfer in more detail.
It allows for the measurement of the accretion efficiency as the samples are
safely recovered after the encounter. Our results are that for very low
velocities we found bouncing as could be expected while the fragmentation
velocity of 20 cm/s was significantly lower than expected. We present the
critical energy for disruptive collisions Q*, which showed up to be at least
two orders of magnitude lower than previous experiments in the literature. In
the wide range between bouncing and disruptive collisions, only one of the
samples fragmented in the encounter while the other gained mass. The accretion
efficiency in the order of a few percent of the particle's mass is depending on
the impact velocity and the sample porosity. Our results will have consequences
for dust evolution models in protoplanetary disks as well as for the strength
of large, porous planetesimal bodies
\'Free Collisions in a Microgravity Many-Particle Experiment. II. The Collision Dynamics of Dust-Coated Chondrules
The formation of planetesimals in the early Solar System is hardly
understood, and in particular the growth of dust aggregates above millimeter
sizes has recently turned out to be a difficult task in our understanding [Zsom
et al. 2010, A&A, 513, A57]. Laboratory experiments have shown that dust
aggregates of these sizes stick to one another only at unreasonably low
velocities. However, in the protoplanetary disk, millimeter-sized particles are
known to have been ubiquitous. One can find relics of them in the form of solid
chondrules as the main constituent of chondrites. Most of these chondrules were
found to feature a fine-grained rim, which is hypothesized to have formed from
accreting dust grains in the solar nebula. To study the influence of these
dust-coated chondrules on the formation of chondrites and possibly
planetesimals, we conducted collision experiments between millimeter-sized,
dust-coated chondrule analogs at velocities of a few cm/s. For 2 and 3 mm
diameter chondrule analogs covered by dusty rims of a volume filling factor of
0.18 and 0.35-0.58, we found sticking velocities of a few cm/s. This velocity
is higher than the sticking velocity of dust aggregates of the same size. We
therefore conclude that chondrules may be an important step towards a deeper
understanding of the collisional growth of larger bodies. Moreover, we analyzed
the collision behavior in an ensemble of dust aggregates and non-coated
chondrule analogs. While neither the dust aggregates nor the solid chondrule
analogs show sticking in collisions among their species, we found an enhanced
sicking efficiency in collisions between the two constituents, which leads us
to the conjecture that chondrules might act as "catalyzers" for the growth of
larger bodies in the young Solar System
The four-populations model: a new classification scheme for pre-planetesimal collisions
Within the collision growth scenario for planetesimal formation, the growth
step from centimetre sized pre-planetesimals to kilometre sized planetesimals
is still unclear. The formation of larger objects from the highly porous
pre-planetesimals may be halted by a combination of fragmentation in disruptive
collisions and mutual rebound with compaction. However, the right amount of
fragmentation is necessary to explain the observed dust features in late T
Tauri discs. Therefore, detailed data on the outcome of pre-planetesimal
collisions is required and has to be presented in a suitable and precise
format. We propose and apply a new classification scheme for pre-planetesimal
collisions based on the quantitative aspects of four fragment populations: the
largest and second largest fragment, a power-law population, and a
sub-resolution population. For the simulations of pre-planetesimal collisions,
we adopt the SPH numerical scheme with extensions for the simulation of porous
solid bodies. By means of laboratory benchmark experiments, this model was
previously calibrated and tested for the correct simulation of the compaction,
bouncing, and fragmentation behaviour of macroscopic highly porous silica dust
aggregates. It is shown that previous attempts to map collision data were much
too oriented on qualitatively categorising into sticking, bouncing, and
fragmentation events. We show that the four-populations model encompasses all
previous categorisations and in addition allows for transitions. This is
because it is based on quantitative characteristic attributes of each
population such as the mass, kinetic energy, and filling factor. As a
demonstration of the applicability and the power of the four-populations model,
we utilise it to present the results of a study on the influence of collision
velocity in head-on collisions of intermediate porosity aggregates.Comment: 14 pages, 11 figures, 5 tables, to be published in Astronomy and
Astrophysic
The Physics of Protoplanetesimal Dust Agglomerates. IV. Towards a Dynamical Collision Model
Recent years have shown many advances in our knowledge of the collisional
evolution of protoplanetary dust. Based on a variety of dust-collision
experiments in the laboratory, our view of the growth of dust aggregates in
protoplanetary disks is now supported by a deeper understanding of the physics
involved in the interaction between dust agglomerates. However, the parameter
space, which determines the collisional outcome, is huge and sometimes
inaccessible to laboratory experiments. Very large or fluffy dust aggregates
and extremely low collision velocities are beyond the boundary of today's
laboratories. It is therefore desirable to augment our empirical knowledge of
dust-collision physics with a numerical method to treat arbitrary aggregate
sizes, porosities and collision velocities. In this article, we implement
experimentally-determined material parameters of highly porous dust aggregates
into a Smooth Particle Hydrodynamics (SPH) code, in particular an
omnidirectional compressive-strength and a tensile-strength relation. We also
give a prescription of calibrating the SPH code with compression and
low-velocity impact experiments. In the process of calibration, we developed a
dynamic compressive-strength relation and estimated a relation for the shear
strength. Finally, we defined and performed a series of benchmark tests and
found the agreement between experimental results and numerical simulations to
be very satisfactory. SPH codes have been used in the past to study collisions
at rather high velocities. At the end of this work, we show examples of future
applications in the low-velocity regime of collisional evolution.Comment: accepted by The astrophysical Journa
Porosities of Protoplanetary Dust Agglomerates from Collision Experiments
Aggregation of dust through sticking collisions is the first step of planet
formation. Basic physical properties of the evolving dust aggregates strongly
depend on the porosity of the aggregates, e.g. mechanical strength, thermal
conductivity, gas-grain coupling time. Also the outcome of further collisions
depends on the porosity of the colliding aggregates. In laboratory experiments
we study the growth of large aggregates of 3 mm to 3 cm through
continuous impacts of small dust agglomerates of 100 m size, consisting of
m grains at different impact velocities. The experiments show that
agglomerates grow by direct sticking as well as gravitational reaccretion. The
latter can be regarded as suitable analog to reaccretion of fragments by gas
drag in protoplanetary disks. Experiments were carried out in the velocity
range between 1.5 m/s and 7 m/s. With increasing impact velocities the volume
filling factor of the resulting agglomerates increases from for
1.5 m/s to for 7 m/s. These values are independent of the target
size. Extrapolation of the measured velocity dependence of the volume filling
factor implies that higher collision velocities will not lead to more compact
aggregates. Therefore, marks a degree of compaction suitable to
describe structures forming at . At small collision velocities
below 1 m/s highly porous structures with will form. For
intermediate collision velocities porosities vary. Depending on the disk model
and resulting relative velocities, objects in protoplanetary disks up to
dm-size might evolve from highly porous () to compact () with a more complex intermediate size range of varying porosity.Comment: Accepted by The Astrophysical Journa
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments
The growth processes from protoplanetary dust to planetesimals are not fully
understood. Laboratory experiments and theoretical models have shown that
collisions among the dust aggregates can lead to sticking, bouncing, and
fragmentation. However, no systematic study on the collisional outcome of
protoplanetary dust has been performed so far so that a physical model of the
dust evolution in protoplanetary disks is still missing. We intend to map the
parameter space for the collisional interaction of arbitrarily porous dust
aggregates. This parameter space encompasses the dust-aggregate masses, their
porosities and the collision velocity. With such a complete mapping of the
collisional outcomes of protoplanetary dust aggregates, it will be possible to
follow the collisional evolution of dust in a protoplanetary disk environment.
We use literature data, perform own laboratory experiments, and apply simple
physical models to get a complete picture of the collisional interaction of
protoplanetary dust aggregates. In our study, we found four different types of
sticking, two types of bouncing, and three types of fragmentation as possible
outcomes in collisions among protoplanetary dust aggregates. We distinguish
between eight combinations of porosity and mass ratio. For each of these cases,
we present a complete collision model for dust-aggregate masses between 10^-12
and 10^2 g and collision velocities in the range 10^-4 to 10^4 cm/s for
arbitrary porosities. This model comprises the collisional outcome, the
mass(es) of the resulting aggregate(s) and their porosities. We present the
first complete collision model for protoplanetary dust. This collision model
can be used for the determination of the dust-growth rate in protoplanetary
disks.Comment: accepted by Astronomy and Astrophysic
Numerical Simulations of Highly Porous Dust Aggregates in the Low-Velocity Collision Regime
A highly favoured mechanism of planetesimal formation is collisional growth.
Single dust grains, which follow gas flows in the protoplanetary disc, hit each
other, stick due to van der Waals forces and form fluffy aggregates up to
centimetre size. The mechanism of further growth is unclear since the outcome
of aggregate collisions in the relevant velocity and size regime cannot be
investigated in the laboratory under protoplanetary disc conditions. Realistic
statistics of the result of dust aggregate collisions beyond decimetre size is
missing for a deeper understanding of planetary growth. Joining experimental
and numerical efforts we want to calibrate and validate a computer program that
is capable of a correct simulation of the macroscopic behaviour of highly
porous dust aggregates. After testing its numerical limitations thoroughly we
will check the program especially for a realistic reproduction of various
benchmark experiments. We adopt the smooth particle hydrodynamics (SPH)
numerical scheme with extensions for the simulation of solid bodies and a
modified version of the Sirono porosity model. Experimentally measured
macroscopic material properties of silica dust are implemented. We calibrate
and test for the compressive strength relation and the bulk modulus. SPH has
already proven to be a suitable tool to simulate collisions at rather high
velocities. In this work we demonstrate that its area of application can not
only be extended to low-velocity experiments and collisions. It can also be
used to simulate the behaviour of highly porous objects in this velocity regime
to a very high accuracy.The result of the calibration process in this work is
an SPH code that can be utilised to investigate the collisional outcome of
porous dust in the low-velocity regime.Comment: accepted by Astronomy & Astrophysic
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases
of planet formation. We developed a laboratory setup to observe collisions of
dust aggregates levitating at mbar pressures and elevated temperatures of 800
K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm
in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller
than 25 \mum in size. We find that for all impact energies in the studied range
sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the
sticking probability decreases with increasing impact parameter. The sticking
probability increases with energy density (impact energy per contact area). We
also observe collisions of aggregates that were formed by a previous sticking
of two larger aggregates. Partners of these aggregates can be detached by a
second collision with a probability of on average 19.8 \pm 4.0%. The measured
accretion efficiencies are remarkably high compared to other experimental
results. We attribute this to the rel. large dust grains used in our
experiments, which make aggregates more susceptible to restructuring and energy
dissipation. Collisional hardening by compaction might not occur as the
aggregates are already very compact with only 54 \pm 1% porosity. The
disassembly of previously grown aggregates in collisions might stall further
aggregate growth. However, owing to the levitation technique and the limited
data statistics, no conclusive statement about this aspect can yet be given. We
find that the detachment efficiency decreases with increasing velocities and
accretion dominates in the higher velocity range. For high accretion
efficiencies, our experiments suggest that continued growth in the mm-range
with larger constituent grains would be a viable way to produce larger
aggregates, which might in turn form the seeds to proceed to growing
planetesimals.Comment: 9 pages, 20 figure
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