Aggregation Of Primary Particles Research Articles

From Cauliflowers to Microspheres: Particle Growth Mechanism in Self-Stabilized Precipitation Polymerization.

Self-stabilized precipitation (2SP) polymerization enables the facile synthesis of uniform polymer spheres with sizes ranging from 100 nm to 3 μm, without the need for stabilizers or cross-linkers, even at high monomer concentrations of up to 30%. While previous studies have extensively explored the influence of reaction conditions on the size and uniformity of microsphere products, their impact on particle morphology has received less attention. In this work, we demonstrate that particle growth in 2SP polymerization primarily occurs via the adsorption of polymers from the solution onto the particle surface, leading to an increase in particle circularity and a smoother surface. The surface roughness of the particles is controlled by the aggregation of primary particles and the subsequent polymer adsorption process during the growth stage. Smooth microspheres are obtained under conditions of moderate monomer concentration, high initiator concentration, and elevated temperature. Our findings highlight the significance of the particle growth process in determining particle morphology in addition to the well-established role of polymer-solvent interactions during precipitation polymerization.

Cerium oxide nanoparticles formation and aggregation dynamics

The conventional synthesis of cerium oxide nanoparticles (CeNPs) relies on the use of trivalent cerium and an oxidizer to ensure a thorough hydrolysis. This study explores CeNPs formation via the hydrolysis of tetravalent cerium, specifically through the spontaneous reaction of ceric ammonium nitrate (Ce(NH4)2(NO3)6) under varied aging conditions, including adjustments in pH, concentration, and heating strategy. This research adopts a thermo-hydrolysis method to synthesize CeNPs with small primary particles (2–3 nm) to loose dendritic clusters (5–8 nm). Additionally, when the starting pH of the solution decreases below 0.8, it favors the formation of clusters with larger diameters (15–20 nm). Analyses using dynamic light scattering (DLS), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) indicate that CeNPs particle size in acidic environments is predominantly determined by the extent of primary particle aggregation, influenced by nitrate adsorption. Overall, our comprehensive analysis of the CeNPs evolution provides fundamental insights for the development of a size/structure controllable synthetic strategy for nanoparticle manufacturing.

Elemental composition of particles formed during welding with stick electrodes

The elemental composition of welding fume (WF) respirable particles formed in carbon and low-alloy steels welding with stick electrodes with rutile (ANO-4) and the basic (UONI 13/45) type of coating was studied by X-ray-spectral electron-probe microanalysis. In the range from 0.25 to 16 µm aerodynamic diameter, all WF particles were divided into three groups according to their iron content: low (Fe≤20%), medium (20%<Fe<40%) and high (Fe≥40%). Each group contains various amounts of elements originating from the welding materials, which reflects their formation mechanism. Their relative abundance in WF depends on the particles aerodynamic diameter, type of electrode coating and the arc power. Particles with the medium iron content (agglomerates of nanosized primary particles) formed the largest group (over 70%) of respirable WF particles. For each electrode type, their average elemental composition does not depend on the size of the agglomerates. The low iron particles contain various combinations of electrode coating elements, and the variety of combinations increases with their aerodynamic diameter grows.These WF particles can be identified as aggregates of primary particles agglomerates and microspatters formed by disintegration of the electrode coating. WF particles with a high iron content are formed by aggregation of molten electrode metal microdroplets and nanosized primary particles agglomerates. They mainly contain iron (60–70%)and oxygen (15–35%). Their average elemental composition does not depend on the size of the aggregates, type of electrode coating and the arc power. The obtained results can be useful for the analysis and assessment of health risks for welders.

Impact of synthesis route on the microstructure and energy-storage properties of (Bi1/2K1/2)TiO3–SrTiO3 relaxor ferroelectric ceramics: Solid-state and hydrothermal approaches

Bismuth potassium titanate (Bi1/2K1/2)TiO3-based relaxor ferroelectrics are promising materials for high-energy-density ceramic capacitors. Herein, we compare the microstructure and energy-storage properties of (Bi1/2K1/2)0.5Sr0.5TiO3 (BKST50) ceramics fabricated via two different routes: solid-state and hydrothermal reactions. A BKST50 fine powder composed of well-dispersed cubic nanoparticles was obtained via the hydrothermal reaction, whereas the conventional solid-state reaction resulted in the aggregation of primary particles. The grain size of the ceramics prepared from the hydrothermal powder could be controlled between 273 ± 24 and 936 ± 69 nm while maintaining a relative density of over 95% by simply varying the sintering temperature. On the other hand, ceramics prepared via the solid-state reaction could not be fully densified even at 1200°C (the highest tested sintering temperature). The hydrothermally derived ceramics withstood higher electric field owing to dense and fine-grained microstructure, leading to a high recoverable energy-storage density of 2.25 J cm−3 at 240 kV cm−1.

Anomalous small-angle X-ray scattering analyses on hierarchical structures of rubber-filler systems.

The hierarchical structures of poly(styrene-ran-butadiene) (SBR) rubber/carbon black (CB) systems vulcanized with sulfur and ZnO have been clarified using anomalous small-angle X-ray scattering (ASAXS) near the Zn absorption edge. In the case of SBR/CB systems vulcanized with peroxide, it has been found previously that the hierarchical structures formed by CB consist of aggregates of primary particles and agglomerates of those aggregates with mass-fractal dimensions. However, to date the hierarchical structures in SBR/CB systems vulcanized with sulfur and ZnO have not been well investigated, despite being commonly used. This is because the strong scattering contrast of Zn prevents the quantitative analyses of the hierarchical structures of CB using X-ray scattering. In this study, the effects of Zn on the scattering intensity were eliminated and the structure factors of CB in SBR/CB systems were obtained using the ASAXS method. By extrapolating to the zero volume fraction of CB, the particle structure factor of the CB aggregates was estimated and it was found that the CB aggregates consist of closely packed CB primary particles. The presence of large particles of ZnO and particles of ZnS on the order of 10 nm in size is confirmed.

Soot-particle core-shell and fractal structures from small-angle X-ray scattering measurements in a flame

We have characterized soot particles measured in situ in a laminar co-flow ethylene-air diffusion flame using small-angle X-ray scattering (SAXS). The analysis includes temperature measurements made with coherent anti-Stokes Raman spectroscopy (CARS) and complements soot volume-fraction and maturity measurements made with laser-induced incandescence (LII). We compared the results of fits to the SAXS measurements using a unified model and a fractal core-shell model. Power-law parameters yielded by the unified model indicate that aggregates of primary particles are in the mass-fractal regime, whereas the primary particles are in the surface-fractal regime in the middle of the flame. Higher and lower in the flame, the primary-particle power-law parameter approaches 4, suggesting smooth primary particles. These trends are consistent with fits using the fractal core-shell model, which indicate that particles have an established core-shell structure in the middle of the flame and are internally homogeneous at higher and lower heights in the flame. Primary-particle size distributions derived using the fractal core-shell model demonstrate excellent agreement with distributions inferred from transmission electron microscopy (TEM) images in the middle of the flame. Higher in the flame, a second small mode appears in the size distributions, suggesting particle fragmentation during oxidation. Surface oxidation would explain (1) aggregate fragmentation and (2) loss of core-shell structure leading to smoother primary-particle surfaces by removal of carbon overlayers. SAXS measurements are much more sensitive to incipient and young soot particles than LII and demonstrate significant volume fraction from particles low in the flame where the LII signal is negligible.

Synthesis and Characterization of Temperature-Responsive N-Cyanomethylacrylamide-Containing Diblock Copolymer Assemblies in Water.

We have previously demonstrated that poly(N-cyanomethylacrylamide) (PCMAm) exhibits a typical upper-critical solution temperature (UCST)-type transition, as long as the molar mass of the polymer is limited, which was made possible through the use of reversible addition-fragmentation chain transfer (RAFT) radical polymerization. In this research article, we use for the first time N-cyanomethylacrylamide (CMAm) in a typical aqueous dispersion polymerization conducted in the presence of poly(N,N-dimethylacrylamide) (PDMAm) macroRAFT agents. After assessing that well-defined PDMAm-b-PCMAm diblock copolymers were formed through this aqueous synthesis pathway, we characterized in depth the colloidal stability, morphology and temperature-responsiveness of the dispersions, notably using cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and turbidimetry. The combined analyses revealed that stable nanometric spheres, worms and vesicles could be prepared when the PDMAm block was sufficiently long. Concerning the thermoresponsiveness, only diblocks with a PCMAm block of a low degree of polymerization (DPn,PCMAm < 100) exhibited a UCST-type dissolution upon heating at low concentration. In contrast, for higher DPn,PCMAm, the diblock copolymer nano-objects did not disassemble. At sufficiently high temperatures, they rather exhibited a temperature-induced secondary aggregation of primary particles. In summary, we demonstrated that various morphologies of nano-objects could be obtained via a typical polymerization-induced self-assembly (PISA) process using PCMAm as the hydrophobic block. We believe that the development of this aqueous synthesis pathway of novel PCMAm-based thermoresponsive polymers will pave the way towards various applications, notably as thermoresponsive coatings and in the biomedical field.

Effects of sub-chronic oral exposure to pyrogenic synthetic amorphous silica (NM-203) in male and female Sprague-Dawley rats: focus on reproductive systems

Synthetic amorphous silica (SAS) consists of agglomerates and aggregates of primary particles in the nanorange (<100 nm) and it is the E551 authorized food additive. The potential risks for human health associated to dietary exposure to SAS are not completely assessed; in particular, data on male and female reproductive systems are lacking. A 90-day oral toxicity study with pyrogenic SAS nanomaterial NM-203 was carried out on the basis of the OECD test guideline 408 in the frame of the NANoREG project. Adult Sprague-Dawley rats of both sexes were orally treated for 90 days with 0, 2, 5, 10, 20 and 50 mg SAS/kg bw per day. Dose levels were selected to be as close as possible to the expected human exposure to food additive E551. The present paper provides specific information on potential effects on male and female reproductive systems, through the evaluation of serum biomarkers, sperm count, histopathological analysis of testis, epididymis, ovary and uterus and real-time PCR on uterus; potential genotoxic alterations were evaluated by comet assay on testis, sperm and ovary. NM-203 did not induce histophatological and genotoxic effects in male reproductive system. In female rats, ovary is not target of NM-203 and only tissue-specific effects on uterus were recorded up to 10 mg/kg bw per day. To our best knowledge, this is the first study providing data on male and female reproductive systems after long-term, repeated oral exposure at dose levels close to dietary human exposure, which identifies a limited concern only for female reproductive health.

Airborne Release Fractions from Surrogate Nuclear Waste Fires Containing Lanthanide Nitrates and Depleted Uranium Nitrate in 30% Tributyl Phosphate in Kerosene

Airborne contaminants from fires containing nuclear waste represent significant health hazards and shape the design and operation of nuclear facilities. Much of the data used to formulate DOE-HDBK-3010-94, “Airborne Release Fractions/Rates and Respirable Fractions for Nonreactor Nuclear Facilities,” from the U.S. Department of Energy, were taken over 40 years ago. The objectives of this study were to reproduce experiments from Pacific Northwest Laboratories conducted in June 1973 employing current aerosol measurement methods and instrumentation, develop an enhanced understanding of particulate formation and transport from fires containing nuclear waste, and provide modeling and experimental capabilities for updating current standards and practices in nuclear facilities. A special chamber was designed to conduct small fires containing 25 mL of flammable waste containing lutetium nitrate, ytterbium nitrate, or depleted uranium nitrate. Carbon soot aerosols showed aggregates of primary particles ranging from 20 to 60 nm in diameter. In scanning electron microscopy, ~200-nm spheroidal particles were also observed dispersed among the fractal aggregates. The 200-nm spherical particles were composed of metal phosphates. Airborne release fractions (ARFs) were characterized by leaching filter deposits and quantifying metal concentrations with mass spectrometry. The average mass-based ARF for 238U experiments was 1.0 × 10−3 with a standard deviation of 7.5 × 10−4. For the original experiments, DOE-HDBK-3010-94 states, “Uranium ARFs range from 2 × 10−4 to 3 × 10−3, an uncertainty of approximately an order of magnitude.” Thus, current measurements were consistent with DOE-HDBK-3010-94 values. ARF values for lutetium and ytterbium were approximately one to two orders of magnitude lower than 238U. Metal nitrate solubility may have varied with elemental composition and temperature, thereby affecting ARF values for uranium surrogates (Yb and Lu). In addition to ARF data, solution boiling temperatures and evaporation rates can also be deduced from experimental data.

Hazard identification of pyrogenic synthetic amorphous silica (NM-203) after sub-chronic oral exposure in rat: A multitarget approach

Food additive E551 consists of synthetic amorphous silica (SAS), comprising agglomerates and aggregates of primary particles in the nanorange (<100 nm), which potential nanospecific risks for humans associated to dietary exposure are not yet completely assessed. In NANoREG project, aim of the study was to identify potential hazards of pyrogenic SAS nanomaterial NM-203 by a 90-day oral toxicity study (OECD test guideline 408). Adult Sprague-Dawley rats of both sexes were orally treated with 0, 2, 5, 10, 20 and 50 mg SAS/kg bw per day; dose levels were selected to be as close as possible to E551 dietary exposure. Several endpoints were investigated, the whole integrative study is presented here along with the results of dispersion characterization, tissue distribution, general toxicity, blood/serum biomarkers, histopathological and immunotoxicity endpoints. No mortality, general toxicity and limited deposition in target tissues were observed. NM-203 affected liver and spleen in both sexes. Proposed NOAEL 5 mg/kg bw per day in male rats for enlarged sinusoids in liver. In female rats, TSH and creatinine levels were affected, proposed LOAEL 2 mg/kg bw per day. Overall, these data provide new insight for a comprehensive risk assessment of SAS exposure by the oral route.

Crystallization by particle attachment is a colloidal assembly process.

The nucleation of crystals has long been thought to occur through the stochastic association of ions, atoms or molecules to form critical nuclei, which will later grow out to crystals1. Only in the past decade has the awareness grown that crystallization can also proceed through the assembly of different types of building blocks2,3, including amorphous precursors4, primary particles5, prenucleation species6,7, dense liquid droplets8,9 or nanocrystals10. However, the forces that control these alternative pathways are still poorly understood. Here, we investigate the crystallization of magnetite (Fe3O4) through the formation and aggregation of primary particles and show that both the thermodynamics and the kinetics of the process can be described in terms of colloidal assembly. This model allows predicting the average crystal size at a given initial Fe concentration, thereby opening the way to the design of crystals with predefined sizes and properties.

In Ukrainian

The effect of the surface modification of fumed silica (SiO 2 ) with poly(dimethylsiloxane) (PDMS) on the rheological properties of its suspensions in vaseline oil has been studied in the current work. The composites with different polymer content (from 5 to 40 wt. %) were synthesized by using the method of liquid phase adsorption modification of silica with linear PDMS-1000 (molecular weight W m ≈ 7960, degree of polymerization d p = 105). It is shown that the structural and morphological characteristics of SiO 2 /PDMS composites are significantly different from those for the initial silica, and depend on the concentration of PDMS. In particular, the specific surface area of the composites, determined by the method of low-temperature desorption of argon, is inversely correlated with the content of polymer. The morphology of SiO 2 /PDMS composites and aggregation of primary particles of fumed silica strongly depends on the method of liquid phase adsorption modification: the initial fumed SiO 2 has a wider distribution of aggregates in size than that in polymer composites according to AFM data. Suspensions with a solid phase concentration (SiO 2 /PDMS) of 5 wt. % in a lyophilic dispersion medium (vaseline oil) demonstrate typical thixotropic behavior with a characteristic hysteresis loop on the rheogram, corresponding to increasing the destruction of the dispersion structure while stepwise shear rate increment and partial its restoration while subsequent stepwise shear rate decrement. Both the effective viscosity values corresponding to the maximum destruction of the suspension structure obtained at high shear rates and the effective viscosity values corresponding to partial restoration of the structure in the subsequent stepwise shear rate decrement mode decrease with increasing polymer content in the studied composites. However, it has been found that the initial values of the effective viscosity for the studied dispersions with a practically almost intact structure (at the lowest shear rates) are higher for dispersions of SiO 2 /PDMS composites with a low concentration of PDMS 10–20 wt. % compared with similar suspensions of the initial silica. A correlation between the effective viscosity and the specific surface area of composites is shown.

Heterogeneous reaction kinetics for oxidation and combustion of boron

Non-isothermal thermogravimetric (TG) analysis was used to study the oxidation kinetics for 95% pure amorphous boron powder. Commercial boron powders were pre-treated with acetonitrile and water to reduce the thickness of their initial hydrated oxide layer and to achieve a narrower particle size distribution. The samples were oxidized at different heating rates in an oxygen-argon gas mixture. Boron particles comprising aggregates of finer primary particles were modeled as spheres with porous shells and solid cores. Experimental TG traces were split among particles of different sizes. The split was based on the measured particle size distribution and accounted for the fraction of the measured mass gain assigned to different powder particle size bins using the core-shell particle model. Further, the reactive shell was assumed to consist of densely packed spherical primary particles. The reaction kinetics was quantified to obtain both activation energy and pre-factor describing oxidation of the primary particles. The early stage of oxidation is well described with the activation energy of 148 ± 6 kJ/mol. The oxidation rate of boron particle of any size can be obtained considering the rate of oxidation of the primary particle and the introduced core-shell particle structure. The obtained oxidation model was extrapolated to the range of temperatures typical of boron combustion. A change in the particle morphology occurring when boron melts and the aggregates of primary particles coalesce into a single droplet was accounted for. Burn times predicted at different temperatures were close to those measured for boron particles burning in room air. Comparisons suggest that the particle surface temperature varies in the range of 3500–4500 K, approaching boron boiling point. It is concluded that the same heterogeneous kinetics governs both low-temperature oxidation of boron when the oxide film thickness is small and its full-fledged combustion occurring at much higher temperatures.

Morphology and Raman spectra of aerodynamically classified soot samples

Abstract. Airborne soot is emitted from combustion processes as aggregates of primary particles. The size of the primary particles and the overall aggregate size control soot transport properties, and prior research shows that these parameters may be related to the soot nanostructure. In this work, a laminar, inverted nonpremixed burner has been used as a source of soot that is almost completely elemental carbon. The inverted burner was connected to an electrical low-pressure impactor, which collected particles on stages according to the aerodynamic diameter, from 0.03 to 10 µm. The morphology was analyzed using a transmission electron microscope followed by image processing to extract projected area and average primary particle size for each aggregate (approximately 1000 aggregates analyzed in total for the nine impactor stages). Carbon nanostructure was analyzed using a Raman spectrometer, and five vibrational bands (D4, D1, D3, G, and D2) were fitted to the spectra to obtain an estimate of the carbon disorder. The average primary particle diameter increases from 15 to 30 nm as the impactor stage aerodynamic diameter increases. The D1, D3, D2, and D4 bands decreased (relative to the G band) with the particle size, suggesting that the larger aggregates have larger graphitic domains.

Lift Off and Breakage of the Structure of Fractal - Like Aggregate

A new mathematical model of aggregate of N identical spherical particles is proposed. Model is able to simulate aggregates with different flexibility. Mutual forces in normal direction between primary particles are described by spring-mass system with parameters related to the material properties. The flexibility of aggregate structure is controlled by harmonic interaction functions. Developed model was used to study the lift-off phenomenon of aggregates in the fluid. Simulation were performed for numerically generated few population of fractal-like aggregates by Diffusion Limited Aggregation algorithm (DLA), which differ with number of primary particles in aggregate’s structure and fractal dimension. Numerical results show that aggregates with more open morphology with fractal dimension in the range of Df = 1.6 - 1.8 are lifted-off easier, compared to aggregates with compacted structure. The effectiveness of the lift off is enhanced by the deformation of the aggregate caused by the flexibility of its structure.

General method of specimen preparation for the quantitative microscopic characterization of nano- and microparticles

Nano-materials are a subject of current research and development due to their special characteristics, based on the size of their constituent particles. A major hurdle is the reliable analysis of the particle size and particle size distribution. This article introduces a new method of specimen preparation for the quantitative analysis of particle systems. Selected examples show the capabilities of computer-assisted image analysis of microscopic images. The problem of agglomeration and aggregation of primary particles is discussed and a strategy for calculating the primary particle distribution based on a mathematical model is presented. The successful, highly controlled deposition of deagglomerated nanoparticles on a microscope carrier enables a reliable particle size distribution measurement, even with strong interacting nanoparticles.

Single‐step process without organic template for the formation of zeolite A from RHA

AbstractDice‐shaped zeolite A (NaA) was prepared via direct dissolution of rice husk ash (96.5% SiO2) in the presence of NaOH and sodium aluminate solution by autoclave process at 90°C/6 h without using any templating agent. The prepared particles were characterized by DTA/TG, XRD, FTIR, N2 adsorption‐desorption physisorption analysis, FESEM, and TEM. XRD results confirmed crystallization of pure NaA zeolite phase. FTIR study shows the characteristic bands at 554 cm−1 for double 4 membered ring (D4R) of NaA zeolite. The total BET surface area of the product was found to be 31.6 m2 g−1. FESEM and TEM images show dice shaped NaA particles of size around 1 μm which is formed via oriented crystalline aggregation of primary particles (30‐40 nm). A tentative mechanism was proposed for the formation of NaA crystals through direct dissolution of rice husk ash. The synthesized NaA zeolite could be used for purification of alcohol and separation of toxic and radioactive ions from waste water via a cost‐effective process.

Effects of fuel structure on structural characteristics of soot aggregates

Two critical structural characteristics of soot aggregates, i.e., the primary particle size and the spatial arrangement of primary particles in aggregates, were investigated for laminar premixed burner-stabilized stagnation propane flames with and without toluene addition under similar temperature profiles. To link the structural characteristics of soot aggregates with the dynamic processes of soot formation, the particle mass and mobility size were measured by using a centrifugal particle mass analyzer and a scanning mobility particle sizer respectively, which enabled us to obtain the particle size distributions and the spatially fractal feature or mass-mobility exponents. The results show that for pure propane flames, the size growth is relatively rapid as indicated from the particle size distributions, and soot aggregates are highly branched with bigger primary particles. By contrast, for toluene-added propane flames, soot nucleation is faster, while the size growth of primary particles is suppressed, resulting in more compact arrangement of smaller primary particles in the soot aggregates. For both types of flames, with the increase of flame temperature, the process of soot inception is accelerated, but the production of soot is suppressed due to the thermodynamic reversibility of soot precursors at higher temperatures.

Experimental and numerical study of the evolution of soot primary particles in a diffusion flame

The evolution of primary soot particles is studied experimentally and numerically along the centreline of a co-flow laminar diffusion flame. Soot samples from a flame fueled with C2H4 are taken thermophoretically at different heights above the burner (HAB), their size and nano-structure are analysed through TEM. The experimental results suggest that after inception, the nascent soot particles coagulate and coalesce to form larger primary particles ( ∼ 5 to 15 nm). As these primary particles travel along the centreline, they grow mainly due coagulation and condensation and a layer of amorphous hydrocarbons (revealed by HRTEM) forms on their surface. This amorphous layer appears to promote the aggregation of primary particles to form fractal structures. Fast carbonisation of the amorphous layer leads to a graphitic-like shell around the particles. Further graphitization compacts the primary particles, resulting in a decrease of their size. Towards the flame tip the primary particles decrease in size due to rapid oxidation. A detailed population balance model is used to investigate the mechanisms that are important for prediction of primary particle size distributions. Suggestions are made regarding future model development efforts. Simulation results indicate that the primary particle size distributions are very sensitive to the parameterization of the coalescence and particle rounding processes. In contrast, the average primary particle size is less sensitive to these parameters. This demonstrates that achieving good predictions for the average primary particle size does not necessarily mean that the distribution has been accurately predicted.

Mechanisms behind overshoots in mean cluster size profiles in aggregation-breakup processes

Aggregation and breakup of small particles in stirred suspensions often shows an overshoot in the time evolution of the mean cluster size: Starting from a suspension of primary particles the mean cluster size first increases before going through a maximum beyond which a slow relaxation sets in. Such behavior was observed in various systems, including polymeric latices, inorganic colloids, asphaltenes, proteins, and, as shown by independent experiments in this work, in the flocculation of microalgae. This work aims at investigating possible mechanism to explain this phenomenon using detailed population balance modeling that incorporates refined rate models for aggregation and breakup of small particles in turbulence. Four mechanisms are considered: (1) restructuring, (2) decay of aggregate strength, (3) deposition of large clusters, and (4) primary particle aggregation where only aggregation events between clusters and primary particles are permitted. We show that all four mechanisms can lead to an overshoot in the mean size profile, while in contrast, aggregation and breakup alone lead to a monotonic, “S”-shaped size evolution profile. In order to distinguish between the different mechanisms simple protocols based on variations of the shear rate during the aggregation-breakup process are proposed.