The structure and strength of depletion force induced particle aggregates

Aggregating fine particulate matter is common practice in many industrial solid-liquid separation processes. Data obtained in this work on dilute aqueous dispersions of model colloidal polystyrene latex spheres indicate that depletion flocculation, which uses non-adsorbing polymer, can yield very compact aggregates. Flocculation of the negatively charged latex particles was induced by the addition of a poly(acrylic acid) at pH 10. The structural compactness of the latex flocs formed in the dilute dispersions was characterised using small-angle static light scattering in terms of mass fractal dimensions. Rheological measurements on the concentrated latex dispersions in the presence of the non-adsorbing polyacid showed Bingham yield stress behaviour. Both the compactness and strength of the latex flocs were found to be significantly dependent upon the level of the polyacid, as well as the concentration of the initial particles. In particular, as the level of the polyacid was raised the floc compactness decreased, whereas its strength increased. They were both seen to level off at high polymer concentrations. Atomic force microscopy measurements were made at varying concentrations of the polyacid to provide a qualitative explanation of the observed floc structural behaviour of the dilute dispersions. By combining the fractal dimension and the Bingham yield stress we were also able to estimate the energy required to separate the flocs into single units in the concentrated dispersions. It was concluded that the interparticle interaction energy is the key to understanding the dependence of both the floc structure and strength on the polymer concentration.     

Using polymer adsorption data and a population balance model to estimate how polymer dosage affects the flocculation of oil sands tailings

Population balance models can describe how particles aggregate and fragment during the flocculation of mineral tailings. We used a new method to estimate some of the parameters in a population balance model describing the flocculation of oil sands mature fine tailings with poly(vinyl benzyl trimethylammonium chloride). Differently from previous population balance models, the polymer adsorption data onto the particles suspended in the tailings were used to estimate a fundamental parameter relating polymer dosage to the mean diameter of the aggregates formed during flocculation. The model could predict the flocculation behaviour of three polymer samples with different molecular weights. This model is another step toward a quantitative understanding of how polymer properties affect the flocculation of mineral tailings.     

In situ line measurement of mean aggregate size and fractal dimension along the flame axis by planar laser light scattering

An in situ laser light scattering method has been developed for line measurement of aggregate size and morphology. Planar multiangular light scattering measurements were interpreted by using Rayleigh–Debye–Gans scattering theory for fractal aggregates to simultaneously obtain the mean radius of gyration and the fractal dimension along the flame axis, which are the parameters characterizing size and morphology of aggregates. The developed system was applied to study the evolution of silica aggregates produced in a methane/air premixed flat flame. To confirm the suitability of the present method as a line measurement of the growth of aggregates, we compared the results with those obtained by conventional point measurements and it was found that the two results agreed well each other. The comparison of size parameters has been also made with those obtained by thermophoretic sampling and TEM observation.     

Controlled aggregation of colloidal particles for toner applications

Micrometer‐sized particles were formed by controlled aggregation of carboxylated polystyrene colloidal spheres having a mean diameter of about 200 nm with a commercial cationic coagulant. To identify the parameters governing the size and structure of the aggregates, the aggregate size distribution was studied over a period of time with dynamic light scattering. The effect of the particle concentration, pH, and ionic strength on the aggregation behavior was investigated. The coagulant concentration used for present studies was 5 parts per hundred on the basis of the polystyrene particles and the particle concentrations used were 10–15%. The particle size distribution for the latex suspensions was also investigated with a 10% aluminum sulfate [Al₂(SO4)₃·14H₂O] solution as a model coagulant. With the commercial coagulant, aggregation was found to be slower at lower pH than at neutral pH. At pH 6, the particles started to aggregate within minutes and form aggregates of about 1000 nm. We expected that lowering the pH would reduce interparticle repulsive forces and enhance the collision efficiency. However, at a lower pH of 2, the aggregation process slowed down. Increasing the ionic strength at neutral pH led to a broader aggregate size distribution, and the population of larger aggregates increased. The suspensions with the model coagulant showed similar behavior. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Characterization and self-assembly of poly[sodium N-(11-acrylamidoundecanoyl)-l-alaninate] in water

A vesicle-forming amino acid derivatized surfactant, sodium N-(11-acrylamidoundecanoyl)-l-alaninate was polymerized to obtain a polysoap. The average molecular weight of the polysoap was determined by gel permeation chromatography. Rheological measurements suggested that the aqueous solution of the polysoap behaves as a non-Newtonian fluid. Fluorescence probe studies using pyrene as probe molecule indicated hydrophobic domain(s) (intra-chain aggregate) formation within a polymer chain. It has been found that the vesicular structure formed by surfactant monomers are retained after polymerization. The polymer forms inter-chain aggregates in concentrated solution. The dependence of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene on temperature, pH, and urea concentration suggested strong inter-chain interactions of the polysoap. The pK_a of the surfactant units of the polymer chain and the phase transition temperature were determined. Dynamic light scattering measurements were performed to determine the mean size of the aggregates. The size of the aggregates was found to increase with the increase in polymer concentration also suggesting formation of inter-chain aggregates. The transmission electron micrographs revealed closed vesicular structures in water.     

Investigation of Absorption and Scattering Properties of Soot Aggregates of Different Fractal Dimension at 532 nm Using RDG and GMM

Radiative properties of numerically generated fractal soot aggregates of different fractal dimensions were studied using the numerically accurate generalized Mie-solution method (GMM) and the Rayleigh-Debye-Gans (RDG) approximate theory. Fractal aggregates of identical prefactor but different fractal dimensions, namely, 1.4, 1.78, and 2.1, were generated numerically using a tunable algorithm of cluster–cluster aggregation for aggregates containing up to 800 primary particles. Radiative properties of these aggregates were calculated at a wavelength of 532 nm assuming a soot refractive index of 1.6 + 0.6i. Four commonly used structure factors in the RDG approximation were used to investigate the effect of structure factor on the differential and total scattering cross-sections and the asymmetry factor. The differential and total scattering properties calculated using the RDG approximation become increasingly sensitive to the structure factor with increasing the fractal dimension. Primary particle interactions are the fundamental mechanism for the aggregate absorption enhancement for small aggregates and the shielding effect for larger aggregates. The extent of these two competing factors is dependent on the fractal dimension and aggregate size. RDG reasonably predicts the effect of fractal dimension on the scattering properties, but fails to account for the effect of aggregation or fractal morphology on the absorption property of fractal soot aggregates, though the error is in general less than 15%.

Copyright 2013 American Association for Aerosol Research     

Poly(methyl methacrylate) and polyacrylonitrile dispersions stabilized by gelatin

Methyl methacrylate was polymerized in an aqueous medium in the presence of gelatin using potassium persulfate as initiator. The dispersion mode of polymerization, when the monomer is completely miscible with water, was investigated and compared with an emulsion process, which proceeds at higher monomer concentration. Spherical and relatively uniform polymer particles were formed. Macroscopic precipitation of polymer is prevented by combination of the steric stabilization by grafted gelatin and of repulsive electrostatic interactions from the initiator residues attached to the particle surface. Static and dynamic light scattering have been used to determine the molar mass (molar mass of the whole dispersion particle, M_wD ～ 10⁸-10⁹ g mol^?1) and hydrodynamic radius (R_hD ～ 50-120 nm) of the particles. The number of particles per unit volume does not depend on overall monomer concentration, and it is higher, and therefore the particle size is smaller, than that observed for the soapless emulsion polymerization. The addition of gelatin may be thus used to modify the particle size. Acrylonitrile dispersions were prepared under similar conditions. Unlike methyl methacrylate, this monomer does not swell the polymer particles. While poly(methyl methacrylate) particles are spherical and relatively uniform, the polyacrylonitrile dispersions consist of polydisperse aggregates of tiny polymer particles.     

Analysis of fractal particles from diesel exhaust using a scanning-mobility particle sizer and laser-induced incandescence

The emission regulations for diesel particulate matter (PM) are becoming increasingly strict. The focus of regulations is turning to reducing the number of nanosized particles as well as the total mass. A more precise measurement technique for particle numbers and mass must be developed to meet these new regulations. In this study, a new method for estimating the mass weighted size distribution of diesel PM was investigated by measuring the size of primary particles and the number concentration distribution of particle aggregates. Time-resolved laser-induced incandescence was used for primary particle size measurement and a scanning-mobility particle sizer was used to quantify the number concentration of aggregates. The results from these two conventional measurement techniques were combined using fractal analysis formulas to relate the electrical mobility diameter, the number of primary particles per aggregate, primary particle size, and fractal dimension. This method, applied to single-cylinder diesel engine exhaust with various engine loads and injection pressures, successfully estimated the mass weighted size distribution of particle aggregates. The procedure is very simple and the estimations are comparable with those based on effective density, making this method a useful and reliable tool for estimating mass weighted size distribution of fractal particles such as diesel PM.     

Transformation between flocs and aggregates in the conditioned sludge suspensions during a rotational test using a torque rheometer

Floccky tester is a typical torque rheometer used to study flocculants and flocculation characteristics of wastewater sludges. This study presents the variations in rheological, geometric characteristics of conditioned anaerobic digested sludge (ADS) to analyze the transformation between flocs and aggregates during a rotational test with a floccky rheometer. The results revealed that the network strength and resistance to shear reach their maximum values at the optimum dose of polymer zetag7557 on the basis of the area under torque-time curves of ADS. Large aggregates, even big sludge lumps, were observed as soon as the optimum polymer dose was injected into the ADS suspensions, and they continued to grow in average size as the shear duration was prolonged. Afterwards, these aggregates broke into smaller ones or were directly transformed into smaller and smaller flocs with the extension of shear application. The corresponding torque values in the torque-time curve showed similar behavior to the above variations. During the above process, the strongest network formed in conditioned ADS suspensions in which large aggregates were generated, while the ADS suspension with the largest single aggregate or the highest average size of aggregates/flocs did not assure the strongest network. The climax of fractal dimensions for conditioned ADS flocs were not coincident with that of the peak height in the torque-time curves for these ADS suspensions.     

Sludge Dewatering and Aggregate Formation Effects through Taylor Vortex Assisted Flocculation

Abstract

For polymer induced flocculation processes, the effects of flow patterns in a gap of a conical stirrer on aggregate formation and subsequent sludge dewatering efficiency were analysed. Different flow regimes were identified by lab scale investigations with model substances and summarized in a Ta and Re number plane. An enhancement of sludge dewaterability for polymer induced flocculation processes was identified through post‐treatment of flocculated sludge aggregates by the specific flow pattern of stable and wavy Taylor vortices. Photo‐optical image analysis of flocculated aggregates shows a clear change of aggregate size distribution with less small particles during aggregate forming by Taylor vortices compared to classical flocculation procedure by stirrer. Results from technical scale dewatering analyses confirmed enhancement of sludge dewatering efficiency for six different dewatering machines using the identified wavy and stable Taylor vortex flow pattern regime.     

Soot aggregates,superaggregates and gel-like networks in laminar diffusion flames

We show that soot formed in laminar diffusion flames of heavily sooting fuels evolves through four distinct growth stages which give rise to four distinct aggregate fractal morphologies. Each successive stage grows upon the previous stage; hence aggregates of one morphology, parameterized by a fractal dimension, form larger superaggregates or gels of a different morphology with a different fractal dimension. These results were inferred from large and small angle static light scattering from the flames, microphotography of the flames, and analysis of soot sampled from the flames. These results and the analysis were substantiated by comparison to computer simulations. The growth stages occur approximately over four successive orders of magnitude in aggregate size. Each growth stage is a fundamental physical process: Stages 1 and 3 are diffusion limited cluster aggregation in three and two dimensions, respectively; Stages 2 and 4 are percolation in three and two dimension, respectively.     

Stability of particle suspensions after fine grinding

The objective of this work was to investigate the physical stability of sub-micron particle suspensions of organic crystalline food compounds after grinding. A Dynomill ball mill was used in combination with zirconium oxide grinding medium beads. The organic product was a poor water soluble product. During grinding the average particle diameter of the particulate product was reduced to a minimum value in the sub-micron range. Forward light scattering was used to analyze the particle size distribution. Dynamic light scattering measurements, on the other hand, showed that there were aggregates present after grinding. The difference in the obtained particle size distributions using both techniques was related to the shear in the measurement device, i.e. in the laser diffraction measurement the shear was higher than in the dynamic light scattering device. Thus in the laser diffraction measurement the aggregates were broken up by shear, while this was not the case in the dynamic light scattering measurements. The difference in the measurements showed that the particles formed aggregates at low to zero shear.The aggregation behaviour of the particles was studied by measuring the sedimentation behaviour of the particles suspension at various pH values. The impact of the pH on the aggregation rate was explained by the zeta potential of the particles. The suspensions were less stable near the iso-electric point of the particles.     

Characterization of Soot Aerosol Produced from Combustion of Propane in a Shock Tube

The knowledge of yields and properties of soot from combustion of hydrocarbon fuels is crucial for accurate evaluation of the impacts of primary aerosols on air quality and climate. This study presents measurements of soot generated from combustion of propane in a shock tube, using independently adjustable fuel equivalence ratio (φ), temperature, and pressure. The characterization of soot yields inside the shock tube by in situ laser extinction is complemented with a set of comprehensive measurements of soot transferred into a fluoropolymer chamber, including particle size distributions, elemental carbon (EC) mass fraction, effective density, mass fractal dimension (Dfm), dynamic shape factor (χ), and optical properties. The properties of soot particles and the soot yield are sensitive to combustion conditions and the duration of the combustion experiment. High-temperature combustion with φ = 2.5 produces small fractal (Dfm = 2) soot particles composed mainly of EC (up to 90%), at a low mass yield. Particles from lower temperature combustion contain a significant fraction of organic material (～50%). Using rich fuel mixtures (φ = 4.0 and 8.0) significantly increases particle size and soot mass yield. At lower temperatures, compact (Dfm = 3) and nearly spherical (χ = 1.1) aggregates with high organic content are formed, whereas at higher temperatures, the particles are fractal and closely resemble those obtained using φ = 2.5. Single scattering albedo (SSA) varies from 0.15 for fractal particles to 0.75 for compact particles. For soot generated at high equivalence ratios, SSA can be used as a proxy for particle morphology and EC content.

Copyright 2012 American Association for Aerosol Research     

Evolution of the fractal dimension for simultaneous coagulation and sintering

Simulation results on the evolution of aggregate structure in aerosol processes with coagulation and sintering as the dominant mechanisms are presented. A model for simulation of the three-dimensional morphology of nano-structured aggregates formed by concurrent coagulation and sintering is applied. The model is based on a stochastic diffusion controlled cluster-cluster aggregation algorithm and sintering is modeled as a successive overlapping of spherical primary particles, which are allowed to grow in order to maintain mass conservation. This leads to computer simulated structured aggregates which are then subject to evaluation. Two different methods to determine the fractal dimension are presented which give comparable results. It is shown that even very small particles show the same fractal behavior. Furthermore, equilibrium structures assuming a constant ratio of the characteristic collision time to the characteristic fusion time are considered as well as the kinetics of structural changes due to a change in the ambient conditions.     

基于介观尺度模拟的污泥絮凝形态及机理

用介观动力学的方法对胶体颗粒与高分子链的絮凝过程进行了模型研究,并根据模型结果对污泥和阳离子聚丙烯酰胺的絮凝过程和污泥脱水机理进行有针对性的探讨。通过建立粗粒化模型和经验势函数力场有效模拟了负电性的胶体颗粒与阳离子型高分子链的絮凝过程;模拟得出了絮凝的外观形态,证明了高分子链对胶体颗粒的桥接作用以及表面电荷的中和作用对絮凝的影响;并对絮凝反应过程的体系势能变化进行了研究。对实际污泥胶体颗粒和阳离子聚丙烯酰胺溶液拍摄了电镜图片并观察了其吸附过程;最后研究了高分子链离子度对溶液体系絮凝的影响,验证了离子度的增加有利于絮凝体的形成,但同时也由于絮凝体结构的复杂而对污泥脱水造成了一定的不利影响。     

Mesoglobules of thermoresponsive polymers in dilute aqueous solutions above the LCST

The colloidal stability and characteristics of particles formed by homopolymers of poly(N-vinyl caprolactam), poly(N-isopropyl acrylamide) and poly(vinyl methyl ether) in dilute aqueous solutions above the lower critical solution temperature, LCST, was followed by means of dynamic and static light scattering. Depending on the solution concentration, the homopolymers precipitate or form stable dispersions of monodisperse spherical particles. To obtain colloidally stable aggregates, also called mesoglobules, no stabilising agent was added. The stability of the mesoglobules upon time and dilution at temperatures above the LCST suggests that the particle surfaces possess a hydrophilic character. The size of the formed particles depends on the concentration and the heating rate of the solutions. However, internal structure and shape of mesoglobules are affected neither by the way, how the mesoglobules were prepared, nor by molar mass of individual macromolecules. Mesoglobules of PNIPAM obey the M∝R^2.7 scaling law. Origin of stability of the dispersions vs. expected precipitation is discussed.     

Impact of Aggregate Size and Structure on Biosolids Settleability

Settling is an important solid–liquid separation process in wastewater treatment. While solids concentration is an important factor influencing settling velocity of particles, other solids characteristics including particle size, shape, and structure also play an important role in settling rate. The “compactness” of bacterial aggregates in particular is recognized to exert a great influence on solid phase dynamic behavior since it has a substantial effect on fluid flow through the aggregate, which, in turn, affects the particles buoyancy. With the recognition that biosolids structure can be described by fractal methods, we now have a convenient means of parameterizing aggregate “compactness.” In this article, we examine the fractal nature of bacterial aggregates (which are the main component of the solids in wastewater treatment processes) using small angle light-scattering methods and assess the impact of aggregate compactness (as described by fractal dimension) on settling velocity of both single aggregates and large aggregate clusters. The results indicate that settling velocity is strongly dependent on both size and aggregate structure, with the larger and less compact flocs settling more quickly as a result of the significant extent of flow through the bacterial assemblages.     

Evaluation of flocs resistance and reflocculation capacity using the LDS technique

In a previous paper we have shown the added value of using LDS to monitor flocculation. It can supply, simultaneously, information on flocs size and structure and enlighten flocculation kinetics and mechanisms. In this paper, LDS is applied to study deflocculation and reflocculation processes of precipitated calcium carbonate (PCC) induced by cationic polyacrylamides, when different types of shear forces are applied. LDS can detect the influence of polymer characteristics and concentration as well as of the type of shearing, on flocs resistance and reflocculation degree, which depend on flocs structure and on the type of bonds between particles. As expected, flocs formed by bridging mechanism reflocculate with difficulty while flocs formed by patching reflocculate to a higher degree. Flocs resulting from reflocculation are more compact than the original ones, as assessed by the mass fractal dimension. Reflocculation is also lower when the flocs are submitted to superficial shearing than when they are submitted to sonication. Shearing induced by sonication is sufficient to break down the flocs in many fragments while the increase of pump speed only detaches particles by erosion, at the flocs surface, where bonds are weaker. Results prove that LDS is useful to monitor deflocculation and reflocculation processes and to predict floc resistance under different conditions. Moreover, the whole study demonstrates the benefit of using LDS for a complete evaluation of flocculants performance in the different stages of flocculation: aggregation, stabilization, deflocculation and reflocculation.     

Comparison of two cationic polymeric flocculant architectures on the destabilization of negatively charged latex suspensions

Flocculation studies between cationic polymers and oppositely charged colloidal particles are reported in which both flocculation kinetics and floc structures are systematically investigated. The flocculation rate constant, stability ratio and kinetics laws are experimentally determined using particle counting for two polymer architectures; a cationic linear polymer and a two-branched polymer. Comparisons are also made using NaCl at different ionic concentrations for the destabilization of the colloidal particles. Detailed measurements of electrophoretic mobility and kinetics rate constants on varying the polymer dosage are reported. Results suggest that the polymer architecture plays important roles on the polymer dosage for the rapid destabilization of the colloidal suspension. The branched polymer at optimal dosage exhibits the highest flocculation rate constant, whereas on the other hand, the linear polymer concentration range of flocculation is larger. In both cases, polymer flocculation is more efficient by a factor of 5-6 than charge screening effects due to the presence of salt. Analysis of the stability ratio indicates that tele-bridging flocculation and electrostatic forces dictate the stability of the charged latex particle suspension. It is shown that the fractal concepts which are valid for aggregation processes are also applicable here and branched polymers as well as linear polymers yield to the formation of compact flocs in comparison to those obtained with salt.