Concentrated Colloidal Suspensions Research Articles

On the absence of structure factors in concentrated colloidal suspensions and nanocomposites.

Small-angle scattering is a commonly used tool to analyze the dispersion of nanoparticles in all kinds of matrices. Besides some obvious cases, the associated structure factor is often complex and cannot be reduced to a simple interparticle interaction, like excluded volume only. In recent experiments, we have encountered a surprising absence of structure factors (S(q) = 1) in scattering from rather concentrated polymer nanocomposites (Genix et al. in ACS Appl Mater Interfaces 11(19):17863-17872, 2019). In this case, quite pure form factor scattering is observed. This somewhat "ideal" structure is further investigated here making use of reverse Monte Carlo simulations in order to shed light on the corresponding nanoparticle structure in space. By fixing the target "experimental" apparent structure factor to one over a given q-range in these simulations, we show that it is possible to find dispersions with this property. The influence of nanoparticle volume fraction and polydispersity has been investigated, and it was found that for high concentrations only a high polydispersity allows reaching a state of S = 1. The underlying structure in real space is discussed in terms of the pair-correlation function, which evidences the importance of attractive interactions between polydisperse nanoparticles. The calculation of partial structure factors shows that there is no specific ordering of large or small particles, but that the presence of attractive interactions together with polydispersity allows reaching an almost "structureless" state.

Aggregation kinetics of a concentrated colloidal suspension under oscillatory flow.

The aggregation behavior of an attractive colloidal silica suspension under oscillatory flow is studied using rheological measurement. We show that the competition between the aggregation of the particles and the aggregate breakup under external stress leads to a non-monotonous evolution of the elastic modulus with time. Remarkably, under certain conditions, the elasticity is not an increasing function of time but exhibits a maximum. The value of the maximum of the elastic modulus depends on the applied shear amplitude and the ionic strength of the suspension. Scaling laws that describes the evolutions of the elastic modulus as a function of the salinity and of the deformation amplitude are proposed and discussed.

Microscopic theory for the pair correlation function of liquidlike colloidal suspensions under shear flow.

We present a theoretical framework to investigate the microscopic structure of concentrated hard-sphere colloidal suspensions under strong shear flows by fully taking into account the boundary-layer structure of convective diffusion. We solve the pair Smoluchowski equationwith shear separately in the compressing and extensional sectors of the solid angle, by means of matched asymptotics. A proper, albeit approximate, treatment of the hydrodynamic interactions in the different sectors allows us to construct a potential of mean force containing the effect of the flow field on pair correlations. We insert the obtained pair potential in the Percus-Yevick relation and use the latter as a closure to solve the Ornstein-Zernike integral equation. For a wide range of either the packing fraction η and the Péclet (Pe) number, we compute the pair correlation function and extract scaling laws for its value at contact. For all the considered values of Pe, we observe a very good agreement between theoretical findings and numerical results from the literature, up to rather large values of η. The theory predicts a consistent enhancement of the structure factor S(k) at k→0, upon increasing the Pe number. We argue this behavior may signal the onset of a phase transition from the isotropic phase to a nonuniform one, induced by the external shear flow.

Scaling of Shear Rheology of Concentrated Charged Colloidal Suspensions across Glass Transition.

Electrostatic interparticle interactions are a key component in controlling and designing the rheological characteristics of concentrated charged colloidal suspensions. Herein, we investigate electroviscous effects on shear rheology using highly charged silica particles. By fixing the volume fraction but varying the salinity, the system undergoes a glass transition as evidenced by the evolution of the yield stress and zero-shear viscosity. We show that the steady shear viscosities obey a critical scaling relation that scales the flow curves into a supercritical branch and a subcritical branch with glass transition salinity serving as the bifurcation point; we also demonstrate an isoviscosity scaling that collapses all isoviscosity lines into a single master curve that exhibits no singularity. On the basis of each scaling relation, in conjunction with common modeling equations, the quantitative relationships between the shear viscosity, stress, and salinity are established. This study demonstrates a new framework to model the steady shear rheology of concentrated charged colloids.

Stress versus strain controlled shear: Yielding and relaxation of concentrated colloidal suspensions

In rheological experiments, the relationship between stress and strain is determined. In the transient regime, this relationship may depend on which of these properties is applied and which is measured. In general, data collected using one or the other as the control parameter are not necessarily equivalent. Moreover, the assumed steady state and the relaxation following this state might depend on whether stress or strain has been applied. We examined colloidal suspensions with concentrations around the glass transition and compared their response to stress and strain, in particular, their transient response after the start-up of shear, their steady state, and their relaxation after cessation of shear. After the start-up of shear, the transient behavior was found to significantly depend on whether the sample is exposed to a constant shear rate or a constant stress. Nevertheless, the transients lead to a rheological steady state that is independent of how it is reached, as long as yielding occurred and the corresponding shear rate or stress is applied. After cessation of shear, the relaxation under strain and stress control shows both similarities and differences. This is quantified based on, e.g., the hydrodynamic, Brownian, and residual stress as well as the recovered strain. In addition, the responses of the rheometers to the abrupt changes have been characterized. The corresponding technical data are presented and taken into account in the data interpretation.

Effects of electrostatic interaction on rheological behavior and microstructure of concentrated colloidal suspensions

This research examined the effect of salt concentration on the viscosity behavior of concentrated colloidal suspensions of monodispersed silica particles in aqueous media and ethylene glycol. Silica particles with a lot of negative surface charge form three-dimensional ordered structures (colloidal crystals) in aqueous media or ethylene glycol due to electrostatic repulsion. These three-dimensional structures are distorted by shear, causing viscosity to decrease and shear thinning to occur. Adding salt increases the ionic strength within the suspension, thereby reducing the Debye length of the particle and weakening the electrostatic repulsion between particles. As a result, the viscosity at low shear rates and the shear thinning gradient decreases. However, if the salt concentration increases further, the ionic strength within the suspension will rise, virtually eliminating the Debye length of the particle. In this event, the electrostatic repulsion between the particles becomes extremely weak, and the microstructures will be formed in the aqueous media by hydrophobic interaction and van der Waals force between the particles. As a result, the viscosity at low shear rates and the shear thinning gradient becomes extremely large. In contrast, the behavior in an ethylene glycol differs from behavior in an aqueous media. In this case, the microstructures are not formed by hydrophobic interaction. Consequently, collisions between particles are promoted, causing shear thickening. This phenomenon provides further support to the hypothesis that shear thickening is caused by collisions between particles and suggests that shear thickening can be avoided by strengthening the electrostatic interaction between particles.

Ultrasonic propagation in concentrated colloidal dispersions: improvements in a hydrodynamic model

Recent theoretical and experimental findings demonstrate that modeling ultrasonic attenuation of a concentrated colloidal suspension requires inclusion of shear-induced contributions, an element being unaccounted for by most scattering models. Herein, we extend a hydrodynamic model from low to high particle volume fraction by effectively relating the single particle dynamic drag to particle concentration to account for hydrodynamic interparticle interactions. We calculate an expression for the complex-valued effective dynamic mass density at high concentrations, which is then combined with a viscosity-corrected effective bulk modulus to estimate ultrasonic velocity and attenuation for a monodisperse suspension of solid spherical particles in a viscous liquid. The effective velocity and attenuation are functions of particle volume fraction, frequency, and physical properties of particles and liquid. We compare our results with those from two recently developed scattering models: a multi-modal multiple scattering model and a core-shell effective medium model, each taking into account the viscosity of the host fluid through shear wave influences. Finally, we find that our extended model predicts experimental attenuation data the best for a silica in water suspension compared to the results of other models.

Multifunctional Properties of Binary Polyrhodanine Manganese Ferrite Nanohybrids-From the Energy Converters to Biological Activity.

The PRHD@MnFe2O4 binary hybrids have shown a potential for applications in the biomedical field. The polymer cover/shell provides sufficient surface protection of magnetic nanoparticles against adverse effects on the biological systems, e.g., it protects against Fenton’s reactions and the generation of highly toxic radicals. The heating ability of the PRHD@MnFe2O4 was measured as a laser optical density (LOD) dependence either for powders as well as nanohybrid dispersions. Dry hybrids exposed to the action of NIR radiation (808 nm) can effectively convert energy into heat that led to the enormous temperature increase ΔT 170 °C (>190 °C). High concentrated colloidal suspensions (5 mg/mL) can generate ΔT of 42 °C (65 °C). Further optimization of the nanohybrids amount and laser parameters provides the possibility of temperature control within a biologically relevant range. Biological interactions of PRHD@MnFe2O4 hybrids were tested using three specific cell lines: macrophages (RAW 264.7), osteosarcoma cells line (UMR-106), and stromal progenitor cells of adipose tissue (ASCs). It was shown that the cell response was strongly dependent on hybrid concentration. Antimicrobial activity of the proposed composites against Escherichia coli and Staphylococcus aureus was confirmed, showing potential in the exploitation of the fabricated materials in this field.

Jet instability of a shear-thickening concentrated suspension.

We investigate the flow of a concentrated suspension of colloidal particles at deformation rates higher than the discontinuous shear-thickening transition shear rate. We show that, under its own weight, a jet of a concentrated enough colloidal suspension, simultaneously flows while it sustains tensile stress and transmits transverse waves. This results in a new flow instability of jets of shear-thickening suspensions: the jet is submitted to rapid transverse oscillations, that we characterize.

Interpreting rheology and electrical conductivity: It all boils down to which particle size

Hypothesis: The electrical charges that develop on the surface of the ceramic particles upon contact with water, due to the interaction with ions in solution, result in a liquid-solid interface, which utterly modifies the properties of individual particles and the way they interact with each other to form a structure. This work explores a new approach to the relationships between structure and stability of suspensions. Experiments: For this purpose, suspensions with a constant 0.35 volume fraction of α-alumina particles, neither spherical nor smooth, and controlled ionic strength (0–90 mM KCl) were prepared and characterized in terms of flow behaviour, electrical conductivity and particle’s electrokinetic mobility. Findings: Electrical conductivity (132 µS/cm < conductivity < 5730 µS/cm) and rheology measurements (10−2 Pa s < viscosity < 104 Pa s) were found to complement each other to produce a more accurate picture of the suspension’s structure. Deviations of experimental data from well-accepted behavioural models were elucidated when the surface area equivalent particle size was used. With the electrical double layer thickness obtained from electrical conductivity measurements, this enabled the interpretation of the relationship between the suspension’s viscosity and the particles electrical conductivity, which provides a criterion for the stability of concentrated colloidal suspensions.

Enhanced Shear Thickening of Silica Colloidal Suspension Using Polystyrene-Polyacrylamide Particles

Shear thickening (ST) refers to a non-Newtonian behavior of concentrated colloidal suspensions, which exhibits significant viscosity increments at high shear stress or high shear rate. A shear thickening fluid (STF) consists of well-dispersed solid particles in a liquid medium. Silica particles are often used as components of STFs due to the abundance of hydroxyl groups on their surfaces. Polystyrene core-polyacrylamide shell particles (PS-PAAm particles) were prepared to enhance the ST of silica particle-based colloidal suspensions. Addition of PS-PAAm particles to various particle weight fractions of silica particle-based STFs resulted in amplification of ST in all instances. These results suggest that PS-PAAm particles enhance inter-particle interactions due to the abundance of hydrogen-bonding donor groups in polyacrylamide.

Experimental test of a frictional contact model for shear thickening in concentrated colloidal suspensions

Rheological experiments on model and industrial colloidal dispersions that exhibit reversible shear thickening are compared to a recent model that correlates simulation results for the shear rheology of bidisperse suspensions of particles with varying levels of contact friction. Comparisons are presented with rheological results for a variety of colloidal dispersions varying from model systems to industrially relevant, commercial dispersions where both shear and first normal stress differences are available. Particle-scale measurements of particle friction coefficients are provided through a survey of the literature. The comparisons show under which conditions the model can quantitatively describe the measured rheology and how the model can be used to correlate data for systems with more complex interactions when allowances are made for effective volume fractions and adjustments in friction coefficients. Discrepancies evident in comparisons for the first normal stress differences highlight the importance of enhanced hydrodynamic friction in many systems, especially for continuous shear thickening, in qualitative agreement with recent simulations that incorporate enhanced lubrication hydrodynamics due to elastohydrodynamics or particle roughness. Finally, a literature survey of measurements of jamming volume fractions and particle friction coefficients indicates that the model overestimates the experimentally observed jamming volume fraction. The comparisons presented in this work provide quantitative information valuable for those modeling suspension processing as well as suggestions for further model improvement.

New analysis and correlation between steady and oscillatory tests in fumed silica-based shear thickening fluids

In recent years, the non-Newtonian behavior of shear thickening fluids (STFs) has motivated a sharp increase in the number of studies related to their use for engineering applications. For this reason, rheological characterization of STFs is crucial in order to develop and design new devices based on these novelsmart fluids. Typically, different experiments have been carried out for the rheological characterization of STFs to prove their shear/strain thickening behavior. In order to find an empirical relationship between steady and oscillatory experiments, the modified Cox-Merz rule has been recently used to correlate the results with enormous controversy between authors due to the disparity in the correlations achieved. In this paper, we present an improvement in the correlation between the results obtained in steady and oscillatory tests in STFs using fumed silica-concentrated suspensions. The proposed method with the use of strain sweep oscillatory tests has improved the correlation in all STF regimes (pre-transition, transition, and post-transition) in comparison with the results obtained using the modified Cox-Merz rule, which uses oscillatory frequency sweep tests. This improvement has been experimentally validated using concentrated colloidal suspensions with different concentrations of fumed silica (from 12.5 to 25 wt%). Rheological experiments with shear thickening fluids performed in this article.

Rapid Dewatering and Consolidation of Concentrated Colloidal Suspensions: Mature Fine Tailings via Self-Healing Composite Hydrogel.

Billions of tonnes of thick waste streams with highly concentrated colloidal suspensions from different origins have accumulated worldwide, exampled as over 220 km2 mature fine tailings (MFT) from oil sands production in north Alberta. Current treatment technologies are limited by slow yet insufficient water release and sludge consolidation. Herein, a self-healing composite hydrogel system is designed to convert concentrated aqueous colloidal suspensions (e.g., MFT with colloidal solid content >30 wt %) into a dynamic double cross-linked network for rapid dewatering and consolidation. The resultant composite hydrogel demonstrates an excellent dewatering performance so that over 50% of water could be rapidly released within 30 min by vacuum filtration. Furthermore, the formed infinite cross-linked network with self-healing ability can effectively trap fine particles of all sizes and capture small flocs during mechanical mixing, thereby enabling a low solid content at the ppm level in the released water. This new strategy outperforms all the previously reported treatment methods; under mechanical compression, over 80% of water is removed from the MFT, thereby generating a stackable material with >70 wt % solids within an hour. These results demonstrate a highly effective approach and provide insight into the development of advanced materials to tackle the challenging environmental slurry issues.

Enhanced shear thickening of polystyrene-poly(acrylamide) and polystyrene-poly(HEMA) particles

Shear thickening (ST) refers to the drastic increment of viscosity that occurs when a high shear force is applied to certain concentrated colloidal suspensions. The hydrocluster mechanism for ST indicates that inter-particle interaction is important for ST behavior. Therefore, we prepared polystyrene particles with polyacrylamide particles (PS-PAAm particles) to control inter-particle interactions. Colloidal suspensions of polystyrene-poly(2-hydroxyethyl methacrylate) (PS-PHEMA) particles in ethylene glycol were prepared and shown to exhibit strong ST behavior. The effect of PS-PAAm particles on the ST behavior of a PS-PHEMA particle colloidal suspension was investigated; ST behavior of shear thickening fluids (STFs) was controlled by doping different amounts of PS-PAAm particles. We suggested inter-particle interaction was enhanced by abundant hydrogen-bonding donor groups in PS-PAAm particles. Based on this study, the applicability of STF can be increased through the STF fabrication exhibiting desired ST behavior. Also, this study will help to understand fundamental ST mechanism.

Modeling mechanical properties of a shear thickening fluid damper based on phase transition theory

Shear thickening fluids (STFs) are highly concentrated colloidal suspensions consisting of monodisperse nano-particles suspended in a carrying fluid, and have the capacity to display both flowable and rigid behaviors, when subjected to sudden stimuli. In that process, the external energy that acts on an STF can be dissipated quickly. The aim of this study is to present a dynamic model of a damper filled with STF that can be directly used in control engineering fields. To this end, shear stress during phase transition of the STF material is chosen as an internal variable. A non-convex function with bifurcation behavior is used to describe the phase transitioning of STF by determining the relationship between the behavioral characteristics of the microscopic phase and macroscopic damping force. This model is able to predict force-velocity and force-displacement relationships as functions of the loading frequency. Efficacy of the model is demonstrated via comparison with experimental results from previous studies. In addition, the results confirm the hypothesis regarding the occurrence of STF phase transitioning when subject to shear stress.

Sedimentation behavior of high-temperature concentrated colloidal suspension based on potassium cryolite

ABSTRACTThe paper describes the sedimentation behavior of concentrated high-temperature oxide-fluoride slurries with alumina particles volume fraction range 0.24 ≤ φ ≤ 0.32 at ∼700 °C. The behavior is of interest due to perspectives of the non-Newtonian fluids usage in the future aluminium reduction technology. To characterize sedimentation behavior several techniques were used: density analysis, X-Ray diffraction, microphotography and electron scanning microscopy. Sample with −63 µm dispersed phase as well as smelter-grade alumina which contains particles in a range of 10–150 µm has been examined. It has been found that particle settling occurs with the initial velocity 0.97·10−5 m·s−1 at φ = 0.24 and gradually reduces reaching zero at φ close to 0.32 which was considered as a maximum packing fraction. MS−1 sedimentation mode has been identified, the type of sedimentation was found to be changed from type 1 to type 3 in the sample with φ = 0.24. The alumina partial density distribution in suspension as a function of time was determined. Alumina average motion velocity during sedimentation was calculated to be 154·10−3 kg·m−2·s−1 at φ = 0.24 in the top layer of the sample within the first 150 s of sedimentation. It had been reducing drastically before it reached zero after 300 s. An alumina flow was higher in the top layers. Some of the rheological properties (such as relaxation time, which increased from 7.2·10−3 to 17.7·10−3 s after the increase of φ from 0.24 to 0.28) has been determined.

Impact-induced solidlike behavior and elasticity in concentrated colloidal suspensions.

Modified drop weight impact tests were performed on SiO_{2}-ethylene glycol concentrated suspensions. Counterintuitive impact-induced solidlike behavior and elasticity, causing significant deceleration and rebound of the impactor, were observed. We provide evidence that the observed large deceleration force on the impactor mainly originates from the hydrodynamic force, and that the elasticity arises from the short-range repulsive force of a solvation layer on the particle surface. This study presents key experimental results to help understand the mechanisms underlying various stress-induced solidification phenomena.

Coupled elasticity in soft solid foams

Elasticity of soft materials can be greatly influenced by the presence of air bubbles. Such a capillary effect is expected for a wide range of materials, from polymer gels to concentrated emulsions and colloidal suspensions. Whereas experimental results and theory exist for describing the elasto-capillary behavior of bubbly materials (i.e. with moderate gas volume fractions), foamy systems still require a dedicated study in order to increase our understanding of elasticity in aerated materials over the full range of gas volume fractions. Here we elaborate well-controlled foams with concentrated emulsion and we measure their shear elastic modulus as a function of gas fraction, bubble size and elastic modulus of the emulsion. Such complex foams possess the elastic features of both the bubble assembly and the interstitial matrix. Moreover, their elastic modulus is shown to be governed by two parameters, namely the gas volume fraction and the elasto-capillary number, defined as the ratio of the emulsion modulus with the bubble capillary pressure. We connect our results for foams with existing data for bubbly systems and we provide a general view for the effect of gas bubbles in soft elastic media. Finally, we suggest that our results could be useful for estimating the shear modulus of aqueous foams and emulsions with multimodal size distributions.

Double layer electrical conductivity as a stability criterion for concentrated colloidal suspensions

The slightly attractive inter–particle equilibrium potential associated with electrostatically stabilized suspensions of minimum viscosity is described by the DLVO theory and commonly gauged by static ζ-potential measurements, plagued with experimental uncertainties. In this work, the electrokinetic mobility of alumina particles was measured in suspensions prepared with selected solids content and ionic strength, as well as was the electrical conductivity of each suspension and suspending liquid. Particles electrical conductivity was then calculated and related to the colloidal stability described by the DLVO theory, enabling the identification of a processing window for the stability control of concentrated suspensions. The maximum repulsive potential and distance between particles (∼46nm) corresponds to the particles maximum conductivity. When the particles conductivity is zero, the diffuse layer is fully collapsed and they stand at the minimum reversible distance (∼7nm). At the equilibrium conductivity, a potential curve is produced with a secondary attractive minimum of ∼1.5 kT at an inter–particle distance of ∼17nm, as suggested by the DLVO theory and the Equipartition of Energy theorem. The condition for accurate measurement of static ζ-potential occurs at the isoconductivity point between particles and suspending liquid.