Dielectric spectroscopy of bidisperse colloidal suspensions

Tuning the luminescence intensity of fluorophores using nanoparticles has shown great potential for the detection of inorganic metal ions, viruses, and proteins. The enhancement or quenching of a dye's fluorescence intensity is strongly dependent on the spatial separation of the dye from the nanoparticle surface. To extend luminescence probing from the solution platform to the solid-state platform, we explored and performed dye quenching assessment using an array format in this study. We report the distance-dependent fluorescence behavior of Au-DNA conjugates prepared by equilibrating phosphine-stabilized gold nanoparticles (AuNPs) of 10-nm size with the designed spacer ds-DNA consisting of thiol-modified target and Cy3-labeled complementary probe of different lengths (5-20 nm). The Cy3-labeled products were immobilized onto MPTMS (3-mercaptopropyltrimethoxysilane)-modified glass substrates and then excited with a 532-nm laser source. Quenching efficiency of AuNPs with increasing Au-to-dye distance was assessed using ligand exchange of the thiolated oligonucleotide by 2-mercaptoethanol (ME) to obtain free Cy3-DNA probe, thus eliminating nanoparticle effect on the dye's luminescence intensity. Effective exchange, revealed by UV-vis absorption and fluorescence profiles, was achieved in a few minutes. It was observed that fluorescence quenching of Au-DNA-Cy3 assessed using the array format was consistent with the result in solution phase for the conjugates with up to 10-nm Au-to-Cy3 separation distance.     

Viscosity of concentrated suspensions: influence of cluster formation

Dispersed particles can form clusters even at low concentrations. Colloidal and hydrodynamic forces are responsible for this phenomenon and these forces determine both structure and size of clusters. We assume that the viscosity of a concentrated suspension is completely determined by cluster size distribution, regardless if clusters form under the action of colloidal, hydrodynamic interactions or applied shear rates. Based on this assumption an equation, which describes dependency of viscosity on a concentration of dispersed particles taking into account cluster formation, is deduced. Under special restrictions the deduced dependency coincides with the well-known Dougherty-Krieger's equation except for a clear physical meaning of parameters entered. Our consideration shows that Dougherty-Krieger's equation has deeper physical background than it has been supposed earlier. Experimental verification of the suggested model shows a good agreement with the theory predictions and proves a presence of clusters even at low concentrations of dispersed particles.     

Viscosity of concentrated colloidal solutions of magnetite

For magnetite colloidal solutions in kerosene stabilized by oleic acid, concentration and temperature dependences of viscosity are determined at parameters varied over wide ranges. The concentration dependence of viscosity is satisfactorily described by the Chong equation (modified to some extent), while the discrepancy between the temperature dependence and the theoretical data can be as large as two orders of magnitude. The observed discrepancy is explained by a decrease in the thickness of the shells composed of surfactant molecules on the particle surface with temperature. At extremely low concentrations, the increment to the effective viscosity linear with respect to concentration is slightly (by 14%) larger than the Einstein value, which may be due to the presence of a small number of aggregates in the solutions.     

Electroviscous effect of moderately concentrated colloidal suspensions: Stern-layer influence

A previous model for the viscosity of moderately concentrated suspensions has been extended. The influence of a dynamic Stern layer (DSL), which produces an additional surface conductance at the electrolyte-particle interface, is included. The theoretical treatment is based on Happel's cell model with Simha's boundary conditions for the interparticle hydrodynamic interactions and on a dynamic Stern-layer model for ionic conduction on the particle surface according to Mangelsdorf and White (ref 39). The results are valid for arbitrary zeta potentials and double-layer thickness. Extensive theoretical predictions are shown and interesting new behaviors are found. The comparison with the results in the absence of additional surface conductance shows a great influence of this mechanism in the energy dissipation during the laminar flow of these suspensions. We conclude that the inclusion of a dynamic Stern layer will be required to match the predictions with the experimental results.     

Electroviscous effect of moderately concentrated colloidal suspensions under overlapping conditions

In this work we present a theoretical model for the calculation of the electroviscous coefficient of a colloidal suspension. The treatment is not limited for dilute suspensions and includes the contribution of the overlapping between adjacent ionic layers. The development here used is based on a cell model, which is applicable to Newtonian suspensions under low shear conditions and without crystalline ordering. Also presented are a complete study of the new numerical results and comparisons with previous results. We find new behaviors for the case of moderate volume fractions that do not appear in the dilute limit.     

Sedimentation of concentrated monodisperse colloidal suspensions: role of collective particle interaction forces

The sedimentation velocities and concentration profiles of low-charge, monodisperse hydroxylate latex particle suspensions were investigated experimentally as a function of the particle concentration to study the effects of the collective particle interactions on suspension stability. We used the Kossel diffraction technique to measure the particle concentration profile and sedimentation rate. We conducted the sedimentation experiments using three different particle sizes. Collective hydrodynamic interactions dominate the particle-particle interactions at particle concentrations up to 6.5 vol%. However, at higher particle concentrations, additional collective particle-particle interactions resulting from the self-depletion attraction cause particle aggregation inside the suspension. The collective particle-particle interaction forces play a much more important role when relatively small particles (500 nm in diameter or less) are used. We developed a theoretical model based on the statistical particle dynamics simulation method to examine the role of the collective particle interactions in concentrated suspensions in the colloidal microstructure formation and sedimentation rates. The theoretical results agree with the experimentally-measured values of the settling velocities and concentration profiles.     

Ion-specific effect on the gelation kinetics of concentrated colloidal silica suspensions

The comparative gelation kinetics of concentrated colloidal silica suspensions at pH 9.8 has been studied in the presence of two different counterions, ammonium as a representative structure-breaker ion and sodium as a representative structure-maker ion. For the same ionic strength of 0.45 M, the experimental results have indicated a pronounced effect of the ion-specificity on the aggregation kinetics and on the microstructure of the gelified networks at the sol-gel transition. This was ascribed to the significant difference of the critical coagulation concentration value in the presence of the two electrolytes.     

Dynamic electrophoretic mobility of spherical colloidal particles in salt-free concentrated suspensions

In this contribution, the dynamic electrophoretic mobility of spherical colloidal particles in a salt-free concentrated suspension subjected to an oscillating electric field is studied theoretically using a cell model approach. Previous calculations focusing the analysis on cases of very low or very high particle surface charge are analyzed and extended to arbitrary conditions regarding particle surface charge, particle radius, volume fraction, counterion properties, and frequency of the applied electric field (sub-GHz range). Because no limit is imposed on the volume fractions of solids considered, the overlap of double layers of adjacent particles is accounted for. Our results display not only the so-called counterion condensation effect for high particle charge, previously described in the literature, but also its relative influence on the dynamic electrophoretic mobility throughout the whole frequency spectrum. Furthermore, we observe a competition between different relaxation processes related to the complex electric dipole moment induced on the particles by the field, as well as the influence of particle inertia at the high-frequency range. In addition, the influences of volume fraction, particle charge, particle radius, and ionic drag coefficient on the dynamic electrophoretic mobility as a function of frequency are extensively analyzed.     

Viscosity of liquid suspensions with fractal aggregates: Magnetic nanoparticles in petroleum colloidal structures

New physical model is presented resulting in a simple formula for the dependence of viscosity η of colloidal liquid solution on the shear rate G applicable to a wide variety of systems including complex natural liquids like petroleum. The principal point of the model is the fractal nature of colloid particle aggregates present in the liquid. Such aggregates are experimentally detected now in non-Newtonian liquids. The model is based on calculation of energy loss on colloidal particle aggregate of fractal structure localized in the flow of liquid with shear rate. We have performed the viscosity measurement experiments which confirmed successfully the developed physical model. Also, we demonstrate experimentally that petroleum colloidal particles and magnetic iron oxide nanoparticles can form composite fractal-like aggregates in natural petroleum materials. Our model can explain both the non-Newtonian properties of petroleum and sensitivity of petroleum viscosity to external magnetic fields.     

Effect of stagnant-layer conductivity on the electric permittivity of concentrated colloidal suspensions

A long-lasting experience in the electrokinetics of suspensions has shown that the so-called standard model may be partly in error in explaining experimental data. In this model, the stagnant layer is considered nonconducting (Ksigmai=0), and only the diffuse layer contributes to the total surface conductivity (Ksigma=Ksigmad). In the present work, the authors analyze the consequences of assuming a nonzero stagnant layer conductivity on the permittivity of concentrated suspensions. Using a cell model to account for the particle-particle interactions, and a well established ion adsorption isotherm on the inner region of the double layer, the authors find the frequency-dependent electric permittivity of suspensions of spherical particles with volume fractions of solids up to above 40%. It is demonstrated that the addition of Ksigmai significantly increases the contributions of the double layer to the polarization of the suspension: the alpha or concentration polarization at low (kilohertz) frequencies, and the Maxwell-Wagner-O'Konski (associated with conductivity mismatch between particle and medium) one at intermediate (megahertz) frequencies. While checking for the possibility that the results obtained in conditions of Ksigmai not equal 0 could be reproduced assuming Ksigmai=0 and raising Ksigmad to reach identical total Ksigma, it is found that this is approximately possible in the calculation of the permittivity. Interestingly, this does not occur in the case of electrophoretic mobility, where the situations Ksigma=Ksigmad and Ksigma=Ksigmad+Ksigmai (for equal Ksigma) can be distinguished for all frequencies. This points to the importance of using more than one electrokinetic technique to properly evaluate not only the zeta potential but other transport properties of concentrated suspensions, particularly Ksigmai.     

Energy dissipation in concentrated monodisperse colloidal suspensions of silica particles in polyethylene glycol

Highly concentrated colloidal suspensions exhibit a discontinuous shear-thickening behaviour. The transition from a low to a high viscosity state is associated to a large energy dissipation. This effect could find applications in structural damping while the viscosity increase brings added stiffness. In the present work, highly concentrated suspensions of monodisperse spherical silica particles in polyethylene glycol were selected for their strong thickening at low critical shear rates. Their damping properties were characterized by measuring the energy dissipated per cycle at low frequency (below 2 Hz) during oscillatory tests using a rheometer. The influence of parameters such as particle concentration, size and frequency was investigated. Damping was found to overcome that of benchmark elastomeric materials only in high frequencies and high strain domains.     

Investigation of particle interactions in concentrated colloidal suspensions using frequency domain photon migration: monodisperse systems

Frequency domain photon migration (FDPM) measurements were conducted to assess particle interactions of concentrated, monodisperse, polystyrene samples obtained directly from industry by using multiple scattering light. The angle-integrated static structure factor, S(q), and static structure factor at small wave vector q, S(0), were obtained from FDPM measurements at high volume fractions ranging from 0.05 to 0.3, and were compared with those obtained from the monodisperse hard sphere Percus-Yevick (HSPY) model. Effects of different colloid sizes on structure factor evaluated at two different wavelengths were also investigated. Results show that the monodisperse HSPY model is suitable for accounting for particle interactions and local microstructures in these colloidal suspensions. Upon using the HSPY model, particle sizes of polystyrene suspensions were recovered from FDPM measurements at high volume fractions (up to 0.3), which agree well with the DLS measurement of diluted sample ( approximately 0.001). The study of polydispersity effect shows that the FDPM method can be successfully used for recovering the mean particle size of polydisperse colloidal suspension with low polydispersity (<16%) under the assumption of monodisperse hard sphere systems.     

Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.     

Rheological peculiarities of concentrated suspensions

The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.     

Viscosity of hard sphere suspensions

A simple functional representation of the concentration dependence of the low-shear viscosity eta of hard sphere suspensions is proposed. The representation, which agrees with published literature at all volume fractions phi, has a hitherto-unremarked transition in its functional form at phi approximately 0.42 identical with phi(t). phi(t) is definitely less than the volume fraction 0.49 of the hard sphere melting transition.     

Electrokinetic parameters of colloidal model systems: analysis and comparison between dilute and concentrated dispersions

In the last decades, the interest of many scientists has been focused on the atypical electrokinetic behavior of charged colloidal systems since several studies have shown, in most cases; it is not so ideal as expected. Particularly, two interesting phenomena have not been clearly explained yet. First, the zeta potential magnitude does not decrease monotonically with increasing ionic strength, as expected according to the Gouy-Chapmann model predicts. Second, the zeta potential obtained from different techniques shows discrepancies. More specifically, the zeta potential obtained from streaming potential is lower (in absolute value) than that measured through electrophoretic mobility. However, a recent work has pointed out that these discrepancies seem to disappear if certain conditions (related with the surface charge density) are satisfied. This work also revealed that unexpected results are found when the electric conductivity was used. Spherical polystyrene particles of appropriate particle size and charge density are employed as polymeric colloidal model in the present work. Common and adequate models are used to make clear and easy our theoretical analysis and its interpretation. To test the surface conductance and ionic mobility effects at the solid-liquid interface, both water medium and alcohol-water mixtures are used.     

Electrorheological fluids as colloidal suspensions

An increase in suspension stress transfer by many orders of magnitude upon application of an external electric field is commonly referred to as the electrorheological response. Suspensions displaying this behavior are often composed of a nonaqueous dispersion of colloidal particles. In this review, the current understanding of the origin of electrorheology is described in the context of a fundamental discussion of the colloidal forces relevant to these suspensions. We show that many of the observed phenomena can be described in terms of colloidal and electric field induced interparticle interactions. The field induced arrangement of a suspension, where columns of particles are formed along field lines, is intimately related to its rheological response. A review of particle interaction potential energies of both colloidal and electric origin provides basic scaling relationshios useful in understanding structural alterations and leads naturally to a discussion of models of a suspension's rheological response. Poorly understood areas such as the effect of charge carriers in the continuous phase and particle size, shape and chemistry are delineated to indicate areas deserving further research.     

The viscoelastic properties of concentrated suspensions

The viscoelastic properties of concentrated, well dispersed, sterically stabilised polymethyl methacrylate particles were studied using a specially developed low frequency, controlled shear stress rheometer. The experimental results show a well defined viscoelastic transition over a narrow frequency range. Evidence is produced to show that the particles act as hard, non-interacting spheres. It is argued that the transition represents a change from a randomly packed assemblage with particle motion dominated by Browniandiffusion processes to a lower packing density shear flow regime.