Tag Archives: Soft Condensed Matter

Transmission of a Seismic Wave generated by impacts on Granular Asteroids [EPA]

Posted on by

http://arxiv.org/abs/2209.11353


In this paper we use a Soft-Sphere Discrete Element method code to simulate the transmission and study the attenuation of a seismic wave. Then, we apply our findings to the different space missions that have had to touch the surface of different small bodies. Additionally, we do the same in regards to the seismic wave generated by the hypervelocity impacts produced by the DART and Hayabusa2 missions once the shock wave transforms into a seismic wave. We find that even at very low pressures, such as those present in the interior of asteroids, the seismic wave speed can still be on the order of hundreds of m/s depending on the velocity of the impact that produces the wave. As expected from experimental measurements, our results show that wave velocity is directly dependent on $P^{1/6}$, where $P$ is the total pressure (confining pressure plus wave induced pressure). Regardless of the pressure of the system and the velocity of the impact (in the investigated range), energy dissipation is extremely high. These results provide us with a way to anticipate the extent to which a seismic wave could have been capable of moving some small particles on the surface of a small body upon contact with a spacecraft. Additionally, this rapid energy dissipation would imply that even hypervelocity impacts should perturb only the external layer of a self-gravitating aggregate on which segregation and other phenomena could take place. This would in turn produce a layered structure of which some evidence has been observed

Read this paper on arXiv…

P. Sánchez, D. Scheeres and A. Quillen
Mon, 26 Sep 22
43/62

Comments: Accepted for publication in The Planetary Sciences Journal

Impact drag force exerting on a projectile penetrating into a hierarchical granular bed [EPA]

Posted on by

http://arxiv.org/abs/2206.01037


Impact of a solid object onto a small-body surface can be modeled by the solid impact onto a hierarchically structured granular target. Impact drag force model for the hierarchically structured granular target is developed based on the experiment. We perform a set of granular impact experiments in which mechanical strength and porosity of target grains are systematically varied. Tiny glass beads ($5$~$\mu$m in diameter) are agglomerated to form porous grains of $2$–$4$~mm in diameter. Then, the grains are sintered to control their strength. A polyethylene sphere ($12.7$~mm in diameter) is dropped onto a hierarchical granular target consisting of these porous grains. Motion of the penetrating sphere is captured by a high-speed camera and analyzed. We find that impact drag force produced by the hierarchically structured granular target can be modeled by the sum of inertial drag and depth-proportional drag. The depth-proportional drag in hierarchical granular impact is much greater than that of the usual granular target consisting of rigid grains. The ratio between grain strength and impact dynamic pressure is a key dimensionless parameter to characterize this extraordinary large depth-proportional drag. Grain fracturing plays an important role in the impact dynamics when the impact dynamic pressure is sufficiently larger than the grain strength. This implies that the effect of grain fracturing should be considered also for the impact on a small body. Perhaps, effective strength of the surface grains can be estimated based on the kinematic observation of the intrusion or touchdown of the planetary explorator.

Read this paper on arXiv…

F. Okubo and H. Katsuragi
Fri, 3 Jun 22
49/57

Comments: 9 pages, 8 figures

Impacts of viscous dissipation on collisional growth and fragmentation of dust aggregates [EPA]

Posted on by

http://arxiv.org/abs/2205.13768


Understanding the collisional behavior of dust aggregates consisting of submicron-sized grains is essential to unveiling how planetesimals formed in protoplanetary disks. It is known that the collisional behavior of individual dust particles strongly depends on the strength of viscous dissipation force; however, impacts of viscous dissipation on the collisional behavior of dust aggregates have not been studied in detail, especially for the cases of oblique collisions. Here we investigated the impacts of viscous dissipation on the collisional behavior of dust aggregates. We performed numerical simulations of collisions between two equal-mass dust aggregates with various collision velocities and impact parameters. We also changed the strength of viscous dissipation force systematically. We found that the threshold collision velocity for the fragmentation of dust aggregates barely depends on the strength of viscous dissipation force when we consider oblique collisions. In contrast, the size distribution of fragments changes significantly when the viscous dissipation force is considered. We obtained the empirical fitting formulae for the size distribution of fragments for the case of strong dissipation, which would be useful to study the evolution of size and spatial distributions of dust aggregates in protoplanetary disks.

Read this paper on arXiv…

S. Arakawa, H. Tanaka and E. Kokubo
Mon, 30 May 22
27/47

Comments: 16 pages, 15 figures. Accepted for publication in ApJ

Scaling and phase diagrams of planetary sediment transport [EPA]

Posted on by

http://arxiv.org/abs/2203.00562


Sediment transport by atmospheric winds shapes the surface and affects the climates of planetary bodies. Reliably predicting the occurrence and rate of sediment transport in the Solar System has been notoriously difficult because fluid density, grain size and soil cohesiveness vary across many orders of magnitude. Here, we use recent advances in analytical and numerical sediment transport modeling to derive general scaling relations for planetary transport. In particular, we show that the equations of motion of rebounding grains predict that the minimum threshold fluid shear velocity needed to sustain transport (transport cessation threshold) scales with the particle-fluid-density ratio ($s$) as $s^{1/3}$, in contrast to the $s^{1/2}$-scaling exhibited by the threshold for transport initiation. The grain size corresponding to this minimum is in the range $80{-}290~\mu\mathrm{m}$ for Solar System bodies. Our results, summarized in phase diagrams for the cessation threshold, mean transport rate and dust emission potential, explain the observed eastward propagation of Titan’s dunes, in spite of a predominantly westward wind circulation, indicate active dust cycles on Earth, Mars and Titan, and suggest marginal but active atmospheric transport on Venus, Triton and Pluto.

Read this paper on arXiv…

T. Pähtz, O. Duŕan and F. Comola
Wed, 2 Mar 22
36/54

Comments: N/A

Propagation and attenuation of pulses driven by low velocity normal impacts in granular media [EPA]

Posted on by

http://arxiv.org/abs/2201.01225


We carry out experiments of low velocity normal impacts into granular materials that fill an approximately cylindrical 11 gallon tub. Motions in the granular medium are tracked with an array of 7 embedded accelerometers. Longitudinal pulses excited by the impact broaden and attenuate as a function of travel distance from the site of impact. Pulse propagation is not spherically symmetric about the site of impact. Peak amplitudes are about twice as large for the pulse propagating downward than at 45 degrees from vertical. An advection-diffusion model is used to estimate the dependence of pulse properties as a function of travel distance from the site of impact. The power law forms for pulse peak pressure, velocity and seismic energy depend on distance from impact to a power of -2.5 and this rapid decay is approximately consistent with our experimental measurements. Our experiments support a seismic jolt model, giving rapid attenuation of impact generated seismic energy into rubble asteroids, rather than a reverberation model, where seismic energy slowly decays. We apply our diffusive model to estimate physical properties of the seismic pulse that will be excited by the forthcoming DART mission impact onto the secondary, Dimorphos, of the asteroid binary (65803) Didymos system. We estimate that the pulse peak acceleration will exceed the surface gravity as it travels through the asteroid.

Read this paper on arXiv…

A. Quillen, M. Neiderbach, B. Suo, et. al.
Wed, 5 Jan 22
9/54

Comments: N/A

Deformation of a rotated granular pile governed by body-force-dependent friction [CL]

Posted on by

http://arxiv.org/abs/2112.02489


Although the gravity dependence of granular friction is crucial to understand various natural phenomena, its precise characterization is difficult. We propose a method to characterize granular friction under various gravity (body force) conditions controlled by centrifugal force; specifically, the deformation of a rotated granular pile was measured. To understand the mechanics governing the observed nontrivial deformation of this pile, we introduced an analytic model considering local force balance. The excellent agreement between the experimental data and theoretical model suggests that the deformation is simply governed by the net body force (sum of gravity and centrifugal force) and friction angle. The body-force dependence of granular friction was precisely measured from the experimental results. The results reveal that the grain shape affects the degree of body-force dependence of the granular friction.

Read this paper on arXiv…

T. Irie, R. Yamaguchi, S. Watanabe, et. al.
Tue, 7 Dec 21
39/91

Comments: 10 pages, 5 figures

Experimentally measuring rolling and sliding in three-dimensional dense granular packings [CL]

Posted on by

http://arxiv.org/abs/2108.11975


Understanding granular materials’ aging poses a substantial challenge: Grain contacts form networks with complex topologies, and granular flow is far from equilibrium. In this letter, we experimentally measure a three-dimensional granular system’s reversibility and aging under cyclic compression. We image the grains using a refractive-index-matched fluid, then analyze the images using the artificial intelligence of variational autoencoders. These techniques allow us to track all the grains’ translations and three-dimensional rotations with accuracy sufficient to infer contact-point sliding and rolling. Our observations reveal unique roles played by three-dimensional rotations in granular flow, aging, and energy dissipation. First, we find that granular rotations dominate the bulk dynamics, penetrating more deeply into the granular material than translations do. Second, sliding and rolling do not exhibit aging across the experiment, unlike translations. Third, aging appears not to minimize energy dissipation, according to our experimental measurements of rotations, combined with soft-sphere simulations. The experimental tools, analytical techniques, and observations that we introduce expose all the degrees of freedom of the far-from-equilibrium dynamics of granular flow.

Read this paper on arXiv…

Z. Benson, A. Peshkov, N. Halpern, et. al.
Mon, 30 Aug 21
30/38

Comments: N/A

Dynamo and the Adiabatic Invariant [EPA]

Posted on by

http://arxiv.org/abs/2108.11548


The paper considers dynamo generated by a shallow fluid layer in a celestial body (planet or star). This dynamo is based on the extra invariant for interacting magnetic Rossby waves. The magnetohydrodynamics (MHD) is linearized on the background of strong toroidal magnetic field. The extra invariant is used to show that the background field is maintained.

Read this paper on arXiv…

A. Balk
Fri, 27 Aug 21
28/67

Comments: 12 pages, 2 figures

Large-scale cortex-core structure formation in brain organoids [CL]

Posted on by

http://arxiv.org/abs/2108.05824


Brain organoids recapitulate a number of brain properties, including neuronal diversity. However, do they recapitulate brain shape? Using a hydrodynamic description for cell nuclei as particles interacting via an attractive field generated by the surrounding active cell cytoskeleton, we quantify shape development in brain organoids. Regions of cell nuclei overdensity in the linear regime drive the initial seeding for cortex-core structures, which emerge in the non-linear regime with elongated cell nuclei and thus, cell shape, in the cortex. We then use an extended version of the buckling without bending morphogenesis model to predict foliations/folds of the cortex in the presence of a nonlinearity due to elongated cells actively regulating strain. In addition to laying new groundwork for the design of more familiar and less familiar brain shapes, our work provides an intriguing quantitative connection with large-scale structure formation in the universe.

Read this paper on arXiv…

A. Borzou and J. Schwarz
Fri, 13 Aug 21
49/64

Comments: 14 pages, 9 figures. Comments are welcome

Relativistic Langevin Equation derived from a particle-bath Lagrangian [CL]

Posted on by

http://arxiv.org/abs/2107.07205


We show how a relativistic Langevin equation can be derived from a Lorentz-covariant version of the Caldeira-Leggett particle-bath Lagrangian. In one of its limits, we identify the obtained equation with the Langevin equation used in contemporary extensions of statistical mechanics to the near-light-speed motion of a Brownian particle in non-relativistic dissipative fluids. The proposed framework provides a more rigorous and first-principles form of the Langevin equation often quoted or postulated as ansatz in previous works. We then refine the aforementioned results by considering more terms in the particle-bath coupling, which improves the precision of the approximation for fully relativistic settings where not only the tagged particle but also the thermal bath motion is relativistic. We discuss the implications of the apparent breaking of space-time translation and parity invariance, showing that these effects are not necessarily in contradiction with the assumptions of statistical mechanics. The intrinsically non-Markovian character of the fully relativistic generalized Langevin equation derived here, and of the associated fluctuation-dissipation theorem, is also discussed.

Read this paper on arXiv…

A. Petrosyan and A. Zaccone
Fri, 16 Jul 21
25/61

Comments: N/A

Craters Formed in Granular Beds by Impinging Jets of Gas [CL]

Posted on by

http://arxiv.org/abs/0905.4851


When a jet of gas impinges vertically on a granular bed and forms a crater, the grains may be moved by several different mechanisms: viscous erosion, diffused gas eruption, bearing capacity failure, and/or diffusion-driven shearing. The relative importance of these mechanisms depends upon the flow regime of the gas, the mechanical state of the granular material, and other physical parameters. Here we report research in two specific regimes: viscous erosion forming scour holes as a function of particle size and gravity; and bearing capacity failure forming deep transient craters as a function of soil compaction.

Read this paper on arXiv…

P. Metzger, R. III, J. Schuler, et. al.
Tue, 13 Apr 2021
48/93

Comments: 4 pages; Powders and Grains 2009, Golden, Colorado, USA

Jet-induced cratering of a granular surface with application to lunar spaceports [CL]

Posted on by

http://arxiv.org/abs/0906.0196


The erosion of lunar soil by rocket exhaust plumes is investigated experimentally. This has identified the diffusion-driven flow in the bulk of the sand as an important but previously unrecognized mechanism for erosion dynamics. It has also shown that slow regime cratering is governed by the recirculation of sand in the widening geometry of the crater. Scaling relationships and erosion mechanisms have been characterized in detail for the slow regime. The diffusion-driven flow occurs in both slow and fast regime cratering. Because diffusion-driven flow had been omitted from the lunar erosion theory and from the pressure cratering theory of the Apollo and Viking era, those theories cannot be entirely correct.

Read this paper on arXiv…

P. Metzger, C. Immer, C. Donahue, et. al.
Tue, 13 Apr 2021
50/93

Comments: 13 pages, link to published version: this http URL

Empirical Scaling Laws of Rocket Exhaust Cratering [EPA]

Posted on by

http://arxiv.org/abs/2104.05176


When launching or landing a spacecraft on the regolith of a terrestrial surface, special attention needs to be paid to the rocket exhaust cratering effects. If the effects are not controlled, the rocket cratering could damage the spacecraft or other surrounding hardware. The cratering effects of a rocket landing a planet’s surface are not understood well, especially for the lunar case with the plume expanding in vacuum. As a result, the blast effects cannot be estimated sufficiently using analytical theories. It is necessary to develop physics-based simulation tools in order to calculate mission-essential parameters. In this work we test out the scaling laws of the physics in regard to growth rate of the crater depth. This will provide the physical insight necessary to begin the physics-based modeling.

Read this paper on arXiv…

C. Donohue, P. Metzger and C. Immer
Tue, 13 Apr 2021
53/93

Comments: 3 pages, 3 figures

Packing fraction of clusters formed in free-falling granular streams using flash X-ray [CL]

Posted on by

http://arxiv.org/abs/2103.04017


We study the packing fraction of clusters in free-falling streams of spherical and irregularly shaped particles using flash X-ray radiography. The estimated packing fraction of clusters is low enough to correspond to coordination numbers less than 6. Such coordination numbers in numerical simulations correspond to aggregates that collide and grow without bouncing. Moreover, the streams of irregular particles evolved faster and formed clusters of larger sizes with lower packing fraction. This result on granular streams suggests that particle shape has a significant effect on the agglomeration process of granular materials.

Read this paper on arXiv…

Y. Nagaashi, A. Nakamura, S. Hasegawa, et. al.
Tue, 9 Mar 21
15/68

Comments: Accepted for publication in Physical Review E

Collision-driven emergence of the cosmic web [CEA]

Posted on by

http://arxiv.org/abs/2103.04935


Gravitational-collapse-based explanations of the cosmic web lead to problems in estimating the total mass in the universe. A first-principles several-scales model is developed here for the structural organisation of cosmic matter in a flat universe, showing that the web formation could be driven by inelastic collisions before gravity took hold, suggesting a possible way to resolve these problems. The following results are derived. (i) The diffusion rate in the particulate gas after recombination is sub-anomalous, with a rapid decay of particle velocities. (ii) The evolution of the particle velocity distribution is calculated explicitly. (iii) The gas density is shown to be unstable, leading to void formation and clusters nucleation. (iv) Rounded clusters are shown to be unstable and tend to elongate. (v) An equation is derived for the growth of long clusters into filaments and solved explicitly. The fast-growing clusters deplete the regions around them and generate large voids, potentially giving rise to the cosmic web before gravity dominated.

Read this paper on arXiv…

R. Blumenfeld
Tue, 9 Mar 21
65/68

Comments: 7 pages, 5 figures, submitted

On the stickiness of CO$_{2}$ and H$_{2}$O ice particles [EPA]

Posted on by

http://arxiv.org/abs/2102.06683


Laboratory experiments revealed that CO${2}$ ice particles stick less efficiently than H${2}$O ice particles, and there is an order of magnitude difference in the threshold velocity for sticking. However, the surface energies and elastic moduli of CO${2}$ and H${2}$O ices are comparable, and the reason why CO${2}$ ice particles were poorly sticky compared to H${2}$O ice particles was unclear. Here we investigate the effects of viscoelastic dissipation on the threshold velocity for sticking of ice particles using the viscoelastic contact model derived by Krijt et al. We find that the threshold velocity for sticking of CO${2}$ ice particles reported in experimental studies is comparable to that predicted for perfectly elastic spheres. In contrast, the threshold velocity for sticking of H${2}$O ice particles is an order of magnitude higher than that predicted for perfectly elastic spheres. Therefore, we conclude that the large difference in stickiness between CO${2}$ and H${2}$O ice particles would mainly originate from the difference in the strength of viscoelastic dissipation, which is controlled by the viscoelastic relaxation time.

Read this paper on arXiv…

S. Arakawa and S. Krijt
Mon, 15 Feb 21
36/53

Comments: 15 pages, 7 figures. Accepted for publication in ApJ

Cohesion of regolith: Measurements of meteorite powders [EPA]

Posted on by

http://arxiv.org/abs/2101.11837


The cohesion of particles has a significant effect on the properties of small bodies. In this study, we measured in open air, the cohesive forces of tens of micron-sized irregularly shaped meteorite, silica sand, glass powder, and spherical glass particles, using a centrifugal method. In addition, we estimated the amount of water vapor adsorbed on the particles under the measurement conditions. The measured cohesive forces of the meteorite particles are tens of times smaller than those of an ideally spherical silica particle and correspond to the submicron-scale effective (or equivalent) curvature radius of the particle surface. Moreover, based on the estimated amount of water vapor adsorbed on the particles, we expect the cohesive forces of the particles in airless bodies to be approximately 10 times larger than those measured in open air. Based on the measurement results, we estimate that the cohesive forces of the particles on asteroids are typically in the sub-micro-Newton range, and that the particles on fast-rotating asteroids are tens of microns in size.

Read this paper on arXiv…

Y. Nagaashi, T. Aoki and A. Nakamura
Fri, 29 Jan 21
9/66

Comments: 32 pages, 9 figures, 7 tables. Accepted for publication in Icarus

Primordial porous structure of chondrite parent bodies due to self-gravity [EPA]

Posted on by

http://arxiv.org/abs/2101.09533


The porosity of an asteroid is important when studying the evolution of our solar system through small bodies and for planning mitigation strategies to avoid disasters due to asteroid impacts. Our knowledge of asteroid porosity largely relies on meteorites sampled on Earth. However, chondrites sampled on Earth are suggested to be sorted by strength. In this study, we obtained an estimate of the most porous structure of primordial “granular” chondrite parent bodies based on measurements of the compaction behavior of chondrite component analogs. We measured compaction curves of dust and dust-beads mixture samples. The dust sample consisted of various spherical and irregular particles with diameters in the order of 10^0-10^1 $\mu$m. The mixture sample consisted of dust and beads with different dust volume fractions (~0.2-1). We used 1.5 and 4.8 $\mu$m particles as dust as a first step although the typical size of materials in matrix may be much smaller. We approximated the compaction curve of each sample with a power-law form and calculated the porosity structure of the primordial chondrite parent bodies using the experimental results. Our results show that the primordial parent bodies are likely to have higher porosity than the chondrites. Moreover, the relatively higher volume fraction of the matrix may be one of the reasons why most meteorites with high porosity are carbonaceous chondrites.

Read this paper on arXiv…

T. Omura and A. Nakamura
Tue, 26 Jan 21
30/84

Comments: 20 pages, 4 tables, 5 figures

A Smoking Gun for Planetesimal Formation: Charge Driven Growth into a New Size Range [EPA]

Posted on by

http://arxiv.org/abs/2101.09124


Collisions electrically charge grains which promotes growth by coagulation. We present aggregation experiments with three large ensembles of basalt beads ($150\,\mu\mathrm{m} – 180\,\mu\mathrm{m})$, two of which are charged, while one remains almost neutral as control system. In microgravity experiments, free collisions within these samples are induced with moderate collision velocities ($0 – 0.2 \,\mathrm{m\,s}^{-1}$). In the control system, coagulation stops at (sub-)mm size while the charged grains continue to grow. A maximum agglomerate size of 5\,cm is reached, limited only by bead depletion in the free volume. For the first time, charge-driven growth well into the centimeter range is directly proven by experiments. In protoplanetary disks, this agglomerate size is well beyond the critical size needed for hydrodynamic particle concentration as, e.g., by the streaming instabilities.

Read this paper on arXiv…

J. Teiser, M. Kruss, F. Jungmann, et. al.
Mon, 25 Jan 21
14/60

Comments: N/A

Centroid migration on an impacted granular slope due to asymmetric ejecta deposition and landsliding [EPA]

Posted on by

http://arxiv.org/abs/2012.10828


For a fundamental understanding of terrain relaxation occurring on sloped surfaces of terrestrial bodies, we analyze the crater shape produced by an impact on an inclined granular (dry-sand) layer. Owing to asymmetric ejecta deposition followed by landsliding, the slope of the impacted inclined surface can be relaxed. Using the experimental results of a solid projectile impact on an inclined dry-sand layer, we measure the distance of centroid migration induced by asymmetric cratering. We find that the centroid migration distance $x_\mathrm{mig}$ normalized to the crater minor-axis diameter $D_\mathrm{cy}$ can be expressed as a function of the initial inclination of the target $\tan\theta$, the effective friction coefficient $\mu$, and two parameters $K$ and $c$ that characterize the asymmetric ejecta deposition and oblique impact effect: $x_\mathrm{mig}/D_\mathrm{cy}=K \tan\theta/(1-(\tan\theta/\mu)^2)+c$, where $K=0.6$, $\mu=0.8$, and $c=-0.1$ to $0.3$. This result is consistent with a previous study that considered the effect of asymmetric ejecta deposition. The obtained results provide fundamental information for analyzing the degradation of sloped terrain on planetary surfaces, such as crater-shape degradation due to the accumulation of micro-impacts.

Read this paper on arXiv…

T. Omura, S. Takizawa and H. Katsuragi
Tue, 22 Dec 20
64/89

Comments: 7 pages, 7 figures

Probe-insertion exhibits gravity-dependent stick-slip dynamics in experiments on a model system for regolith surfaces [EPA]

Posted on by

http://arxiv.org/abs/2011.12890


The surfaces of many planetary bodies, including asteroids and small moons, are covered with dust to pebble-sized grains held weakly to the surface by gravity and contact forces. The Hayabusa2 and OSIRIS-REx missions have both confirmed that this is the case for the asteroids (162173) Ryugu and (101955) Bennu, respectively, raising the question of how surface disturbances propagate in low-gravity environments. Instruments including sensors and anchoring mechanisms for use on such surfaces will require efficient and effective design principles. We analyze the behavior of a flexible probe inserted into loose regolith as a function of speed and gravitational acceleration as a prototypical example exploring the relevant dynamics. The EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) flew on several parabolic flights. It employs a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible probe during its insertion into a laboratory system of bi-disperse, cm-sized model grains. We identify the grain-scale forces throughout the system for probe insertion at a variety of speeds and for four different levels of gravity: terrestrial, martian, lunar, and microgravity. We demonstrate that the photoelastic techniques provide results that complement traditional load cell measurements, with both methods identifying discrete, stick-slip failure events that increase in both magnitude and frequency as a function of the gravitational acceleration. For microgravity experiments, stick-slip behaviors are negligible. We additionally find that faster probe insertion can suppress stick-slip behaviors where they are present. We conclude that the behavior of regolith on rubble pile asteroids is likely quite distinct from the environments found on larger objects.

Read this paper on arXiv…

J. Featherstone, R. Bullard, T. Emm, et. al.
Thu, 26 Nov 20
19/65

Comments: N/A

Probe-insertion exhibits gravity-dependent stick-slip dynamics in experiments on a model system for regolith surfaces [EPA]

Posted on by

http://arxiv.org/abs/2011.12890


The surfaces of many planetary bodies, including asteroids and small moons, are covered with dust to pebble-sized grains held weakly to the surface by gravity and contact forces. The Hayabusa2 and OSIRIS-REx missions have both confirmed that this is the case for the asteroids (162173) Ryugu and (101955) Bennu, respectively, raising the question of how surface disturbances propagate in low-gravity environments. Instruments including sensors and anchoring mechanisms for use on such surfaces will require efficient and effective design principles. We analyze the behavior of a flexible probe inserted into loose regolith as a function of speed and gravitational acceleration as a prototypical example exploring the relevant dynamics. The EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) flew on several parabolic flights. It employs a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible probe during its insertion into a laboratory system of bi-disperse, cm-sized model grains. We identify the grain-scale forces throughout the system for probe insertion at a variety of speeds and for four different levels of gravity: terrestrial, martian, lunar, and microgravity. We demonstrate that the photoelastic techniques provide results that complement traditional load cell measurements, with both methods identifying discrete, stick-slip failure events that increase in both magnitude and frequency as a function of the gravitational acceleration. For microgravity experiments, stick-slip behaviors are negligible. We additionally find that faster probe insertion can suppress stick-slip behaviors where they are present. We conclude that the behavior of regolith on rubble pile asteroids is likely quite distinct from the environments found on larger objects.

Read this paper on arXiv…

J. Featherstone, R. Bullard, T. Emm, et. al.
Thu, 26 Nov 20
42/65

Comments: N/A

Validating N-body code Chrono for granular DEM simulations in reduced-gravity environments [EPA]

Posted on by

http://arxiv.org/abs/2009.10448


The Discrete Element Method (DEM) is frequently used to model complex granular systems and to augment the knowledge that we obtain through theory, experimentation, and real-world observations. Numerical simulations are a particularly powerful tool for studying the regolith-covered surfaces of asteroids, comets, and small moons, where reduced-gravity environments produce ill-defined flow behaviors. In this work, we present a method for validating soft-sphere DEM codes for both terrestrial and small-body granular environments. The open-source code Chrono is modified and evaluated first with a series of simple two-body-collision tests, and then, with a set of piling and tumbler tests. In the piling tests, we vary the coefficient of rolling friction to calibrate the simulations against experiments with 1 mm glass beads. Then, we use the friction coefficient to model the flow of 1 mm glass beads in a rotating drum, using a drum configuration from a previous experimental study. We measure the dynamic angle of repose, the flowing layer thickness, and the flowing layer velocity for tests with different particle sizes, contact force models, coefficients of rolling friction, cohesion levels, drum rotation speeds and gravity levels. The tests show that the same flow patterns can be observed at Earth and reduced-gravity levels if the drum rotation speed and the gravity-level are set according to the dimensionless parameter known as the Froude number. Chrono is successfully validated against known flow behaviors at different gravity and cohesion levels, and will be used to study small-body regolith dynamics in future works.

Read this paper on arXiv…

C. Sunday, N. Murdoch, S. Tardivel, et. al.
Wed, 23 Sep 20
-1734/86

Comments: N/A

Laboratory Impact Splash Experiments to Simulate Asteroid Surfaces [EPA]

Posted on by

http://arxiv.org/abs/2007.01130


Granular material that is bound by the low gravity of a small asteroid is mobilized by slow velocity impacts. These splashes generated by impacts might play an important role in sculpting the asteroid’s surface. In laboratory experiments we characterize the ejecta generated by spherical 150 $\rm \mu m$ diameter basalt grains impacting a granular bed at 0.8 m/s. We find that such an impact typically leads to less than 10 particles being ejected from the granular bed, with typical ejecta trajectories rising to less than one particle diameter above the surface. That is, the observed impacts are highly dissipative and only a small fraction of the impact energy is imparted onto the ejecta. While the impactor itself still rebounds, it typically slows down significantly to an average of about 20 % of its impact velocity. Scaled to asteroids, impactor and ejecta generated from impacts of sand sized grains are not able to spread over the asteroid’s surface but will stay close to the impact site. Therefore these highly inelastic impacts into soft granular beds efficiently trap grains, in contrast to more elastic impacts on bare, rocky surfaces confirming suggestions by Shinbrot (2017). This is also in agreement to observed features on asteroids as this topological elasticity bias suggests that redistribution of grains leads to a size segregation.

Read this paper on arXiv…

T. Bogdan, J. Kollmer, J. Teiser, et. al.
Fri, 3 Jul 20
42/57

Comments: N/A

Thermal Fluctuations in Nuclear Pasta [CL]

Posted on by

http://arxiv.org/abs/2005.04766


Despite their astrophysical relevance, nuclear pasta phases are relatively unstudied at high temperatures. We present molecular dynamics simulations of symmetric nuclear matter with several topologies of `lasagna’ at a range of temperatures to study the pasta-uniform transition. Using the Minkowski functionals we quantify trends in the occupied volume, surface area, mean breadth, and Euler characteristic. The amplitude of surface displacements of the pasta increase with temperature which produce short lived topological defects such as holes and filaments near melting, resulting in power laws for increasing surface curvature with temperature. We calculate the static structure factor and report the shear viscosity and thermal conductivity of pasta, finding that the shear viscosity is minimized at the melting temperature. These results may have implications for the thermoelastic properties of nuclear pasta and finite temperature corrections to the equation of state at pasta densities.

Read this paper on arXiv…

M. Caplan, C. Forsman and A. Schneider
Tue, 12 May 20
19/64

Comments: 12 pages, 8 figures

Generation of chiral asymmetry via helical magnetic fields [CL]

Posted on by

http://arxiv.org/abs/2002.09501


It is well known that helical magnetic fields undergo a so-called inverse cascade by which their correlation length grows due to the conservation of magnetic helicity in classical ideal magnetohydrodynamics (MHD). At high energies above approximately $10$ MeV, however, classical MHD is necessarily extended to chiral MHD and then the conserved quantity is $\langle\mathcal{H}\rangle + 2 \langle\mu_5\rangle / \lambda$ with $\langle\mathcal{H}\rangle$ being the mean magnetic helicity and $\langle\mu_5\rangle$ being the mean chiral chemical potential of charged fermions. Here, $\lambda$ is a (phenomenological) chiral feedback parameter. In this paper, we study the evolution of the chiral MHD system with the initial condition of nonzero $\langle\mathcal{H}\rangle$ and vanishing $\mu_5$. We present analytic derivations for the time evolution of $\langle\mathcal{H}\rangle$ and $\langle\mu_5\rangle$ that we compare to a series of laminar and turbulent three-dimensional direct numerical simulations. We find that the late-time evolution of $\langle\mathcal{H}\rangle$ depends on the magnetic and kinetic Reynolds numbers ${\rm Re}{\mathrm{M}}$ and ${\rm Re}{\mathrm{K}}$. For a high ${\rm Re}{\mathrm{M}}$ and ${\rm Re}{\mathrm{K}}$ where turbulence occurs, $\langle\mathcal{H}\rangle$ eventually evolves in the same way as in classical ideal MHD where the inverse correlation length of the helical magnetic field scales with time $t$ as $k_\mathrm{p} \propto t^{-2/3}$. For a low Reynolds numbers where the velocity field is negligible, the scaling is changed to $k_\mathrm{p} \propto t^{-1/2}\mathrm{ln}\left(t/t_\mathrm{log}\right)$. After being rapidly generated, $\langle\mu_5\rangle$ always decays together with $k_\mathrm{p}$, i.e. $\langle\mu_5\rangle \approx k_\mathrm{p}$, with a time evolution that depends on whether the system is in the limit of low or high Reynolds numbers.

Read this paper on arXiv…

J. Schober, T. Fujita and R. Durrer
Tue, 25 Feb 20
8/76

Comments: 16 pages, 11 figures, submitted to PRD

A novel experimental setup for an oblique impact onto an inclined granular layer [EPA]

Posted on by

http://arxiv.org/abs/2001.01857


We develop an original apparatus of the granular impact experiment by which the incident angle of the solid projectile and inclination angle of the target granular layer can be systematically varied. Whereas most of the natural cratering events occur on inclined surfaces with various incident angles, there have not been any experiments on oblique impacts on an inclined target surface. To perform systematic impact experiments, a novel experimental apparatus has to be developed. Therefore, we build an apparatus for impact experiments where both the incident angle and the inclination angle can be independently varied. The projectile-injection unit accelerates a plastic ball (6~mm in diameter) up to $v_i\simeq 100$~m~s$^{-1}$ impact velocity. The barrel of the injection unit is made with a three-dimensional printer. The impact dynamics is captured by high-speed cameras to directly measure the impact velocity and incident angle. The rebound dynamics of the projectile (restitution coefficient and rebound angle) is also measured. The final crater shapes are measured using a line-laser profiler mounted on the electric stages. By scanning the surface using this system, a three-dimensional crater shape (height map) can be constructed. From the measured result, we can define and measure the characteristic quantities of the crater. The analyzed result on the restitution dynamics is presented as an example of systematic experiments using the developed system.

Read this paper on arXiv…

S. Takizawa, R. Yamaguchi and H. Katsuragi
Wed, 8 Jan 20
37/64

Comments: 9 pages, 8 figures

Linearly forced fluid flow on a rotating sphere [CL]

Posted on by

http://arxiv.org/abs/1912.02131


Motivated in part by the complex flow patterns observed in planetary atmospheres, we investigate generalized Navier-Stokes (GNS) equations that couple nonlinear advection with a generic linear instability. This analytically tractable minimal model for fluid flows driven by internal active stresses has recently been shown to permit exact solutions on a stationary 2D sphere. Here, we extend the analysis to linearly driven flows on rotating spheres, as relevant to quasi-2D atmospheres. We derive exact solutions of the GNS equations corresponding to time-independent zonal jets and superposed westward-propagating Rossby waves. Direct numerical simulations with large rotation rates obtain statistically stationary states close to these exact solutions. The measured phase speeds of waves in the GNS simulations agree with analytical predictions for Rossby waves.

Read this paper on arXiv…

R. Supekar, V. Heinonen, K. Burns, et. al.
Thu, 5 Dec 19
22/71

Comments: 12 pages, 5 figures

Single particle triboelectrification of Titan sand analogs [EPA]

Posted on by

http://arxiv.org/abs/1911.12923


Sand electrification is important for aeolian sediment transportation on terrestrial bodies with silicate sand as the main sediment composition. However, it has not been thoroughly studied for icy bodies such as Titan with organic sand as the main dune-forming material. We used the colloidal probe atomic force microscopy (AFM) technique to study triboelectric charging processes using Titan and Earth sand analogs. We found that it is easy to generate triboelectric charges between naphthalene (a simple aromatic hydrocarbon), polystyrene (an aromatic hydrocarbon polymer), and borosilicate glass (Earth silicate sand analog). Strong electrostatic forces can be measured after contact and/or tribocharging. In contrast, tholin, a complex organic material, does not generate any detectable electrostatic forces with contact or tribocharging within the detection limit of the instrument. If Titan sand behaves more like tholin, this indicates that the tribocharging capacity of Titan sand is much weaker than Earth silicate sand and much less than previously measured by Mendez-Harper et al., (2017), where only simple organics were used for Titan sand analogs. Thus, triboelectrification may not contribute to increasing interparticle forces between sand particles on Titan as much as on Earth. Interparticle forces generated by other electrostatic processes or other interparticle forces such as van der Waals and capillary cohesion forces could be the dominant interparticle forces that govern Titan sand formation and sediment transportation on the surface. Titan sand is also unlikely to produce large electrical discharge through tribocharging to affect future missions to Titan’s surface.

Read this paper on arXiv…

X. Yu, S. Horst, C. He, et. al.
Mon, 2 Dec 19
84/91

Comments: 25 pages, 5 figures

Challenges in fluid flow simulations using Exascale computing [CL]

Posted on by

http://arxiv.org/abs/1911.10020


In this paper, I discuss the challenges in porting hydrodynamic codes to futuristic exascale HPC systems. In particular, we describe the computational complexities of finite difference method, pseudo-spectral method, and Fast Fourier Transform (FFT). We show how global data communication among the processors brings down the efficiency of pseudo-spectral codes and FFT. It is argued that FFT scaling may saturate at 1/2 million processors. However, finite difference and finite volume codes scale well beyond million processors, hence they are likely candidates to be tried on exascale systems. The codes based on spectral-element and Fourier continuation, that are more accurate than finite difference, could also scale well on such systems.

Read this paper on arXiv…

M. Verma
Mon, 25 Nov 19
55/55

Comments: N/A

Thermal conductivity and coordination number of compressed dust aggregates [EPA]

Posted on by

http://arxiv.org/abs/1901.09700


Understanding the heat transfer mechanism within dust aggregates is of great importance for many subjects in planetary science. We calculated the coordination number and the thermal conductivity through the solid network of compressed dust aggregates. We found a simple relationship between the coordination number and the filling factor and revealed that the thermal conductivity through the solid network of aggregates is represented by a power-law function of the filling factor and the coordination number.

Read this paper on arXiv…

S. Arakawa, M. Tatsuuma, N. Sakatani, et. al.
Tue, 29 Jan 19
26/62

Comments: 8 pages, 7 figures. Accepted for publication in Icarus. arXiv admin note: text overlap with arXiv:1711.06268

Boulder Stranding in Ejecta Launched by an Impact Generated Seismic Pulse [EPA]

Posted on by

http://arxiv.org/abs/1812.01670


We consider how an impact generated seismic pulse affects the surface of an asteroid distant from the impact site. With laboratory experiments on dry polydisperse gravel mixtures, we track the trajectories of particles ejected from the surface by a single strong upward propagating pressure pulse. High speed video images show that ejecta trajectories are independent of particle size, and collisions primarily take place upon landing. When they land particles are ballistically sorted, as proposed by Shinbrot et al. (2017), leaving larger particles on the surface and smaller particles more widely dispersed. A single strong pulse can leave previously buried boulders stranded on the surface. Boulder stranding due to an impact excited seismic pulse is an additional mechanism that could leave large boulders present on the surface of rubble asteroids such as 162173 Ryugu, 101955 Bennu and 25143 Itokawa.

Read this paper on arXiv…

E. Wright, A. Quillen, J. South, et. al.
Thu, 6 Dec 18
45/52

Comments: Links to experiment videos: this https URL , this https URL , this https URL , this https URL

Impact-induced energy transfer and dissipation in granular clusters under microgravity conditions [CL]

Posted on by

http://arxiv.org/abs/1810.11146


The impact-induced energy transfer and dissipation in granular targets without any confining walls are studied by microgravity experiments. A solid projectile impacts into a granular target at low impact speed ($0.045 \leq v_p \leq 1.6$~m~s$^{-1}$) in a laboratory drop tower. Granular clusters consisting of soft or hard particles are used as targets. Porous dust agglomerates and glass beads are used for soft and hard particles, respectively. The expansion of the granular target cluster is recorded by a high-speed camera. Using the experimental data, we find that (i)~a simple energy scaling can explain the energy transfer in both, soft- and hard-particles granular targets, (ii)~the kinetic impact energy is isotropically transferred to the target from the impact point, and (iii)~the transferred kinetic energy is $2$~-~$7$\% of the projectile’s initial kinetic energy. The dissipative-diffusion model of energy transfer can quantitatively explain these behaviors.

Read this paper on arXiv…

H. Katsuragi and J. Blum
Mon, 29 Oct 18
44/45

Comments: 7 pages 9 figures

Thermal conductivity of porous aggregates [EPA]

Posted on by

http://arxiv.org/abs/1711.06268


$\mathit{Context.}$ The thermal conductivity of highly porous dust aggregates is a key parameter for many subjects in planetary science; however, it is not yet fully understood. $\mathit{Aims.}$ In this study, we investigate the thermal conductivity of fluffy dust aggregates with filling factors of less than $10^{-1}$. $\mathit{Methods.}$ We determine the temperature structure and heat flux of the porous dust aggregates calculated by $N$-body simulations of static compression in the periodic boundary condition. $\mathit{Results.}$ We derive an empirical formula for the thermal conductivity through the solid network $k_{\rm sol}$ as a function of the filling factor of dust aggregates $\phi$. The results reveal that $k_{\rm sol}$ is approximately proportional to ${\phi}^{2}$, and the thermal conductivity through the solid network is significantly lower than previously assumed. In light of these findings, we must reconsider the thermal histories of small planetary bodies.

Read this paper on arXiv…

S. Arakawa, H. Tanaka, A. Kataoka, et. al.
Mon, 20 Nov 17
5/56

Comments: 4 pages, 4 figures. Accepted for publication in Astronomy & Astrophysics

The Physics of Protoplanetesimal Dust Agglomerates. X. Mechanical properties of dust aggregates probed by a solid-projectile impact [EPA]

Posted on by

http://arxiv.org/abs/1709.03118


Dynamic characterization of mechanical properties of dust aggregates has been one of the most important problems to quantitatively discuss the dust growth in protoplanetary disks. We experimentally investigate the dynamic properties of dust aggregates by low-speed ($\lesssim 3.2$ m s$^{-1}$) impacts of solid projectiles. Spherical impactors made of glass, steel, or lead are dropped onto a dust aggregate of packing fraction $\phi=0.35$ under vacuum conditions. The impact results in cratering or fragmentation of the dust aggregate, depending on the impact energy. The crater shape can be approximated by a spherical segment and no ejecta are observed. To understand the underlying physics of impacts into dust aggregates, the motion of the solid projectile is acquired by a high-speed camera. Using the obtained position data of the impactor, we analyze the drag-force law and dynamic pressure induced by the impact. We find that there are two characteristic strengths. One is defined by the ratio between impact energy and crater volume and is $\simeq 120$ kPa. The other strength indicates the fragmentation threshold of dynamic pressure and is $\simeq 10$ kPa. The former characterizes the apparent plastic deformation and is consistent with the drag force responsible for impactor deceleration. The latter corresponds to the dynamic tensile strength to create cracks. Using these results, a simple model for the compaction and fragmentation threshold of dust aggregates is proposed. In addition, the comparison of drag-force laws for dust aggregates and loose granular matter reveals the similarities and differences between the two materials.

Read this paper on arXiv…

H. Katsuragi and J. Blum
Tue, 12 Sep 17
32/71

Comments: 10 pages, 7 figures

Collisions between sintered icy aggregates [EPA]

Posted on by

http://arxiv.org/abs/1705.04778


Collisions between sintered icy dust aggregates are numerically simulated. If the temperature of an icy aggregate is sufficiently high, sintering promotes molecular transport and a neck between adjacent grains grows. This growth changes the mechanical responses of the neck. We included this effect to a simulation code, and conducted collisional simulations. For porous aggregates, the critical velocity for growth, below which the mass of an aggregate increases, decreased from 50\,m\,s$^{-1}$ for the non-sintered case to 20\,m\,s$^{-1}$. For compacted aggregates, the main collisional outcome is bouncing. These results come from the fact that the strength of the neck is increased by sintering. The numerical results suggest that the collisional growth of icy grain aggregates is strongly affected by sintering.

Read this paper on arXiv…

S. Sirono and H. Ueno
Tue, 16 May 17
18/78

Comments: N/A

Giant ripples on comet 67P/Churyumov-Gerasimenko sculpted by sunset thermal wind [CL]

Posted on by

http://arxiv.org/abs/1703.02592


Explaining the unexpected presence of dune-like patterns at the surface of the comet 67P/Churyumov-Gerasimenko requires conceptual and quantitative advances in the understanding of surface and outgassing processes. We show here that vapor flow emitted by the comet around its perihelion spreads laterally in a surface layer, due to the strong pressure difference between zones illuminated by sunlight and those in shadow. For such thermal winds to be dense enough to transport grains — ten times greater than previous estimates — outgassing must take place through a surface porous granular layer, and that layer must be composed of grains whose roughness lowers cohesion consistently with contact mechanics. The linear stability analysis of the problem, entirely tested against laboratory experiments, quantitatively predicts the emergence of bedforms in the observed wavelength range, and their propagation at the scale of a comet revolution. Although generated by a rarefied atmosphere, they are paradoxically analogous to ripples emerging on granular beds submitted to viscous shear flows. This quantitative agreement shows that our understanding of the coupling between hydrodynamics and sediment transport is able to account for bedform emergence in extreme conditions and provides a reliable tool to predict the erosion and accretion processes controlling the evolution of small solar system bodies.

Read this paper on arXiv…

P. Jia, B. Andreotti and P. Claudin
Thu, 9 Mar 17
11/54

Comments: 37 pages, 13 figures, 1 table

Visco-rotational shear instability of Keplerian granular flows [EPA]

Posted on by

http://arxiv.org/abs/1702.07271


We present the linear rheological instability triggered by the interplay of the shear rheology and Keplerian differential rotation of incompressible dense granular fluids. Instability sets in granular fluids, where the viscosity parameter grows faster than the square of the local shear rate (strain rate) at constant pressure. Found instability can play a crucial role in the formation of observed structures in planetary rings, as well as promote structure formation in protoplanetary disks dense granular material.

Read this paper on arXiv…

L. Poniatowski and A. Tevzadze
Fri, 24 Feb 17
38/50

Comments: 5 pages, 3 figures. Comments welcome

An experimental study of low-velocity impacts into granular material in reduced gravity [EPA]

Posted on by

http://arxiv.org/abs/1702.05980


In order to improve our understanding of landing on small bodies and of asteroid evolution, we use our novel drop tower facility to perform low-velocity (2-40 cm s^-1), shallow impact experiments of a 10 cm diameter aluminum sphere into quartz sand in low effective gravities (~0.2-1 m s^-2). Using in situ accelerometers, we measure the acceleration profile during the impacts and determine the peak accelerations, collision durations and maximum penetration depth. We find that the penetration depth scales linearly with the collision velocity but is independent of the effective gravity for the experimental range tested, and that the collision duration is independent of both the effective gravity and the collision velocity. No rebounds are observed in any of the experiments. Our low-gravity experimental results indicate that the transition from the quasi-static regime to the inertial regime occurs for impact energies two orders of magnitude smaller than in similar impact experiments under terrestrial gravity. The lower energy regime change may be due to the increased hydrodynamic drag of the surface material in our experiments, but may also support the notion that the quasi-static regime reduces as the effective gravity becomes lower.

Read this paper on arXiv…

N. Murdoch, I. Martinez, C. Sunday, et. al.
Tue, 21 Feb 17
48/70

Comments: Advance Access publication: January 4 2017

A pure hydrodynamic instability in shear flows and its application to astrophysical accretion disks [HEAP]

Posted on by

http://arxiv.org/abs/1608.00980


We provide the possible resolution for the century old problem of hydrodynamic shear flows, which are apparently stable in linear analysis but shown to be turbulent in astrophysically observed data and experiments. This mismatch is noticed in a variety of systems, from laboratory to astrophysical flows. There are so many uncountable attempts made so far to resolve this mismatch, beginning with the early work of Kelvin, Rayleigh, and Reynolds towards the end of the nineteenth century. Here we show that the presence of stochastic noise, whose inevitable presence should not be neglected in the stability analysis of shear flows, leads to pure hydrodynamic linear instability therein. This explains the origin of turbulence, which has been observed/interpreted in astrophysical accretion disks, laboratory experiments and direct numerical simulations. This is, to the best of our knowledge, the first solution to the long standing problem of hydrodynamic instability of Rayleigh stable flows.

Read this paper on arXiv…

S. Nath and B. Mukhopadhyay
Thu, 4 Aug 16
70/70

Comments: 14 pages including 10 figures; accepted for publication in ApJ

Astromaterial Science and Nuclear Pasta [HEAP]

Posted on by

http://arxiv.org/abs/1606.03646


The heavens contain a variety of materials that range from conventional to extraordinary and extreme. For this colloquium, we define Astromaterial Science as the study of materials, in astronomical objects, that are qualitatively denser than materials on earth. Astromaterials can have unique properties, related to their density, such as extraordinary mechanical strength, or alternatively be organized in ways similar to more conventional materials. The study of astromaterials may suggest ways to improve terrestrial materials. Likewise, advances in the science of conventional materials may allow new insights into astromaterials. We discuss Coulomb crystals in the interior of cold white dwarfs and in the crust of neutron stars and review the limited observations of how stars freeze. We apply astromaterial science to the generation of gravitational waves. According to Einstein’s Theory of General Relativity accelerating masses radiate gravitational waves. However, very strong materials may be needed to vigorously accelerate large masses in order to produce continuous gravitational waves that are observable in present detectors. We review large-scale molecular dynamics simulations of the breaking stress of neutron star crust that suggest it is the strongest material known, some ten billion times stronger than steel. Nuclear pasta is an example of a soft astromaterial. It is expected near the base of the neutron star crust at densities of ten to the fourteen grams per cubic centimeter. Competition between nuclear attraction and Coulomb repulsion rearrange neutrons and protons into complex non-spherical shapes such as flat plates (lasagna) or thin rods (spaghetti). We review semi-classical molecular dynamics simulations of nuclear pasta. We illustrate some of the shapes that are possible and discuss transport properties including shear viscosity and thermal and electrical conductivities.

Read this paper on arXiv…

M. Caplan and C. Horowitz
Tue, 14 Jun 16
50/67

Comments: 13 pages, 7 figures

Cohesion of Amorphous Silica Spheres: Toward a Better Understanding of the Coagulation Growth of Silicate Dust Aggregates [EPA]

Posted on by

http://arxiv.org/abs/1603.03168


Adhesion forces between submicrometer-sized silicate grains play a crucial role in the formation of silicate dust agglomerates, rocky planetesimals, and terrestrial planets. The surface energy of silicate dust particles is the key to their adhesion and rolling forces in a theoretical model based on the contact mechanics. Here we revisit the cohesion of amorphous silica spheres by compiling available data on the surface energy for hydrophilic amorphous silica in various circumstances. It turned out that the surface energy for hydrophilic amorphous silica in a vacuum is a factor of 10 higher than previously assumed. Therefore, the previous theoretical models underestimated the critical velocity for the sticking of amorphous silica spheres, as well as the rolling friction forces between them. With the most plausible value of the surface energy for amorphous silica spheres, theoretical models based on the contact mechanics are in harmony with laboratory experiments. Consequently, we conclude that silicate grains with a radius of $0.1~\mu$m could grow to planetesimals via coagulation in a protoplanetary disk. We argue that the coagulation growth of silicate grains in a molecular cloud is advanced either by organic mantles rather than icy mantles or, if there are no mantles, by nanometer-sized grain radius.

Read this paper on arXiv…

H. Kimura, K. Wada, H. Senshu, et. al.
Fri, 11 Mar 16
23/59

Comments: in The Astrophysical Journal, 812:67 (12pp), 2015 October 10

Charge-Swapping Q-balls [CL]

Posted on by

http://arxiv.org/abs/1409.3232


Q-balls are non-topological solitonic solutions to a wide class of field theories that possess global symmetries. Here we show that in these same theories there also exists a tower of novel composite Q-ball solutions where, within one composite Q-ball, positive and negative charges co-exist and swap at a frequency lower than the natural frequency of an individual Q-ball. These charge-swapping Q-balls are constructed by assembling Q-balls and anti-Q-balls tightly such that their nonlinear cores overlap. We explain why charge-swapping Q-balls can form and why they swap charges.

Read this paper on arXiv…

E. Copeland, P. Saffin and S. Zhou
Wed, 6 Jan 16
3/43

Comments: 5 pages, 5 figures

The characterisation of irregularly-shaped particles: a re-consideration of finite-sized, porous and fractal grains [CL]

Posted on by

http://arxiv.org/abs/1511.01663


Context. A porous and/or fractal description can generally be applied where particles have undergone coagulation into aggregates. Aims. To characterise finite-sized, porous and fractal particles and to understand the possible limitations of these descriptions. Methods. We use simple structure, lattice and network considerations to determine the structural properties of irregular particles. Results. We find that, for finite-sized aggregates, the terms porosity and fractal dimension may be of limited usefulness and show with some critical and limiting assumptions, that highly-porous aggregates (porosity > 80%) may not be constructable. We also investigate their effective cross-sections using a simple cubic model. Conclusions. In place of the terms porosity and fractal dimension, for finite-sized aggregates, we propose the readily-determinable quantities of inflation, I (a measure of the solid filling factor and size), and dimensionality, D (a measure of the shape). These terms can be applied to characterise any form of particle, be it an irregular, homogeneous solid or a highly-extended aggregate.

Read this paper on arXiv…

A. Jones
Mon, 9 Nov 15
50/55

Comments: 13 pages, 8 figures

Parking-garage structures in astrophysics and biophysics [CL]

Posted on by

http://arxiv.org/abs/1509.00410


A striking shape was recently observed for the cellular organelle endoplasmic reticulum consisting of stacked sheets connected by helical ramps. This shape is interesting both for its biological function, to synthesize proteins using an increased surface area for ribosome factories, and its geometric properties that may be insensitive to details of the microscopic interactions. In the present work, we find very similar shapes in our molecular dynamics simulations of the nuclear pasta phases of dense nuclear matter that are expected deep in the crust of neutron stars. There are dramatic differences between nuclear pasta and terrestrial cell biology. Nuclear pasta is 14 orders of magnitude denser than the aqueous environs of the cell nucleus and involves strong interactions between protons and neutrons, while cellular scale biology is dominated by the entropy of water and complex assemblies of biomolecules. Nonetheless the very similar geometry suggests both systems may have similar coarse-grained dynamics and that the shapes are indeed determined by geometrical considerations, independent of microscopic details. Many of our simulations self-assemble into flat sheets connected by helical ramps. These ramps may impact the thermal and electrical conductivities, viscosity, shear modulus, and breaking strain of neutron star crust. The interaction we use, with Coulomb frustration, may provide a simple model system that reproduces many biologically important shapes.

Read this paper on arXiv…

C. Horowitz, D. Berry, M. Caplan, et. al.
Wed, 2 Sep 15
28/87

Comments: 5 pages, 3 figures

Adiabatic Invariance of Oscillons/I-balls [CL]

Posted on by

http://arxiv.org/abs/1508.01028


Real scalar fields are known to fragment into spatially localized and long-lived solitons called oscillons or $I$-balls. We prove the adiabatic invariance of the oscillons/$I$-balls for a potential that allows periodic motion even in the presence of non-negligible spatial gradient energy. We show that such potential is uniquely determined to be the quadratic one with a logarithmic correction, for which the oscillons/$I$-balls are absolutely stable. For slightly different forms of the scalar potential dominated by the quadratic one, the oscillons/$I$-balls are only quasi-stable, because the adiabatic charge is only approximately conserved. We check the conservation of the adiabatic charge of the $I$-balls in numerical simulation by slowly varying the coefficient of logarithmic corrections. This unambiguously shows that the longevity of oscillons/$I$-balls is due to the adiabatic invariance.

Read this paper on arXiv…

M. Kawasaki, F. Takahashi and N. Takeda
Thu, 6 Aug 15
10/48

Comments: 26 papes, 4 figures

Flux saturation length of sediment transport [CL]

Posted on by

http://arxiv.org/abs/1311.0661


Sediment transport along the surface drives geophysical phenomena as diverse as wind erosion and dune formation. The main length-scale controlling the dynamics of sediment erosion and deposition is the saturation length $L_\mathrm{s}$, which characterizes the flux response to a change in transport conditions. Here we derive, for the first time, an expression predicting $L_\mathrm{s}$ as a function of the average sediment velocity under different physical environments. Our expression accounts for both the characteristics of sediment entrainment and the saturation of particle and fluid velocities, and has only two physical parameters which can be estimated directly from independent experiments. We show that our expression is consistent with measurements of $L_\mathrm{s}$ in both aeolian and subaqueous transport regimes over at least five orders of magnitude in the ratio of fluid and particle density, including on Mars.

Read this paper on arXiv…

T. Pahtz, J. Kok, E. Parteli, et. al.
Thu, 25 Dec 14
26/29

Comments: 5 pages, 3 figures