Transient Processes in the Disperse System with Coagulation and Aggregate Disintegration

The kinetics of transition of the coagulating disperse system with aggregate disintegration into a new equilibrium state was considered in the course of the disturbance of the earlier existing equilibrium, which is caused by a change in the flow shear rate. An analysis carried out on the basis of a two-fraction model of dispersed phase [B.M. Dolgonosov, Kolloid. Zh., 2001, vol. 63, no. 1, p. 32] has shown that the spectrum of fine-fraction particles is a superposition of the initial and final states with the superposition coefficients dependent on the current value of the average mass of coarse-fraction particles (large-size aggregates). The spectrum of large aggregates in the form of a quasi-equilibrium distribution of a Gaussian type with time-dependent parameters was obtained. The variation of superposition coefficients and parameters of the spectrum of large aggregates with time was calculated. The conditions of applicability of the suggested approach were discussed.     

Evolution of the Size Spectrum of Aggregates in the Disperse System with Reversible Coagulation

Kinetics of coagulation–fragmentation in a disperse system represented by different-scale fractions was studied using two-fraction model (Dolgonosov, B.M., Kolloidn. Zh., 2001, vol. 63, no. 1, p. 27). A model accounting originally only for the interaction between different fractions was extended, by introducing interactions between the particles of coarse fraction, to describe considerable deviations from the equilibrium. The threshold dependence of the coagulation efficiency on the sizes of colliding aggregates was introduced. According to this dependence, the efficiency vanishes when the aggregate sizes exceed a certain threshold value. The scheme of discretization of integro-differential equations of a model was proposed to numerically solve these equations. The results of calculations demonstrated the evolution of dispersed phase to the equilibrium, which is accompanied by (1) the transformation of initial bimodal spectrum into polymodal, (2) complex evolution of the latter with the change in the position and height of peaks and their merging, and, finally, (3) a return to bimodal spectrum with changed characteristics.     

Kinetics of Reversible Gravitational Coagulation in a Spatially Uniform Disperse System

Spatially uniform disperse system where the processes of coagulation, fragmentation, and sedimentation of fractal aggregates occur simultaneously, was discussed. Kinetics of the passage of such system from the initial state, where the dispersed phase was already separated into fine and coarse fractions, to a new equilibrium state was studied. The enlargement of aggregates takes place both due to the capture of the particles of fine fraction and their adherence during collisions. Disintegration of aggregates occurs by the detachment of fragments from the aggregate surface under the action of hydrodynamic stresses arising during aggregate sedimentation. The solution of the coagulation–fragmentation equations indicates that, at the intermediate stage of a process, the aggregate mass distribution density is characterized by polymodal structure, which is degenerated into bimodal by the end of the passage.     

Effect of sol-gel transition on shear-induced drop deformation in aqueous mixtures of gellan and kappa-carrageenan

In this work, we present an experimental methodology to investigate the dynamics under shear flow of a drop that is gelling as a consequence of a temperature quench. The experiments were carried out on the system water/gellan/kappa-carrageenan in the biphasic region of the phase diagram, the gellan-rich phase being used as the dispersed phase. Gelation was brought about by lowering the temperature during flow after steady state drop deformation had been reached. Simple shear flow was applied by using a parallel plate apparatus equipped with optical microscopy and image analysis, which made it possible to monitor drop shape evolution before, during, and after gelation. The onset of gelation trapped drop deformation, thus producing anisotropic particles. The fingerprint of gelation was the simultaneous tumbling of the drops, which rotated as rigid ellipsoids under the action of shear flow. Interfacial tension between the two equilibrium phases was determined at different times during the temperature quench by analyzing drop retraction upon cessation of flow. Up to gelation, no significant change was observed in the measured values.     

Mesoscale constitutive modeling of magnetic dispersions

A constitutive model for dispersions of acicular magnetic particles has been developed by modeling the particles as rigid dumbbells dispersed in a solvent. The effects of Brownian motion, anisotropic hydrodynamic drag, a steric force in the form of the Maier-Saupe potential, and, most importantly, a mean-field magnetic potential are included in the model. The development is similar to previous models for liquid-crystalline polymers. The model predicts multiple orientational states for the dispersion, and this phase behavior is described in terms of an orientational order parameter S and an average alignment parameter J; the latter is introduced because the magnetic particles have distinguishable direction due to polarity. A transition from isotropic to nematic phases at equilibrium is predicted. Multiple nematic phases-both prolate and oblate-are predicted in the presence of steady shear flow and external magnetic field parallel to the flow. The effect of increasing magnetic interparticle interactions and particle concentration is also presented. Comparisons with experimental data for the steady shear viscosity show very good agreement.     

Migration of nanosilica particles in polymer blends

Hydrophilic pyrogenic silica melt mixed in immiscible polypropylene/poly (ethylene‐co‐vinyl acetate) (PP/EVA) blend was found to migrate from the PP matrix to the EVA dispersed domains and remained confined inside them. Surprisingly, it was shown than silica was also able to migrate from a dispersed PP phase to an EVA matrix but this transfer was slower and not complete. The same silica with a hydrophobic surface treatment moved and accumulated to the blend interface and in PP. The mechanisms from which this migration proceeds are discussed. Whereas self diffusion of the particles was shown to have almost no effect, shear induced movements and collisions with dispersed drops is believed to be the most efficient mechanism. The possible trapping of silica aggregates during droplet–droplet coalescence was impossible to observe but is thought to be a possible additional mechanism. No quantification on the relative importance of the latter phenomenon can be drawn at the moment. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1976–1983, 2008     

Experimental and modeling study of breakage and restructuring of open and dense colloidal aggregates

In this work we present experimental and simulation analysis of the breakage and restructuring of colloidal aggregates in dilute conditions under shear. In order to cover a broad range of hydrodynamic and interparticle forces, aggregates composed of primary particles with two sizes, d(p) = 90 and 810 nm, were generated. Moreover, to understand the dependence of breakage and restructuring on the cluster structure, aggregates grown under stagnant and turbulent conditions, having substantially different initial internal structures with fractal dimension d(f) equal to 1.7 and 2.7, respectively, were used. The aggregates were broken by exposing them to a well-defined elongational flow produced in a nozzle positioned between two syringes. To investigate the evolution of aggregate size and morphology, respectively, the mean radius of gyration, , and d(f) were monitored during the breakup process using light scattering and confocal laser scanning microscopy. It was found that the evolution of aggregates' fractal dimension during breakage is solely controlled by their initial structure and is independent of the primary particles size. Similarly, the scaling of the steady-state vs the applied hydrodynamic stress is independent of primary particle size, however, depends on the history of aggregate structure. To quantitatively explain these observations, the breakage process was modeled using stokesian dynamics simulations incorporating DLVO and contact interactions among particles. The required flow-field for these simulations was obtained from computational fluid dynamics. The complex flow pattern was simplified by considering a characteristic stream line passing through the zone with the highest hydrodynamic stress inside the nozzle, this being the most critical flow condition experienced by the clusters. As the flow-field along this streamline was found to be neither pure simple shear nor pure extensional flow, the real flow was approximated as an elongational flow followed by a simple shear flow, with a stepwise transition between them. Using this approach, very good agreement between the measured and simulated aggregate size values and structure evolution was obtained. The results of this study show that the process of cluster breakup is very complex and strongly depends on the initial aggregate structure and flow-field conditions.     

Stoichiometry and physical chemistry of promiscuous aggregate-based inhibitors

Many false positives in early drug discovery owe to nonspecific inhibition by colloid-like aggregates of organic molecules. Despite their prevalence, little is known about aggregate concentration, structure, or dynamic equilibrium; the binding mechanism, stoichiometry with, and affinity for enzymes remain uncertain. To investigate the elementary question of concentration, we counted aggregate particles using flow cytometry. For seven aggregate-forming molecules, aggregates were not observed until the concentration of monomer crossed a threshold, indicating a "critical aggregation concentration" (CAC). Above the CAC, aggregate count increased linearly with added organic material, while the particles dispersed when diluted below the CAC. The concentration of monomeric organic molecule is constant above the CAC, as is the size of the aggregate particles. For two compounds that form large aggregates, nicardipine and miconazole, we measured particle numbers directly by flow cytometry, determining that the aggregate concentration just above the CAC ranged from 5 to 30 fM. By correlating inhibition of an enzyme with aggregate count for these two drugs, we determined that the stoichiometry of binding is about 10,000 enzyme molecules per aggregate particle. Using measured volumes for nicardipine and miconazole aggregate particles (2.1 x 10(11) and 4.7 x 10(10) A(3), respectively), computed monomer volumes, and the observation that past the CAC all additional monomer forms aggregate particles, we find that aggregates are densely packed particles. Finally, given their size and enzyme stoichiometry, all sequestered enzyme can be comfortably accommodated on the surface of the aggregate.     

The controlled flocculation of submicron emulsion particles. I. A three-step synthetic route to supermicron polymer particles

The basic features of a three-step experimental process to produce supermicron polymer particles are described. First, a submicron emulsifier-free latex is prepared by a well-known technique. Second, the latex is aggregated by destabilizing with cetyl pyridinium chloride under constant stirring conditions, to yield roughly spherical clusters of 6-12 μ diameter. Third, the aggregates are stabilized with poly(vinyl alcohol) and internally coalesced by heating at or above the glass transition temperature. The final product particles have relatively smooth surfaces. Results are qualitatively interpreted in terms of a dynamic equilibrium where the aggregate size is determined by a balance between attractive interparticle potentials and stirring shear forces. Bimodal aggregate size distributions suggest the aggregate break-up mechanism may involve the erosion of individual latex particles and small fragments from the surface of aggregates. © 1996 John Wiley & Sons, Inc.     

利用高压水射流技术制备天然橡胶复合材料

首先利用超声空化作用将炭黑团聚体破碎、切割、分散在水中制得炭黑悬浮液,然后在高速射流场中,炭黑悬浮液被高速射流卷吸到天然胶乳中,在射流边界,由于二者存在极大的速度差,而形成一个湍流混合层,炭黑在湍流拉伸、剪切作用下微观分散到天然胶乳中.结果表明,与传统干法工艺相比,射流工艺可以使炭黑更均匀的分散到天然橡胶基体中.Payne效应结果表明射流工艺减弱了炭黑与炭黑之间的相互作用,增强了炭黑与橡胶之间的相互作用.同时射流工艺制备的复合材料硫化时间变短,硫化程度增加,硫化胶的撕裂强度提高了78%,回弹性提高了20%,DIN磨耗减小了33%.动态力学性能结果表明,射流工艺制备的复合材料在60℃左右具有更低的损耗因子.     

Kinetics of Structurization Processes in Three-Phase Disperse Systems under the Dynamic Conditions (Vibration) during Mixing

Regularities of the formation kinetics of the three-phase disperse systems of the solid–liquid–gas type under dynamic conditions were considered. The mechanism of the action of continuous shear and vibration (with various frequencies and amplitudes) on the formation and breakage of dispersed structures in such systems was studied. The interrelation of contact interactions between the particles and particle aggregates with the bulk rheological properties of three-phase disperse systems during mixing of finely dispersed solid phases with liquid medium and consequent compaction of dispersions was established. The revealed regularities of the formation dynamics of three-phase disperse structures provide the basis for chemical technology of highly concentrated disperse systems and highly filled disperse composites.     

Mesoscale constitutive modeling of magnetic dispersions: material functions for shear flows

We recently developed a constitutive model for magnetic dispersions by modeling the magnetic particles as rigid dumbbells dispersed in a solvent. The theory yielded a constitutive equation in which the stress tensor could be expressed as a function of the velocity gradient, an orientational order tensor, S, an average alignment vector, J, and any imposed external magnetic field, H. The constitutive equation is used here to predict material functions for steady shear flow (shear-rate dependent viscosity and first normal stress coefficient) as well as those for unsteady shear flows (stress growth upon inception of steady shear and small-amplitude oscillatory shear). The importance of effects of concentration, equilibrium nematic ordering in the dispersion, and anisotropy in the hydrodynamic drag are emphasized. Comparisons with available experimental data on viscosity for magnetic inks under steady shear flow and inception of steady shear flow show reasonably good agreement.     

新型乳液聚合制备疏松PMMA树脂及成粒机理

采用以水相为分散相、甲基丙烯酸甲酯 (MMA) 环己烷混合物为连续相的新型乳液聚合制备PMMA树脂 .发现 ,在未加乳化剂和加入少量Tween2 0乳化剂时 ,均可制备由初级粒子凝聚而成、无明显皮膜结构的疏松PMMA粒子 ,初级粒子粒径小于环己烷存在下MMA悬浮聚合得到的PMMA粒子的初级粒子 .根据聚合体系相构成、PMMA在MMA 环己烷混合液的溶解性及PMMA粒子粒径分布和形态的演变 ,提出了在分散水滴内乳液聚合形成初级粒子 生长 凝聚的新型乳液聚合成粒机理     

The scaling behavior of a highly aggregated colloidal suspension microstructure and its change in shear flow

The nature of the network structure and the evolution of structural change in shear flow were investigated for metal particle dispersions in terms of fractal aggregation of colloidal particles. Polymer-stabilized metal particle inks were prepared via a polyvinyl chloride coating dispersed in solvent. The fractal dimension of 1.74 was calculated with the scaling model based on the power law relationship between the elastic modulus and volume fraction. This scaling behavior can be explained by considering the deformable network structure of soft materials. While the elastic property of the floc was dominant, the limit of linearity was found at the inter-floc link, which is relatively weak and brittle. The steady shear results reveal two mechanisms that contribute to the breakdown of the microstructure in metal particle inks at increasing shear rate. Scaling of steady shear viscosity shows that these mechanisms are related to both inter-floc interactions and the elasticity of the floc itself. Further, these results suggest that individual flocs deform with weak inter-floc interactions and rupture into smaller flocs or aggregates at high shear stress, which is associated with the increased shear rate.     

Restructuring and break-up of two-dimensional aggregates in shear flow

We consider single two-dimensional aggregates, containing glass particles, placed at a water/air interface. We have investigated the critical shear rate for break-up of aggregates with different sizes in a simple shear flow. All aggregates break-up nearly at the same shear rate (1.8 +/- 0.2 s(-)(1)) independent of their size. The evolution of the aggregate structure before break-up was also investigated. With increasing shear rate, the aggregates adopt a more circular shape, and the particles order in a more dense, hexagonal structure. A simple theoretical model was developed to explain the experimentally observed break-up. In the model, the aggregate is considered as a solid circular disk that will break near its diameter. The capillary and drag force on the two parts of the aggregate were calculated, and from this force balance, the critical shear rate was found. The model shows a weak size dependence of the critical shear rate for the considered aggregates. This is consistent with the experimental observations.     

Effect of Electrode Pattern on the Column Structure and Yield Stress of Electrorheological Fluids

For electrorheological (ER) suspensions, the aggregate structures of particles were observed in electric fields by the use of transparent cells with different electrode patterns. Although the suspension is dispersed to noninteracting particles without electric fields, many aggregates are formed on the electrode surface in electric fields. Since the dipole–dipole interactions cause chain structures of particles and equilibrium conformations of chains are always aligned with electric field, the aggregates indicate the presence of columns spanning the electrode gap. The particle concentration in columns which are developed between parallel-plate electrodes is about 22 vol %. In striped electrodes, the particles construct striped aggregates along the electrodes and no particles remain in the insulating region. The particle concentration in striped aggregates is about 35 vol %. The nonuniformity of electric field is responsible for the high particle concentration. The increase in particle concentration of column lead to the high yield stress of electrified suspension. Therefore, the ER performance of suspension as an overall response can be improved by the electrode design.     

Zinc oxide colloids with controlled size, shape, and structure

Highly dispersed uniform ZnO particles of different sizes and shapes were prepared by slowly adding zinc salt and sodium hydroxide solutions in parallel into aqueous solutions of Arabic gum. Except for the very early stages, the precipitated solids consisted of a well-defined zinc oxide phase. Depending on the experimental conditions, the size of the final polycrystalline particles formed by the aggregation of nanosize entities varied from 100 to 300 nm. The reaction temperature affected both the size of the nanosize precursors and their arrangement in the final particles. At ambient temperature the primary nanoparticles, approximately 10 nm in size, formed spherical aggregates, while at 600 degrees C they were much larger (44 nm) and combined to form rather uniform hexagonal ZnO prisms. The aspect ratio and the internal structure of the latter could be altered by changing the nature of the zinc salt, the addition rate, and the initial concentration of the reactants. Based on the findings of the study a two-stage mechanism for the formation of uniform polycrystalline particles with well-defined geometric shapes is proposed.     

Flow electrification in nonaqueous colloidal suspensions, studied with video microscopy

Flow electrification in nonaqueous suspensions has been scarcely reported in the literature but can significantly affect colloidal stability and (phase) behavior, perhaps even without being recognized. We have observed it in shear flow experiments on concentrated binary suspensions of hydrophobized silica particles in chloroform. In this low-polarity solvent, electrical charges on the large-particles' surfaces manifest themselves via long-ranged forces, because hardly any screening can take place through counterions. By shearing the suspension for a prolonged time, we could demonstrate that the effective interactions between the large particles change from weakly attractive (due to the small particles) to strongly repulsive (due to acquired Coulomb interactions). One of the conditions required for flow electrification was the presence of a glass surface in the shear cell. A spectacular manifestation of the phenomenon was observed with confocal video microscopy. First, the formation of large-particle aggregates was seen, while subsequently (over a long shearing time) the aggregates disintegrated into small entities, mostly primary particles. The spatial distribution of these entities in the quiescent state after stopping the flow showed evidence for acquired long-range repulsion. The occurrence of flow electrification was further corroborated by control experiments, where no flow was imposed, antistatic agent was added, or the glass bottom was coated with a conducting (indium tin oxide, ITO) layer: here, the aggregates kept growing until they became very large. To further diagnose the phenomenon, we have also done experiments in which an external electric field was applied (via the ITO layer) to an aggregated suspension. When the lower electrode was given the lowest potential, the aggregates were found to move away from the bottom and disintegrate. The qualitative similarity hereof with the flow electrification experiment suggests that in the latter, the glass acquired negative charges. After prolonged application of an external electric field, we observed segregation into regions enriched in large particles and regions completely depleted of them. In the quiescent fluid these regions exist as isolated units, but in shear flow they merge into bands, a behavior which resembles shear banding.     

The structure recovery capacity of highly concentrated emulsions under shear flow via studying their rheopexy

An investigation was performed into the structure recovery of highly concentrated water-in-oil emulsions (HCEs) under shear flow via studying their rheopexy. Experiments with the shear rate sweep in the up and down modes demonstrate that HCE has rheopexy. Restoration of the initial structure after cessation of shearing needs a period of time. The recovery time and ratio depend on the shear rate and the droplet size of the dispersed phase. A high shear rate results in a high probability of structure break of HCE. Thus, it is difficult to return to its initial structure. The structure of HCE that underwent shearing is closer to its original situation when the droplet size is small.