Interparticle Adhesion Research Articles

Improvement of Flowability of Fine Cohesive Powders by Flow Additives

AbstractMost minerals and concentrates, auxiliaries, agricultural and synthetic particulate solid products are produced with particle sizes of less than 100 μm. These fine powders show similar problematic product properties. Their interparticle adhesion forces, especially van der Waals forces, exceed the gravitational forces by orders of magnitude. Thus, the trouble‐free treatment of these products presents large problems as well as scientific and technological challenges. The physical properties of such materials, especially flowability, are essential for many industrial applications including product generation, processing, application and consumption, e.g., in the pharmaceutical, chemical, food and manufacturing industries or plant engineering. In recent decades, one of the most frequently used solutions to this problem has been to add small amounts of flow additives that improve the flowability of these fine cohesive powders. This paper provides an overview of the physico‐chemically active principles of flow additives, model approaches and useful flowability evaluation methods. The efficiency of several flow additives, i.e., nanoparticles and surfactants, is critically discussed by the provision of selected test results from the literature. Conclusions are drawn with respect to the application of this flow additive method in process industries.

Fabrication of efficient solar cells on plastic substrates using binder-free ball milled titania slurries

Mechanically stable, well adhered TiO 2 films based on nanocrystalline P-25 TiO 2 slurries were developed using ball milling as a part of the processing stage, without the need for binders or high temperature annealing. The strength of the TiO 2 films was examined by a novel nano-scratch technique which was developed to assess inter-particle adhesion. Interfacial and photovoltaic properties of flexible dye sensitized solar cells with a ruthenium dye involving two tetra-butyl-ammonium carboxylate groups (N719) were studied. The maximum power conversion efficiency of 4.2% is obtained under illumination of 100 mW cm −2, for the electrodes fabricated using 20 h milled slurry, shorter or longer milling times were less optimal.

Shear strength of granular materials

ABSTRACT The shear strength properties of granular materials reflect their inherent force and fabric anisotropy. We analyze the role of fabric and force anisotropies with respect to the critical-state shear strength. Then, a model of accessible geometrical states in terms of particle connectivity and contact anisotropy is presented. This model incorporates in a simple way the fact that, due to steric exclusions, the highest levels of connectivity and anisotropy cannot be reached simultaneously, a property that affects seriously the shear strength. We also analyze the force anisotropy in the light of the specific role of weak forces in sustaining strong force chains and thus the main mechanism that underlies anisotropic force patterns. Finally, we briefly discuss the effect of interparticle friction, particle shape, and adhesion.

Factors influencing dust suppressant effectiveness

Water sprays are a common method used to reduce particulate matter (PM) emissions. Various factors such as wettability, surface area coverage, fine particle engulfment rates, interparticle adhesion forces, suppressant penetration and suppressant longevity have all been suggested as critical factors in achieving effective PM control. However, it has not been established which of these factors are the most important. Experimental work indicated that suppressant penetration is the most critical of these factors. The length of time after application that suppressants were effective was also improved by using hygroscopic reagents that retained moisture to prevent evaporation. Maximizing suppressant penetration and improving suppressant longevity led to an average 86% reduction in PM10 concentrations in laboratory dust tower tests.

Analysis of the influence of relative humidity on the moisture sorption of particles and the aerosolization process in a dry powder inhaler

The purpose of the present work was to analyze the moisture sorption and distribution of pharmaceutical powders at various relative humidities (RHs), to investigate its impact on the inter-particle adhesion force as well as the performance of drug delivery through a dry powder inhaler (DPI). Moisture sorption isotherms were measured and the moisture distribution was analyzed using the GAB method and the Young–Nelson equations for Inhalac 230 ® , salbutamol sulfate, and the binary mixture. Adhesion force of the mixture decreased with RH initially, and then increased upon further increase in RH. An increase in the fine particle fraction of emitted dose ( FPF em ) with RH at low moisture contents appeared to be the result of a reduction in electrostatic force due to charge dissipation associated with moisture sorption, while a decrease in the FPF em at high moisture contents arose from the liquid bridge force. It appeared that the emission efficiency was significantly affected by the electrostatic force. The results suggested that electrostatic force not only influenced the drug FPF em but the drug emission efficiency as well, while the liquid bridge force mainly affected the aerosol FPF em for the DPI system investigated. The results indicated that the influence of the RH on the performance of DPI drug delivery was through its impact on the fundamental electrostatic and liquid bridge forces depending on the distribution of the adsorbed moisture content.

Chemical and leaching properties of paper mill sludge

Paper mills produce large amounts of paper mill sludge in the treatment of process water. According to overburden studies, the hydraulic conductivity of the paper mill sludge originating from a paper mill in Northern Finland at a pressure of 30kPa was 4.4×10∓10 ms∓1, and 1.7×10∓ 10 ms∓1at a pressure of 100kPa. These values well meet the generally required values of between 1.0×10∓7 ms∓1 and 1.0×10∓9 ms-1 for a geological barrier in landfill base and sides at landfills in the European Union. Paper mill sludge can also be used in the artificially constructed geological barrier layer of landfill cover structures. The angle of internal friction of the paper mill sludge was 34.8° and the cohesion of inter-particle adhesion 23 kPa, which are important measures for assessing the shear strength of paper mill sludge and thus the stability of the landfill layer in which it is used. During a 28-day period, the biodegradability of the paper mill sludge in soil was ca 0.4% and in ground water under 1%, whereas according to the OECD 301F standard concentrations (BOD28), it was ca 8%. For the determination of total element concentrations in the paper mill sludge, the dried sample was digested using USEPA method 3052. A five-stage sequential leaching procedure was also used to fractionate trace elements in the paper mill sludge between the water-soluble (H2O), exchangeable (CH3COOH), easily reduced (HONH3CD, oxidizable (H2O2 + CH3COONH4), and (5) the residual (HF + HNO3 + HCl) fractions. This paper covers also examples of case studies how paper mill sludge is utilized in Finland.

Fluidization of nanoparticles: A simple equation for estimating the size of agglomerates

In this work, we revise data published in the last decade on the size of agglomerates in gas-fluidized beds of nanoparticles. Experimental measurements reviewed are based on non-invasive techniques, mainly consisting of laser-based planar visualization of agglomerates in the splash zone and indirect derivation from the fit of bed expansion, settling, and/or minimum fluidization velocity data to empirical correlations. Special attention is focused on the effect of fluidization aids such as vibration, magnetic assistance, sound excitation or centrifugation. Independent measurements performed by diverse authors or by the same authors using different techniques are confronted. Empirical models proposed to predict agglomerate size are also reviewed. Most of these models are difficult to apply in practice because they rely on parameters that need to be measured in the fluidization experiment or assumed. We propose a simple equation to estimate agglomerate size derived from the balance between the local shear force on the particle attached at the outer layer of the agglomerate and the interparticle adhesion force. In general, the results predicted by this equation are in satisfactory agreement with the reviewed experimental data.

Fluidization, bubbling and jamming of nanoparticle agglomerates

Primary nanoparticles in gas-fluidized beds are agglomerated in multi-stage complex-agglomerates formed by several steps. The size of the complex-agglomerates can be estimated by a simple equation obtained from the balance between the local shear force on the particle attached at the outer layer of the agglomerate and the interparticle adhesion force. Usually the bed of complex-agglomerates can be fluidized homogeneously, with large bed expansion and absence of appreciable gas bubbles. In the nonbubbling regime the expansion of the bed can be described by a modified Richardson–Zaki equation. At high gas flows large bubbles can develop that curtail further bed expansion. Yet in some cases the fluidized bed transits directly to elutriation. We provide a criterion to predict either the onset of gas bubbles or the suppression of bubbles. As the gas flow supply to the fluidized bed is decreased there is a point at which the nanoparticles are jammed in a loose configuration characterized by a particle volume fraction that we relate to the ratio of interparticle adhesion to particle weight.

Self-Assembly of “Paint-On” Colloidal Crystals Using Poly(styrene-co-N-isopropylacrylamide) Spheres

We report on the synthesis of monodispersed, spherical colloidal particles composed of poly(styrene-co-N-isopropylacrylamide) (pS-co-NIPAm) that rapidly self-assemble via simple solvent evaporation to form 3-D colloidal crystals that are both robust and uniformly diffractive. As a suspension of the pS-co-NIPAm particles dries, the interactions of their soft, flexible peripheries assist in particle self-assembly to form a close-packed ordered structure. Interparticle adhesion occurs at these contacts as the final amount of solvent evaporates, enhancing the durability of the assembly. The dense core provides stability of the ordered structure subsequent to drying. These dried, crystalline films exhibit Bragg diffraction, which can be modified by changing the particle size through varying the monomer feed concentrations. The facile drying process used for crystalline assembly presents a unique approach to photonic inks that can be used as “paint-on” (i.e., processable) dispersions, which are advantageous for the fabrication of materials for optical applications.

Effect of compaction history on the fluidization behavior of fine cohesive powders

Fine particles agglomerate in the fluidized state due to the strength of interparticle attractive forces as compared to particle weight. Interparticle adhesion can be largely increased by consolidation stresses applied during powder handling. As a consequence, fragments of the consolidated powder may persist when the powder is fluidized, which gives rise to large agglomerates of strongly adhered particles in fluidization. This history-dependent effect can be minimized by coating the particles with surface additives such as silica nanoparticles. In this paper, we investigate the effect of high consolidation stresses sigma(c) previously applied to samples of silica-coated fine particles on their fluidization behavior. Our experimental measurements show that, even though homogeneous fluidization is still observed, the average agglomerate size and fractal dimension of the agglomerates increase as sigma(c) is increased. Bed expansion in the fluidized state is hindered by previously applied high consolidations, which we attribute to an increase of the largest stable size of mesoscopic fluid pockets. As a consequence, we observe that the initiation of macroscopic bubbling is delayed up to larger values of the fluid velocity.

Mechanistic analysis and computer simulation of impact breakage of agglomerates: Effect of surface energy

Agglomerates are ubiquitous as intermediate or manufactured products in chemical, pharmaceutical and food industries. During handling and processing they may suffer breakage if they are weak. On the other hand, if they are too strong, their dispersion and disintegration could be difficult. The control of their mechanical strength is therefore highly desirable. However, the analysis of agglomerate strength is complex due to the large number of parameters that influence agglomerate behaviour, such as the primary particle size, density and elastic modulus, and the interparticle bond strength. A simple mechanistic model is presented here which relates the number of broken contacts in agglomerate due to impact velocity, interparticle adhesion energy and the particle properties of the particles forming the agglomerate. The model is based on the hypothesis that the energy used to break contacts during impact is proportional to the incident kinetic energy of the agglomerate. The damage ratio defined as the ratio of broken contacts to the initial number of bonds is shown to depend on the dimensionless group, Δ , in the form ( ρ V 2 D 5 / 3 E 2 / 3 ) / Γ 5 / 3 , where V is the impact velocity, E the elastic modulus, D the particle diameter, ρ the particle density and Γ the interface energy. This dimensionless group, Δ , incorporates the Weber number, ( ρ DV 2 / Γ ) , which was previously shown to be influential in agglomerate breakage, and may be presented in the form, Δ = WeI e 2 / 3 , where I e = ED / Γ . The predicted dependency of the damage ratio on the surface energy has been tested using distinct element method (DEM). Four different agglomerates have been formed and impacted against a target for three different values of the surface energy of the primary particles. The simulation results show that the effect of surface energy is better described by the above mechanistic model than by the Weber number alone, as previously used to characterise the impact strength of agglomerates.

Modelling of cohesive granular flow in powder processing

Several polymer processing operations such as rotational moulding and powder coating involve avalanching flow of granular materials. The objective of this work is to develop a model and simulate mixing and segregation of granular particles during such flow. The movement of particles has been modelled using the Monte Carlo approach. Important forces such as interparticle friction, adhesion and electrostatic forces have been incorporated in the simulation. Simulation results show segregation in the bed as a result of size differences and are consistent with published results for cohesionless systems. The simulation allows for the quantitative evaluation of the effects of particle characteristics and material properties on the flow behaviour of granular materials during processing, which, otherwise, could not easily be determined experimentally. Results showed that the gradual development of cohesive forces inhibits the movement of particles and can lead to the development of reverse segregation patterns.

From colloidal dispersions to colloidal pastes through solid-liquid separation processes

Abstract Solid-liquid separation is an operation that starts with a dispersion of solid particles in a liquid and removes some of the liquid from the particles, producing a concentrated solid paste and a clean liquid phase. It is similar to thermodynamic processes where pressure is applied to a system in order to reduce its volume. In dispersions, the resistance to this osmotic compression depends on interactions between the dispersed particles. The first part of this work deals with dispersions of repelling particles, which are either silica nanoparticles or synthetic clay platelets, dispersed in aqueous solutions. In these conditions, each particle is surrounded by an ionic layer, which repels other ionic layers. This results in a structure with strong short-range order. At high particle volume fractions, the overlap of ionic layers generates large osmotic pressures; these pressures may be calculated, through the cell model, as the cost of reducing the volume of each cell. The variation of osmotic pressure with volume fraction is the equation of state of the dispersion. The second part of this work deals with dispersions of aggregated particles, which are silica nanoparticles, dispersed in water and flocculated by multivalent cations. This produces large bushy aggregates, with fractal structures that are maintained through interparticle surface-surface bonds. As the paste is submitted to osmotic pressures, small relative displacements of the aggregated particles lead to structural collapse. The final structure is made of a dense skeleton immersed in a nearly homogeneous matrix of aggregated particles. The variation of osmotic resistance with volume fraction is the compression law of the paste; it may be calculated through a numerical model that takes into account the noncentral interparticle forces. According to this model, the response of aggregated pastes to applied stress may be controlled through the manipulation of interparticle adhesion.

Evaluation of the flowability of surface-modified preparations by the measurement of the inter-particle adhesive force

In our previous study, we succeeded in manufacturing rapidly disintegrating tablets by a simple method of direct compression of a mixture of surface-modified ibuprofen using a high-speed agitating granulator. In direct compression, the flowability of the mixture of powdery ingredients is a very important qualitative factor such as for the consistency of the tabletting process and the uniformity of the drug contents. We evaluated the flowability of the surface-modified powders according to the angle of repose in the previous study. However, the angle of repose represents a characteristic of a powder as a mass of particles, and does not enable accurate evaluation of characteristics of individual particles. Also, as the flowability of powder is affected by the adhesive-agglomerative property of particles, the adhesive-agglomerative property is expected to affect the measured value of the angle of repose. In this study, we measured inter-particle adhesion force of surface-modified ibuprofen particles under various conditions, using a newly developed apparatus for the measurement of adhesion force among microparticles. Since the measurement of inter-particle adhesion force was found to enable not only more accurate and quantitative evaluation of the flowability than the measurement of the angle of repose but also an evaluation of the uniformity of surface modification, which was impossible according to the angle of repose, a new method was established for evaluating the flowability of surface-modified particles.

Interparticle contact forces in fine cohesive powders. Theory and experiments

AbstractThe flow of fine powders is strongly influenced by interparticle contact forces. Depending on the interparticle load force the contact behavior can be elastic, fully plastic or elasto‐plastic. We propose a model for the elasto‐plastic loading regime that yields a nonlinear dependence of the interparticle adhesion force on the interparticle load force. Interparticle forces are also obtained experimentally. Theoretical results are in reasonable agreement with experimental data.

Study on char deposition characteristics on the heat exchanger tube in a coal gasifier—relationship between char formation and deposition characteristics

In a coal gasifier and heat exchanger, adhered and deposited ash and char inhibit heat transfer making the stable operation of the gasifier difficult. Since heat transfer performance of the heat exchanger directly affects the output of Integrated Coal Gasification Combined Cycle plants, it is important to understand the effect of char on the characteristics of adhesion and deposition on the heat exchanger tube beforehand. However, very few studies have been conducted concerning the relationship between char's property and its adherability, which varies depending on the coal type and the operating conditions. In this study, we first conducted an analysis of the characteristics of heat transfer in the heat exchanger of the pilot plant gasifier to understand the effects of the properties of the char on the heat transfer in the heat exchanger. Using a bench-scale gasifier at CRIEPI, a transformation of the char morphology took place as the gasification reaction progressed. In the experiment, char from three distinct coal types was made to adhere on a plate and then allowed to re-entrain, so that the inter-particle adhesion strength of the char could be measured. Using the char from the same coals, deposition experiments on a horizontal tube placed in a drop tube furnace were also conducted to examine the mechanism of char adhesion on the heat exchanger tube.

Electrostatic Microencapsulation Technique for Producing Composite Particles

The electrostatic microencapsulation process and the various means of achieving such encapsulation for developing composite particles with two or more powders is briefly reviewed, with a particular focus on the dual-function sorbent/scintillation particles for use in radionuclide selective sensing. In preparing the composite particles, two different types of particles, an ion-exchange resin and scintillating microbeads, were used. The sorbent particles capture and preconcentrate the radionuclide of interest (e.g., 99 Tc) from solution. The scintillating microbeads are used to convert the energy of radioactive decay into that detectable light that can be deducted by a photomultiplier tube arrangement. To simulate the electrostatic microencapsulation of resin and scintillators, several surrogate materials such as acrylic powder (mean particle diameter, d 50 =23 µm), red toner (d 50 =16 µm), resin (d 50 =134 µm), and fluorescent latex spheres (d 50 =2 µm) were used. A microencapsulation tower was constructed to use corona guns operating at high voltages of opposite polarity to charge the "host" and "guest" particles. Experimental arrangements and test results are presented. The results show that if polymer particles are used and if one of the two powders is smaller than 10 µm in diameter, the electrostatic and van der Waals forces provide enough interparticle adhesion to bond the guest and host particles through plastic deformation. Interparticles adhesion between resin (d 50 =133 µm) and toner (d 50 =15 µm) showed that the composite particles were stable in water suspension. For larger particles, such as resin and scintillators, the use of a binding agent is necessary to form stable composite particles. electrostatic microencapsulation particle coating groundwater monitoring composite particles radionuclide detection

Solid Third Body Analysis Using a Discrete Approach: Influence of Adhesion and Particle Size on Macroscopic Properties

The dynamics of solid third bodies sheared between two rubbing bodies is far from being understood. Yet, this interface plays a prominent role in the velocity accommodation and in the load transmission. In the present paper, a simple model, which uses the Distinct Element Method, is operated in order to understand phenomena occurring in dry contact. In this model, the solid third body is considered as an aggregate of discrete interacting particles. Inter-particle forces are determined by force-displacement law and trajectories are calculated using the Newton’s second law. The global behavior of the simulated contact can be analyzed through the evolution versus time of characteristic parameters calculated by averaging over all the particles. The model is used to study the effect of particle size and inter-particle forces. The influence of particle size is studied in presence of repulsive force (based on Hertz contact model), and in presence of adhesive force (based on JKR contact model). Some promising results are highlighted. In particular, with the boundary conditions chosen in this paper, it is shown that the particle size has a weak influence when inter-particle forces are repulsive but has a dramatic influence when inter-particle adhesion is considered: solid third body goes from a quasi-fluid to a quasi-solid behavior.

Adhesion and aggregation of fine particles

The influence of interparticle adhesion on the formation and properties of aggregates is reviewed. Increasing the adhesive forces between particles in an aggregate should raise the strength of the aggregate because each particle contact then requires more force for fracture. However, it is well known experimentally that strongly adhesive fine particles lead to fluffy structures which contain fewer contacts and which are therefore weak, even though each individual particle contact may be stronger. Thus, adhesion can both increase and decrease the strength of aggregates, since the process of aggregation is inhibited by adhesion, whereas, the strength of the final aggregate is proportional to adhesion. This paper reviews the background to this problem and then gives two well-defined examples of opposing behaviours: one where there is an exact correlation between adhesion and aggregates in the case of red blood cells; the other where adhesion reduces aggregation in a computer simulation of adhering spheres.

Looking for self-organized critical behavior in avalanches of slightly cohesive powders.

We report results from a statistical analysis of avalanches of cohesive powders in a slowly rotated drum. Interparticle adhesion, which diminishes the effect of inertia and whose magnitude strongly fluctuates in a local scale, makes avalanches in slightly cohesive powders eligible for displaying self-organized criticality. However, the results show that avalanche sizes, time interval between avalanches, and maximum stable angle do not follow a power-law distribution. Otherwise, these parameters scale with powder cohesiveness.