50 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
\'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
Method measuring oxygen tension and transport within subcutaneous devices
Cellular therapies hold promise to replace the implantation of whole organs in the treatment of disease. For most cell types, in vivo viability depends on oxygen delivery to avoid the toxic effects of hypoxia. A promising approach is the in situ vascularization of implantable devices which can mediate hypoxia and improve both the lifetime and utility of implanted cells and tissues. Although mathematical models and bulk measurements of oxygenation in surrounding tissue have been used to estimate oxygenation within devices, such estimates are insufficient in determining if supplied oxygen is sufficient for the entire thickness of the implanted cells and tissues. We have developed a technique in which oxygen-sensitive microparticles (OSMs) are incorporated into the volume of subcutaneously implantable devices. Oxygen partial pressure within these devices can be measured directly in vivo by an optical probe placed on the skin surface. As validation, OSMs have been incorporated into alginate beads, commonly used as immunoisolation devices to encapsulate pancreatic islet cells. Alginate beads were implanted into the subcutaneous space of Sprague–Dawley rats. Oxygen transport through beads was characterized from dynamic OSM signals in response to changes in inhaled oxygen. Changes in oxygen dynamics over days demonstrate the utility of our technology
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
Compression Behaviour of Porous Dust Agglomerates
The early planetesimal growth proceeds through a sequence of sticking
collisions of dust agglomerates. Very uncertain is still the relative velocity
regime in which growth rather than destruction can take place. The outcome of a
collision depends on the bulk properties of the porous dust agglomerates.
Continuum models of dust agglomerates require a set of material parameters that
are often difficult to obtain from laboratory experiments. Here, we aim at
determining those parameters from ab-initio molecular dynamics simulations. Our
goal is to improveon the existing model that describe the interaction of
individual monomers. We use a molecular dynamics approach featuring a detailed
micro-physical model of the interaction of spherical grains. The model includes
normal forces, rolling, twisting and sliding between the dust grains. We
present a new treatment of wall-particle interaction that allows us to perform
customized simulations that directly correspond to laboratory experiments. We
find that the existing interaction model by Dominik & Tielens leads to a too
soft compressive strength behavior for uni and omni-directional compression.
Upon making the rolling and sliding coefficients stiffer we find excellent
agreement in both cases. Additionally, we find that the compressive strength
curve depends on the velocity with which the sample is compressed. The modified
interaction strengths between two individual dust grains will lead to a
different behaviour of the whole dust agglomerate. This will influences the
sticking probabilities and hence the growth of planetesimals. The new parameter
set might possibly lead to an enhanced sticking as more energy can be stored in
the system before breakup.Comment: 11 pages, 14 figures, accepted for publication in A&
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
Measurements and Analysis of Secondary User Device Effects on Digital Television Receivers
This is the published version. Copyright © 2009 Newman et al.This article presents results from a study of the potential effects of secondary users operating in unoccupied television spectrum. Television spectrum is known within the wireless communications community as being underutilized, making it a prime candidate for dynamic spectrum access. The proposed use of this open spectrum has prompted questions concerning the quantity of available channel space that could be used without negative impact on consumers who view digital television broadcasts and the viability of secondary use of open channels immediately adjacent to a digital television broadcast channel. In this work, we investigate secondary device operation in the channels directly adjacent to a desired television channel, and the effects upon a selection of consumer digital television (DTV) receivers. Our observations strongly suggest that secondary users could operate "White Space Devices" (WSDs) in unoccupied channel bandwidth directly adjacent to a desired digital television (DTV) channel, with no observable adverse impact upon the reception of the desired channel content
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