Interparticle Cohesive Forces Research Articles

Experimental Method for Measuring Cohesion of Regolith via Electrostatic Lofting

The hypothesized electrostatic lofting of individual regolith grains on the Moon and asteroids has been investigated extensively in laboratory studies. Cohesion may dominate how regolith behaves on these small, airless bodies, yet the magnitude of this force remains uncertain. We induce the electrostatic detachment of dust as a mechanism to break cohesive bonds between individual zirconia-silica microspheres in order to measure the interparticle cohesive force between them, likely dominated by capillary bridges. A high-speed camera imaged centroid positions of the lofted microspheres over time. Using the centroids from the initial detachment, we numerically calculated initial accelerations to solve for the cohesion that had been restraining the microspheres. Unexpectedly, the electrostatic lofting of clumps of particles was observed and experimental results showed that clumps were a nonnegligible portion of the lofted object population.

Discrete element method model calibration and validation for the spreading step of the powder bed fusion process to predict the quality of the layer surface

A Discrete Element Method model, including interparticle cohesive forces, was calibrated and validated to develop a tool to predict the powder layer’s quality in the powder bed fusion process. An elastic contact model was used to describe cohesive interparticle interactions. The surface energy of the model particles was estimated by assuming that the pull-off force should provide the strength of the material evaluated at low consolidation with shear test experiments. The particle rolling friction was calibrated considering the bulk density of the layer produced by the spreading tool. The model was validated with the experiments by comparing the wavelet power spectra obtained with the simulations with those of the experimental layers illuminated by grazing light. The calibration proposed in this study demonstrated superior performance compared to our previous methods, which relied on measuring the angle of repose and unconfined yield strength.

Role of gravity magnitude on flowability and powder spreading in the powder bed fusion additive manufacturing process: Towards additive manufacturing in space

Understanding powder spreading under low gravity conditions is essential for optimizing final products using additive manufacturing in space. In this study, we investigated the role of gravity on flowability and spreading mechanisms through combined experimental and discrete element method (DEM) studies. Three powders with different theoretical densities were used to reenact low compressive conditions resembling those in a low-gravity environment. The influence of low compressive conditions on flowability and spreading behavior was examined using the Hall flowmeter, rotating drum, and spreading experiments. In the experimental result, the static flowability was primarily affected by the presence of elongated particles rather than the compressive conditions. The dynamic AoR of TD_4 powder increased compared to that of TD_8 powder, despite the presence of spherical particles with a smooth surface finish. A DEM simulation study was conducted using TD_8 powder to investigate the impact of different gravity levels on dynamic flowability. The DEM studies revealed that the dynamic flowability under rotation was decreased under low gravity owing to the promoted cohesive interactions. The powder spreading experiment was performed using the three powders with different theoretical densities. The in-situ observation with particle image velocimetry analysis revealed that kinetic energy dissipation in the spreading process was accelerated in the TD_8 powder pile, despite its high interparticle friction and cohesive force. The powder spreading simulation was conducted using TD_8 powder to clarify the effect of low gravity on the powder spreading process. In TD_8 powder under 1 G, particle supply was facilitated by a synergistic effect of free-falling and deposited particles. However, increased cohesive interactions under 0.5 G and 0.16 G restricted particle supply via free-falling, consequently reducing the powder bed density by about 2 % and 6.2 %, respectively. These findings prove that the cohesive force predominantly controls dynamic flowability and powder bed quality in the spreading process under low gravity conditions.

Experimental study of particle impact on cohesive granular packing.

We investigate experimentally the impact process of sand particles onto a cohesive granular packing made of similar particles. We use a sand-oil mixture with varying liquid content to tune the cohesive strength of the packing. The outcome of the impact is analyzed in terms of the production of ejected particles from the packing. We quantify this production as a function of the impact velocity of the particles for increasing cohesion strength. We identified three different regimes depending on the cohesion number Co, defined as the ratio of the interparticle cohesive force to the particle weight. For small cohesion (i.e., Co⪅1), the ejection process is not modified by the cohesion. For intermediate cohesion (i.e., 1⪅Co⪅20), the ejection process becomes less efficient: the number of ejected particles per impact for a given impact velocity is decreased but the critical impact velocity to trigger the ejection process remains unchanged. Finally, for strong cohesion (i.e., Co⪆20), we observed a progressive increase of the critical impact velocity. These experimental results confirm spectacularly the outcomes of recent numerical simulations on the collision process of a particle onto a cohesive packing and open avenues to model the aeolian transport of moist sand.

Factors Influencing Food Powder Flowability

The flowability of food powders is a critical determinant of their processing efficiency, product quality, and overall operational success. This review delves into the intricacies of powder flowability, elucidating the factors that govern it and exploring various methods for its evaluation and enhancement. Particle size and distribution, particle shape, surface properties, moisture content, and storage conditions stand as the key determinants of powder flowability. Finer powders, with their increased interparticle cohesive forces, tend to exhibit poorer flowability. Particle shape also plays a role, with irregular or elongated particles flowing less readily than spherical ones. Surface properties influence interparticle friction, thereby impacting flow behavior. Moisture content significantly affects flowability, as increased moisture can lead to liquid bridge formation, hindering powder movement. Storage temperature, on the other hand, generally enhances powder flow due to reduced interparticle cohesive forces at higher temperatures. This highlights the need to understand the factors influencing food powder flowability and to employ appropriate evaluation strategies for optimizing food powder processing efficiency, product quality, and overall production success.

On sediment incipient motion of marine cohesive sediments with different bulk densities

This paper aims to methodically examine the incipient characteristics of natural marine fine sediment with particle size less than 0.012 mm for a variety range of bulk densities. Theoretical derivation and experimental data analysis are appropriately combined to explore the effect of bulk density on the inter-particle cohesive force and to deduce the incipient shear stress (ISS) of fine sediment. Under the same particle size condition, the achieved results indicate that the ISS of sediment intensifies with the growth of the bulk density. In addition, the larger the bulk density, the faster the increase rate, revealing a power exponential growth trend. The ISS of sediment also magnifies with the reduction of the particle size under the same bulk density condition; however, as the particle size decreases to 0.007 mm, the ISS of sediment would no longer vary. The influential factors on the cohesive force, including intermolecular gravitational force, film water pressure, and bulk density, are thoroughly analyzed, and a rational expression for the cohesive force is also proposed. The modified shields parameter represents the ratio between the sediment driving force and the stabilization force (e.g., gravitational and cohesive forces) and is still a function of the sediment Reynolds number. The modified shields curve still follows the shape of the original shields curve and satisfies both cohesive and non-cohesive sediment conditions but with differences in the form of shields parameter.

Study of Fluidization Behavior Transition from Geldart B to A Induced by High Temperature Using Electrical Capacitance Tomography

The transition of the fluidization behavior of Geldart B particles to that of A particles induced by temperature change was investigated by a developed high-temperature electrical capacitance tomography (ECT) sensor. Silica particles with a Sauter mean diameter of 237 μm and density of 2650 kg/m3, typically Geldart B particles under ambient conditions, were fluidized in a column of 5.5 cm from 20 to 600 °C. With the increase in temperature, ECT measurements showed a decrease in minimum bubbling velocity (Umb), no bed expansion characteristic in the homogeneous fluidization regime, an absence of multiple-bubbles regime, and a larger bubble size. The pressure drop against superficial gas velocity curves at elevated temperatures confirmed homogeneous fluidization between the minimum fluidization velocity (Umf) and Umb. Our analysis demonstrates that cohesive interparticle forces, which increase linearly with temperature, are responsible for the fluidization behavior transition of the silica particles.

CFD-DEM investigation on the agglomeration behavior of micron-sized combusted iron fines

The occurrence of agglomeration due to particle sintering is one of the most significant technical issues hindering the direct reduction process of iron oxide fines (DRI) in a high-temperature fluidized bed. To get a better understanding of the agglomeration mechanism, the (de-)fluidization behavior of micron-sized iron oxide particles in a 3-D fluidized bed is investigated using Computational Fluid Dynamics coupled with a coarse-grained Discrete Element Method (CFD-cgDEM). The inter-particle cohesive force is considered as a temperature-dependent solid bridge force. The coarse-grained method is first verified by comparing results for different scaling factors. Subsequently, the effects of temperature on the bed pressure drop, void fraction distribution, particle velocity distribution, and agglomerate size are investigated. In conclusion, the developed model shows the capacity for reliable prediction of particle agglomeration behavior, which can shed light on the design of the DRI process in high-temperature fluidized beds.

Viscosity of ice-in-oil slurries

Ice slurries are phase change materials extensively used in refrigeration technology. This work describes an experimental study and empirical modeling that was carried out to characterize the rheological behavior of ice-in-oil slurries. Decane and crushed ice were mixed to prepare test samples with ice volume percentages ranging from 3.5 to 17.7%. The size of the particles was 0.27±0.13mm. The viscosity measurements are performed at −2.5, −5.0, and −10.0°C using a rotational viscometer with a three-bladed impeller. The maximum relative velocity was ∼3.1 for 17.7% vol. concentration. A Bingham viscoplastic model was used to predict the rheological behavior of ice-in-oil slurries. The fractal dimension, packing limit, size of ice particles, and inter-particle cohesive forces were all considered in rheological calculations to make the model extensively applicable. The model accuracy is then examined using third-party experiments and experimental findings from the current study. The model appears to be a viable tool for predicting the viscosity of ice-in-oil slurries.

Minimum fluidization velocity and reduction behavior of combusted iron powder in a fluidized bed

The fluidization and reduction behavior of micron-sized iron oxide powder, produced by iron combustion, is studied in a lab-scale cylindrical fluidized bed. The minimum fluidization velocity umf is found to stabilize above normalized static bed heights of 0.5 H/D (static bed height divided by the bed diameter). umf is measured as a function of temperature between 280 and 860 K for both H2 and N2 as fluidizing gas. The experimental results start to deviate from the Ergun correlation at temperatures above 560 K, both for N2 and H2. A new correlation, taking the cohesive inter-particle solid bridge force into account, is proposed in this work to predict the minimum fluidization velocity at high temperature. Reduction experiments are carried out for a total time of 5 h at constant excess velocity with 50, 75 and 100 vol% of H2 and temperatures between 623 and 823 K. Gradual defluidization occurs when the operating temperature exceeds 800 K. A maximum reduction degree of 61% is obtained at 807 K and 100 vol% H2.

Microscale evaluation of natural anti-agglomeration behavior of oils via gas hydrate interparticle cohesive force measurements

Gas hydrate formation in flowlines presents one of the major threats to flow assurance. Exploitation of a non-plugging oil, which contains hydrate anti-agglomerant (AA) properties is an attractive hydrate management strategy. Hence, it is important to evaluate an oil for its potential as a non-plugging oil, considering its significant economic savings, since the properties to prevent hydrate plugging already exist in the oil without additional cost. In this work, a High-Pressure Micromechanical Force (HP-MMF) apparatus was used to quantify the cohesive forces between hydrate particles to provide insight into the natural anti-agglomeration tendencies of oils. Investigation was conducted using seven oils (A – G). Based on the significant reductions ranging from 81 to 99.8% to the cohesive force observed for oils A – E, these oils were categorized as potential non-plugging oils. The cohesive force reductions are attributed to the presence of natural surfactants in these oils that take the role as AAs. Increase in water content in the presence of non-plugging oils led to higher cohesive force, which weakened the non-plugging oil’s performance in preventing hydrate agglomeration, but can be overcome by salt. Oils F and G showed no significant reductions to the hydrate cohesive force, therefore are classified as plugging oils. These results show that the microscale evaluation in the HP-MMF can be applied to identify natural anti-agglomeration behavior of different oils, providing important insights to optimize hydrate flow assurance strategy in the field.

High pressure micromechanical force method to assess the non-plugging potential of crude oils and the detection of asphaltene-hydrate mixed agglomerates

A hydrate prevention strategy taking advantage of crude oil that exhibit hydrate non-plugging tendency is a viable option in oilfield production conditions, or during transient operations. For that reason, it is important to accurately evaluate crude oils for their potential as non-plugging oils in a small-scale apparatus, such as a High-Pressure Micromechanical Force (HP-MMF) system, before continuing to large-scale testing. HP-MMF is an interfacial method that has been applied to measure gas hydrate interparticle cohesive force to gain insight into the hydrate agglomeration tendencies of crude oils. Herein, methane/ethane hydrate cohesive force measurements in the presence of small concentrations of crude oil were performed in the HP-MMF. Direct observations from the HP-MMF revealed, for the first time, the appearance of asphaltene precipitates that interact with the gas hydrate particles to produce an asphaltene-hydrate mixed agglomerate. A new HP-MMF method has been developed that enables representative sampling and evaluation of the effect of crude oil samples on the cohesive force between two gas hydrate particles in the HP-MMF. The method has been tested with two crude oils, where lower cohesive force results are demonstrated by the presence of small concentrations (<5 vol% in model oil) of Crude A and C compared to the baseline system of pure model oil. Hence, indicating the presence of natural anti-agglomerants (AAs) that contribute to the non-plugging behavior of these crude oils. These studies show that the HP-MMF is a convenient tool to assess the non-plugging potential of crude oils at realistic (high pressure, low temperature) conditions, requiring only small oil sample volumes. The HP-MMF method developed in this work is demonstrated to be a unique tool that can be utilized for the detection of asphaltene-hydrate mixed agglomerates. These new findings provide important insights for asphaltene precipitation in the presence of gas hydrates, that can create catastrophic agglomeration in the flowlines.

Improved Flowability and Wettability of Whey Protein-Fortified Skim Milk Powder via Fluidized Bed Agglomeration.

Recently, protein-fortified milk powders are being widely consumed in Korea to prevent sarcopenia, and the demand for high-protein food powders is continuously increasing in the Korean market. However, spray-dried milk proteins have poor flowability and wettability owing to their fine particle sizes and high inter-particle cohesive forces. Fluidized bed agglomeration is widely used to improve the instant properties of food powders. This study investigated the effect of fluidized bed agglomeration on whey protein isolate (WPI)-fortified skim milk powder (SMP) at different SMP/WPI ratios. The fluidized bed process increased the particle size distribution, and agglomerated particles with grape-like structures were observed in the SEM images. As the size increased, the Carr index (CI) and Hausner ratio (HR) values of the agglomerated WPI-fortified SMP particles exhibited excellent flowability (CI: <15) and low cohesiveness (HR: <1.2). In addition, agglomerated WPI-fortified SMP particles exhibited the faster wetting time than the instant criterion (<20 s). As a result, the rheological and physical properties of the WPI-fortified SMP particles were effectively improved by fluidized bed agglomeration. However, the fluidized bed agglomeration process led to a slight change in the color properties. The CIE L* decreased, and the CIE b* increased because of the Maillard reaction. The apparent viscosity (ηa,10) and consistency index (K) values of the rehydrated solutions (60 g/180 mL water) increased with the increasing WPI ratio. These results may be useful for formulating protein-fortified milk powder with better instant properties.

Numerical investigation of the particle circulation characteristics in a tapered fluidized bed

PurposeThe objective of this study is to investigate several issues related to particle circulation within the TFB, including exploring an appropriate method to quantify particle circulation time, the effects of different operational parameters on particle circulation time, and the relationship between particle mixing and particle circulation.Design/methodology/approachThe computational fluid dynamics coupled with the discrete element method (CFD-DEM) is applied to investigate the particle circulation characteristics of a tapered fluidized bed (TFB). An approach for defining particle circulation, which accounts for the horizontal motion of each particle, is proposed to estimate particle circulation time.FindingsIt is found that the overall particle circulation in a TFB could be accelerated by increasing air velocity and wall inclination angle, while an increase in particle size and an increase in inter-particle cohesive forces decelerate particle circulation; the increase in the open area ratio of the central region of the air distributor would decelerate the particle circulation. Moreover, the particle circulation time and mixing rate are independent variables that describe the flow dynamics of particles from different perspectives.Practical implicationsA large part of fluidized beds in industrial applications can be classified as TFB. This study presents a numerical method to obtain detailed knowledge about particle circulation in a TFB, which is essential for the design, optimization, and control of related processes.Originality/valueThe particle circulation in a TFB is important but rarely investigated, and it is hard to be quantified using existing experimental approaches. The proposed numerical workflow reveals the characteristics of particle circulation from a particle-scale perspective.

Second-virial theory for shape-persistent living polymers templated by disks.

Living polymers composed of noncovalently bonded building blocks with weak backbone flexibility may self-assemble into thermoresponsive lyotropic liquid crystals. We demonstrate that the reversible polymer assembly and phase behavior can be controlled by the addition of (nonadsorbing) rigid colloidal disks which act as an entropic reorienting "template" onto the supramolecular polymers. Using a particle-based second-virial theory that correlates the various entropies associated with the polymers and disks, we demonstrate that small fractions of discotic additives promote the formation of a polymer nematic phase. At larger disk concentrations, however, the phase is disrupted by collective disk alignment in favor of a discotic nematic fluid in which the polymers are dispersed antinematically. We show that the antinematic arrangement of the polymers generates a nonexponential molecular-weight distribution and stimulates the formation of oligomeric species. At sufficient concentrations the disks facilitate a liquid-liquid phase separation which can be brought into simultaneously coexistence with the two fractionated nematic phases, providing evidence for a four-fluid coexistence in reversible shape-dissimilar hard-core mixtures without cohesive interparticle forces. We stipulate the conditions under which such a phenomenon could be found in experiment.

Micromechanical description of adsorptive-capillary stress in wet fine-grained media

The aim of this paper is to investigate the effect of an adsorbed water layer on the mechanical behavior of fine-grained wet granular materials in the pendular regime with isolated capillary liquid bridges. The adsorbed water forms a thin liquid film tightly bound to a particle’s surface equilibrated by a so-called “disjoining pressure”. In a stress transmission analysis, this disjoining pressure concept is embedded in the so-called Augmented Young-Laplace equation to account for thin film interfacial interactions. Using a homogenization technique for upscaling the micro-scale physics, an adsorptive-capillary stress tensor is derived whose discrete representation reveals a new interparticle cohesive force. In the presence of adsorbed layers, it is shown that the new liquid bridge profile, as numerically solved from the Young-Laplace equation, leads to a higher cohesive interparticle force and rupture distance. The proposed adsorptive-capillary stress tensor is further implemented within a discrete element modeling framework. As such, the evolutions of microstructure, stress tensors, and shear strength are illustrated during suction-controlled triaxial simulations. Our numerical results demonstrate that adsorbed layers have a notable effect on the mechanical behavior of fine-grained materials, particularly at higher suctions.

Critical spin periods of sub-km-sized cohesive rubble-pile asteroids: dependences on material parameters

ABSTRACT In this work, we employ a soft-sphere discrete element method with a cohesion implementation to model the dynamical process of sub-km-sized cohesive rubble piles under continuous spin-up. The dependences of the critical spin periods Tc on several material parameters for oblate rubble piles with different diameters were explored. Our simulations show that the interparticle cohesive force can strengthen the bodies as expected, especially for the smaller ones. The simulated results of Tc were fitted with the continuum theory developed by Holsapple, through which we find the interparticle cohesion is proportional to the best-fitting bulk cohesion and the ratio shows no dependence on the density. In addition, we find Tc decreases as the density increases in the compressive regime, while the trend reverses when transitioning to the tensile regime. Besides, though a higher friction angle can strengthen the bodies, its influence on Tc is minimized near the separation between the two regimes. Our numerical findings are generally consistent with the continuum theory, except that the latter predicts that Tc should increase as the friction angle increases in the tensile regime, which is contrary to the numerical results. This remarkable difference reminds us to take caution when applying the continuum theory to critically spinning cohesive rubble piles in the tensile regime, especially when dealing with the effect of the friction angle. Finally, we emphasize that the separation between the regimes can be specified by a characteristic period, which is only a function of density for a given shape.

Structural Effects of Gas Hydrate Antiagglomerant Molecules on Interfacial Interparticle Force Interactions.

Gas hydrate interparticle cohesive forces are important to determine the hydrate crystal particle agglomeration behavior and subsequent hydrate slurry transport that is critical to preventing potentially catastrophic consequences of subsea oil/gas pipeline blockages. A unique high-pressure micromechanical force apparatus has been employed to investigate the effect of the molecular structure of industrially relevant hydrate antiagglomerant (AA) inhibitors on gas hydrate crystal interparticle interactions. Four AA molecules with known detailed structures [quaternary ammonium salts with two long tails (R1) and one short tail (R2)] in which the R1 has 12 carbon (C12) and 8 carbon (C8) and saturated (C-C) versus unsaturated (C═C) bonding are used in this work to investigate their interfacial activity to suppress hydrate crystal interparticle interactions in the presence of two liquid hydrocarbons (n-dodecane and n-heptane). All AAs were able to reduce the interparticle cohesive force from the baseline (23.5 ± 2.5 mN m-1), but AA-C12 shows superior performance in both liquid hydrocarbons compared to the other AAs. The interfacial measurements indicate that the AA with an R1 longer alkyl chain length can provide a denser barrier, and the AA molecules may have higher packing density when the AA R1 alkyl tail length is comparable to that of the liquid hydrocarbon chain on the gas hydrate crystal surface. Increasing the salinity can promote the effectiveness of an AA molecule and can also eliminate the effect of longer particle contact times, which typically increases the interparticle cohesive force. This work reports the first experimental investigation of high-performance known molecular structure AAs under industrially relevant conditions, showing that these molecules can reduce the interfacial tension and increase the gas hydrate-water contact angle, thereby minimizing the gas hydrate interparticle interactions. The structure-performance relation reported in this work can be used to help in the design of improved AA inhibitor molecules that will be critical to industrial hydrate crystal slurry transport.

Anisotropic nature of the capillary stress tensor

The paper describes a micromechanical approach that explores the anisotropic nature of the capillary stress tensor and its evolution in pendular granular materials via Discrete Element Modeling (DEM) simulations. Dimensionless parameters are used to address the conditions under which the contribution of capillarity (or cohesive interparticle forces) to the stress transmission within a Representative Elementary Volume (REV) is expected to be considerable. From a series of suction-controlled conventional triaxial tests, numerical results show that the significance of the capillary stress and the relative magnitude of its mean to deviatoric components is directly connected to the characteristic particle size and applied stress. In addition, it is shown that the anisotropic character of the capillary stress tensor intensifies with increasing suction. Furthermore, a simple shear test is conducted at constant mean stress to reveal the development of deviatoric capillary stresses in the absence of any change in mean stress, which cannot be captured by the commonly used Bishop’s stress expression.

Laboratory measurements of initial launch velocities of electrostatically lofted dust on airless planetary bodies

Electrostatic dust lofting may play an important role in surface evolution of airless bodies as indicated from a number of unexplained observations, including the lunar horizon glow and dust ponding on asteroids, for example. The initial launch velocity of a charged dust particle determines its range of motion, hence it is a critical quantity to assess the dynamics of electrostatic dust transport. Here we report on laboratory measurements of the launch velocity of electrostatically lofted dust and its relationship with the dust size. For irregularly-shaped dust, similar-sized particles show a large velocity dispersion, indicating a large variation in the inter-particle cohesive force. The maximum launch velocity is shown to decrease with the increase in the dust radius and it is on the order of ~0.7 m/s for 15 μm radius particles. Derived from the patched charge model (Wang et al., 2016a. Geophys. Res. Lett. 43, 6103–6110), the theoretical expectation of an inverse velocity-size relationship of lofted dust is shown to be consistent with the maximum launch velocity measurements. The dust shape is found to have a significant effect on the lofting process, showing that irregularly-shaped particles are lofted with much larger velocities than spherical particles. The initial launch angle relative to the surface shows a broad distribution.