Particle-particle Aggregation Research Articles

Effects of Tween 20 addition on electrokinetic transport in a polydimethylsiloxane microchannel.

Tween 20 is frequently added to particle suspensions for reducing the particle-wall adhesion and particle-particle aggregation in microfluidic devices. However, the influences of Tween 20 on the fluid and particle behaviors have been largely ignored. We present in this work the first experimental study of the effects of Tween 20 addition on the electrokinetic transport of fluids and particles in a polydimethylsiloxane microchannel. We find that adding 0.1% v/v Tween 20 to a buffer solution can significantly reduce the electroosmotic mobility as well as the electrokinetic and electrophoretic mobilities of polystyrene particles and yeast cells. Further increasing the Tween 20 concentration within the range typically used in microfluidic applications continues reducing these mobility values, but at a smaller rate. Our finding suggests that Tween 20 should be used with care in electrokinetic microdevices when the flow rate or particle/cell throughput is an important parameter.

Hybrid microstructure of smectite clay gels revealed using neutron and synchrotron X-ray scattering

Aqueous suspensions of swelling clays display a nematic sol-gel transition at very low solid concentrations. The underlying microstructure of the gel has remained a point of contention since the time of Irving Langmuir and has been a major obstacle to fully realizing the potential of clays for practical applications. Here, we comprehensively probe the microstructure of a smectite clay suspension using ultra-small angle neutron/X-ray scattering and find that the nematic gel is structurally ordered and contains entities that are at least an order of magnitude larger than the individual particles. Complementary cryo-electron microscopy shows the presence of domains having particle-particle ordering responsible for nematic texture and regions of particle-particle aggregation responsible for gel-like behavior. We find that the smectic clay gels have a hybrid microstructure with co-existing repulsive nematic domains and attractive disordered domains.

Surfactant effects on microfluidic extensional flow of water and polymer solutions

Surfactants are often added to particle suspensions in the flow of Newtonian or non-Newtonian fluids for the purpose of reducing particle-particle aggregation and particle-wall adhesion. However, the impact on the flow behavior of such surfactant additions is often overlooked. We experimentally investigate the effect of the addition of a frequently used neutral surfactant, Tween 20, at the concentration pertaining to microfluidic applications on the entry flow of water and three common polymer solutions through a planar cavity microchannel. We find that the addition of Tween 20 has no significant influence on the shear viscosity or extensional flow of Newtonian water and Boger polyethylene oxide solution. However, such a surfactant addition reduces both the shear viscosity and shear-thinning behavior of xanthan gum and polyacrylamide solutions that each exhibit a strong shear-thinning effect. It also stabilizes the cavity flow and delays the onset of flow instability in both cases. The findings of this work can directly benefit microfluidic applications of particle and cell manipulation in Newtonian and non-Newtonian fluids.

Modeling bacteriophage-induced inactivation of Escherichia coli utilizing particle aggregation kinetics

Targeted inactivation of bacteria using bacteriophages has been proposed in applications ranging from bioengineering and biofuel production to medical treatments. The ability to differentiate between desirable and undesirable organisms, such as in targeting filamentous bacteria in activated sludge, is a potential advantage over conventional disinfectants. Like conventional disinfectants, bacteriophages exhibit non-linear concentration-time (Ct) dynamics in achieving bacterial inactivation. However, there is currently no workable model for predicting these observed non-linear inactivation rates. This work considers an approach to predicting bacteriophage-induced inactivation rates by utilizing classical particle aggregation theory. Bacteriophage-bacteria interactions are represented as a two-step process of transport by Brownian motion, differential settling, and shear, followed by attachment. Modifying classical expressions for particle-particle aggregation to include bacterial growth, death, and bacteriophage reproduction, the model was calibrated and validated using literature data. The calibrated model captures much of the observed non-linearity in inactivation rates and reasonably predicts the final host concentration. This model was shown to be most useful in systems more likely to reflect an industrial setting, where the initial multiplicity of infection, or MOI (the ratio of bacteriophage to host organisms), was 1 or greater. For systems of an initial MOI of less than 1 the model showed increased sensitivity to changes in input parameters and a less pronounced ability to reasonably predict inactivation rates.

Synthesis and application of PEGylated tracer particles for measuring protein solution viscosities using Dynamic Light Scattering-based microrheology

The measurement of flow properties, such as the zero shear viscosity, of protein solutions is of paramount importance for many applications such as pharmaceutical formulations, where the syringeability of physiologically effective doses is a key property. However, the determination of these properties with classical rheological methods is often challenging due to e.g. detrimental surface effects or simply the lack of sufficient material. A possible alternative is Dynamic Light Scattering-based microrheology, where the Brownian motion of tracer particles embedded in the protein solution is monitored to access the zero shear viscosity of the sample. The prime advantages of this method compared to classical rheology are the absence of disturbing surface effects and the up to two orders of magnitude smaller protein quantities needed for an entire concentration series. This Protocol provides a detailed description of the synthesis of sterically stabilized tracer particles with surface and overall particle properties specifically designed to investigate the viscosity of protein solutions up to concentrations close to the arrest transition. These particles are tailored to avoid protein-particle as well as particle-particle aggregation at various sample conditions and thus allow for an artifact-free application of Dynamic Light Scattering-based tracer microrheology to determine the flow behaviour of biological samples. The Protocol concludes with step by step instructions for the characterization of protein solutions using a combination of the tracer particles and an advanced dynamic light scattering technique yielding the concentration-dependent zero shear viscosity.

Application of population balance equation in modeling of asphaltene particle size distribution and characterization of aggregation mechanisms under miscible gas Injection

Abstract Particle size distribution (PSD) is an important factor that determines how asphaltene instability can damage porous media during natural depletion and enhanced oil recovery processes. In this work, aggregate size distribution under natural depletion and miscible nitrogen injection processes are determined via image analysis techniques and the results are modelled by population balance equation. Unimodal distribution curves in natural depletion show the dominance of particle-particle aggregation mechanism and the clustering is detected only around crude oil bubble point pressure. It is also observed that miscible nitrogen injection considerably increases the number and size of asphaltene flocs and directs the agglomeration process towards cluster-cluster aggregation (i.e. bimodal distribution curves) which can severely damage porous media. Results of population balance modeling characterize dominant aggregation mechanisms providing one optimum collision factor for unimodal curves and two optimum collision factors for bimodal distributions which confirms the alteration of aggregation mechanism due to miscible gas injection.

AC susceptibility as a tool to probe the dipolar interaction in magnetic nanoparticles

The dipolar interaction is known to substantially affect the properties of magnetic nanoparticles. This is particularly important when the particles are kept in a fluid suspension or packed within nano-carriers. In addition to its usual long-range nature, in these cases the dipolar interaction may also induce the formation of clusters of particles, thereby strongly modifying their magnetic anisotropies. In this paper we show how AC susceptibility may be used to obtain information regarding the influence of the dipolar interaction in a sample. We develop a model which includes both aspects of the dipolar interaction and may be fitted directly to the susceptibility data. The usual long-range nature of the interaction is implemented using a mean-field approximation, whereas the particle-particle aggregation is modeled using a distribution of anisotropy constants. The model is then applied to two samples studied at different concentrations. One consists of spherical magnetite nanoparticles dispersed in oil and the other of cubic magnetite nanoparticles embedded on polymeric nanospheres. We also introduce a simple technique to address the presence of the dipolar interaction in a given sample, based on the height of the AC susceptibility peaks for different driving frequencies.

Experimental and Theoretical Investigations of PAN Molecular Weight Increase in Precipitation Polymerization as a Function of H2O/DMSO Ratio

The precipitation polymerization of acrylonitrile (AN) was carried out in a mixture solution of dimethyl sulfoxide (DMSO) and water at <TEX>$50{\sim}65^{\circ}C$</TEX> using <TEX>${\alpha},{\alpha}'$</TEX>-azobisisobutyronitrile (AIBN) as an initiator. The increased molecular weight polyacrylonitrile (PAN) was prepared with increasing the <TEX>$H_2O$</TEX>/DMSO ratio from 10/90 to 80/20. The viscosity average molecular weight of <TEX>$H_2O$</TEX>/DMSO solvent was 4.4 times larger than that of <TEX>$H_2O$</TEX>/DMF solvent, and precipitation polymerization was accelerlated due to the far decreased chain transfer effect of DMSO. Based on the experimental results, the increased PAN molecular weight was regarded as the summation of two mechanisms: i) particle-particle aggregation and ii) particle-radical attachment. The theoretical equation derived from the mechanisms was well coincided with the experimental results showing the linear relationship between the viscosity average molecular weight and the H2O/DMSO ratio.

An in-situ Observation on Initial Aggregation Process of Colloidal Particles near Three-Phase Contact Line of Air, Water and Vertical Substrate

The self-assembling process near the three-phase contact line of air, water and vertical substrate is widely used to produce various kinds of nanostructured materials and devices. We perform an in-situ observation on the self-assembling process in the vicinity of the three phase contact line. Three kinds of aggregations, i.e. particle-particle aggregation, particle-chain aggregation and chain-chain aggregation, in the initial stage of vertical deposition process are revealed by our experiments. It is found that the particle-particle aggregation and the particle-chain aggregation can be qualitatively explained by the theory of the capillary immersion force and mirror image force, while the chain-chain aggregation leaves an opening question for the further studies. The present study may provide more deep insight into the self-assembling process of colloidal particles.

Small-Angle Neutron Scattering Study of Natural Aquatic Nanocolloids

We examine the potential of small-angle neutron scattering (SANS) as a quantitative tool for studying nanostructure and length scales in natural freshwater aquatic colloidal dispersions (river water, river bed sediments, agricultural field drainage and slurry). Our results show that natural aquatic colloids are generally fractal with 3D network-type structures. None of these structures are consistent with diffusion-limited (DLCA) or reaction-limited (RLCA) particleparticle aggregation processes. The results also suggest three characteristic length scales: one ca. 3-10 nm, which we identify with "primary particle" sizes, another of ca. 20-50 nm suggestive of small aggregates, and the last ca. 50-200 nm which we postulate arises from transient networks of the aggregates. The role of organic matter in mediating colloid structure in aquatic dispersions was examined by neutron contrast variation and by measuring samples both before and after treatment with hydrogen peroxide. The results suggest that the aggregate network structure is mediated by organic matter.

Rapid Gel-Like Latex Film Formation through Controlled Ionic Coacervation of Latex Polymer Particles Containing Strong Cationic and Protonated Weak Acid Functionalities

We introduce a controlled ionic coacervation (CIC) process that rapidly forms uniform, gel-like latex films with significant mechanical integrity without loss of water from the film. This process uses latex particles that contain both strong cationic charges and weak protonated acid groups. An increase in pH ionizes the weak acid and triggers the rapid setting of the latex films. The necessary increase in pH can be achieved by coating the latex onto an alkaline surface (such as concrete) or by controlled release of a fugitive acid (such as carbon dioxide). We explore the effect of latex composition and concentration on this process. We show that the CIC process does not require a water-soluble polymer to obtain the rapid-set film properties. Our proposed mechanism for CIC process is consistent with models for rapid, irreversible, particle-particle aggregation.

Studies of Lead Zirconate Titanate Sol Aging Part II: Particle Growth Mechanisms and Kinetics

The kinetics of particle growth in lead zirconate titanate sol gel precursor solutions has been investigated. It was found that chemical reaction limited aggregation was responsible for most of the sol aging, followed by diffusion limited aggregation in the later stage. At the beginning, particles grow by reaction between initial polymers and particles. As the particle number-density increases, a particle-particle aggregation characterised by an exponential growth law becomes the predominant mechanism. When particles grow larger, towards gel formation, the aggregation changes to diffusion limited. Mathematical models derived according to the above mechanisms agreed well with experimentally measured particle growth profiles.

Influence of surface chemistry on particle-particle aggregation as measured by a Coulter Counter in the low concentration regime

The link between particle concentration, surface properties, and particle aggregation has been investigated for polystyrene latex in the low particle concentration regime, volume fractions less than 1 × 10-6. The particles used in this model study were 9.14-μm diameter polystyrene beads, approximately the same size as red blood cells, using a Coulter Counter with a 100-μm aperture as a gauge of clustering. The lower concentrations of beads displayed the same trend as reported in other publications. Albumin and fibrinogen were then attached to the beads and the effect of non-specific binding by the proteins on particle agglomeration was studied. Albumin showed little effect on the number of aggregates encountered whereas fibrinogen increased the number of aggregates, indicating that stronger particle interactions occurred between the fibrinogen coated particles. The addition of stearic acid had no marked effect on the aggregation properties of the beads, coated or uncoated. The addition of sodium stearate, however, increased the number of doublets, triplets, and multiplets formed. These results may be relevant to adhesion of cells. Our approach is a plausible way to consider the roles that proteins and fatty acid molecules have on the relative binding potential without the constraints of using live cells.

Kinetics of lead zirconate titanate sol aging

The growth of polymeric particles during the hydrolysis and condensation of PbZr0.3Ti0.7O3 (PZT 30/70) precursor solutions has been studied. At the beginning, particles grow by reaction between monomers and particles. As the particle number-density increases, particle-particle aggregation characterised by an exponential growth law becomes the predominant mechanism. When particles grow larger, toward gel formation, the aggregation changes to diffusion limited. Mathematical models derived according to the above mechanisms agreed well with measured particle growth profiles.

Particle-particle aggregation with 1/r2 forces in reduced gravity environments.

Particle-particle aggregation with 1/r2 forces in reduced gravity environments.

Magnetic marbles as a model for ferromagnetic particle aggregation: fractal dimensions

The author investigates from a heuristic point of view the role of magnetic interactions in single domain ferromagnetic particle-particle aggregation. A simple experimental model consisting of magnetic marbles in a box leads to scale invariant aggregation with a fractal dimension D=1.46+or-0.15 lower than expected values for non-magnetic particles. The results are in good agreement with experimental results on small Co particles a few nanometers in diameter and computer simulations of cluster aggregation in the presence of magnetic dipolar interactions.

Study of particle behaviour in high field magnetic flocculation

A recently developed dynamic theory of magnetic field-induced particle-particle aggregation in suspensoids is further extended in terms of characteristic velocity coefficients in the manner of the inertialess theory of H.G.M.S. This is done by expressing the flocculation process in terms of familiar parameters such as magnetic velocity and capture radius. Hybrid flocculation of diamagnetic-paramagnetic pairs is described. Also demonstrated here is the importance of the suspensoid settling velocity following the occurrence of binary flocculation.

Studies in the rheology of clay-water systems

The rheological properties of sodium Attapulgite sols and gels have been studied as a function of clay concentration, etc.; and the effects of peptising and coagulating electrolytes on these properties have been investigated. These systems show similar properties to those of mixed calcium-sodium Montmorillonite (and Hectorite) clays of equivalent exchangeable sodium content. The intrinsic viscosity and viscosities of the heterocoagulated sols are low, despite the high anisometry of the individual Attapulgite laths (or needles), since disaggregation is incomplete: these sols set to gels of moderate strength. Heterocoagulation is eliminated (as in other clays) by adsorption of peptisors (polyphosphates, etc.) onto the positively charged edges of the dispersed clay particles. Coagulation of the sols dispersed in sodium carbonate solution by electrolytes leads to the formation of highly dissymmetric structures with high viscosity and yield stress. The peptised sols are more stable to electrolyte; and when coagulated, the particle-particle aggregation and structure formation is very much reduced.