Rheological Properties of Silica Particle Suspensions in Mineral Oil

The rheological properties of particles suspended in a non‐polar mineral oil have been investigated as a function of volume fraction of particles, particle size, surface properties and shear rate. Three different types of particles were investigated; glass microspheres, monodisperse silica particles and fumed silica. The suspensions showed shear thinning behavior at higher volume fractions, and the viscosity increased with decreasing particle size. The hydrophobic particles display lass shear thinning effects. The relative viscosity of all the suspensions was well fitted to the Krieger and Dougherty model.     

Rheology of polystyrene latexes with adsorbed and free gelatin

The rheology of monodisperse polystyrene latex particles of two different particle radii (26 and 67 nm) has been studied with a range of concentrations of the polyampholyte gelatin. Gelatin contributes to the rheology by adsorption to the particles and by thickening the continuous phase. High viscosities and strong shear thinning are measured for low volume fractions of latex. A procedure is presented to deconvolute the effects of free and bound gelatin by applying simple hard-sphere models. This procedure allows us to estimate the effective size of the gelatin-covered particles as well as the continuous-phase gelatin concentration and viscosity. The layer thicknesses from rheology agree well with those from PCS. The effect of varying particle volume fraction, ionic strength, pH and gelatin and surfactant concentration on the rheology of these suspensions is presented. For the smaller latex, the adsorbed layer occupies a greater fraction of the effective volume. Increasing free polymer concentration reduces the adsorbed-layer thickness. The reduced critical shear stress increases with the suspension viscosity for suspensions of the 26 nm latex but is constant for the 67 nm latex. At very high shear (>2000 s⁻¹), the suspensions show excess shear thinning over that expected from a hard-sphere model. This excess thinning is attributed to deformation of the adsorbed gelatin layer under high shear stress and interpreted in terms of an empirical interparticle potential.     

Effect of Particle Size Distribution on the Rheology of Dispersed Systems

The rheological properties of aqueous polystyrene latex dispersions from three synthetic batches, with nearly the same z-average particle sizes, 400 nm, but varying degrees of polydispersity, 0.085, 0.301, and 0.485, respectively, were systematically investigated using steady-state shear and oscillatory shear measurements. The particles were sized with photon correlation spectroscopy and transmission electron microscopy and were stabilized sterically with PEO–PPO–PEO triblock copolymer (Synperonic F127). Results from steady-state shear measurements show that the viscosities of the systems exhibit shear-thinning behavior at high solid fractions. However, the degree of shear thinning depends on the breadth of particle size distribution, with the narrowest distribution suspension exhibiting the highest degree of shear thinning. The Herschel–Bulkley relationship best describes the flow curves. The relative viscosities as a function of volume fraction data were compared, and it was found that the broadest distribution suspension had the lowest viscosity for a given volume fraction. In addition, the data were fitted to the Krieger–Dougherty equation for hard spheres. A reasonable agreement of theory with experiment is observed, particularly and surprisingly for the very broad distribution. However, when the contribution to the volume due to the adsorbed polymer layer is considered, the agreement between experiment and theory becomes closer for all the suspensions, although the agreement for the broad distribution suspension is now worse. Fitting the Dougherty–Krieger theory to the experimental data based on our experimental maximum packing fractions gives very good agreement for all the systems studied. From oscillatory shear measurements, the moduli were obtained as a function of frequency at various latex volume fractions. The results show general change of the dispersions from viscous (G" > G′) at low volume fractions (0.25–0.30) to moderately elastic (G′ > G") at moderately high volume fractions (0.41–0.45). The change at this concentration level is likely due to some compression and interpenetration of the stabilizing polymer chain at the periphery, indicating the dominance of the interparticle forces. Overall, the very broad distribution was found to have the lowest elastic modulus for a given volume fraction.     

Effect of pH and Salt on Rheological Properties of Aerosil Suspensions

Ionic strength and pH will influence the zeta potential of suspended particles, and consequently particle interactions and rheological properties as well. In this study the rheological properties and aggregation behaviour of Aerosil particles dispersed in aqueous solutions with various pH and salt concentration were studied. The potential energy was estimated by the DLVO theory and short range hydration forces and compared to the experimentally determined zeta potential. The strongest attraction between particles occurs at the isoelectric point (pH 4) and resulted in large aggregates, which gave relatively higher values of viscosity, yield stress, moduli, and shear thinning effects. The relative viscosity as a function of volume fraction was fitted to the Krieger and Dougherty model for all the suspensions. Oscillation measurements showed that the suspensions display elastic behaviour at low pH and viscous behavior at high pH. Furthermore, suspensions with high salt content had higher storage moduli. A power law dependency of storage moduli with volume fraction could be used to indicate the interaction strength between particles.     

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier‐free emulsion polymerization. Using a stress‐controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999     

Rheology of binary colloidal structures assembled via specific biological cross-linking

The selectivity and range of energies offered by specific biological interactions serve as valuable tools for engineering the assembly of colloidal particles into novel materials. In this investigation, high affinity biological interactions between biotin-coated "A" particles (RA = 0.475 microm) and streptavidin-coated "B" particles (RB = 2.75 microm) drive the self-assembly of a series of binary colloidal structures, from colloidal micelles (a large B particle coated by smaller A particles) to elongated chain microstructures (alternating A and B particles), as the relative number of small (A) to large (B) particles (2 < or = NA/NB < or = 200) is decreased at a low total volume fraction (10(-4) < or = phiT < or = 10(-3)). At a significantly higher total volume fraction (phiT > or = 10(-1)) and a low number ratio (NA/NB = 2), the rheological behavior of volume-filling particle networks connected by streptavidin-biotin bonds is characterized. The apparent viscosity (eta) as a function of the shear rate gamma, measured for networks at phiT = 0.1 and 0.2, exhibits shear-rate-dependent flow behavior, and both the apparent viscosity and the extent of shear thinning increase upon an increase of a factor of 2 in the total volume fraction. Micrographs taken before and after shearing show a structural breakdown of the flocculated binary particle network into smaller flocs, and ultimately a fluidlike suspension, with increasing shear rate. Rheological measurements provide further proof that suspension microstructure is governed by specific biomolecular interactions, as control experiments in which the streptavidin molecules on particles were blocked displayed Newtonian flow behavior. This investigation represents the first attempt at measuring the rheology of colloidal suspensions where assembly is driven by biomolecular cross-linking.     

Dispersion stability and rheological behavior of suspensions of polystyrene coated fumed silica particles in polystyrene solutions

Polystyrene coated silica（SiO₂@PS） core-shell composite particles with averaged diameter of about 290 nm were prepared by in situ emulsion polymerization of styrene on the surface ofγ-methacryloxypropyltrimethoxysilane grafted SiO₂ nanoparticles of 20-50 nm in diameter.Rheological behavior and dispersion stability of SiO₂@PS suspension in 10 wt%PS solution were compared with suspensions of untreated SiO₂ and silane modified SiO₂ nanoparticles.Suspensions of the untreated and the silane modified SiO₂ exhibited obvious shear thinning.The SiO-2@PS suspension exhibits shear viscosity considerably smaller than suspensions of untreated and silane modified SiO₂ at low shear rates.Transmission electron microscopy showed that the composite particles can uniformly and stably disperse in PS solution compared to other suspensions,implying that the PS shell can effectively enhance the particle compatibility with PS macromolecules in solution.     

Flow curves of dense colloidal dispersions: schematic model analysis of the shear-dependent viscosity near the colloidal glass transition

A recently proposed schematic model for the nonlinear rheology of dense colloidal dispersions is compared to flow curves measured in suspensions that consist of thermosensitive particles. The volume fraction of this purely repulsive model system can be adjusted by changing temperature. Hence, high volume fractions (phi相似文献   

Flow-Induced Anisotropy in Mixtures of Associative Polymers and Latex Particles

The effect of associative polymers on the structure and rheological behavior of colloidal suspensions is discussed. Adding associative polymer is known to increase the viscosity of the suspensions. At high shear rates the increase is close to what could be expected on the basis of the hydrodynamic effects of the added polymer. At low shear rates the viscosity increases much more. Small-angle light scattering (SALS) during flow is used here to investigate the underlying structural mechanisms. The SALS patterns indicate that the associative polymer changes the particulate structure: characteristic butterfly patterns appear even at relatively low particle volume fractions. They are not present in the suspensions without associative polymer. The patterns indicate that fluctuations in particle concentration are more pronounced in the flow direction than in the vorticity direction and that anisotropic particulate structures with an orientation along the vorticity direction develop. The evolution of their characteristic length scale during flow has been followed over time. Changing the hydrophilic part of the polymer from polyacrylamide to polyacrylic acid induces stronger associative interactions. In the suspensions this results in a reduction of the relative viscosity rather than an increase. The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspensions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competition between adsorption of the associative polymer on the particles with the intermolecular associations between the polymer chains seems to be responsible for the observed differences. Copyright 2000 Academic Press.     

Viscosity modification of lubricating oil based on high-concentration silica nanoparticle colloidal system

In this study, colloidal systems with SiO₂ nanoparticle as viscosity modifier additive were synthesized in the lubricating oil via an in situ Stober sol-gel method. The fluid characters of lubricating oil and viscosity variation were carefully investigated via rheological methods. The results showed that the lubricating oil transformed from Newtonian fluid to non-Newtonian fluid with increasing the concentration of nanoparticles, and smaller particles displayed better thickening effect toward lubricating oil. For the system with highly concentrated nanoparticle (20?wt%), the rheological behavior mainly depends on the size of nano-SiO₂. The lubricating oil with smaller nano-SiO₂ particles displayed higher structural strength and response rate, resulting in good recoverability after high-speed shear. The viscosity changed with temperature and also displayed a thermo-responsive behavior, which significantly alleviated the effect of shear thinning on the viscosity under high temperature. This study presented a new strategy for effectively tuning the fluid characters and modifying the viscosity of lubricating oils by adding highly concentrated inorganic nanoparticles.     

Influence of poly(ethylene oxide) brushes on the rheological properties of MgO colloidal suspensions in water

A combined experimental and multiscale simulation study of the influence of polymer brush modification on interactions of colloidal particles and rheological properties of dense colloidal suspensions has been conducted. Our colloidal suspension is comprised of polydisperse MgO colloidal particles modified with poly(ethylene oxide) (PEO) brushes in water. The shear stress as a function of shear rate was determined experimentally and from multiscale simulations for a suspension of 0.48 volume fraction colloids at room temperature for both bare and PEO-modified MgO colloids. Bare MgO particles exhibited strong shear thinning behavior and a yield stress on the order of several Pascals in both experiments and simulations. In contrast, simulations of PEO-modified colloids revealed no significant yielding or shear thinning and viscosity only a few times larger than solvent viscosity. This behavior is inconsistent with results obtained from experiments where modification of colloids with PEO brushes formed by adsorption of PEO-based comb-branched chains resulted in relatively little change in suspension rheology compared to bare colloids over the range of concentration of comb-branch additives investigated. We attribute this discrepancy in rheological properties between simulation and experiment for PEO-modified colloidal suspensions to heterogeneous adsorption of the comb-branch polymers.     

Dynamic yielding, shear thinning, and stress rheology of polymer-particle suspensions and gels

The nonlinear rheological version of our barrier hopping theory for particle-polymer suspensions and gels has been employed to study the effect of steady shear and constant stress on the alpha relaxation time, yielding process, viscosity, and non-Newtonian flow curves. The role of particle volume fraction, polymer-particle size asymmetry ratio, and polymer concentration have been systematically explored. The dynamic yield stress decreases in a polymer-concentration- and volume-fraction-dependent manner that can be described as apparent power laws with effective exponents that monotonically increase with observation time. Stress- or shear-induced thinning of the viscosity becomes more abrupt with increasing magnitude of the quiescent viscosity. Flow curves show an intermediate shear rate dependence of an effective power-law form, becoming more solidlike with increasing depletion attraction. The influence of polymer concentration, particle volume fraction, and polymer-particle size asymmetry ratio on all properties is controlled to a first approximation by how far the system is from the gelation boundary of ideal mode-coupling theory (MCT). This emphasizes the importance of the MCT nonergodicity transition despite its ultimate destruction by activated barrier hopping processes. Comparison of the theoretical results with limited experimental studies is encouraging.     

Glassy dynamics and mechanical response in dense fluids of soft repulsive spheres. II. Shear modulus, relaxation-elasticity connections, and rheology

We apply the quiescent and mechanically driven versions of nonlinear Langevin equation theory to study how particle softness influences the shear modulus, the connection between shear elasticity and activated relaxation, and nonlinear rheology of the repulsive Hertzian contact model of dense soft sphere fluids. Below the soft jamming threshold, the shear modulus follows a power law dependence on volume fraction over a narrow interval with an apparent exponent that grows with particle stiffness. To a first approximation, the elastic modulus and transient localization length are controlled by a single coupling constant determined by local fluid structure. In contrast to the behavior of hard spheres, an approximately linear relation between the shear modulus and activation barrier is predicted. This connection has recently been observed for microgel suspensions and provides a microscopic realization of the elastic shoving model. Yielding, shear and stress thinning of the alpha relaxation time and viscosity, and flow curves are also studied. Yield strains are relatively weakly dependent on volume fraction and particle stiffness. Shear thinning commences at values of the effective Peclet number far less than unity, a signature of stress-assisted activated relaxation when barriers are high. Apparent power law reduction of the viscosity with shear rate is predicted with a thinning exponent less than unity. In the vicinity of the soft jamming threshold, a power law flow curve occurs over an intermediate reduced shear rate range with an apparent exponent that decreases as fluid volume fraction and/or repulsion strength increase.     

Rheological Behavior of Titanium Dioxide Suspensions

The rheological properties of titanium dioxide dispersed in water are measured over a wide range of powder concentrations, temperatures, and pH values. The value of intrinsic viscosity of titanium dioxide measured with an Ubbelohde capillary viscometer is 3.55, which is useful for determining the shape and aggregation property of the particles. The yield stress and steady shear viscosity of titanium dioxide with broad and narrow particle size distributions were measured over a wide range of solid volume fractions on a Brabender rheometer. It is observed that the rheological properties of the suspensions are quite different due to the difference in particle size distributions. Quemada, Casson, and Zhou's models were used to fit the experimental data and useful parameters were obtained. Calculated data are also in good agreement with the experimental data. As expected, the shear viscosity and yield stress decrease with increasing temperature. But when the temperature is around 50 degrees C, yield stress increases with increasing temperature while shear viscosity exhibits a complex behavior. The phenomena are very interesting and special. The Peclet number was used to analyze the shear thickening behavior. Models were also used to describe the shear viscosity under different temperatures and the master plots of the reduced variables eta/eta(infinity) vs t(c)gamma; at different temperatures are superimposed, which means the agreement is fair and the models are suitable to describe the rheological properties of titanium dioxide suspensions. pH effects were investigated on a Rheometrics RFS-II rheometer and it was found that pH can change the surface charge of the particles, which also affects the rheological behavior. The pH at which maximum shear viscosity and yield stress occur is in concordance with the isoelectric point. Copyright 2001 Academic Press.     

Influence of interparticle interaction effects on the rheological properties of low density polyethylene filled with glass beads

The rheological properties of Low-density polyethylene (LDPE) compounds with different fractions of glass beads have been investigated by means of high pressure capillary rheometry. The purpose of this study is to find a functional approach for describing the flow behaviour of suspensions as a function of the volumetric filler content and the applied shear stress or shear rate, respectively. The flow behaviour of suspensions is influenced by interaction effects, which are dependent on the filler particle, its volume content and particle size. While small glass beads exhibit pronounced interparticle interaction effects even at low volumetric filler concentrations, large glass beads show a plateau of negligibly interactions up to approx. 20% volumetric filler content. With introducing a generalized interaction function the flow behaviour of the tested suspensions could be described with reasonable accuracy in consideration of the transition from negligible to pronounced interactions.     

Investigation on the interaction between C16TAB and NaCMC in semidilute aqueous solution based on rheological measurement

The rheological behavior of unentangled and entangled semidilute solution of anionic polyelectrolyte sodium carboxymethyl cellulose (NaCMC) containing cationic surfactant cetyltrimethylammonium bromide (C₁₆TAB) was investigated. The results reveal that the rheological properties of these semidilute NaCMC solutions depend on the amount of C₁₆TAB added. In the unentangled semidilute NaCMC solution (0.5 g/L), the viscosity decreases with the increase of C₁₆TAB amount in the low surfactant concentration region (below the critical aggregation concentration, CAC). However, in high surfactant concentrations (above CAC), the viscosity decreases sharply with the increase in C₁₆TAB amount. It is found that viscosity change of NaCMC solution could be described using Colby’s model when surfactant concentrations are between CAC and saturated concentration (C_s), suggesting that no inter-polymer interaction exists between C₁₆TAB and NaCMC in the unentangled semidilute solution. However, for the entangled semidilute NaCMC solution (5 g/L), the addition of C₁₆TAB leads to an increase in viscosity. Meanwhile, the solution exhibits an enhanced shear thinning behavior due to adding more C₁₆TAB than 1 mM. The viscosity increase is ascribed to the physical cross linking of surfactant micelles with NaCMC chains. Furthermore, it is suggested that the enhanced shear thinning behavior results from weak interaction between NaCMC chains and C₁₆TAB micelles.     

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.     

Rheological Modeling of Dispersion System of Nano/Microsized Polymer Particles Considering Swelling Behavior

Rheological behavior of dispersion system containing nano/microsized cross-linked polymer particle was studied considering particle hydration and swelling. Viscosity of the dispersion system depends on swelling kinetics of polymer particles. Under shear flow, dispersion of swollen polymer particles is shear thinning. According to experimental results, kinetics of particle swelling and hydration was described well by second-order kinetic equation. Relational expression between equilibrium particle size and influencing factors of swelling such as salt concentration and temperature was presented. Assume that swollen polymer particles are uniform and have a simple core-shell structure, interacting through a repulsive steric potential. The rheological modeling of such dispersion system at low shear rate was presented using the concept of effective volume fraction, which depends on swelling kinetics and interparticle potential. Cross model was introduced to describe shear-thinning behavior. The viscosity equation allows correlation of experimental data of relative viscosity versus shear rate or hydration time; accounting for effect of temperature and salt concentration on viscosity. Predictions of the model have a good agreement with experimental results.     

Influence of concentration on the particle size analysis of polymer latexes using diffusing-wave spectroscopy

The use of a diffusing-wave spectroscopy (DWS) technique for the particle size analysis of a series of suspensions of polymer latexes with diameters ranging between 90 nm and 1300 nm and volume fractions from 0.02 to 0.18 has been investigated. Particle sizes from DWS were in reasonable agreement with those from transmission electron microscopy and disc centrifuge photosedimentometry. Photon correlation spectroscopy was applicable only to the latexes with particle sizes less than 500 nm. For polymer latexes with volume fractions (V_f) up to 0.09, the decay rate of the autocorrelation function from DWS was related to the particle size over the range of particle sizes examined. At the highest volume fraction (V_f = 0.18), it was possible to distinguish between particles with diameters of 740 and 1,300 nm from their autocorrelation functions, but not between particles with diameters of 90 and 430 nm.     

Effects of the degree of surface modification on the rheological responses of precipitated silica suspensions in benzyl alcohol

Two types of precipitated silica powders modified by poly (dimethylsiloxane) (PDMS) were suspended in benzyl alcohol and their rheological properties were investigated as a function of silica volume fraction, φ. The suspensions were classified into sol, pre-gel, and gel states based on the increase in φ. An increase in the degree of surface modification by PDMS caused gelation at higher φ. Plots of apparent shear viscosity against shear rate in the sol and pre-gel states of highly modified silica suspensions showed weak shear thickening behavior, while the same plots for silica suspensions with a low modification level exhibited shear thinning behavior. The dynamic moduli of hydrophobic suspensions in the pre-gel and gel states were dependent on the surface modification: the storage modulus G′ was larger than the loss modulus G″ in the linear region and these moduli increased with increasing φ, irrespective of the silica powder. The linear region of the φ range for the precipitated silica suspensions was wider than that for the fumed silica powders modified by PDMS suspended in benzyl alcohol, while the G′ value in the linear region for the precipitated silica suspensions was less than those for the fumed silica suspensions.