Millimeter-sized Particles Research Articles

Numerical investigation on the magnetostrictive effect of magneto-sensitive elastomers based on a magneto-structural coupling algorithm

Magneto-sensitive elastomers (MSEs) are composite materials with ferromagnetic particles embedded in rubber matrices. Their mechanical properties can be changed by applying an external magnetic field. Although their stiffness and damper properties have been extensively studied, only a few studies have been involved with their magnetostriction behaviors, which have potential applications in sensors. To observe the interaction mechanisms between mechanical and magnetic fields and to investigate the magnetostrictive effect numerically, a novel magneto-structural coupling algorithm was developed. A magnetostrictive test system was also developed and fabricated for validating the simulation method. Several MSE samples embedded with millimeter-sized particles were fabricated and tested. The simulation results agreed well with the experimental results. Also both of them showed negative magnetostrictive strains for the specified samples and test conditions in this study. The contributions of four influencing factors were evaluated, and some results were concluded. Before magnetic saturation, the bigger the magnetic field strength is, the stronger the magnetostrictive effect is, and their relationship follows a quadratic polynomial expression. The closer the distance between two adjacent particles is, the stronger the magnetostrictive effect is, and their relationship satisfies a cubic polynomial equation. The higher the particle volume fraction is, the stronger the magnetostrictive effect is, and there is a linear relationship between them. The particle diameter has little influence on the magnetostrictive effect.

Explaining millimeter-sized particles in brown dwarf disks

Planets have been detected around a variety of stars, including low-mass objects, such as brown dwarfs. However, such extreme cases are challenging for planet formation models. Recent sub-millimeter observations of disks around brown dwarf measured low spectral indices of the continuum emission that suggest that dust grains grow to mm-sizes even in these very low mass environments. To understand the first steps of planet formation in scaled-down versions of T-Tauri disks, we investigate the physical conditions that can theoretically explain the growth from interstellar dust to millimeter-sized grains in disks around brown dwarf. We modeled the evolution of dust particles under conditions of low-mass disks around brown dwarfs. We used coagulation, fragmentation and disk-structure models to simulate the evolution of dust, with zero and non-zero radial drift. For the non-zero radial drift, we considered strong inhomogeneities in the gas surface density profile that mimic long-lived pressure bumps in the disk. We studied different scenarios that could lead to an agreement between theoretical models and the spectral slope found by millimeter observations. We find that fragmentation is less likely and rapid inward drift is more significant for particles in brown dwarf disks than in T-Tauri disks. We present different scenarios that can nevertheless explain millimeter-sized grains. As an example, a model that combines the following parameters can fit the millimeter fluxes measured for brown dwarf disks: strong pressure inhomogeneities of $\sim$ 40% of amplitude, a small radial extent $\sim$ 15 AU, a moderate turbulence strength $\alpha_{\mathrm{turb}}= 10^{-3}$, and average fragmentation velocities for ices $v_f = 10 m s^{-1}$.

Ring shaped dust accumulation in transition disks

Context.Transition disks are believed to be the final stages of protoplanetary disks, during which a forming planetary system or photoevaporation processes open a gap in the inner disk, drastically changing the disk structure. From theoretical arguments it is expected that dust growth, fragmentation and radial drift are strongly influenced by gas disk structure, and pressure bumps in disks have been suggested as key features that may allow grains to converge and grow efficiently. Aims. We want to study how the presence of a large planet in a disk influences the growth and radial distribution of dust grains, and how observable properties are linked to the mass of the planet. Methods. We combine two-dimensional hydrodynamical disk simulations of disk-planet interactions with state-of-the-art coagulation/fragmentation models to simulate the evolution of dust in a disk which has a gap created by a massive planet. We compute images at different wavelengths and illustrate our results using the example of the transition disk LkCa15. Results. The gap opened by a planet and the long-range interaction between the planet and the outer disk create a single large pressure bump outside the planetary orbit. Millimeter-sized particles form and accumulate at the pressure maximum and naturally produce ring-shaped sub-millimeter emission that is long-lived because radial drift no longer depletes the large grain population of the disk. For large planet masses around 9 $M_{\mathrm{Jup}}$, the pressure maximum and, therefore, the ring of millimeter particles is located at distances that can be more than twice the star-planet separation, creating a large spatial separation between the gas inner edge of the outer disk and the peak millimeter emission. Smaller grains do get closer to the gap and we predict how the surface brightness varies at different wavelengths.

Experimental study on the ignition and combustion characteristics of a magnesium particle in water vapor

Magnesium is of interest for underwater propulsion due to the superior ignition behavior of magnesium particles and the highly exothermic Mg-water reaction. In this work, the ignition and combustion characteristics of an individual millimeter-sized magnesium particle in water vapor were studied. In order to build an atmosphere of water vapor, an experiment system was established and validated by the experiments of magnesium particle in air. The ignition and combustion of a single magnesium particle were accomplished in a combustor filled with water vapor. The surface changes of the particle during the ignition and a steady-state vapor phase combustion were observed. Based on the data obtained, ignition mechanism was analyzed and ignition temperature was determined. The steady-state combustion of the sample was controlled by diffusion in gas phase, and a one-dimensional, spherically symmetric quasi-steady model was adopted to describe the process. The dependence of burning time on the diameter was investigated, and the conclusion that burning time is proportional to the square of the metal sample diameter was drawn.

Radar Scattering from Ice Aggregates Using the Horizontally Aligned Oblate Spheroid Approximation

AbstractThe assumed relationship between ice particle mass and size is profoundly important in radar retrievals of ice clouds, but, for millimeter-wave radars, shape and preferred orientation are important as well. In this paper the authors first examine the consequences of the fact that the widely used “Brown and Francis” mass–size relationship has often been applied to maximum particle dimension observed by aircraftDmaxrather than to the mean of the particle dimensions in two orthogonal directionsDmean, which was originally used by Brown and Francis. Analysis of particle images reveals thatDmax≃ 1.25Dmean, and therefore, for clouds for which this mass–size relationship holds, the consequences are overestimates of ice water content by around 53% and of Rayleigh-scattering radar reflectivity factor by 3.7 dB. Simultaneous radar and aircraft measurements demonstrate that much better agreement in reflectivity factor is provided by using this mass–size relationship withDmean. The authors then examine the importance of particle shape and fall orientation for millimeter-wave radars. Simultaneous radar measurements and aircraft calculations of differential reflectivity and dual-wavelength ratio are presented to demonstrate that ice particles may usually be treated as horizontally aligned oblate spheroids with an axial ratio of 0.6, consistent with them being aggregates. An accurate formula is presented for the backscatter cross section apparent to a vertically pointing millimeter-wave radar on the basis of a modified version of Rayleigh–Gans theory. It is then shown that the consequence of treating ice particles as Mie-scattering spheres is to substantially underestimate millimeter-wave reflectivity factor when millimeter-sized particles are present, which can lead to retrieved ice water content being overestimated by a factor of 4.

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, A., Ormel, C.W., Güttler, C., Blum, J., Dullemond, C.P. [2010]. Astron. Astrophys., 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 cms−1. For 2 and 3mm 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 cms−1. 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.

Attraction-driven aggregation of dipolar particles in an external magnetic field

We present a detailed experimental study of the aggregation of particles of permanent magnetic moment in an external magnetic field. The experiments are performed with millimeter-sized particles floating on the surface of water. Due to the large size of the particles, thermal noise does not have any relevance in the system; the particles undergo deterministic motion. Experiments are carried out varying the concentration which also controls the relative importance of the dipole-external field and the dipole-dipole interactions. We determined the exponents characterizing the aggregation process and found that the attraction driven aggregation of dipolar particles obeys the Vicsek-Family dynamic scaling. The exponents are found to have a concentration dependence which can be attributed to the change of mobility of clusters and their interaction at higher concentrations.

Combined effect of YORP and collisions on the rotation rate of small Main Belt asteroids

The rotation rate distribution of small Main Belt asteroids is dominated by YORP and collisions. These mechanism act differently depending on the size of the bodies and give rise to non-linear effects when they both operate. Using a Monte Carlo method we model the formation of a steady state population of small asteroids under the influence of both mechanisms and the rotation rate distribution is compared to the observed one as derived from Pravec et al. (Pravec, P. et al. [2008]. Icarus 197, 497–504). A better match to observations is obtained with respect to the case in which only YORP is considered. In particular, an excess of slow rotators is produced in the model with both collisions and YORP because bodies driven to slow rotation by YORP have a random walk-like evolution of the spin induced by repeated collisions with small projectiles. This is a dynamical evolution different from tumbling and it lasts until a large impact takes the body to a faster rotation rate. According to our model, the rotational fission of small asteroids is a very frequent event and might explain objects like P/2010 A2 and its associated tail of millimeter-sized dust particles. The mass loss during fission of small asteroids might significantly influence the overall collisional evolution of the belt. Fission can in fact be considered as an additional erosion mechanism, besides cratering and fragmentation, acting only at small diameters.

Sequential Reactions Directed by Core/Shell Catalytic Reactors

Millimeter-sized reactor particles made of permeable polymer doped with catalysts arranged in a core/shell fashion direct sequences of chemical reactions (e.g., alkyne coupling followed by hydrogenation or hydrosilylation followed by hydrogenation). Spatial compartmentalization of catalysts coupled with the diffusion of substrates controls reaction order and avoids formation of byproducts. The experimentally observed yields of reaction sequences are reproduced by a theoretical model, which accounts for the reaction kinetics and the diffusion of the species involved.

Towards initial mass functions for asteroids and Kuiper Belt Objects

Our goal is to understand primary accretion of the first planetesimals. Some examples are seen today in the asteroid belt, providing the parent bodies for the primitive meteorites. The primitive meteorite record suggests that sizeable planetesimals formed over a period longer than a million years, each of which being composed entirely of an unusual, but homogeneous, mixture of millimeter-size particles. We sketch a scenario that might help explain how this occurred, in which primary accretion of 10–100 km size planetesimals proceeds directly, if sporadically, from aerodynamically-sorted millimeter-size particles (generically “chondrules”). These planetesimal sizes are in general agreement with the currently observed asteroid mass peak near 100 km diameter, which has been identified as a “fossil” property of the pre-erosion, pre-depletion population. We extend our primary accretion theory to make predictions for outer Solar System planetesimals, which may also have a preferred size in the 100 km diameter range. We estimate formation rates of planetesimals and explore parameter space to assess the conditions needed to match estimates of both asteroid and Kuiper Belt Object (KBO) formation rates. For parameters that satisfy observed mass accretion rates of Myr-old protoplanetary nebulae, the scenario is roughly consistent with not only the “fossil” sizes of the asteroids, and their estimated production rates, but also with the observed spread in formation ages of chondrules in a given chondrite, and with a tolerably small radial diffusive mixing during this time between formation and accretion. As previously noted, the model naturally helps explain the peculiar size distribution of chondrules within such objects. The optimum range of parameters, however, represents a higher gas density and fractional abundance of solids, and a smaller difference between Keplerian and pressure-supported orbital velocities, than “canonical” models of the solar nebula. We discuss several potential explanations for these differences. The scenario also produces 10–100 km diameter primary KBOs, and also requires an enhanced abundance of solids to match the mass production rate estimates for KBOs (and presumably the planetesimal precursors of the ice giants themselves). We discuss the advantages and plausibility of the scenario, outstanding issues, and future directions of research.

Experiments on the photophoretic motion of chondrules and dust aggregates—Indications for the transport of matter in protoplanetary disks

In a set of 16 drop tower experiments the motion of sub-millimeter to millimeter-sized particles under microgravity was observed. Illumination by a halogen lamp induced acceleration of the particles due to photophoresis. Photophoresis on dust-free chondrules, on chondrules, glass spheres and metal spheres covered with SiC dust and on pure SiC dust aggregates was studied. This is the first time that photophoretic motion of millimeter-sized particles has been studied experimentally. The absolute values for the photophoretic force are consistent with theoretical expectations for spherical particles. The strength of the photophoretic force varies for chondrules, dust covered particles and pure dust from low to strong, respectively. The measurements support the idea that photophoresis in the early Solar System can be efficient to transport solid particles outward.

Chemical Degradation of Drinking Water Disinfection Byproducts by Millimeter-Sized Particles of Iron−Silicon and Magnesium−Aluminum Alloys

The candidature of Fe-Si and Mg-Al alloys at millimeter-scale particle sizes for chemical degradation of disinfection byproducts (DBPs) in drinking water systems was substantiated by their enhanced corrosion resistance and catalytic effect on the degradation. The Mg-Al particles supplied electrons for reductive degradation, and the Fe-Si particles acted as a catalyst and provided the sites for the reaction. The alloy particles are obtained by mechanical milling and stable under ambient conditions. The proposed method for chemical degradation of DBPs possesses the advantages of relatively constant degradation performance, long-term durability, no secondary contamination, and ease of handling, storage and maintenance in comparison with nanoparticle systems.

Model of thermal conductivity in powder beds

Heat transfer between contacting solid particles in gas is localized about the points of contact because of the low thermal conductivity of gas. This suggests the model of a network of discrete thermal resistances. Thermal interaction of two particles in the zone of contact between them includes thermal conductivity in the solid phase as well as heat transfer through a gas gap between the particles, where the conductive heat transfer at higher distances from the center of the contact changes to the transition transfer and free molecular transfer near the contact center. In the framework of this approach the effective thermal conductivity depends on three dimensionless morphological parameters: the volume fraction of solid phase, the mean coordination number, and the ratio of the size of necks between particles to the size of particles. The physical properties of the phases are specified by the thermal conductivities of the solid and the gas, the adiabatic exponent of the gas, and the Knudsen number. The proposed model agrees with the experimental S curves of the effective thermal conductivity versus the logarithm of gas pressure. It also describes the experimental tendencies of increasing the effective thermal conductivity with the particle size and the volume fraction of solid. The model can be applied to powder beds with micron-sized particles as well as to packed beds with millimeter-sized particles in gas. Free molecular and transient regimes of heat transfer in the gas filling pores can be important even for millimeter-sized particles at atmospheric pressure when the Knudsen number is as low as ${10}^{\ensuremath{-}5}$. These rarefied gas phenomena are responsible for such a complicated behavior of the considered heterogeneous media. Their quantitative description gives the model, which does not contain fitting parameters.

A zero-gravity instrument to study low velocity collisions of fragile particles at low temperatures

We discuss the design, operation, and performance of a vacuum setup constructed for use in zero (or reduced) gravity conditions to initiate collisions of fragile millimeter-sized particles at low velocity and temperature. Such particles are typically found in many astronomical settings and in regions of planet formation. The instrument has participated in four parabolic flight campaigns to date, operating for a total of 2.4 h in reduced-gravity conditions and successfully recording over 300 separate collisions of loosely packed dust aggregates and ice samples. The imparted particle velocities achieved range from 0.03 to 0.28 m s(-1) and a high-speed, high-resolution camera captures the events at 107 frames/s from two viewing angles separated by either 48.8 degrees or 60.0 degrees. The particles can be stored inside the experiment vacuum chamber at temperatures of 80-300 K for several uninterrupted hours using a built-in thermal accumulation system. The copper structure allows cooling down to cryogenic temperatures before commencement of the experiments. Throughout the parabolic flight campaigns, add-ons and modifications have been made, illustrating the instrument flexibility in the study of small particle collisions.

Simulated rainfall-driven dissolution of TNT, Tritonal, Comp B and Octol particles

Live-fire military training can deposit millimeter-sized particles of high explosives (HE) on surface soils when rounds do not explode as intended. Rainfall-driven dissolution of the particles then begins a process whereby aqueous HE solutions can enter the soil and groundwater as contaminants. We dripped water onto individual particles of TNT, Tritonal, Comp B and Octol to simulate how surface-deposited HE particles might dissolve under the action of rainfall and to use the data to verify a model that predicts HE dissolution as a function of particle size, particle composition and rainfall rate. Particle masses ranged from 1.1 to 17 mg and drip rates corresponded to nominal rainfall rates of 6 and 12 mm h −1. For the TNT and Tritonal particles, TNT solubility governed dissolution time scales, whereas the lower-solubility of RDX controlled the dissolution time of both RDX and TNT in Comp B. The large, low-solubility crystals of HMX slowed but did not control the dissolution of TNT in Octol. Predictions from a drop-impingement dissolution model agree well with dissolved-mass timeseries for TNT, Tritonal and Comp B, providing some confidence that the model will also work well when applied to the rainfall-driven, outdoor dissolution of these HE particles.

Physicochemical studies of a newly synthesized molecule, 6-methyl-3-phenethyl-3,4-dihydro-1H-quinazoline-2-thione (JSH18) for topical formulations

A new molecule having the structure of 6-methyl-3-phenethyl-3,4-dihydro-1H-quinazoline-2-thione (JSH18) was synthesized and possibly presupposed to show depigmentation through the inhibition of tyrosinase which is involved in formation of melanin. Therefore, we are going to develop JSH18 as an inhibitor of melanin synthesis with topical formulations to show its optimal efficiency for skin whitening based on preformulation studies. The preformulation to figure out the physicochemical properties was done by solubility measurements, differential scanning calorimetry study, scanning electron microscopy as well as cell viability assay and skin retention study. Through solubility test, glycofurol, N-methyl pyrrolidones, and isopropanol showed best solubilizing effects. JSH18 showed a strong single endothermic peak at 180.6 degrees C and its millimeter-sized particles could be reduced by grinding suggesting the improvement of solubility. Interestingly, the cytotoxicity of JSH18 was different according to the cell origin type. JSH 18 showed the cell viability at the concentration of 4 uM in human keratinocyte HacaT cells but that was suddenly dropped at the concentration of 400 uM in murine keratinocyte PAM212 cells. Based on above preformulation results, formulation studies of JSH18 will be performed for the development of topical formulations in future.

Dynamic Self-Assembly in Ensembles of Camphor Boats

Millimeter-sized gel particles loaded with camphor and floating at the interface between water and air generate convective flows around them. These flows give rise to repulsive interparticle interactions, and mediate dynamic self-assembly of nonequilibrium particle formations. When the numbers of particles, N, are small, particle motions are uncorrelated. When, however, N exceeds a threshold value, particles organize into ordered lattices. The nature of hydrodynamic forces underlying these effects and the dynamics of the self-assembling system are modeled numerically using Navier-Stokes equations as well as analytically using scaling arguments.

The effect of thermal radiation on the solidification dynamics of metal oxide melt droplets

Cooling and solidification of metal oxide droplets in water are considered, using a single-particle model which takes into account heat conduction and thermal radiation transfer within the particle. It is shown that, for millimeter-size particles, near-infrared absorption of the particle's substance determines the solidification pattern and dynamics. For semi-transparent aluminum oxide particles, the rate of surface solidification is controlled by convective heat transfer. For opaque corium particles, thermal radiation from the particle surface leads to fast surface solidification. The impact of so-formed crust layer on subsequent particle fragmentation is discussed with respect to its influence on steam explosion.

Orally administered, colon‐specific mucoadhesive azopolymer particles for the treatment of inflammatory bowel disease: An in vivo study

Radiolabeled congeners of a series of azopolymers have been synthesized and characterized. The in vivo (rat) gastrointestinal transit profile of millimeter-sized particles of these azopolymers has been determined and used to facilitate the selection of a candidate material for therapeutic applications. The efficacy of the selected material as a protective coating for the colonic mucosa has been tested in a hapten-reactivated, in vivo model of inflammatory bowel disease: 7 days after reactivation of the condition, the myeloperoxidase activity of animals that had received doses of the selected azopolymer was determined to be at the same level as that of healthy animals or that of the negative control group, highlighting the therapeutic promise of this material.

Characteristics of Composition B particles from blow-in-place detonations

We sampled residues from high-order and low-order blow-in-place detonations of mortars and projectiles filled with Composition B (Comp B), a TNT and RDX mixture. Our goals were to (1) characterize the types of explosive particles, (2) estimate the explosive ‘footprint’ for different munitions, and (3) estimate the mass of Comp B remaining after each detonation. The aerial deposition of Comp B particles helps estimate how large of an area is contaminated by a low-order detonation and how best to sample residue resulting from different rounds. We found that the high-order detonations deposited microgram to milligram quantities whereas the low-order detonations deposited gram quantities of Comp B. For the high-order detonations the concentration of Comp B in the residue decreased as a function of distance from the blast. The low-order tests scattered centimeter-sized chunks and millimeter-sized or smaller particles of Comp B. The chunks were randomly scattered whereas the number of millimeter-sized particles decreased with distance from the detonation. For both high- and low-order detonations we found that the smaller munitions deposited less Comp B than the larger munitions and deposited it closer to the detonation point.