Contact Points Of Particles Research Articles

Digital image analysis of gas bypassing and mixing in gas‐fluidized bed: Effect of particle shape

AbstractThe study investigates effect of particle shape on gas bypassing and mixing of gas‐fluidized Geldart A particles. A shallow fluidized bed (FB), configured at benchscale, was used with digital image analysis (DIA) for the investigation. The extent of scatter of tracer particles throughout the bed was assessed from DIA images of defluidized powder. A novel method employing Jupyter notebook software, was used to directly determine Mixing Index from digital images. Remarkably, platelet‐shaped China clay powder displayed the best mixing characteristics (Mixing Index: 0.79) with no significant bypassing. Angular shaped Quartz displayed moderate mixing (Mixing Index: 0.67), but high bypassing (Bypassing Index: 0.75). Contrary to conventional assumptions, spherical‐shaped diatomite exhibited poor mixing (Mixing Index: 0.61) with the highest bypassing (Bypassing Index: 0.82). Platelet particles performed well even with fines removal. Most likely, particle shape significantly influenced the number of available particle contact points, tracer migration, and traceronparticle binding.

Hard Particle Mask Electrochemical Machining of Micro-Textures

The efficient and cost-effective preparation of masks has always been a challenging issue in mask-based electrochemical machining. In this paper, an electrochemical machining process of micro-textures is proposed using hard particle masks such as titanium and zirconia particles. Numerical simulations were conducted to analyze the formation mechanisms of micro-protrusion structures with insulating and conductive hard particle masks, followed by experimental verification of the process. The results indicate that when the hard particles are electrically insulating, metal material preferentially dissolves at the center of the particle gap, and the dissolution then expands over time in depth and towards the particle contact points. Conversely, using the conductive particles as the masks, such as titanium particles, dissolution initially occurs in a ring region centered at the contact point between the hard particle and the anode, with a radius approximately one-quarter of the chosen particle’s diameter (200 μm), and then continues to expand outward.

Flow behavior of polypropylene reactor powder in horizontal stirred bed reactors characterized by X-ray imaging

Horizontal stirred bed reactors (HSBRs) are widely used in the commercial production of polypropylene (PP). Despite their commercial significance, a comprehensive understanding of the flow behavior in HSBRs remains elusive, primarily due to the lack of detailed experimental data. This study investigates the influence of operating parameters on the particle flow behavior of two types of PP reactor powder in a laboratory-scale HSBR using X-ray imaging. Our results indicate that the overall flow behavior and phase holdup in the HSBR are dominated by agitation. Moreover, gas injection through the inlet points at the bottom of the HSBR results in spouting behavior, which can lead to reduced gas-solid contacting and, in extreme cases, complete bypass. Finally, the presence of liquid (in this study, isopropyl alcohol) adversely affects the flow behavior of the PP reactor powder due to liquid bridging at the contact points of particles. Powders that comprise particles with relatively small sizes and dense surface morphology are particularly prone to reduced flow behavior when exposed to liquid.

Superheater ash deposit ageing – Impact of melt fraction on morphology and chemistry

The initial melting behaviour of synthetic ash deposits, with a focus on how the amount of melt formed, affects the final deposit morphology, was studied. Binary and ternary reciprocal deposits were exposed to a temperature gradient with varying exposure time, steel temperature, and furnace temperature. Two different archetype deposit morphologies emerged. The final deposit morphology was identified to depend on the amount of melt formed during the experiment. A skeletal deposit morphology developed in systems with less than 30 wt-% melt. Short-term experiments proved that the formation of the skeletal structure is accelerated by the presence of melt, accumulating at contact points of distinct particles. If melt fractions exceeded 30 wt-%, a molten morphology developed, as the amount of melt present was sufficient to fill the pores within the deposit. For molten deposits of ternary reciprocal systems, enrichment of K in the formed melt and concurrent movement toward the steel was observed. The results confirmed a deposit ageing mechanism that has been proposed earlier, based on full-scale superheater deposit samples. The experiments showed that deposit melting and ageing behaviour are strongly connected, as the two identified morphologies differed in their ageing behaviour. The presented results improve the understanding of the initial melting behaviour of deposits and illustrate the connection between deposit morphology and deposit ageing. The results can be utilised as input data for deposition models and help in predicting the final deposit morphology and its associated ageing behaviour, aiding boiler operators and manufacturers dealing with deposit-related issues.

Ultra-low shrinkage and high-strength porous TiB/TiB2 ceramics via low temperature in-situ reaction sintering combined with the freeze-drying method

Ultra-low shrinkage porous TiB 2 -based ceramics reinforced by the TiB whiskers are firstly fabricated through the in-situ reaction between TiB 2 and Ti at a low temperature (1450 °C). The growth of TiB whiskers with a high aspect ratio at pore channels is achieved through a vapor-solid growth mechanism, while low aspect ratio TiB whiskers at pore walls are dominated by a solid-state reaction diffusion growth, forming bimodal distribution whiskers in porous TiB 2 -based ceramics. The overlapping TiB whiskers with low-speed growth at particle contact points can significantly inhibit the shrinkage and improve the strength of porous TiB 2 -based ceramics. When the solid content is fixed at 20 vol% and target TiB content changes from 0 to 80 vol%, the porous ceramics show slight sintering shrinkage (from 1.1 to 4.7%) and high porosity (from 79.3 to 73.7%) while keeping high compressive strength of 1.8–18.2 MPa, which is higher than most reported porous ceramics at the same porosity.

Effect of a Heterogeneous Distribution of the Conductive Additives and Binder Domain on the Impedances of Lithium-Ion Battery Electrodes

The conductive carbon and binder are essential components of Lithium-ion battery electrodes. Their main task is to enhance the electrical connectivity and mechanical stability. Therefore, a homogeneous distribution of the carbon binder domain (CBD) throughout the electrode is desired. However, inhomogeneities can arise in the production process. At high drying rates of the wet coating, the CBD migrates to the surface of the electrode due to the drag of the evaporating solvent. This effect causes a gradient of the CBD through the thickness of the electrode. This process has two effects: (a) depletion of the CBD at the current collector, diminishing the electronic pathways and the adhesion of the electrode layer, and (b) accumulation of the CBD at the electrode surface, thus blocking pathways for Lithium transport. This is especially prominent in thick electrodes. Therefore, we investigate the effect of binder migration on the electrochemical performance of NMC622 electrodes of Lithium-ion batteries.In our simulations we use virtual electrode microstructures of NMC622 electrodes which are informed by tomography data. As it is challenging to resolve the CBD using tomographic techniques, it is distributed on a model-basis at the contact points of active material particles using an in-house structure generator[1].The valuable information derived from combining microstructure-resolved simulations with electrochemical impedance spectroscopy (EIS) on symmetric cells is used to characterize the Lithium-ion transport in the electrode pore space, including the contributions of the CBD. Additionally, half-cell discharge simulations are also conducted to analyze the effect of CBD heterogeneity on electrode performance.Through our work, we demonstrate the significance of the CBD distribution and enable predictive simulations for future battery design.[1] Hein S. et al, J. Electrochem. Soc. 2020, 167 013546 Figure 1

Simulation of a solid-phase oxygen control scheme in lead-bismuth eutectic system

It is known that solid-phase oxygen control scheme is one of the important methods to solve the corrosion problem when we choose lead–bismuth eutectic (LBE) as coolant in lead-cooled fast reactor (LFR) or Accelerator Driven System (ADS). The mass-exchanger (MX), a porous chamber filled with mechanically stable lead oxide (PbO spheroids), is the key component of solid-phase oxygen control. In order to study the characteristics of oxygen mass transfer, pressure drop, flow field and oxygen concentration field in MX, a randomly packed bed model is established using DEM method. The fluid domain is meshed with computational fluid dynamics (CFD) code STAR-CCM +. The particle–particle and wall–particle contact points are modified with short cylinder bridges to improve the cell qualities. The mass transfer boundary layer theory is applied to explain the oxygen supply process of PbO sphere dissolution to LBE. The results of the average porosity, radial porosity distribution and pressure drop match the previous literature pretty well. The CFD results related to Sherwood number (Sh) are validated by CRAFT experimental data with an acceptable error. The law of oxygen transfer and distribution is obtained by studying the CFD contours and the spatial distribution of oxygen concentration and velocity data. The temperature dependence and velocity dependence of Sherwood number are obtained through multiple linear regression processing of CFD data. Finally, the effects of different temperatures and mass flow rates on oxygen concentration and pressure drop are evaluated to determine the suitable operating domain for MX.

A unified level set method for simulating mixed granular flows involving multiple non-spherical DEM models in complex structures

Since the mixed granular flows containing multiple non-spherical DEM models in complex structures are still challenging for DEM applications, a unified method for creating level set functions of arbitrarily shaped particles constructed by different non-spherical DEM models was developed, and a particle–structure contact algorithm was proposed. At the initial moment, particles of arbitrary shapes constructed by spheres, superquadric equations, spherical harmonic functions, and polyhedral models were discretized using spatial level set functions, and complex structures were discretized into a series of triangular elements. Hence, the contact problem between arbitrarily shaped particles and complex structures was solved using the proposed contact algorithm between level set functions and triangular elements. Meanwhile, particle–particle contact points in mixed granular systems containing multiple non-spherical DEM models were calculated using the level set method . To examine the conservation, accuracy, and robustness of the proposed model, four sets of tests were performed and compared with the theoretical and experimental results, including a plane impacted by a single particle, the elastic collision between two particles, the accumulation of multiple particles, and the mixed granular flow in complex structures. The corresponding numerical results are in good agreement with the theoretical and experimental results, verifying that the present DEM model can accurately predict the motion characteristics of different non-spherical DEM models and can be widely applied to mixed granular flows involving multiple DEM models in complex structures.

Lattice–Boltzmann simulations for analysing the detachment of micron-sized spherical particles from surfaces with large-scale roughness structures

Fully resolved numerical simulations of a micron-sized spherical particle residing on a surface with large-scale roughness are performed by using the Lattice–Boltzmann method. The aim is to investigate the influence of surface roughness on the detachment of fine drug particles from larger carrier particles for transporting fine drug particles in a DPI (dry powder inhaler). Often the carrier surface is modified by mechanical treatments for modifying the surface roughness in order to reduce the adhesion force of drug particles. Therefore, drug particle removal from the carrier surface is equivalent to the detachment of a sphere from a rough plane surface. Here a sphere with a diameter of 5 μm at a particle Reynolds number of 1.0, 3.5 and 10 are considered. The surface roughness is described as regularly spaced semi-cylindrical asperities (with the axes oriented normal to the flow direction) on a smooth surface. The influence of asperity distance and size ratio (i.e. the radius of the semi-cylinder to the particle radius, Rc/Rd) on particle adhesion and detachment are studied. The asperity distance is varied in the range 1.2 <L/Rd < 2 and the semi-cylinder radius between 0.5 <Rc/Rd < 0.75. The required particle resolution and domain size are appropriately selected based on numerical studies, and a parametric analysis is performed to investigate the relationship between the contact distance (i.e. half the distance between the particle contact points on two neighbouring semi-cylinders), the asperity distance, the size ratio, and the height of the particle centroid from the plane wall. The drag, lift and torque acting on the spherical particle are measured for different particle Reynolds numbers, asperity distances and sizes or diameters. The detachment of particles from rough surfaces can occur through lift-off, sliding and rolling, and the corresponding detachment models are constructed for the case of rough surfaces. These studies will be the basis for developing Lagrangian detachment models that eventually should allow the optimisation of dry powder inhaler performance through computational fluid dynamics.

Neighborhood Relationships of Widely Distributed and Irregularly Shaped Particles in Partially Dewatered Filter Cakes

A more thorough understanding of the properties of bulk material structures in solid–liquid separation processes is essential to understand better and optimize industrially established processes, such as cake filtration, whose process outcome is mainly dependent on the properties of the bulk material structure. Here, changes of bulk properties like porosity and permeability can originate from local variations in particle size, especially for non-spherical particles. In this study, we mix self-similar fractions of crushed, irregularly shaped Al2O3 particles (20 to 90 µm and 55 to 300 µm) to bimodal distributions. These mixtures vary in volume fraction of fines (0, 20, 30, 40, 50, 60 and 100 vol.%). The self-similarity of both systems serves the improved parameter correlation in the case of multimodal distributed particle systems. We use nondestructive 3D X-ray microscopy to capture the filter cake microstructure directly after mechanical dewatering, whereby we give particular attention to packing structure and particle–particle relationships (porosity, coordination number, particle size and corresponding hydraulic isolated liquid areas). Our results reveal widely varying distributions of local porosity and particle contact points. An average coordination number (here 5.84 to 6.04) is no longer a sufficient measure to describe the significant bulk porosity variation (in our case, 40 and 49%). Therefore, the explanation of the correlation is provided on a discrete particle level. While individual particles < 90 µm had only two or three contacts, others > 100 µm took up to 25. Due to this higher local coordination number, the liquid load of corresponding particles (liquid volume/particle volume) after mechanical dewatering increases from 0.48 to 1.47.

Structure forming of Cu–W pseudoalloys prepared in different routes

The microstructures of alloys formed during the sintering of tungsten powder mixtures (PV2, 3.8–6.0 μm average particle size) and copper (PMS-11, 45–60 μm fraction) prepared by various methods were compared. The methods included simple metal powder mixing, mechanical activation (MA) of metal powders, copper precipitation from the solution of its sulfate (CuSO4·5H2O) on tungsten powder with simultaneous mechanical activation. The molar ratio of metals in mixtures Cu/W = 1. An aqueous solution for copper deposition included diethylene glycol (up to 30 %), glycerin (up to 8 %), hydrofluoric acid (up to 0.1 %), wetting agent OP-10 (up to 0.8 %). Mechanical activation was carried out in an AGO-2 planetary mill with 200 g of steel balls charged into the drums rotating at 2220 rpm for 5 min. Reduced copper in the solution and in the air rapidly oxidizes to the Cu2O oxide, so the composite powders obtained were washed, dried, and stored in an argon atmosphere. Samples pressed from the powders obtained (tablets 3 mm in diameter, 1.5–2.0 mm in height with a density of 7.7–8.0 g/cm3) were sintered in argon at atmospheric pressure and temperatures from 1000 to 1500 °C. During the sintering of Cu–W composite particles, several areas of the process can be distinguished. «Solid phase» sintering occurs at the contact points of composite particles at temperatures lower than the copper melting point. When samples are heated from the melting point to 1200 °C, samples are sintered by the liquid-phase mechanism from the conventional mixture of metal powders to form a low-porous cake. When composite powders obtained by MA during the copper deposition and MA of metal powder mixtures are sintered, samples are delaminated with the formation of large pores elongated perpendicular to the pressing axis and partially filled with copper melt. When samples obtained by powder MA are heated above 1400 °C, phase separation occurs and almost all copper is displaced from the sample to the surface.

Experimental Study on Mechanism of Wave-Induced Liquefaction of Sand-Clay Seabed

In this study, a series of laboratory experiments for the response of wave induced clay-sand seabed were carried out to clarify the mechanism of liquefaction of clayey seabed. The experiments were conducted in an 80 m long wave flume. In the tests, the sand-clay beds were mixed with various clay contents (CC) from 0.5% to 15% and were tested for given wave conditions. The pore water pressure and the water elevation were measured in each test. Soil properties tests and scanning electron microscope (SEM) experiments on different seabed samples were carried out to further explore the mechanism of liquefaction. The experimental results indicated that the amplitude and accumulation of the excess pore water pressure (EPP) varied with different CC in the sand-clay bed. With the introduction of CC, micro-structure and properties (such as permeability and compressibility) of bed soils changed. Sand-clay bed presented more susceptibility to liquefy compared with pure sand bed. CC promoted seabed liquefaction, even if the added amount was very small (CC is 0.5%), however when CC exceeded a certain value (10% in this study), the mixed bed will not be liquefied. This phenomenon can be well explained by the micro-structure of sand-clay bed. CC within a sandy seabed, does not only affect the permeability, but also change the compressibility of seabed soils. For example, the microfabric of seabed vulnerable to liquefaction is loose. Clay aggregations generally gathered at the sand particle contact points. This microfabric is easily compressed under wave loads and allowed pore water to flow, resulting in the accumulation of pore water pressure. On the other hand, the microfabric of seabed that was resistant to liquefaction appeared to be more compact. Due to clay-filled gaps between the sand particles, the pore water is more difficult to flow when seabed was compressed. Furthermore, the tendency of seabed liquefaction is closely related to CC.

Effect of admixed solid inertants on dispersibility of combustible dust clouds in a modified hartmann tube

Reduction of dust explosion hazards can be achieved by processing materials in less hazardous forms. Two principles are often employed to moderate explosion potential. One approach is to increase dust particle size so as to decrease its reactivity. The other involves altering the dust composition by admixture with solid inertants. An essential prerequisite for dust explosions is the formation of airborne dust clouds. Dust dispersibility (i.e., the ease of dispersion of a dust and the tendency of the particulate matter to remain airborne once a dust cloud has been formed) is therefore an important safety characteristic, certainly requiring research attention. This paper presents experimental results using a modified Hartmann tube to determine the effect of admixed solid inertants on particle size distribution of dust clouds. Nine types of combustible dust and two sizes of inert Al2O3 were tested repeatedly five times. Generally, admixing small amounts of micro-sized solid inertants caused relative improvement on the dispersibility of combustible dust, forming a better dispersed cloud, and sometimes resulted in increased ignition sensitivity. However, the addition of agglomerate nano inert particles caused a multilayer coating of combustible particles, causing an increase in the effective surface area at the particle contact points, and thereby increasing cohesion in the powder mixtures. Higher inter-particle forces in turn enhanced the combustible particle agglomeration during dispersion, leading to an increase in effective particle size distribution.

The contact detection for heart-shaped particles

The contact detection between non-spherical particles is an important issue in the discrete element method (DEM). The non-spherical particles commonly used in DEM are ellipsoids or super-ellipsoids. Such particles possess convex shape functions. The determination of contact points can be reduced to a set of nonlinear partial differential equations by the Lagrange method. Then, the Newton-Raphson method is used to solve these equations. The Lagrange method's premise is that the shape function is convex. For heart-shaped particles with concave shape functions, Lagrange method is not applicable. In this study, the contact point between heart-shaped particles is determined by grid method. This method does not need to provide the initial guess point and does not need to solve the Jacobian inverse matrix. There is no divergence in the solution process, and no limit to the concavity or convexity of the shape function. The algorithm is stable and is a general method. In the MATLAB and C compiler environment, the contact points between super-ellipsoid particles are calculated by the grid method and Newton-Raphson method, respectively. The grid method takes 60 and 0.14 ms, respectively, and its computational efficiency is better than that of the Newton-Raphson method. The time consumption for contact point of heart-shaped particles is less than that of super-ellipsoid particles. The number of contact point between two concave particles may be bigger than one; for this case, accurate results can still be obtained via the grid method. The method given in this study is applicable to the simulation of soft or hard particle systems. In addition, the method is used to simulate the evolution of the super-ellipsoid system with boundary constraints under gravity, and the stability and applicability of grid method are verified.

Enhancement of Fixed-bed Flow Reactions under Microwave Irradiation by Local Heating at the Vicinal Contact Points of Catalyst Particles

The formation of local high temperature regions, or so-called “hot spots”, in heterogeneous reaction systems has been suggested as a critical factor in the enhancement of chemical reactions using microwave heating. In this paper, we report the generation of local high temperature regions between catalyst particles under microwave heating. First, we demonstrated that reaction rate of the dehydrogenation of 2-propanol over a magnetite catalyst was enhanced 17- (250 °C) to 38- (200 °C) fold when heated with microwave irradiation rather than an electrical furnace. Subsequently, the existence of microwave-generated specific local heating was demonstrated using a coupled simulation of the electromagnetic fields and heat transfer as well as in situ emission spectroscopy. Specific high-temperature regions were generated at the vicinal contact points of the catalyst particles due to the concentrated microwave electric field. We also directly observed local high temperature regions at the contact points of the particles during microwave heating of a model silicon carbide spherical material using in situ emission spectroscopy. We conclude that the generation of local heating at the contact points between the catalyst particles is a key factor for enhancing fixed-bed flow reactions under microwave irradiation.

A new collision model for ellipsoidal particles in shear flow

All natural and a growing number of manufactured solid particles are non-spherical. Interesting fluid–particle dynamics applications include the transport of granular material, piling of seeds or grains, inhalation of toxic aerosols, use of nanofluids for enhanced cooling or improved lubrication, and optimal drug-targeting of tumors. A popular approach for computer simulations of such scenarios is the multi-sphere (MS) method, where any non-spherical particle is represented by an assemblage of spheres. However, the MS approach may lead to multiple sphere-to-sphere contact points during collision, and subsequently to erroneous particle transport and deposition. In cases where non-spherical particles can be approximated as ellipsoids with arbitrary aspect ratios, a new theory for particle transport, collision and wall interaction is presented which is more accurate computationally and more efficient than the MS method. In general, with the new ellipsoidal particle interaction (EPI) model, contact points and planes of ellipsoids, rather than spheres, are obtained based on a geometric potential algorithm. Then, interaction forces and torques of the colliding particles are determined via inscribed ‘pseudo-spheres’, employing the soft-particle approach. The off-center forces and moments are then transferred to the mass center of the ellipsoids to solve the appropriate translatory and angular equations of motion. Considering ellipses to illustrate the workings and predictive power of the new collision model, turbulent fluid–particle flow with the EPI model in a 2-D channel is simulated and compared with 3-D numerical benchmark results which relied on the MS method. The 2-D concentrations of micron particles with different aspect ratios matched closely with the 3-D cases. However, interesting differences occurred when comparing the particle-velocity profiles for which the 2-D EPI model generated somewhat larger particle velocities due to out-of-plane collisions, slightly higher particle–wall interactions, and two-way coupling effects.

A multi-core ready discrete element method with triangles using dynamically adaptive multiscale grids.

SummaryThe simulation of vast numbers of rigid bodies of non‐analytical shapes and of tremendously different sizes that collide with each other is computationally challenging. A bottleneck is the identification of all particle contact points per time step. We propose a tree‐based multilevel meta data structure to administer the particles. The data structure plus a purpose‐made tree traversal identifying the contact points introduce concurrency to the particle comparisons, whilst they keep the absolute number of particle‐to‐particle comparisons low. Furthermore, a novel adaptivity criterion allows explicit time stepping to work with comparably large time steps. It optimises both toward low algorithmic complexity per time step and low numbers of time steps. We study three different parallelisation strategies exploiting our traversal's concurrency. The fusion of two of them yields promising speedups once we rely on maximally asynchronous task‐based realisations. Our work shows that new computer architecture can push the boundary of rigid particle computability, yet if and only if the right data structures and data processing schemes are chosen.

THE EFFECT OF MATRIC SUCTION ON THE SHEAR STRENGTH OF UNSATURATED SANDY CLAY

The shear strength parameters of the soil is the key engineering property required for the designof geotechnical structures and stability analysis. The shear strength of an unsaturated soil is controlled by matricsuction. This research examined the effect of matric suction on the shear strength behavior of sandy clay usinga series of experimental tests in the laboratory. The contact filter paper method was used to measure matricsuction and the SoilVision knowledge-based system was used to give an approximation of the entire soil watercharacteristic curve (SWCC). The shear strength of the unsaturated sandy clay was examined using a series ofunconsolidated undrained (UU) triaxial tests to identify the relationship between shear strength, cohesion,internal friction angle, and matric suction. The result shows that the shear strength of the soil increases asmatric suction increases. It was found that when the matric suction is smaller than air entry value (AEV), theshear strength of the soil increases as matric suction increases and the increment is linear because the soil is insaturated conditions. At a certain matric suction value (on soil samples occurred at 40-600 kPa matric suctionvalues), there is a significant increase in the shear strength of soil associated with the inter-particle forceproduced due to negative pore water pressure. Furthermore, at a high matric suction, the relationship betweenshear strength and matric suction tend to be stable as in a low water content, the matric suction is not transmittedeffectively to the contact point of soil particles.

Oligotrophic and eutrophic MICP treatment for silica and carbonate sands

The terms ‘oligotrophic treatment’ and ‘eutrophic treatment’ have been introduced to differentiate between whether fixation liquid or nutrients have been administered during the on-site process for microbially induced precipitation of calcite. Results from scanning electron microscopy images for small-scale soil column tests have shown the different treatments to have markedly different precipitation patterns, crystal morphology and bond formation and eventual bond failure in silica sand. In the case of oligotrophic treatment, the fixation phase led to a more widespread pattern of precipitation in silica sand particles, which resulted in large single rhombohedral crystals being formed on the surface of particles or at particle contact points when nutrients were withheld on site. In contrast, eutrophic treatment involving the on-site delivery of nutrients promoted preferential precipitation at particle contact points in the form of micritic dome-like structures of vaterite that acquired a new layer of crystal with each delivery of nutrients. The different crystal morphologies were found to influence particle-bond failure mechanism in the form of either a particle-bond interface mode of failure or an internal failure of the carbonate crystal along a suture.

Experimental study on compression property of regolith analogues

The compression property of regolith reflects the strength and porosity of the regolith layer on small bodies and their variations in the layer that largely influence the collisional and thermal evolution of the bodies. We conducted compression experiments and investigated the relationship between the porosity and the compression using fluffy granular samples. We focused on a low-pressure and high-porosity regime. We used tens of μm-sized irregular and spherical powders as analogs of porous regolith. The initial porosity of the samples ranged from 0.80 to 0.53. The uniaxial pressure applied to the samples lays in the range from 30 to 4 × 105 Pa. The porosity of the samples remained at their initial values below a threshold pressure and then decreased when the pressure exceeded the threshold. We defined this uniaxial pressure at the threshold as “yield strength”. The yield strength increased as the initial porosity of a sample decreased. The yield strengths of samples consisting of irregular particles did not significantly depend on their size distributions when the samples had the same initial porosity. We compared the results of our experiments with a previously proposed theoretical model. We calculated the average interparticle force acting on contact points of constituent particles under the uniaxial pressure of yield strength using the theoretical model and compared it with theoretically estimated forces required to roll or slide the particles. The calculated interparticle force was larger than the rolling friction force and smaller than the sliding friction force. The yield strength of regolith may be constrained by these forces. Our results may be useful for planetary scientists to estimate the depth above which the porosity of a regolith layer is almost equal to that of the regolith surface and to interpret the compression property of an asteroid surface obtained by a lander.