Changes in the nanoparticle aggregation rate due to the additional effect of electrostatic and magnetic forces on mass transport coefficients

The need may arise to be able to simulate the migration of groundwater nanoparticles through the ground. Transportation velocities of nanoparticles are different from that of water and depend on many processes that occur during migration. Unstable nanoparticles, such as zero-valent iron nanoparticles, are especially slowed down by aggregation between them. The aggregation occurs when attracting forces outweigh repulsive forces between the particles. In the case of iron nanoparticles that are used for remediation, magnetic forces between particles contribute to attractive forces and nanoparticles aggregate rapidly. This paper describes the addition of attractive magnetic forces and repulsive electrostatic forces between particles (by ‘particle’, we mean both single nanoparticles and created aggregates) into a basic model of aggregation which is commonly used. This model is created on the basis of the flow of particles in the proximity of observed particles that gives the rate of aggregation of the observed particle. By using a limit distance that has been described in our previous work, the flow of particles around one particle is observed in larger spacing between the particles. Attractive magnetic forces between particles draw the particles into closer proximity and result in aggregation. This model fits more closely with rapid aggregation which occurs between magnetic nanoparticles.     

Brownian dynamics simulation of the aggregation of submicron particles in static gas

A Brownian dynamics simulation was conducted to investigate the formation of aggregates that are composed of submicron particles such as soot. Three models were considered for aggregation: a diffusion-limited aggregation model, in which an aggregate grows around a fixed particle; a particle–cluster aggregation model, in which a single aggregate grows by collisions between particles and the aggregate; and a cluster–cluster aggregation (CCA) model, in which many particles and aggregates form multiple aggregates. A comparison of the three aggregation models showed that the CCA model resulted in a soot-like branching shape. The aggregation was investigated by employing the CCA model; it was determined that increase in gas temperature affected the shielding effect of the aggregate branch by changing the displacement and velocity of Brownian particles. Furthermore, these simulations demonstrated that the size and aspect ratio of the field and the particle density also affected aggregation shape.     

Magnetite nanoparticles: Electrochemical synthesis and characterization

Magnetite (Fe₃O₄) nanoparticles (NP) with sizes between 20 and 30 nm have been obtained by Fe electrooxidation in the presence of an amine surfactant, which acted as a supporting electrolyte and coating agent, controlling particle size and aggregation during the synthesis. The effect of different parameters on the nature and size of the particles as well as the mechanism of formation of the particles have been studied by different techniques. It was concluded that, under the electrochemical conditions used in this work, the NP mean size was found to be constant at around 20 nm when the electrooxidation current density is increased from 10 to 200 mA cm⁻². However, when the potential is over 6 V, particle size decreases from 30 to 20 nm and metallic iron appears as an impurity. The mechanism of particles formation has being clarified and the critical effect of the distance between electrodes for obtaining magnetic iron oxide nanoparticles has been understood. Finally, the presence of an electrostatic adsorbed surfactant coating the particles allows the functionalization of the particles easily by exchange reaction with biomolecules of interest, which makes this material very promising for future application in biotechnology.     

Synthesis and characterization of carbon-encapsulated iron/iron carbide nanoparticles by a detonation method

Carbon-encapsulated iron-based nanoparticles with a core-shell structure were produced by detonation decomposition of explosive mixture precursors containing iron ion components. The size and magnetic properties of the as-prepared composite particles were revealed by X-ray powder diffraction, transmission electron microscopy and magnetic measurements. Results showed that the different sizes of the iron-based nanocrystal core and the thickness of the carbon shell could be yielded by adjusting the component materials of the explosive precursors during the course of these detonation chemical reactions. The composite particles had a body-centered cubic iron or iron carbide core with a coating of graphitic or amorphous carbon layers. Magnetic measurements indicated these composite nanoparticles were magnetic at the room temperature, with some variation in the values of saturation magnetization, remanences and coercive forces that depend on the size and grain composition.     

Variable blocking temperature of a porous silicon/Fe3O4 composite due to different interactions of the magnetic nanoparticles

ABSTRACT: In the frame of this work, the aim was to create a superparamagnetic nanocomposite system with a maximized magnetic moment when magnetized by an external field and a blocking temperature far below room temperature. For this purpose, iron oxide nanoparticles of 3.8-, 5- and 8-nm size have been infiltrated into the pores of porous silicon. To fabricate tailored magnetic properties of the system, the particle size and the magnetic interactions among the particles play a crucial role. Different concentrations of the particles dispersed in hexane have been used for the infiltration to vary the blocking temperature TB, which indicates the transition between the superparamagnetic behavior and blocked state. TB is not only dependent on the particle size but also on the magnetic interactions between them, which can be varied by the particle-particle distance. Thus, a modification of the pore loading on the one hand and of the porous silicon morphology on the other hand results in a composite material with a desired blocking temperature. Because both materials, the mesoporous silicon matrices as well as the Fe3O4 nanoparticles, offer low toxicity, the system is a promising candidate for biomedical applications.     

RANDOM AGGREGATION OF UNIVARIATE AND MULTIVARIATE LINEAR PROCESSES

Abstract. The paper is devoted to random aggregation of multivariate autoregressive moving-average (ARMA) processes. We derive second-order characteristics of random aggregate models. We show that random aggregation preserves the ARMA structure. Moreover, we specify a functional relation between the initial model poles and aggregate ones. We then examine the case of univariate ARMA processes. Theorem 4 shows that, if the initial process is ARMA( p, q ), the random aggregate process is an ARMA( p*, q* ) model with p* at most equal to p ; * depends, among other things, on the sampling distribution L . This theorem generalizes the well-known results on the topic of time interval aggregation without overlapping.     

Magnetically driven hydrodynamic interactions of magnetic and non-magnetic particles

The influence of magnetic forces on (i) the motion of magnetisable particles in a non-magnetisable fluid and (ii) the motion of non-magnetisable particles in a magnetisable fluid was investigated. A non-uniform magnetic field was used to induce a strong magnetic field gradient. The study was conducted at low particle Reynolds numbers allowing independent evaluation of the hydrodynamic forces along the tangential and normal directions, which in turn were used to deduce the strength and variation of the magnetic forces. Two iron spheres placed in a non-magnetisable fluid showed strong mutual attraction in the normal direction, consistent with an inverse distance to the power four law. The velocity of each sphere in the tangential direction prior to aggregation was constant. Upon aggregation, the velocity of the dumbbell in the tangential direction was approximately two times higher. This sudden increase in velocity was attributed to an increase in the magnetisation following aggregation of the two spheres. It was concluded that a dumbbell has a larger concentration of matter along the direction of the magnetic field and hence, with respect to motion in the tangential direction, a higher magnetisation. For the two acrylic spheres in a magnetised fluid the attractive force was found to be negligible, most probably because of the low magnetisation of the paramagnetic salt solution used. The two spheres did migrate towards each other because of the local field gradients that develop in the normal direction. Interestingly, the spheres developed a constant velocity prior to aggregation, which was also maintained after aggregation. Two acrylic spheres glued together also travelled in the tangential direction at the velocity observed for the individual spheres. It was concluded that there was no change in the magnetisation of the fluid following the aggregation of the spheres.     

Effect of characteristics of media on cobalt and iron nanoparticles prepared by arc discharge method

Above a critical size, cobalt (Co) and iron (Fe) nanoparticles (NPs) aggregate due to magnetic dipole–dipole interaction into chains. In this paper, the effect of liquid environment on nucleation, growth and aggregation of Fe and Co nanoparticles is reported. The iron NPs were prepared by novel arc discharge method in ethylene glycol and 1-propanol and the cobalt NPs were prepared by that method in five liquid environments: ethylene glycol, 1-propanol, ethanol, methanol, and deionized water. SEM and FE-SEM observations were employed for morphology of the NPs. Meanwhile, in this case, the effect of media was discussed by considering the characteristics of the solution such as size, shape and dipole moment of its molecules. Preparation of Fe results shows that compared to 1-propanol a denser agglomeration can be seen among the NPs dispersed in ethylene glycol and the NPs possess a broader size distribution. Preparation of Co NPs in five solvent results if a solvent has large molecules with small dipole moment then the NPs will be larger in size. When three different amounts of surfactant PVP is added to prepared Fe nanofluid, it can be concluded that a low PVP content leads to more uniform NPs with rather clear boundaries.     

Aggregation mechanism of fine fly ash particles in uniform magnetic field

Aggregation of fly ash particles with size range of 0.023-9.314 μm in a uniform magnetic field was investigated for removing them. A binary collision-aggregation model evaluating particle aggregation coefficient was developed. Based on the model, particle removal efficiency was calculated by solving the General Dynamic Equation. The comparison was done between the calculated and experimental data. The modeling aggregation coefficient shows that the aggregation coefficient increases with particle size, and the bigger the size difference between two particles is, the more strongly the gravity difference promotes aggregation. For the mid-sized particles, their removal efficiencies are higher than those of the smaller and bigger ones. The effect of the magnetic flux density on total particle removal efficiency is similar to that on aggregation coefficient. Before particles are saturatedly magnetized, their total removal efficiency increases with an increase in the magnetic flux density. Higher removal efficiency can also be caused by prolonging the particle residence time in the magnetic field or increasing their mass concentration. The particle number median diameter decreases with an increase in the total removal efficiency. Calculation results are found to coincide essentially with the experimental data.     

Aggregation behavior of nanoparticles in fluidized beds

The fluidization behavior of fumed silica, zirconia, and iron oxide nanopowders was studied at atmospheric and reduced pressures. Using a high-speed laser imaging system, the characteristics of fluidized aggregates of nanoparticles were studied in real time. The effect of different particle interactions such as London-van der Waals, liquid bridging and electrostatic on different fluidization parameters was studied at atmospheric pressure. The reduction of interparticle forces resulted in a reduced aggregate size and minimum fluidization velocity (U_mf) and an increased bed expansion. Nanoparticles were also fluidized at reduced pressure (∼ 16 Pa) with vibration to study the effect of low pressure on the minimum fluidization velocity. Aggregate properties (size, density) instead of primary nanoparticle properties were found to govern the minimum fluidization velocity and expansion of the fluidized bed. An important consideration is the relative strength of intra-aggregate interparticle forces (forces within the aggregate holding nanoparticles together) to inter-aggregate interparticle forces (forces between aggregates). This relative strength may be inferred from the sphericity of the aggregates during fluidization.     

Frequency-Dependent Magnetic Susceptibility of Magnetite and Cobalt Ferrite Nanoparticles Embedded in PAA Hydrogel

Chemically responsive hydrogels with embedded magnetic nanoparticles are of interest for biosensors that magnetically detect chemical changes. A crucial point is the irreversible linkage of nanoparticles to the hydrogel network, preventing loss of nanoparticles upon repeated swelling and shrinking of the gel. Here, acrylic acid monomers are adsorbed onto ferrite nanoparticles, which subsequently participate in polymerization during synthesis of poly(acrylic acid)-based hydrogels (PAA). To demonstrate the fixation of the nanoparticles to the polymer, our original approach is to measure low-field AC magnetic susceptibility spectra in the 0.1 Hz to 1 MHz range. In the hydrogel, the magnetization dynamics of small iron oxide nanoparticles are comparable to those of the particles dispersed in a liquid, due to fast Néel relaxation inside the particles; this renders the ferrogel useful for chemical sensing at frequencies of several kHz. However, ferrogels holding thermally blocked iron oxide or cobalt ferrite nanoparticles show significant decrease of the magnetic susceptibility resulting from a frozen magnetic structure. This confirms that the nanoparticles are unable to rotate thermally inside the hydrogel, in agreement with their irreversible fixation to the polymer network.     

Design principles for spray-roasted iron oxides for the manufacturing of ferrites

A range of high purity iron oxides are prepared by varying basic operation parameters of an industrial spray roasting process. These iron oxides are investigated in relation to their morphology and subsequently evaluated as raw materials for MnZn-ferrite preparation. It appears that the most important morphological parameters for determining the reactivity (defined as firing shrinkage at equal compaction density) of the high purity iron oxide, and consequently the final density and magnetic properties of the ferrite specimens, are the primary particle size and the number of primary particles per aggregate. As found, the specific surface area of the iron oxide is of no predictive value for the behavior of the iron oxide in a MnZn-ferrite manufacturing process. A small primary particle size is important for a high reactivity; however, when particles are packed together in large aggregates, they are not available for the prefiring reactions. As a result, reactive sintering takes place leading to high porosity and bad microstructure. As found by the characterization methods employed in this article, the optimum iron oxides for MnZn-ferrite preparation should have a primary particle size between 0.45 and 0.55 μm with an aggregate size below 1.60 μm.     

DEM simulation of aggregation of suspended nanoparticles

A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively.Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4-10, solid volume fraction of 0.02-0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point.     

Controlling transport and chemical functionality of magnetic nanoparticles

A wide range of metal, magnetic, semiconductor, and polymer nanoparticles with tunable sizes and properties can be synthesized by wet-chemical techniques. Magnetic nanoparticles are particularly attractive because their inherent superparamagnetic properties make them desirable for medical imaging, magnetic field assisted transport, and separations and analyses. With such applications on the horizon, synthetic routes for quickly and reliably rendering magnetic nanoparticle surfaces chemically functional have become an increasingly important focus. This Account describes common synthetic routes for making and functionalizing magnetic nanoparticles and discusses initial applications in magnetic field induced separations. The most widely studied magnetic nanoparticles are iron oxide (Fe2O3 and Fe3O4), cobalt ferrite (CoFe 2O4), iron platinum (FePt), and manganese ferrite (MnFe 2O4), although others have been investigated. Magnetic nanoparticles are typically prepared under either high-temperature organic phase or aqueous conditions, producing particles with surfaces that are stabilized by attached surfactants or associated ions. Although it requires more specialized glassware, high-temperature routes are generally preferred when a high degree of stability and low particle size dispersity is desired. Particles can be further modified with a secondary metal or polymer to create core-shell structures. The outer shells function as protective layers for the inner metal cores and alter the surface chemistry to enable postsynthetic modification of the surfactant chemistry. Efforts by our group as well as others have centered on pathways to yield nanoparticles with surfaces that are both easily functionalized and tunable in terms of the number and variety of attached species. Ligand place-exchange reactions have been shown quite successful for exchanging silanes, acids, thiols, and dopamine ligands onto the surfaces of some magnetic particles. Poly(ethylene oxide)-modified phospholipids can be inserted into nonpolar surface monolayers of as-prepared nanoparticles as a method for modifying the surface chemistry that induces water solubility. In general, reactive termini can subsequently be used to append a range of chemical groups. For example, surfactants with trifluoroethylester or azide termini have been used to attach a range of amine- or alkyne-containing species, respectively. Chemically functionalized magnetic nanoparticles are promising as advanced materials for analytical separations and analysis. Magnetic field flow fractionation leverages the size-dependent magnetic moments to purify and separate the components of a complex mixture of particles. Similarly, magnetic field gradients are useful for manipulating transport and separation in simple microfluidic devices. By either approach, magnet-induced transport of the particles is a simple method in which an attached reagent, catalyst, or bioanalytical tag can be moved between flow streams within a lab on a chip device.     

Modification of interparticle forces for nanoparticles using atomic layer deposition

Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating.     

Irreversible changes in protein conformation due to interaction with superparamagnetic iron oxide nanoparticles

The understanding of the interactions between nanomaterials and proteins is of extreme importance in medicine. In a biological fluid, proteins can adsorb and associate with nanoparticles, which can have significant impact on the biological behavior of the proteins and the nanoparticles. We report here on the interactions of iron saturated human transferrin protein with both bare and polyvinyl alcohol coated superparamagnetic iron oxide nanoparticles (SPIONs). The exposure of human transferrin to SPIONs results in the release of iron, which changes the main function of the protein, which is the transport of iron among cells. After removal of the magnetic nanoparticles, the original protein conformation is not recovered, indicating irreversible changes in transferrin conformation: from a compact to an open structure.     

Effect of glidants in binary powder mixtures

The intention of this study is to investigate on a particulate level the flow properties of dry powder mixtures consisting of cornstarch and a second nanoscaled material. Special attention is paid to the question on the working mechanism of glidants. In 1974, Rumpf showed that a roughness on the surface of a smooth particle leads to a reduction of its forces of interaction with another particle. The interaction forces are reduced as the surface roughness increases the distance between the centers of gravity of the two interacting particles. Agglomerates as well as the primary particles of materials used as glidants are characterized by diameters in the lower nanometer range. In consequence they are strongly adsorbed at the surface of larger particles and act as a surface roughness. If the effect of a glidant would be due to its ability to act as a surface roughness then all nanoparticles being small enough to reduce the net interaction forces could be used as glidants almost irrespective of their chemical nature. Indeed we have been able to demonstrate that nanoparticles of titanium dioxide, aluminum oxide, silicon dioxide or of carbon black act as glidants. Mixing time directly influences the efficiency of a nanomaterial to act as a glidant. Due to increasing ratio of adhesive force to particle weight with decreasing particle radius, nanomaterials tend to aggregate and agglomerate. With increasing mixing time the size of agglomerates decreases. At the same time the number of primary particles available for adsorption on the surfaces of the cornstarch particles increases. An optimum in flow properties is achieved at a characteristic mixing time. At a further increase in mixing time, the size of agglomerates decreases and the coverage of the cornstarch particles by nanoparticles increases. Eventually cornstarch particles are obtained being completely coated with nanoparticles. The surfaces of these coated particles are smooth. Accordingly they show a poor flow behavior. The property of the nanomaterial to act as a glidant is lost.     

Rheology of Concentrated Suspensions Containing Weakly Attractive Alumina Nanoparticles

The use of nanoparticles for the fabrication of new functional ceramics and composites often requires the preparation of concentrated fluid suspensions. However, suspensions containing nanoparticles are limited in solids content because of the excluded volume formed by the dispersant adlayer around the particles. We investigated the effect of the adlayer thickness on the rheological behavior of suspensions containing model alumina nanoparticles, using dispersant molecules with deliberately tailored chain length. The apparent viscosity and yield stress of the particle suspensions were markedly decreased by increasing the dispersant length, mainly due to a reduction of the attractive forces among particles. Fluid suspensions with solids content up to 35 vol% were prepared in toluene using a dispersant length of 2.5 nm. Our experimental results and viscosity predictions based on a hard sphere model indicate that fluid suspensions with up to 43 vol% of 65 nm alumina particles could be prepared using an optimum dispersant length of about 3.6 nm.