Suspensions Of Colloidal Particles Research Articles

Effect of Asphaltene Dispersants on Aggregate Size Distribution and Growth

A crude oil system can become unstable because of changes in hydrocarbon composition, pressure, or temperature during normal production from the reservoir or during commingling with dissimilar crude oils. These changes can generate asphaltene particles that can result in significant production and refining problems. The generation of these particles is a two-step process: phase separation and asphaltene particle growth. Phase separation occurs when nanosize asphaltene particles from the crude oil precipitate and grow into large aggregates. Our study on asphaltene precipitation shows that large asphaltene particles are aggregates consisting of very small (sub-micrometer) size asphaltene particles. One mechanism to control asphaltenes is to kinetically inhibit the phase separation of asphaltenes by adding a small amount of a chemical that interferes with the phase separation processes. Another mechanism to control asphaltenes is to inhibit growth by stabilizing the colloidal suspension of the sub-micrometer asphaltene particles to significantly slow the flocculation and settling processes. Asphaltene chemical additives of known molecular structures as well as proprietary chemical blends were selected for this study. None of chemicals studied inhibited phase separation; however, some of the dispersants did slow or stop flocculation and growth. Four different analytical techniques have been used to study the effect of chemical additives on asphaltene aggregation and settling and to evaluate the effectiveness of different asphaltene chemicals in keeping asphaltene particles suspended/dispersed in crude oils. From the turbidity measurement, asphaltene dispersants can be classified into three categories based on their performance. The particle size distribution measurement showed three different types of asphaltenes: stable asphaltenes, colloidal asphaltenes, and flocculated asphaltenes, on the basis of aggregate sizes. Asphaltene dispersants can stabilize colloidal asphaltenes and slow the growth and formation of flocculated asphaltenes. These results can be used to select the best chemical treatment plan for preventing/reducing asphaltene settling and deposition.

Direct measurements of the frequency-dependent dielectrophoresis force.

Dielectrophoresis (DEP), the phenomenon of directed motion of electrically polarizable particles in a nonuniform electric field, is promising for applications in biochemical separation and filtration. For colloidal particles in suspension, the relaxation of the ionic species in the shear layer gives rise to a frequency-dependent, bidirectional DEP force in the radio frequency range. However, quantification methods of the DEP force on individual particles with the pico-Newton resolution required for the development of theories and design of device applications are lacking. We report the use of optical tweezers as a force sensor and a lock-in phase-sensitive technique for analysis of the particle motion in an amplitude modulated DEP force. The coherent detection and sensing scheme yielded not only unprecedented sensitivity for DEP force measurements, but also provided a selectivity that clearly distinguishes the pure DEP force from all the other forces in the system, including electrophoresis, electro-osmosis, heat-induced convection, and Brownian forces, all of which can hamper accurate measurements through other existing methods. Using optical tweezers-based force transducers already developed in our laboratory, we have results that quantify the frequency-dependent DEP force and the crossover frequency of individual particles with this new experimental method.

The Effect of Nanoparticle Coating on the Thermal Conductivity of Nanofluids

AbstractNanofluids are engineered colloidal suspensions of nanometer-sized particles in a carrier fluid and are receiving significant attention because of their potential applications in heat transfer area. Theoretical investigations have shown that the enhanced thermal conductivity observed in nanofluids is due to nanoparticle clustering and networking. This provides a low resistance path to the heat flowing through the fluid. However, the surface coating of the nanoparticles, which is often used to provide stable dispersion over the long term, may act as a thermal barrier, reducing the effective thermal conductivity of the nanofluid. Moreover, nanofluids with the same type of nanoparticles may exhibit different effective thermal conductivities, depending upon the thermal properties and thickness of the coating. In this context, thermal conductivity characterization of well dispersed iron oxide nanoparticles with two different surface coatings was carried out employing the transient hot wire technique. The diameter of the iron oxide core was 35 nm and the coatings used were aminosilane and carboxymethyl-dextran (CMX) of 7nm in thickness. Preliminary results suggest that effective thermal conductivity of CMX coated nanoparticle suspensions is slightly higher than that of aminosilane coated nanoparticles. In both cases, the effective thermal conductivity is higher than that predicted by the Maxwell model for composite media.

Inductive heating of ferrimagnetic particles and magnetic fluids: Physical evaluation of their potential for hyperthermia

The potential of colloidal subdomain ferrite particle suspensions (SDP) (‘magnetic fluids’), exposed to an alternating magnetic field, is evaluated for hyperthermia. Power absorption measurements of different magnetic fluids are presented in comparison to multidomain ferrite particles (MDP). Variations with frequency as well as magnetic field strength have been investigated. The experimental results clearly indicate a definite superiority of even non-optimized magnetic fluids over MDP ferrites regarding their specific absorption rate (SAR). Based on the work of [Shliomis, Pshenichnikov, Morozov, Shurubor. Magnetic properties of ferrocolloids. J Magn Magn Mater 1990;85:40–46 and [Hanson The frequency dependence of the complex susceptibility of magnetic fluids. J Magn Magn Mater, 1991;96 (In press).], a solid-state physical model is applied to explain the specific properties of magnetic fluids with respect to a possible use in hyperthermia. The experimentally determined SAR data on magnetic fluids are used to estimate the heating capabilities of a magnetic induction heating technique assuming typical human dimensions and tissue parameters. It is considered that for a moderate concentration of 5 mg ferrite per gram tumour (i.e. 0.5% w/w) and clinically acceptable magnetic fields, intratumoral power absorption is comparable to RF heating with local applicators and superior to regional RF heating (by comparison with clinical SAR measurements from regional and local hyperthermia treatments). Owing to the high particle density per volume, inductive heating by magnetic fluids can improve temperature distributions in critical regions. Furthermore, localized application of magnetic fluids in a tumour might be easier and less traumatic than interstitial implantation techniques.

Optical absorption under total internal reflection in the characterization of soot in diesel engine oils

Optical absorption of infrared radiation under total internal reflection in a novel sensor has been utilized to investigate the soot contamination of diesel engine oil by response surface methodology in factorial experiments. Sensor response showed highly significant dependence on oil soot concentration and temperature, in which the effect of the soot was greatest. The soot contamination of the optical rod in engine oil was found to be a surface phenomenon which showed little or no dependence on bulk oil shearing displacement below 500 rpm. The quadratic effect of sensor response to soot concentration was very high due to the agglomeration of soot particles, derived from the high surface energy of carbon soot. Test results of this optical absorption technique were in conformance with those other oil analysis techniques such as UV spectrophotometry, total acid number, viscosimetry, optical microscopy and EPMA. The technique proved to be more reliable than RDE emission spectrometry which showed ambiguous results due to colloidal suspension of soot particles in oil. Optical absorption proved to be an effective criterion in characterizing the soot contamination of diesel engine oil.

Elastic constants of hard thin platelets by Monte Carlo simulation and virial expansion

In this paper we present an investigation into the calculation of the Frank elastic constants of hard platelets via molecular simulation and virial expansion beyond second order. Monte Carlo simulations were carried out and director fluctuations measured as a function of wave vector k, giving the elastic constants through a fit in the low-k limit. Additionally, the virial expansion coefficients of the elastic constants up to sixth order were calculated, and the validity of the theory determined by comparison with the simulation results. The simulation results are also compared with experimental measurements on colloidal suspensions of platelike particles.

Hydrodynamic Interactions in Colloidal Ferrofluids: A Lattice Boltzmann Study

We use lattice Boltzmann simulations, in conjunction with Ewald summation methods, to investigate the role of hydrodynamic interactions in colloidal suspensions of dipolar particles, such as ferrofluids. Our work addresses volume fractions phi of up to 0.20 and dimensionless dipolar interaction parameters lambda of up to 8. We compare quantitatively with Brownian dynamics simulations, in which many-body hydrodynamic interactions are absent. Monte Carlo data are also used to check the accuracy of static properties measured with the lattice Boltzmann technique. At equilibrium, hydrodynamic interactions slow down both the long-time and the short-time decays of the intermediate scattering function S(q, t), for wavevectors close to the peak of the static structure factor S(q), by a factor of roughly two. The long-time slowing is diminished at high interaction strengths, whereas the short-time slowing (quantified via the hydrodynamic factor H(q)) is less affected by the dipolar interactions, despite their strong effect on the pair distribution function arising from cluster formation. Cluster formation is also studied in transient data following a quench from lambda = 0; hydrodynamic interactions slow the formation rate, again by a factor of roughly two.

Dynamical and structural insights into the smectic phase of rod-like particles

Self-diffusion in a model system of rod-like particles is studied in the smectic (or lamellar)phase. The experimental system is formed by a colloidal suspension of filamentous fdvirus particles, which allows the direct visualization at the scale of the singleparticle of mass transport between the smectic layers. Self-diffusion takes placepreferentially in the direction normal to the smectic layers and occurs in steps of one rodlength, reminiscent of a hopping-type of transport. The probability density functionis obtained experimentally at different times and is found to be in qualitativeagreement with theoretical predictions based on a dynamical density functional theory.

Experimental investigation on the magneto-optic effects of ferrofluids via dynamic measurements

An experimental investigation on the origin of the magneto-optic effects of ferrofluids is presented. Dynamic measurements of their transverse and longitudinal magneto-optic effects and their magnetic properties have been performed. As ferrofluids are colloidal suspensions of magnetic particles in a liquid, the influence of two parameters has been studied: the magnetic anisotropy energy of particles and the carrier liquid viscosity for four ferrofluid samples. The interpretation and the comparison of results give some clues for a better understanding of magneto-optical effects of ferrofluids.

From particles to spins: Eulerian formulation of supercooled liquids and glasses.

The dynamics of supercooled liquid and glassy systems are usually studied within the Lagrangian representation, in which the positions and velocities of distinguishable interacting particles are followed. Within this representation, however, it is difficult to define measures of spatial heterogeneities in the dynamics, as particles move in and out of any one given region within long enough times. It is also nontransparent how to make connections between the structural glass and the spin glass problems within the Lagrangian formulation. We propose an Eulerian formulation of supercooled liquids and glasses that allows for a simple connection between particle and spin systems, and that permits the study of dynamical heterogeneities within a fixed frame of reference similar to the one used for spin glasses. We apply this framework to the study of the dynamics of colloidal particle suspensions for packing fractions corresponding to the supercooled and glassy regimes, which are probed via confocal microscopy.

Gelation on the microscopic scale

Particle-tracking methods are used to study gelation in a colloidal suspension of Laponite clay particles. We track the motion of small fluorescent polystyrene spheres added to the suspension, and obtain the micron-scale viscous and elastic moduli of the material from their mean-squared displacement. The fluorescent spheres move subdiffusively due to the microstructure of the suspension, with the diffusive exponent decreasing from close to one at early times to near zero as the material gels. The particle-tracking data show that the system becomes more heterogeneous on the microscopic scale as gelation proceeds. We also determine the bulk-scale moduli using small-amplitude oscillatory shear rheometry. Both the macroscopic and microscopic moduli increase with time, and on both scales we observe a transition from a primarily viscous fluid to an elastic gel. We find that the gel point, determined as the time at which the viscous and elastic moduli are equal, is length-scale dependent--gelation occurs earlier on the bulk scale than on the microscopic scale.

Experimental characterization and population balance modelling of the dense silica suspensions aggregation process

Concentrated suspensions of nanoparticles subjected to transport or shear forces are commonly encountered in many processes where particles are likely to undergo processes of aggregation and fragmentation under physico-chemical interactions and hydrodynamic forces. This study is focused on the analysis of the behavior of colloidal silica in dense suspensions subjected to hydrodynamic forces in conditions of destabilization. A colloidal silica suspension of particles with an initial size of about 80 nm was used. The silica suspension concentration was varied between 3% and 20% of weight. The phenomenon of aggregation was observed in the absence of any other process such as precipitation and the destabilization of the colloidal suspensions was obtained by adding sodium chloride salt. The experiments were performed in a batch agitated vessel. The evolution of the particle size distributions versus time during the process of aggregation was particularly followed on-line by acoustic spectroscopy in dense conditions. Samples were also analyzed after an appropriate dilution by laser diffraction. The results show the different stages of the silica aggregation process whose kinetic rates depend either on physico-chemical parameters or on hydrodynamic conditions. Then, the study is completed by a numerical study based on the population balance approach. By the fixed pivot technique of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51 (8), 1311–1332], the hypothesis on the mechanisms of the aggregation and breakage processes were justified. Finally, it allows a better understanding of the mechanisms of the aggregation process under flowing conditions.

New Method of Water Purification Based on the Particle-Exclusion Phenomenon

Colloidal particles in suspension are excluded from the vicinity of various hydrophilic surfaces. On the basis of this phenomenon, a novel method of water purification is proposed and tested. Proof-of-concept is demonstrated using a custom-made extractor that collected clean water from the annular "exclusion zone" within a Nafion tube. Up to 99.6% of particles could be removed from the suspension. The experimental results suggest that particle exclusion may provide a new framework for water purification from both organic and inorganic matter, as well as from harmful pathogens.

Particle deposition study during sessile drop evaporation

AbstractThe focus of this article is the numerical study of particle deposition profiles on a solid substrate during the evaporation of a sessile drop of a colloidal particle suspension. The evaporation flux along the drop interface, the induced fluid dynamics inside the drop, and the particle deposition profile on the solid substrate are solved simultaneously. The governing equations are solved numerically using the Galerkin/finite element method (G/FEM) for discretization of the spatial domain and an adaptive finite difference method for discretization in the time domain. Several particle deposition profiles, such as a ring‐shaped deposit and uniform particle distribution, are obtained from the numerical simulations. The particle deposition profile is found to be influenced by the mass transfer (both convective and diffusive mass transfer) of the particles in the bulk liquid and by the deposition rate along the substrate. © 2008 American Institute of Chemical Engineers AIChE J, 2008

A novel inverse method for determining the refractive indices of medium and dispersed particles simultaneously by turbidity measurement

The refractive indices of particles and dispersion medium are important parameters in many colloidal experiments using optical techniques, such as turbidity and light scattering measurements. These data are in general wavelength-dependent and may not be available at some wavelengths fitting to the experimental requirement. In this study we present a novel approach to inversely determine the refractive indices of particles and dispersion medium by examining the consistency of measured extinction cross sections of particles with their theoretical values using a series of trial values of the refractive indices. The colloidal suspension of polystyrene particles dispersed in water was used as an example to demonstrate how this approach works and the data obtained via such a method are compared with those reported in literature, showing a good agreement between both. Furthermore, the factors that affect the accuracy of measurements are discussed. We also present some data of the refractive indices of polystyrene over a range of wavelengths smaller than 400 nm that have been not reported in the available literature.

Ultrasonic determination of the particle size distribution in water-based magnetic liquid

Magnetic liquids are stable colloidal suspensions of nano-sized magnetic particles in a carrier liquid medium. In the present paper the determination of the particle size distribution function using ultrasonic spectroscopy is described. The ultrasonic spectra of water-based magnetic fluid measured in the 3.5–50 MHz frequency range are analyzed using formulas for the velocity and absorption of sound in dispersion media obtained by Vinogradov. The results of the ultrasonic studies are compared with the particles size distribution function evaluated from the processing of the magnetic susceptibility data

Ultra-Micro-Thermal Field-Flow Fractionation

Micro-thermal field-flow fractionation, proposed conceptually and implemented experimentally several years ago, has developed rapidly in terms of theory, instrumentation, and numerous applications for the analysis and characterization of polymers and particles of synthetic, natural, and biological origin. Although the advances have been important, achieving the ultimate limits of miniaturization imposed by the physics as well as by recent technologies represents a challenge that was explored. The result of the reported experimental study is a new separation channel for ultra-micro-thermal field-flow fractionation, which was compared, in terms of performance, with the existing compact micro-thermal field-flow fractionation unit. The limits of the miniaturization are experimentally demonstrated with the use of suspensions of colloidal particles, which represent a more difficult case of separation than polymers in solution.

Frequency-Dependent Electrical Conductivity of Concentrated Dispersions of Spherical Colloidal Particles

This paper outlines the application of a self-consistent cell-model theory of electrokinetics to the problem of determining the electrical conductivity of a dense suspension of spherical colloidal particles. Numerical solutions of the standard electrokinetic equations, subject to self-consistent boundary conditions, are implemented in formulas for the electrical conductivity appropriate to the particle-averaged cell model of the suspension. Results of calculations as a function of frequency, zeta potential, volume fraction, and electrolyte composition, are presented and discussed.

Concentration gradients in mixed magnetic and nonmagnetic colloidal suspensions

An approximate analytic model is derived to describe the equilibrium concentration profile within mixed suspensions of magnetic and nonmagnetic colloidal particles, and a strong qualitative agreement is shown between theory and experiment. This work extends previous theoretical and experimental investigations of density profiles within magnetic nanoparticle fluids, known as ferrofluid.

Shear thickening in electrically-stabilized colloidal suspensions

A theory is presented for the onset of shear thickening in colloidal suspensions of particles, stabilized by an electrostatic repulsion. Based on an activation model, a critical shear stress can be derived for the onset of shear thickening in dense suspensions for a constant potential and a constant charge approach of the spheres. Unlike previous models, the total interaction potential is taken into account (sum of attraction and repulsion). The critical shear stress is related to the maximum of the total interaction potential scaled by the free volume per particle. A comparison with experimental investigations shows the applicability of the theory.