Suspensions Of Colloidal Particles Research Articles

Alignment of Plate-Like Particles in a Colloidal Dispersion under Flow in a Uniform Pipe Studied by High-Energy X-ray Diffraction

High-energy angle-dispersive X-ray diffraction has been used to study the alignment of colloidal suspension of kaolinite particles in water as they flow along a pipe. X-rays with energies above 25 keV have a major advantage, as they can penetrate through thick samples and walls of containers and permit investigation of samples under realistic flow conditions. As an example of the method, flow through a circular cross-section pipe with an internal diameter of 5 mm has been studied: this is typical of industrial applications. The angular distribution of intensities of peaks in the diffraction pattern as a function of the location of the pipe in the X-ray beam provides information about the alignment of particles under flow. Order parameters have been calculated to describe the alignment and direction of orientation. It is observed that the particles align in the direction of flow with their flat faces parallel to the flow. The experimental results are compared with the calculations of the local strain rate that help to explain the onset of alignment of the particles.

Monte Carlo cluster algorithm for fluid phase transitions in highly size-asymmetrical binary mixtures

Highly size-asymmetrical fluid mixtures arise in a variety of physical contexts, notably in suspensions of colloidal particles to which much smaller particles have been added in the form of polymers or nanoparticles. Conventional schemes for simulating models of such systems are hamstrung by the difficulty of relaxing the large species in the presence of the small one. Here we describe how the rejection-free geometrical cluster algorithm of Liu and Luijten [J. Liu and E. Luijten, Phys. Rev. Lett. 92, 035504 (2004)] can be embedded within a restricted Gibbs ensemble to facilitate efficient and accurate studies of fluid phase behavior of highly size-asymmetrical mixtures. After providing a detailed description of the algorithm, we summarize the bespoke analysis techniques of [Ashton et al., J. Chem. Phys. 132, 074111 (2010)] that permit accurate estimates of coexisting densities and critical-point parameters. We apply our methods to study the liquid-vapor phase diagram of a particular mixture of Lennard-Jones particles having a 10:1 size ratio. As the reservoir volume fraction of small particles is increased in the range of 0%-5%, the critical temperature decreases by approximately 50%, while the critical density drops by some 30%. These trends imply that in our system, adding small particles decreases the net attraction between large particles, a situation that contrasts with hard-sphere mixtures where an attractive depletion force occurs.

Double layer polarization in “realistic” aqueous salt-free suspensions

The analysis of the frequency dependent electric permittivity of colloidal suspensions as a function of the frequency of the externally applied field is a powerful tool, very sensitive to the particle–solution interface and can provide rich information on the dynamics of electrical double layers. Among the different physical situations that can manifest themselves in the permittivity spectrum, attention will be paid in this paper to the case of concentrated suspensions of spherical colloidal particles in “realistic” salt-free conditions. This means that the ionic species in the liquid phase will be: (1) the “released counterions” coming from the dissociation of surface groups of the particles, (2) the H + and OH − ions from water dissociation, and (3) the ions generated by the environmental CO 2 dissolved in the suspension if it is open to the atmosphere. Theoretical predictions based on the cell model indicate the presence of two relaxations in the permittivity spectrum when the particle charge is large enough. The lowest frequency peak (appearing at frequencies around 10 5 Hz) changes with the overall conductivity of the medium, while the faster relaxation (characteristic frequency in the range 10 7–10 8 Hz) only depends on the surface charge of the particle. This suggests that the first relaxation corresponds to the classical Maxwell–Wagner (MW) relaxation associated to interfacial polarization of the diffuse layer, whereas the second, higher-frequency one, relates to the polarization of a compact layer of counterions developed very close to the particle surface at high particle charges (ionic condensation effect). In order to really prove the physical origin of these dispersions we have compared the results with equivalent salt-added (or low salt) suspensions. These have been built using the same surface charge density values for the particles and an added salt concentration such that the DC conductivities of the salt-free systems are mimicked. Interestingly, it has been found for the first time that a clear alpha-process (observed at the kHz frequency region, and linked to the freezing of the concentration polarization of the double layer) is present together with the above mentioned two MW processes. The alpha relaxation is, however, absent in the case of the realistic salt-free suspensions. In these, the existence of links between the local concentrations of ions due to the weak character of the main electrolyte in solution, carbonic acid, apparently prevents or largely hinders the formation of concentration polarization clouds.

Origins of the anomalous stress behavior in charged colloidal suspensions under shear

Numerical simulations are conducted to determine microstructure and rheology of sheared suspensions of charged colloidal particles at a volume fraction of ϕ=0.33. Over broad ranges of repulsive force strength F0 and Péclet number Pe, dynamic simulations show coexistence of ordered and disordered stable states with the state dependent on the initial condition. In contrast to the common view, at low shear rates, the disordered phase exhibits a lower viscosity (μ(r)) than the ordered phase, while this behavior is reversed at higher shear rates. Analysis shows the stress reversal is associated with different shear induced microstructural distortions in the ordered and disordered systems. Viscosity vs shear rate data over a wide range of F0 and Pe collapses well upon rescaling with the long-time self-diffusivity. Shear thinning viscosity in the ordered phase scaled as μ(r)∼Pe(-0.81) at low shear rates. The microstructural dynamics revealed in these studies explains the anomalous behavior and hysteresis loops in stress data reported in the literature.

Equilibrium structure of ferrofluid aggregates

We study the equilibrium structure of large but finite aggregates of magnetic dipoles,representing a colloidal suspension of magnetite particles in a ferrofluid. With increasingsystem size, the structural motif evolves from chains and rings to multi-chain andmulti-ring assemblies. Very large systems form single- and multi-wall coils, tubes andscrolls. These structural changes result from a competition between various energyterms, which can be approximated analytically within a continuum model. We alsostudy the effect of external parameters such as magnetic field on the relativestability of these structures. Our results may give insight into experimental dataobtained during solidification of ferrofluid aggregates at temperatures where thermalfluctuations become negligible in comparison to inter-particle interactions. These datamay also help to experimentally control the aggregation of magnetic particles.

PH-Sensitive Ionomeric Particles Obtained via Chemical Conjugation of Silk with Poly(amino acid)s

Silk-fibroin-based biomaterials have been widely utilized for a range of biomaterial-related systems. For all these previously reported systems, the β-sheet forming feature of the silk was the key stabilizing element of the final material structure. Herein, we describe a different strategy, based on the engineering of silk-based ionomers that can yield stable colloidal composites or particle suspensions through electrostatic interactions. These silk-based ionomers were obtained by carbodiimide-mediated coupling of silk fibroin with polylysine hydrobromide and polyglutamic acid sodium salts, respectively. Colloidal composites could be obtained by mixing the ionomeric pair at high concentration (i.e., 25% w/v), while combining them at lower concentrations (i.e., 5% w/v) yielded particle suspensions. The assembly of the ionomers was driven by electrostatic interactions, pH-dependent, and reversible. The network assembly appeared to be polarized, with the interacting poly(amino acid) chains clustered to the core of the particles and the silk backbone oriented outward. In agreement with this assembly mode, doxorubicin, a hydrophilic antitumor drug, could be released at a slow rate, in a pH-dependent manner, indicating that the inside of the ionomeric particles was mainly hydrophilic in nature.

Pair interaction potentials of colloids by extrapolation of confocal microscopy measurements of collective suspension structure

A method for measuring the pair interaction potential between colloidal particles by extrapolation measurement of collective structure to infinite dilution is presented and explored using simulation and experiment. The method is particularly well suited to systems in which the colloid is fluorescent and refractive index matched with the solvent. The method involves characterizing the potential of mean force between colloidal particles in suspension by measurement of the radial distribution function using 3D direct visualization. The potentials of mean force are extrapolated to infinite dilution to yield an estimate of the pair interaction potential, U(r). We use Monte Carlo simulation to test and establish our methodology as well as to explore the effects of polydispersity on the accuracy. We use poly-12-hydroxystearic acid-stabilized poly(methyl methacrylate) particles dispersed in the solvent dioctyl phthalate to test the method and assess its accuracy for three different repulsive systems for which the range has been manipulated by addition of electrolyte.

Application of Magnetic Nanoparticles in Pharmaceutical Sciences

Micro- and nano-sized magnetic support is a recently developed revolutionary technology for bioseparation, especially for ligand fishing, protein, enzyme, DNA, RNA and cell isolation or purification. Magnetic nanoparticles (MPs) are referred to by various synonyms, such as magnetic beads (MB) or micro- and nano-sized magnetic beads. They are also called ferrofluids or magnetic fluids, meaning colloidal suspensions of magnetic particles in a liquid carrier. Generally, these particles are part of nanotechnology, which can be defined as engineering of functional systems on a molecular scale. It has the potential to create many new materials and devices with a wide range of applications across the biomedical, chemical, electronic and mechanical fields. Their superparamagnetic properties have opened promising new perspectives for their application in several areas. The pioneering “medical” application in the treatment of lymphatic nodes and metastases based on injecting “metallic particles” preheated in a magnetic field was first published by Gilchrist et al. in 1957 (1). Since then, magnetic particles have been modified by coating with antibodies, enzymes, proteins or specific ligands that enable them to bind to other biologically active compounds or receptors on the cell surface. In the last decade, magnetic particles have been increasingly used as a promising technique for a wide spectrum of biomedical applications. The number of publications presenting original studies on using magnetic beads is increasing year by year (Fig. 1). The growing interest in the magnetic carrier is focused on biochemistry, molecular biology and medical specialties. Because of the superparamagnetic properties and micro- and nano-dimensions, magnetic nanoparticles can be used to isolate any target and linked to various manual and automated applications (2–4). Innovative research has produced a new application for magnetic nanoparticles that carries an exciting biomedical and bioengineering potential, for instance, cell molecule and nucleic acid separation, immunoassays, pathogen detection, protein purification, gen mutation analysis and magnetic-force-based tissue engineering (5,6). Also, the interest in the potential application of the magnetic technique in pharmacy is significantly growing. It is currently being recognised that this magnetic nanotechnology could play an important role in this area. Fig. 1 The number of publications on the subject “Magnetic Beads” and “Biochemistry and Molecular Biology, Medicine or Pharmacology, Toxicology and Pharmaceutics” identified by Scopus (http://www.scopus.com/home.url). DRUG DISCOVERY The process of drug discovery in the modern scientific aspect is very complex. It integrates many disciplines, including biotechnology, medicine and pharmacology. For years, drug discovery has also focused on identification of unknown biomolecular interactions with known targets. A number of established methods are used in evaluating the binding of ligands/drugs to receptors and proteins, such as equilibrium dialysis, ultrafiltration, ultracentrifugation, bioaffinity chromatography and other spectroscopic methods. Also, the BIAcore system and surface plasmon resonance (SPR) are the two most popular recently technologies for real-time biomolecule interaction analysis. Because of the different modifications of multifunctional surface of the magnetic particles, these approaches are practically useful in this area. The objective of the recent study was to adapt immobilized human serum albumin (HSA) onto the surface of magnetic beads for the purpose of “ligand fishing” (3). These beads correctly isolated a known binder from a mixture of known compounds, and the whole experiment was carried out manually using a magnet and automatically using the Magtration System. Furthermore, the magnetic separation technique was extended to the protein-ligand and protein–protein interaction using magnetic beads with immobilized heat-shock protein 90α (Hsp90α) (2). The advantage of this system is that the Hsp90α-coated magnetic beads can isolate interacting partners (ligand and proteins) from complex mixtures and cellular extracts (e.g. from KU-812 cells) in less than 15 min, unlike co-immunoprecipitation methods, which take several hours to recover the protein of interest. Presently, the extension of the magnetic beads method to multiple complex cellular extracts and complex plant extracts is being investigated. The most recent study demonstrates that ligand fishing based on biological functionalised MB is an effective and convenient way to identify and isolate bioactive small molecules from botanical extracts (Camptotheca acuminata and Dioscorea nipponica), and the process has a significant structure specifity (7,8). Using a similar concept, protein-coated magnetic beads were used as a tool for a rapid drug-protein binding study (4). Knowledge of serum protein binding behaviour as a significant component of blood is very important for the rational use of drugs. An extensive understanding of the ligand/drug-protein interaction may be of special importance for the modelling of pharmacokinetics, including possible drug–drug interaction. In this context, magnetic beads with immobilized HSA have been successfully used to determine the affinity of known drugs for the protein (4). Moreover, the preliminary competition experiments suggest that protein-coated magnetic beads can be employed in drug–drug interaction studies. However, the validation of the proposed method is being investigated.

A powerful structure-directing agent for the synthesis of nanosized Al- and high-silica zeolite Beta in alkaline medium

Purely siliceous and Al-containing nano-zeolite Beta (BEA) crystals were synthesized using 4,4′-trimethylenebis( N-methyl, N-benzyl-piperidinium) cations as structure-directing agent (SDA). The influence of the synthesis parameters (synthesis time and temperature, heating source) as well as the chemical composition of the precursor gel on the nature of the product and its particle size was investigated. Purely siliceous nano-zeolite Beta with a particle size of around 140 nm and single domains of around 30 nm in size was synthesized with a high level of reproducibility and stabilized in colloidal suspension. Aluminium-containing Beta zeolite (Al-BEA) crystals were prepared either under conventional hydrothermal treatment or under microwave irradiation. Stable colloidal suspensions of Al-BEA particles with a size of 200 nm were obtained, consisting of single domains in the range of 5–15 nm, depending on the nature of the heating mode.

Suspensions of repulsive colloidal particles near the glass transition: Time and frequency domain descriptions

We study the relaxation of both spontaneous and shear-induced fluctuations in suspensions of charged-stabilized colloidal particles near the glass transition by dynamic light scattering and rheology. Both observations are here understood in terms of a common structural relaxation process under a hard-sphere mode-coupling formalism. For ergodic systems, we show that the descriptions of the relaxation dynamics in time and frequency domains are governed by a common set of dynamic parameters. It is further shown that the microscopic ergodicity break-up induces the emergence of the macroscopic glass elasticity.

Short-time diffusion of charge-stabilized colloidal particles: generic features

Analytical theory and Stokesian dynamics simulations are used in conjunction with dynamic light scattering to investigate the role of hydrodynamic interactions in short-time diffusion in suspensions of charge-stabilized colloidal particles. The particles are modeled as solvent-impermeable charged spheres, repelling each otherviaa screened Coulomb potential. Numerical results for self-diffusion and sedimentation coefficients, as well as hydrodynamic and short-time diffusion functions, are compared with experimental data for a wide range of volume fractions. The theoretical predictions for the generic behavior of short-time properties obtained from this model are shown to be in full accord with experimental data. In addition, the effects of microion kinetics, nonzero particle porosity and residual attractive forces on the form of the hydrodynamic function are estimated. This serves to rule out possible causes for the strikingly small hydrodynamic function values determined in certain synchrotron radiation experiments.

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.

Invagination process induced by 2D desiccation of colloidal solutions

The drying process of a suspension of hard colloidal particles is investigated in a circular thin cell which ensures a two-dimensional radial drying. During solvent removal the concentration of particles accumulates to the liquid–gas interface and reaches the close packing concentration: a rigid skin forms. During the evaporation of water, the length of this rigid envelope keeps on decreasing till it stops shrinking. Then a buckling process occurs that leads to an inversion of the curvature. At this time and after a steady decrease, the envelope periphery starts to increase while its inner volume keeps decreasing. This deformation continues as the depressed region extends and invaginates inside the film. An experimental procedure coupling imaging of the skin formation and drying kinetics measurements provides a complete description of the successive mechanisms: drying process, formation of the rigid skin and invagination process.

Particle size effects in colloidal gelatin particle suspensions

This paper describes the effects of simple shear flow on the formation and properties of colloidal gelatin particle suspensions. Microscopy and light scattering show that simple shear flow of a phase-separating gelatin–dextran mixture gave smaller particles with a narrower size distribution. Upon gelation due to a temperature decrease, the viscosity of the gelatin increased, which altered the coalescence and break-up behaviour of the particles formed. The small particles obtained by a high shear during processing aggregated into larger particle clusters, once particle solidified upon gelation. The particle size can be predicted using correlation with droplet break-up and coalescence considering the properties before gelation. The sizes of the clusters can be predicted with the coalescence behaviour using the properties after gelation. Clusters originating from small particles resist more deformation, resulting in pronounced rheological effects (e.g. increase viscosity, increased strain softening point).

The Actual Dielectric Response Function for a Colloidal Suspension of Spherical Particles

In this paper, we present a theoretical analysis of the dielectric response of a dense suspension of spherical colloidal particles based on a self-consistent cell model. Particular attention is paid to (a) the relationship between the dielectric response and the conductivity response and (b) the connection between the real and imaginary parts of these responses based on the Kramers-Kronig relations. We have thus clarified the analysis of Carrique et al. (Carrique, F.; Criado, C.; Delgado, A. V. J. Colloid Interface Sci. 1993, 156, 117). We have shown that both the conduction and displacement current components are complex quantities with both real and imaginary parts being frequency dependent. The dielectric response exhibits characteristics of two relaxation phenomena: the Maxwell-Wagner and the alpha-relaxations, with the imaginary part being the more sensitive instrument. The inverse Fourier transform of the simulated dielectric response is compared with a phenomenological, two-exponential response function with good agreement obtained. The two fitted decay times also compare well with times extracted from the explicit simulations.

Dielectric spectra of ionic water-in-oil microemulsions below percolation: Frequency dependence behavior

We have investigated the dielectric properties of water-in-oil microemulsions composed of sodium bis(2-ethyl-hexyl)sulfosuccinate, water, and decane, using radiofrequency impedance spectroscopy, below the percolation threshold, where the system behaves as surfactant-coated individual water droplets dispersed in a continuous oil phase. The analysis of the dielectric spectra has evidenced that the whole dielectric response below percolation is due to two different contributions, which give rise to two partially overlapping dielectric relaxations, approximately in the frequency range from 10 to 500 MHz. The first of these mechanisms is originated by the bulk polarization of counterions distributed in the electrical double layer of the droplet interior. The second mechanism is associated with a correlated motion of the anionic head groups SO3- at the surfactant-water interface. The introduction of this latter contribution allows us to justify the experimentally observed increase in the low-frequency permittivity as a function of temperature up to temperatures very close to percolation. The present study shows that deviations from the expected values on the basis of dielectric theories of heterogeneous systems (Maxwell-Wagner effect) observed when percolation is approaching can be accounted for, in a reasonable way, by the introduction of a further polarization mechanism, which involves the anionic surfactant groups. Only very close to percolation, when microemulsions undergo a scaling behavior, deviations of the permittivity (and electrical conductivity as well) are a print of the structural rearrangement of the whole system and models based on colloidal particle suspension theories fail. Even if the whole picture of the dielectric properties of microemulsion systems does not change in deep, nevertheless, the refinement introduced in this paper demonstrates how different polarization mechanisms could be simultaneously present in these rather complex systems and, above all, how the individual particle colloidal properties are maintained up to very close to the percolation threshold.

Subcooled flow boiling heat transfer of dilute alumina, zinc oxide, and diamond nanofluids at atmospheric pressure

A nanofluid is a colloidal suspension of nano-scale particles in water, or other base fluids. Previous pool boiling studies have shown that nanofluids can improve the critical heat flux (CHF) by as much as 200%. In a previous paper, we reported on subcooled flow boiling CHF experiments with low concentrations of alumina, zinc oxide, and diamond nanoparticles in water (≤0.1% by volume) at atmospheric pressure, which revealed a substantial CHF enhancement (∼40–50%) at the highest mass flux ( G = 2500 kg/m 2 s) and concentration (0.1 vol.%) for all nanoparticle materials ( Kim et al., 2009). In this paper, we focus on the flow boiling heat transfer coefficient data collected in the same tests. It was found that for comparable test conditions the values of the nanofluid and water heat transfer coefficient are similar (within ±20%). The heat transfer coefficient increased with mass flux and heat flux for water and nanofluids alike, as expected in flow boiling. A confocal microscopy-based examination of the test section revealed that nanoparticle deposition on the boiling surface occurred during nanofluid boiling. Such deposition changes the number of micro-cavities on the surface, but also changes the surface wettability. A simple model was used to estimate the ensuing nucleation site density changes, but no definitive correlation between the nucleation site density and the heat transfer coefficient data could be found.

Dynamics of permeable particles in concentrated suspensions

We calculate short-time diffusion properties of suspensions of porous colloidal particles as a function of their permeability, for the full fluid-phase concentration range. The particles are modeled as spheres of uniform permeability with excluded volume interactions. Using a precise multipole method encoded in the HYDROMULTIPOLE program, results are presented for the hydrodynamic function, H(q) , sedimentation coefficient, and self-diffusion coefficients with a full account of many-body hydrodynamic interactions. While self-diffusion and sedimentation are strongly permeability dependent, the wave-number dependence of the hydrodynamic function can be reduced by appropriate shifting and scaling, to a single master curve, independent of permeability. Generic features of the permeable sphere model are discussed.

Electrostatics in liquids: From electrolytes and suspensions towards emulsions and patchy surfaces

These lecture notes describe ionic screening of liquid-immersed charged surfaces within the linear and the nonlinear Poisson–Boltzmann theory. The classic Gouy–Chapman description of a single charged planar surface is extensively described, as well as its linearised version and the connection with charge renormalisation. We also consider effective ion-mediated electrostatic interactions between suspended colloidal particles in some detail, and in addition we study Debye–Hückel theory for bulk electrolytes. We argue that relatively straightforward extensions of these basic notions can be used to describe systems of recent interest, e.g. patchy surfaces, oil–water interfaces, and Pickering emulsions. A number of references to recent literature is included.

Short-time dynamics of permeable particles in concentrated suspensions

We study short-time diffusion properties of colloidal suspensions of neutral permeable particles. An individual particle is modeled as a solvent-permeable sphere of interaction radius a and uniform permeability k, with the fluid flow inside the particle described by the Debye-Bueche-Brinkman equation, and outside by the Stokes equation. Using a precise multipole method and the corresponding numerical code HYDROMULTIPOLE that account for higher-order hydrodynamic multipole moments, numerical results are presented for the hydrodynamic function, H(q), the short-time self-diffusion coefficient, D(s), the sedimentation coefficient K, the collective diffusion coefficient, D(c), and the principal peak value H(q(m)), associated with the short-time cage diffusion coefficient, as functions of porosity and volume fraction. Our results cover the full fluid phase regime. Generic features of the permeable sphere model are discussed. An approximate method by Pusey to determine D(s) is shown to agree well with our accurate results. It is found that for a given volume fraction, the wavenumber dependence of a reduced hydrodynamic function can be estimated by a single master curve, independent of the particle permeability, given by the hard-sphere model. The reduced form is obtained by an appropriate shift and rescaling of H(q), parametrized by the self-diffusion and sedimentation coefficients. To improve precision, another reduced hydrodynamic function, h(m)(q), is also constructed, now with the self-diffusion coefficient and the peak value, H(q(m)), of the hydrodynamic function as the parameters. For wavenumbers qa>2, this function is permeability independent to an excellent accuracy. The hydrodynamic function of permeable particles is thus well represented in its q-dependence by a permeability-independent master curve, and three coefficients, D(s), K, and H(q(m)), that do depend on the permeability. The master curve and its coefficients are evaluated as functions of concentration and permeability.