Microns In Radius Research Articles

A light scattering analysis of the cryovolcano plumes on enceladus

Observations by NASA's Cassini spacecraft show that Enceladus, the sixth largest moon of Saturn, has cryovolcanoes on its south pole as well as a global subsurface ocean beneath Enceladus's icy crust. Photographs from the Imaging Science Subsystem (ISS) cameras aboard Cassini during flybys of Enceladus show cryovolcano plumes. The images are taken in several filters and with linear polarizers at various angles and orbital positions. Light scattered by the plume particles and observed by Cassini may provide information about the plume composition and potentially the dynamics of the ocean below. Here, we remove the coupling between orbital position and scattered plume intensity as well as analyze ISS data using light scattering models and the Q-space method. We suggest that the plume particles are generally larger than two microns in radius and nonspherical in shape.

Триплеттриплетная аннигиляция молекулярных электронных возбуждений вблизи диамагнитной металлической микрочастицы в переменном магнитном поле

A mathematical model of the spinselective reaction of triplettriplet annihilation of molecules on the surface of a spherical microparticle or a cylindrical rod of micrometer radius in an alternating magnetic field transformed by conducting microparticles is presented. Based on these models, the calculations of the integral and local rates of magnetically sensitive molecular processes were performed. It has been shown that in the local nearsurface regions of the conducting microparticles in an alternating magnetic field, the rate of spinselective bimolecular reactions changes. The comparison of the nanoand microsphere showed that with the external magnetic field induction value B0 = 1 T, the maximum resulting field at the equator of the particle was equal to 12 mT and 100 mT, respectively. It was also revealed that for nanoand microcylinders with a similar value of the external magnetic field induction B0 = 1 T, the maximum resulting field at the point (R,0) of the 2d magnetic pole was equal to 12 mT and 1.5 T, respectively. The amplitude of the magnetic effect for different times and particle sizes was equal to +1 % and 2 %. For a silver microcylinder with a radius R = 25 μm, there was switching from a purely negative magnetic effect at the time t = 0 ns to an effect with positive and negative parts. For the case of a silver cylinder of radius R = 30 μm and external magnetic field frequency ω = 5∙108 s1, an increase in the amplitude of the positive part of the magnetic effect curve from 0.25 % to 1 % was observed when the time instant changed from t = 0 ns, which corresponds to the maximum of the external magnetic field, up to t = 3.68 ns when the resulting field of the microwire reaches its maximum. This work may be of interest to physicists and chemists involved in science in the field of chemical physics, physics of magnetic phenomena, spinselective reactions and spintronics.

A new experimental set-up for aerosol stability investigations in microgravity conditions

The temporal and spatial evolution of dispersed media is a fundamental problem in a wide range of physicochemical systems, such as emulsions, suspensions and aerosols. These systems are multiphasic and involve compounds of different densities. They are therefore subject to the influence of gravity which determines the sedimentation rate of their dispersed phase. This effect can be dominant and prevent a detailed study of the phenomena occurring between the constituents themselves, such as the coalescence of drops in emulsions, the evaporation of droplets or the flocculation in suspensions. In this context, the Centre National d’Etudes Spatiales (CNES) has recently supported the development of a new instrument to produce populations of droplets, a few micrometers in radius, under controlled conditions with the objective of allowing a detailed study of their properties in microgravity conditions. The principle of this instrument is to generate, by a fast compression/expansion of air, populations of water droplets and to track their evolution by optical scanning tomography in transmission mode within a volume of approximately 2 mm 3 . Parabolic flight experiments have shown the possibility to generate and accurately follow the evolution of populations of several hundred droplets for more than 20 s. The first experimental results show that it is possible to study their evaporation kinetics or their motion when imposing von Karman swirling flows. This work is part of the AEROSOL project of DECLIC-EVO supported by CNES and aims to help the understanding of cloud microphysics which remains a critical open problem in the context of global warming.

Particle sorting method based on swirl induction.

Fluid-based methods for particle sorting demonstrate increasing appeal in many areas of biosciences due to their biocompatibility and cost-effectiveness. Herein, we construct a microfluidic sorting system based on a swirl microchip. The impact of microchannel velocity on the swirl stagnation point as well as particle movement is analyzed through simulation and experiment. Moreover, the quantitative mapping relationship between flow velocity and particle position distribution is established. With this foundation established, a particle sorting method based on swirl induction is proposed. Initially, the particle is captured by a swirl. Then, the Sorting Region into which the particle aims to enter is determined according to the sorting condition and particle characteristic. Subsequently, the velocities of the microchannels are adjusted to control the swirl, which will induce the particle to enter its corresponding Induction Region. Thereafter, the velocities are adjusted again to change the fluid field and drive the particle into a predetermined Sorting Region, hence the sorting is accomplished. We have extensively conducted experiments taking particle size or color as a sorting condition. An outstanding sorting success rate of 98.75% is achieved when dealing with particles within the size range of tens to hundreds of micrometers in radius, which certifies the effectiveness of the proposed sorting method. Compared to the existing sorting techniques, the proposed method offers greater flexibility. The adjustment of sorting conditions or particle parameters no longer requires complex chip redesign, because such sorting tasks can be successfully realized through simple microchannel velocities control.

Broadband measurements of the surface impedance in ferromagnetic wires as a boundary condition for scattering problems

In the cm-wave range, the skin depth is comparable to the micron radius of ferromagnetic wires and thus their magnetic and conductive properties will influence the scattered electromagnetic field. Solving the scattering problem by considering the complicated internal magnetic structure of the wire becomes unpractical. There is thus a need to develop hybrid methods when the external problem of scattering on an inclusion is solved together with the experimental impedance conditions on its surface. Herein, we propose an accurate broadband impedance measurement technique operating up to GHz. The printed circuit board measurement and calibration cells designed in this work allow flexibility in choosing external stimuli such as magnetic field, tensile stress, and heat. The experimental impedance can be then used as the boundary condition in electromagnetic simulations making it possible to model, predict and design wire-based composites using numerical solvers such as CST Studio Suite.

Probing neuronal functions with precise and targeted laser ablation in the living cortex: comment

The paper [Optica 8, 1559 (2021)OPTIC82334-253610.1364/OPTICA.433562] is devoted to probing neuronal functions by targeted cell ablation in living cortex. It demonstrates the advantage of cell ablation by single high-energy pulses from an amplified femtosecond (fs) laser system, which leaves adjacent structures intact. The single-pulse effects rely on mechanical disruption by laser-induced microcavitation, which goes along with a lack of diffusive collateral damage. In comparison, cell damage by low-energy pulses from fs laser oscillators is shown to be associated with a damage zone with tens of micrometer radius around the targeted cell. The collateral damage is attributed to heat accumulation and diffusion. We demonstrate through numerical simulations of plasma formation and temperature evolution for the experimentally employed irradiation conditions that a large number of free electrons is produced but the temperature rise at focus is only 0.3 K. Therefore, the damage is caused by nonlinear photochemistry and plasma-mediated chemical effects rather than by thermal effects.

Pore-Scale Modeling of Mineral Growth and Nucleation in Reactive Flow

A fundamental understanding of mineral precipitation kinetics relies largely on microscopic observations of the dynamics of mineral surfaces exposed to supersaturated solutions. Deconvolution of tightly bound transport, surface reaction, and crystal nucleation phenomena still remains one of the main challenges. Particularly, the influence of these processes on texture and morphology of mineral precipitate remains unclear. This study presents a coupling of pore-scale reactive transport modeling with the Arbitrary Lagrangian-Eulerian approach for tracking evolution of explicit solid interface during mineral precipitation. It incorporates a heterogeneous nucleation mechanism according to Classical Nucleation Theory which can be turned “on” or “off.” This approach allows us to demonstrate the role of nucleation on precipitate texture with a focus at micrometer scale. In this work precipitate formation is modeled on a 10 micrometer radius particle in reactive flow. The evolution of explicit interface accounts for the surface curvature which is crucial at this scale in the regime of emerging instabilities. The results illustrate how the surface reaction and reactive fluid flow affect the shape of precipitate on a solid particle. It is shown that nucleation promotes the formation of irregularly shaped precipitate and diminishes the effect of the flow on the asymmetry of precipitation around the particle. The observed differences in precipitate structure are expected to be an important benchmark for reaction-driven precipitation in natural environments.

Generating micro grooves with a semicircular cross-section using wire electrochemical micromachining

Micro grooves with a semicircular cross-section of tens of microns in radius on metal foils have posed a challenge for machining. This paper proposed stepwise manipulating process parameters in wire electrochemical micromachining (WECMM) to improve the profile accuracy of micro grooves with a semicircular cross-section via evenly distributing the machining gap. Firstly, a two-dimensional model of the electric field was established to investigate the groove shaping process including the changes of machining gap distribution at all normal directions of a cylindrical wire electrode. It indicated that the groove machining process ended before it reached the equilibrium status of WECMM. The corresponding machining gap at the wire electrode feeding direction is much smaller than those at other normal directions. Thereafter, a non-uniform machining gap distribution brings the semicircular profile error when using a cylindrical wire electrode. In this study, stepwise manipulations of the applied voltage and electrode feed rate is proposed to apply at the moment when the machining allowances at all directions are at a comparative range. According to simulations, an optimized stepwise parametric manipulation of increasing the pulse voltage from 12.5 to 14 V and decreasing the wire electrode feed rate from 0.5 to 0.1 μm/s was activated at 113 s and proceeded till 145 s. In this way, the machining gap distribution uniformity is improved and the machining allowances at major normal directions of the cylindrical wire electrode are reduced to zero. Experiments verified that the groove profile error was reduced from $$ {3.39}_{-0.45}^{+0.22} $$ μm to $$ {3.38}_{-0.13}^{+0.16} $$ μm with optimized stepwise-manipulation. Finally, the feasibility of simultaneously machining 20 micro grooves with a semicircular cross-section of 50 μm in radius was verified in experiments.

Porous silica beads produced by nanofluid emulsion freezing

Abstract It is shown that porous spherical particles can be obtained via the freezing of silica nanoparticle aqueous suspensions emulsified in a continuous oil phase. After two freeze-thaw cycles, nanoparticles turn aggregated into flocculated objects with microstructure that depends upon emulsion volume fraction and droplet size. For low volume fractions, regular microspheres are produced while for large ones, irregular beads with several tens of micrometer radius are formed. Electronic microscopy, mercury porosimetry and nitrogen adsorption are used to get insights into these porous particles typical radius, pore size distribution, surface area and pore network structure. All exhibit mesopores that result from inter-nanoparticle spacing after flocculation. An unexpected macroporous domain appears which is not observed when drying non-emulsified suspensions. This macroporosity is interpreted as the signature of dendrite formation during the undercooled period, right before freezing occurs. Beside this additional macroporosity, the protocols presented in this article constitute also promising emulsion-based routes for porous material synthesis with original geometry, chemical composition and porosity.

Compact Optical Spectroscopy for Identification of Colloidal Metals

Optical spectroscopy has provided new capabilities and opportunities in materials science, structural engineering, chemistry, biology, and medicine. The expense of spectroscopic instrumentation can be prohibitive—often in the range of tens of thousands of dollars. It would be useful to have a compact, practical alternative that is more cost-effective in specific cases. One example is the case of detecting colloidal metals in solution, in which the metal is in nanoparticle or microparticle form; normally suspended in a dielectric material or fluid, such as water.In this project, we provide a practical, low-cost alternative to more traditional spectroscopic techniques for identifying metals in colloidal form. The basis for the method is the interaction of light with materials, as described by the plasma frequency and band gaps in metals. Metals reflect electromagnetic radiation whose frequency is below the plasma frequency. A glance at the plasma-frequency expression, ωp = √((Ne q 2)/(m* ε 0)), provides an explanation of why the plasma frequency can be a useful tool for identifying a given metal. In this expression, ωp is the plasma frequency in radians per second; Ne is the number of electrons per unit volume (numerical volume density); q is the charge of an electron; m* is the effective mass of an electron; and ε 0 is the permittivity of free space. Now, the quantity Ne can be found in reference tables, or it can be readily calculated for a particular metal. As a consequence, a prominent spectral output band can be a clear indicator of a metal’s “identity.”When a metal sample is in bulk form, it will have a plasma frequency in the ultraviolet range. As a result, a strong spectral response will be observed at the wavelength corresponding to that frequency (referred to as the plasma wavelength). Now, the plasma frequency does not change when a material is in colloidal form—but, due to interband transitions within the metal, the response wavelength will be at a lower value than the plasma wavelength. This wavelength may be in the visible or near infrared (NIR) region of the spectrum. This is the case with many metals, such as gold, silver, copper, lead, and magnesium. The presence of the response wavelength in the optical region is the basis for the method we are proposing.In the compact system we have developed, dual-convex lenses are used to collimate a white-light LED source, and then focus this light onto a sample. The sample is a glass slide populated with silver, copper, or magnesium particles of micrometer radius. Upon passing through the metal sample, the white light interacts with the sample, causing a variation in the intensities of the white light’s constituent components. This amounts to a variation in intensity as a function of wavelength, which is based on that particular metal’s plasma frequency and interband transitions. In our work, we have focused on the intensities of the red, green, and blue light that emerge from the sample—all three of which can be measured easily, after they have been spatially separated by a diffraction grating. To measure the intensities, we used low-cost optical sensors (photodiode arrays) which provided real-time measurements of the intensities of the red, green, and blue light. We were able to experimentally differentiate among silver, copper, magnesium, and iron using this method. Figure 1

Hot exozodiacal dust: an exocometary origin?

Context. Near- and mid-infrared interferometric observations have revealed populations of hot and warm dust grains populating the inner regions of extrasolar planetary systems. These are known as exozodiacal dust clouds, or exozodis, reflecting the similarity with the solar system’s zodiacal cloud. Radiative transfer models have constrained the dust to be dominated by tiny submicron-sized, carbon-rich grains that are accumulated very close to the sublimation radius. The origin of this dust is an unsolved issue. Aims. We explore two exozodiacal dust production mechanisms, first re-investigating the Poynting-Robertson drag pile-up scenario, and then elaborating on the less explored but promising exocometary dust delivery scenario. Methods. We developed a new, versatile numerical model that calculates the dust dynamics, with non-orbit-averaged equations for the grains close to the star. The model includes dust sublimation and incorporates a radiative transfer code for direct comparison to the observations. We consider in this study four stellar types, three dust compositions, and we assume a parent belt at 50 au. Results. In the case of the Poynting-Robertson drag pile-up scenario, we find that it is impossible to produce long-lived submicron-sized grains close to the star. The inward drifting grains fill in the region between the parent belt and the sublimation distance, producing an unrealistically strong mid-infrared excess compared to the near-infrared excess. The dust pile-up at the sublimation radius is by far insufficient to boost the near-IR flux of the exozodi to the point where it dominates over the mid-infrared excess. In the case of the exocometary dust delivery scenario, we find that a narrow ring can form close to the sublimation zone, populated with large grains from several tens to several hundreds of micrometers in radius. Although not perfect, this scenario provides a better match to the observations, especially if the grains are carbon-rich. We also find that the number of active exocomets required to sustain the observed dust level is reasonable. Conclusions. We conclude that the hot exozodiacal dust detected by near-infrared interferometry is unlikely to result from inward grain migration by Poynting-Robertson drag from a distant parent belt, but could instead have an exocometary origin.

Assembly of Spindles and Asters in Xenopus Egg Extracts.

Here, we provide methods for assembly of mitotic spindles and interphase asters in Xenopus laevis egg extract, and compare them to spindles and asters in the egg and zygote. Classic "cycled" spindles are made by adding sperm nuclei to metaphase-arrested cytostatic factor (CSF) extract and inducing entry into interphase extract to promote nucleus formation and DNA replication. Interphase nuclei are then converted to cycled spindles arrested in metaphase by addition of CSF extract. Kinetochores assemble in this reaction and these spindles can segregate chromosomes. CSF spindles are made by addition of sperm nuclei to CSF extract. They are less physiological and lack functional kinetochores but suffice for some applications. Large interphase asters are prepared by addition of artificial centrosomes or sperm nuclei to actin-intact egg extract. These asters grow rapidly to hundreds of microns in radius by branching microtubule nucleation at the periphery, so the aster as a whole is a network of short, dynamic microtubules. They resemble the sperm aster after fertilization, and the asters that grow out of the poles of the mitotic spindle at anaphase. When interphase asters grow into each other they interact and assemble aster interaction zones at their shared boundary. These zones consist of a line (in extract) or disc (in zygotes) of antiparallel microtubule bundles coated with cytokinesis midzone proteins. Interaction zones block interpenetration of microtubules from the two asters, and signal to the cortex to induce cleavage furrows. Their reconstitution in extract allows dissection of the biophysics of spatially regulated cytokinesis signaling.

Transient thermal stress wave and vibrational analyses of a thin diamond crystal for X-ray free-electron lasers under high-repetition-rate operation.

High-brightness X-ray free-electron lasers (FELs) are perceived as fourth-generation light sources providing unprecedented capabilities for frontier scientific researches in many fields. Thin crystals are important to generate coherent seeds in the self-seeding configuration, provide precise spectral measurements, and split X-ray FEL pulses, etc. In all of these applications a high-intensity X-ray FEL pulse impinges on the thin crystal and deposits a certain amount of heat load, potentially impairing the performance. In the present paper, transient thermal stress wave and vibrational analyses as well as transient thermal analysis are carried out to address the thermomechanical issues for thin diamond crystals, especially under high-repetition-rate operation of an X-ray FEL. The material properties at elevated temperatures are considered. It is shown that, for a typical FEL pulse depositing tens of microjoules energy over a spot of tens of micrometers in radius, the stress wave emission is completed on the tens of nanoseconds scale. The amount of kinetic energy converted from a FEL pulse can reach up to ∼10 nJ depending on the layer thickness. Natural frequencies of a diamond plate are also computed. The potential vibrational amplitude is estimated as a function of frequency. Due to the decreasing heat conductivity with increasing temperature, a runaway temperature rise is predicted for high repetition rates where the temperature rises abruptly after ratcheting up to a point of trivial heat damping rate relative to heat deposition rate.

Insight into the Pore Structure of Tight Gas Sandstones: A Case Study in the Ordos Basin, NW China

A wide spectrum of pore size distributions (PSD) exists in tight gas sandstones, ranging from several nanometers to several hundred micrometers in radius, which controls both the physical rock-flow capacity and storage capacity. Thin-section, scanning electron microscope (SEM), field emission scanning electron microscope (FE-SEM), high-pressure mercury intrusion (HPMI), constant-rate mercury intrusion (CRMI) and micro-CT scanning experiments are performed on tight sandstone samples from the eighth member of the Middle Permian Shihezi Formation (P2h8) in the Ordos Basin to better understand the pore-system characteristics of a tight gas sandstone. The results of this case study show that various types of pores exist in the P2h8 sandstones: residual intergranular pores, intraparticle dissolution pores, intercrystalline pores, and small micro-cracks are observed. We combine HPMI and CRMI to determine the PSD; the pore sizes range from 3.7 nm to 600 μm in radius. The multimodal PSD is characterized by two bro...

Comparison of Osteoconduction, cytocompatibility and corrosion protection performance of hydroxyapatite-calcium hydrogen phosphate composite coating synthesized in-situ through pulsed electro-deposition with varying amount of phase and crystallinity

In order to improve the surface biocompatibility, a composite coating of hydroxyapatite and calcium hydrogen phosphate was synthesized over 316 L grade of stainless steel through pulsed electro deposition. It was observed that relative phase composition and corresponding crystallinity vary with the deposition current density. Thus a detailed study with the help of simulated body fluid and MG63 cell line was carried out in order to compare their respective biocompatibility in terms osteoconduction and cytocompatibility. A detailed electrochemical behavior study was carried out to assess the corrosion behavior under contact with body fluid.Cell proliferation studies indicate that the coating deposited with a current density of 10 mA/cm2, which is having the highest weight percentage and crystallinity of hydroxyapatite phase, is the most preferable one for continuous, faster and interconnected cell growth. Also cell cytoskeletons spread in bigger areas around their nucleus with a strong interconnection between each other. The bio-mineralization study shows that on immersion of these coatings in simulated body fluid, coating obtained through 10 mA/cm2, trends to build up hydroxyapatite on its surface with 200 nm pores at initial 1–3 days. After 14 days of immersion, hydroxyapatite deposition trends to form a spherical shape of around 2 micron in radius which can accommodate larger surface area for tissue growth vis-à-vis better bonding, as compared to other coating surfaces. The polarization resistance of 10 mA/cm2 coating is almost 10 times higher than the others and thus indicates its slower rate of corrosion under contact with simulated body fluid.

Positive Streamer Simulation in Air Using Finite Element Method

A new methodology for positive streamer process is presented. This process depends on the critical electric field value that required for streamer initiation. So, an accurate computational of the electric field distribution across needle-to-plane gap is required. A finite element method using Comsol Multiphysics program is adopted for this simulation. The streamer path has been considered as a conducting path of 300 micro-meter in length and 30 micro-meter in radius with 10⁸ electrons on its head. The results have been verified with others.

Carbon in Solidphase Reduction of Oxides

The gasphase-solidphase model is used to explain the results of solid chromite ore carbothermic reduction. The reducing agent is carbon atoms. Carbon is brought to the surface of chromite grains by gaseous carbon-containing molecules and radicals, such as C3O2, CH4, CH3, CH2, CH, C2H2 and other carbon containing particles. Gas particles penetrate a piece of ore through the pores and cracks. Active carbon atoms C are formed by the dissociation of gaseous particles. Reduction by carbon is carried out on the chromite grain surface due to the cations and anions diffusion in the oxide lattice towards the grain surface. Porosity of chromite ores is experimentally defined under reducing conditions; it is estimated at up to 20 – 25 vol.%. Up to 1/4 of porosity is accounted for macropores and macrocracks that have the radius of over 50 microns, the remaining porosity share is accounted for the pore dimensions that are estimated at 0.5 – 50 microns in radius. The carbon presence in the pores of partially reduced ores is deduced based on experiment. There is also carbon in the reduced metal that is situated deep in the lump ore. The experimental results agree with the gasphase-solidphase reduction model.

Biogeography of a human oral microbiome at the micron scale

The spatial organization of complex natural microbiomes is critical to understanding the interactions of the individual taxa that comprise a community. Although the revolution in DNA sequencing has provided an abundance of genomic-level information, the biogeography of microbiomes is almost entirely uncharted at the micron scale. Using spectral imaging fluorescence in situ hybridization as guided by metagenomic sequence analysis, we have discovered a distinctive, multigenus consortium in the microbiome of supragingival dental plaque. The consortium consists of a radially arranged, nine-taxon structure organized around cells of filamentous corynebacteria. The consortium ranges in size from a few tens to a few hundreds of microns in radius and is spatially differentiated. Within the structure, individual taxa are localized at the micron scale in ways suggestive of their functional niche in the consortium. For example, anaerobic taxa tend to be in the interior, whereas facultative or obligate aerobes tend to be at the periphery of the consortium. Consumers and producers of certain metabolites, such as lactate, tend to be near each other. Based on our observations and the literature, we propose a model for plaque microbiome development and maintenance consistent with known metabolic, adherence, and environmental considerations. The consortium illustrates how complex structural organization can emerge from the micron-scale interactions of its constituent organisms. The understanding that plaque community organization is an emergent phenomenon offers a perspective that is general in nature and applicable to other microbiomes.

Comet 67P/Churyumov–Gerasimenko: Hardening of the sub-surface layer

From the operation of the MUPUS thermal probe Spohn et al. (2015) concluded that the material of the nucleus of 67P/Churyumov–Gerasimenko is likely to have a high strength, at least locally at the Philae landing site. In this work we consider the derived strength of the material in order to constrain its granulation. For this purpose we performed numerical simulations of the long-term sintering of ice–dust granular mixtures of different granulation, covered by a dust mantle. The dust mantle has a thickness of 0–16cm, and a (pore size and temperature-dependent) thermal conductivity. According to our simulations a hardened layer at least a meter thick forms beneath the dust only when the grains are tens of microns in radius, or smaller.

Rotation-induced evolution of far-field emission patterns of deformed microdisk cavities

We study rotation-induced asymmetry of far-field emission from optical microcavities, based on which a new scheme of rotation detection may be developed. It is free from the "dead zone" caused by the frequency splitting of standing-wave resonances at rest, in contrast to the Sagnac effect. A coupled-mode theory is employed to provide a quantitative explanation and guidance on the optimization of the far-field sensitivity to rotation. We estimate that a 10^4 enhancement of the minimal detectable rotation speed can be achieved by measuring the far-field asymmetry, instead of the Sagnac effect, in microcavities 5 microns in radius and with distinct emission directions for clockwise and counterclockwise waves.