Rheological Behavior Of Aqueous Suspensions Research Articles

Nonlinear oscillatory rheology of aqueous suspensions of cellulose nanocrystals and nanofibrils

In this work, the nonlinear rheological behavior of aqueous suspensions composed of two typical nanocellulose [rod-like cellulose nanocrystals (CNCs) and filamentous cellulose nanofibrils (CNFs)] was examined and compared by using various large-amplitude oscillatory shear (LAOS) analysis methods, such as Fourier-transform rheology, stress decomposition, Chebyshev polynomials, and the sequence of physical processes. From our analysis, the nonlinear rheological parameters of higher harmonics, dissipation ratio, strain hardening ratio, shear thickening ratio, transient modulus, and cage modulus were obtained and quantitatively analyzed. CNCs tend to assemble to form anisotropic structures in an aqueous medium while the CNFs are entangled to form gels. The CNF suspensions demonstrated a significant viscous modulus overshoot and had stronger yield stresses, but the yield of CNC suspensions was more ductile. In the case of low concentrations, the CNF suspensions demonstrated stronger intracycle shear thickening behavior in medium-amplitude oscillatory shear region and lower dissipation ratios at small strain amplitudes. Although both nanocellulose suspensions revealed the existence of four intracycle rheological transition processes (viscoplastic deformation, structural recovery, early-stage yielding, and late-stage yielding), the CNF suspensions exhibited a stronger structural recovery ability. Larger strain amplitudes did not invariably result in a broader range of intracycle rheological transitions, which are also affected by the excitation frequency. The application of the various LAOS analysis methods provided valuable intracycle nonlinear rheological insights into nanocellulose suspensions, which are of great importance for enhancing their industrial perspectives.

Rheophysical study of dispersed alumina suspensions

This paper studies the rheological behavior of aqueous suspensions of fine grained (d50 = 200 nm) alumina. Rheological measurements were performed on suspensions containing various amounts of solid and 0.26 wt% of ammonium polymethacrylate in order to ensure a good state of dispersion. Brownian motions, colloidal interactions and hydrodynamic interactions dictate the behavior. The viscosity is mainly influenced by the shear rate and by the solid volume fraction. A Maron and Pierce model was used to describe this last effect in the hydrodynamic regime. The maximum packing fraction was found to be 40 vol%. This value is related to the colloidal stability as described by the DLVO theory. In fact, the maximum packing fraction leads to a surface-to-surface separation distance of about 46 nm because of the repulsive potential. Finally, a dimensionless approach was achieved to quantitatively identify, on the rheogram, the different flow regimes associated to each dominant interaction type.

Coupled Effects of Fibril Width, Residual and Mechanically Liberated Lignin on the Flow, Viscoelasticity, and Dewatering of Cellulosic Nanomaterials.

The rheological behavior of aqueous suspensions of lignocellulose nanofibrils (LCNFs) is investigated systematically by considering the coupled effect of residual lignin and LCNF morphology. The LCNF was obtained by high-energy fluidization of TEMPO-oxidized mechanical fibers, followed by size fractionation (fibril widths of ∼5, ∼9, and ∼18 nm). The nanofibril width and the corresponding fibril-fibril interactions are strongly influenced by the presence and distribution of lignin in the respective fractions, either retained on the fibril surface or as free structures present in the finest size fraction. All samples containing lignin display dilatancy, typifying gel suspensions with aggregated hydrophobic particles. Fine fractionated samples display strong gel behavior. The coarse fractionated sample, by contrast, shows a greater tendency to flocculate via entanglement and displays less gel-like characteristics; hence, it dewaters more freely.

Direct Visualization of the Hydration Layer on Alumina Nanoparticles with the Fluid Cell STEM in situ.

Rheological behavior of aqueous suspensions containing nanometer-sized powders is of relevance to many branches of industry. Unusually high viscosities observed for suspensions of nanoparticles compared to those of micron size powders cannot be explained by current viscosity models. Formation of so-called hydration layer on alumina nanoparticles in water was hypothesized, but never observed experimentally. We report here on the direct visualization of aqueous suspensions of alumina with the fluid cell in situ. We observe the hydration layer formed over the particle aggregates and show that such hydrated aggregates constitute new particle assemblies and affect the flow behavior of the suspensions. We discuss how these hydrated nanoclusters alter the effective solid content and the viscosity of nanostructured suspensions. Our findings elucidate the source of high viscosity observed for nanoparticle suspensions and are of direct relevance to many industrial sectors including materials, food, cosmetics, pharmaceutical among others employing colloidal slurries with nanometer-scale particles.

Experimental Investigation of Rheological Behavior and Pressure Drop of Aqueous Suspensions of Carbon Nanotubes in a Horizontal Tube

A new engineering medium, called nanofluid, have attracted a wide range of researches on many processes in engineering applications. Many experimental and theoretical studies on viscosity of nanofluids are controversial and the shear rate dependent viscosity is frequently neglected. The aim of the present work is to investigate the effect of the rheological behavior of aqueous suspensions of carbon nanotubes on the friction factor and the pressure drop in a horizontal tube under laminar and turbulent flow. The particles weight concentration used here is 0.01%. The rheological behavior was measured for a range of temperature from 0 °C to 40 °C. The results showed that the nanofluids exhibit a shear thinning behavior at very low shear stress. The analysis results of the flow characteristics showed a non-Newtonian behavior under laminar flow and a Newtonian behavior under turbulent flow.

Rheological behaviour and spectroscopic investigations of cerium-modified AlO(OH) colloidal suspensions

The rheological behaviour of aqueous suspensions of boehmite (AlO(OH)) modified with different Ce-salts (Ce(NO3)3, CeCl3, Ce(CH3COO)3 and Ce2(SO4)3) was investigated at a fixed Ce/Al molar ratio (0.05). Freshly prepared boehmite suspensions were near-Newtonian and time-independent. A shear-sensitive thixotropic network developed when Ce-salts with monovalent anions were introduced in the nanoparticle sols. The extent of particle aggregation dramatically increased with ageing for Ce(NO3)3 and CeCl3 whereas an equilibrium value was reached with Ce(CH3COO)3. The addition of Ce2(SO4)3 with divalent anions involved no thixotropy but rather a sudden phase separation.The combined data set of IRTF and DRIFT spectra indicated that free NO3- anions of peptized boehmite adsorb on the nanoparticle surface by H-bond. The introduction of Ce-salts in the boehmite sol led to the coordination between Ce3+ ions and NO3- anions adsorbed on boehmite i.e. to [Ce(NO3)4(H2O)x]− complex. Such coordination led to a thixotropic behaviour which was lower with Ce(NO3)3 compared to CeCl3 and Ce(CH3COO)3. In contrast, Ce2(SO4)3 formed insoluble complexes with dissolved aluminium species. The formation of H-bonded surface nitrate complexes was found to play a decisive role on the particle–particle interactions and consequently on the rheological behaviour of the sols.

Dispersion and Rheology of Aqueous Suspensions of Nanosized BaTiO3

The zeta potential and the rheological behavior of aqueous suspensions of a commercial nanosized BaTiO3 powder were studied. The influence of the volume fraction of solids, the kind and concentration of dispersant, the homogenization technique, and times were investigated. Because the as‐received powder contained ∼5 wt% organics, the powder was calcined to 350°C/0.5 h. After calcination, a good dispersion was achieved for suspensions with up to 80 wt% solids using a polyacrylic‐based dispersant combined with tetramethylammonium hydroxide and 10 min of ultrasound. Green bodies obtained by slip casting had relatively low densities (2.6 g/cm3, 43% TD), but homogeneous, agglomerate‐free microstructures.

Rheological behavior of aqueous suspensions containing cationic starch and aluminum magnesium hydrotalcite-like compound in the presence of different electrolytes

The rheological properties of aqueous suspensions consisting of cationic starch (CS) and positively charged aluminum magnesium hydrotalcite-like compound (HTlc) in the presence of different electrolytes (NaCl, CaCl 2 and AlCl 3) were investigated. It is found that the network-like structure of pure CS solution is formed by the interaction between CS molecules. Both the equilibrium viscosities and the elastic response of CS solution decrease with the addition of NaCl, CaCl 2 and AlCl 3. Small amplitude sinusoidal oscillation tests show that a three-dimensional network of the HTlc/CS suspension can be formed through the bridge effect between CS molecules and HTlc particles due to the hydrogen bonding between the ether groups or hydroxyl groups of CS and the hydroxyl groups of HTlc. Both the equilibrium viscosity and dynamic property of the HTlc/CS suspensions indicate that the network-like structural strength of the suspensions increases firstly and then decreases with increasing HTlc content. The equilibrium viscosity and the elastic response of the HTlc/CS suspensions decrease gradually with the addition of NaCl or CaCl 2, but decrease firstly then increase and then decrease with increasing AlCl 3 concentration, i.e., the structural strength of the HTlc/CS suspensions can be strengthened by the addition of appropriate amount of AlCl 3 content.

Rheological behaviour of aqueous suspensions of laponite: new insights into the ageing phenomena

In this paper, the ageing behaviour of suspensions of laponite with varying salt concentration is investigated using rheological tools. It is observed that ageing is accompanied by an increase in the complex viscosity. The creep experiments performed at various ages show damped oscillations in the strain. The characteristic time scale of the damped oscillations, the retardation time, shows a prominent decrease with increasing age of the system. However, this dependence weakens with an increase in the salt concentration, which is known to change the microstructure of the system from glass like to gel like. We postulate that a decrease in the retardation time can be represented as a decrease in the viscosity (friction) of the dissipative environment surrounding the arrested entities that oppose elastic deformation of the system. We believe that ageing in colloidal glass leads to a greater ordering that enhances relative spacing between the constituents, thereby reducing the frictional resistance. However, since a gel state is inherently different in structure (fractal network) to that of a glass state (disordered), ageing in the gel does not induce ordering. Consequently, we observe an inverse dependence of retardation time on age, which becomes weaker with an increase in the salt concentration. We analyse these results from the perspective of ageing dynamics of both glass and gel states of laponite suspensions.

Effect of pH on the rheological behavior of pure and interstratified smectite clays

AbstractThe effects of pH and ionic strength on the rheological behavior of aqueous suspensions of both crude and purified Wyoming bentonite and natural interstratified illite-smectite from Tunisia were investigated. Flow tests were performed on the four clay suspensions at a 10% clay concentration at different pH values. They showed that the rheological properties were highly sensitive to pH and the nature of the clay. The evolution of the yield stress as a function of pH is characterized by the presence of maxima and minima attributed to changes in the mode of association of the particles. The effect of pH on the behavior of clay suspended in NaCl solutions was also studied. The results show that the presence of NaCl has very little effect on the pH and yield stress values of the four materials.

Slip casting of barium zirconate aqueous concentrated suspensions

The rheological behaviour of aqueous suspensions of barium zirconate was studied focusing on the effect of the volume fraction of particles, leading to the determination of the maximum packing fraction. Optimised suspensions dispersed with both polymethacrylic acid and tetramethylammonium hydroxide were slip cast on plaster moulds to produce discs and crucibles, leading to relative green densities greater than 62% of theoretical ones. Densification was studied at temperatures ranging from 1450 to 1700 °C by static and dynamic sintering experiments. Scanning electron microscopy studies demonstrate that slip cast parts are dense and very homogeneous, without any significant processing-related defects or abnormal grain growth.

Reaction sintered Al 2O 3/Al 2TiO 5 microcrack-free composites obtained by colloidal filtration

Dense and microcrack-free Al 2O 3/Al 2TiO 5 composites (10, 30 and 40 vol.% of Al 2TiO 5) have been obtained by colloidal filtration and reaction sintering, using alumina and titania as starting powders. The processing of the composites has been studied focusing on the rheological behaviour of aqueous suspensions of each powder and of mixtures. Colloidal filtration of optimised suspensions, with a solid loading as high as 50 vol.%, and a thermal treatment at 1450 °C, lead to completely reacted and uncracked sintered materials with homogeneously distributed aluminium titanate contents up to 40 vol.% and high density. Thermal diffusivity values from 25 to 800 °C are coincident on heating and cooling for the three studied composites, and decrease with temperature and with aluminium titanate content.

Influence of aluminum doping on titania pigment structural and dispersion properties

The influence of aluminum concentration on the structural properties and rheological behavior of aqueous suspensions of aluminum-doped titania pigment from the chloride process was investigated. The variation in rheological properties correlates with the change in the pigment surface properties, determined from electrophoresis measurements and atomic surface concentrations. Pigment suspensions exhibited a maximum yield stress and viscosity at or near the isoelectric point (iep). The pH of the maximum yield value of the pigment suspension increases with increasing aluminum hydroxyl group density at the particle surface. For pigments with a high aluminum surface concentration, at pH values where the magnitude of the zeta potential was high, a low-viscosity, dispersed suspension was obtained. The pigment with the lowest aluminum concentration, however, retained high yield stresses over a large pH range even when the zeta potential was of considerable magnitude. Pigment particle interactions are chiefly dictated by van der Waals forces and electrostatic repulsive forces, likely to be influenced by heteroaggregation. The aggregate strength would therefore depend upon the proportion and distribution of aluminum and titanium surface groups of the heterogeneous pigment, which will influence both the Hamaker constant and the degree of heteroaggregation. Overall, very small additions to the total aluminum concentration translate to significant aluminum surface concentration disparities and subsequently to large particle interaction differences.

Rheological behavior of aqueous suspensions of hydroxyapatite (HAP).

The aim of this study is to characterize the influence of organic dispersants on the stability of hydroxyapatite (HAP) dispersions, using viscosity measurements, electrokinetic sonic amplitude (ESA) measurements and adsorption isotherms. Colloidal stability of aqueous suspensions of HAP as a function of solids loading and of the type and concentration of deflocculant has been studied. The interaction between particles and dispersant molecules was characterized. Adsorption measurements of the amount of dispersants on the particles surface, viscosity and electrophoretic measurements as a function of the dispersant concentration are correlated. Suspension parameters have been adjusted in order to obtain a high solids loading slurry with a plastic viscosity which will allow its use in colloidal processing.

Influence of mullite additions on thermal shock resistance of dense alumina materials. Part 1: Processing studies

Homogeneous, fine grained (≈ 0·5–2 µm), and dense (≥ 97% of theoretical density) alumina–mullite composites have been prepared by a colloidal filtration route. In order to improve the uniformity of the composites, the rheological behaviour of aqueous suspensions of alumina, mullite, and alumina–mullite mixtures was studied, focusing on the optimum concentration of deflocculant, the mixing procedure, and milling time. The sintered materials were characterised (scanning electron microscopy, dynamic Young's modulus, Vickers indentation hardness, and toughness) and compared with an alumina material with similar density and microstructure. Small mullite additions (5–15 vol.-%) allow desirable values of hardness and toughness of alumina to be maintained while reducing the Young's modulus below that of alumina, so that it may be expected that the thermal shock behaviour will be improved.

Influence of mullite additions on thermal shock resistance of dense alumina materials. Part 1: Processing studies

AbstractHomogeneous, fine grained (≈ 0·5–2 µm), and dense (≥ 97% of theoretical density) alumina–mullite composites have been prepared by a colloidal filtration route. In order to improve the uniformity of the composites, the rheological behaviour of aqueous suspensions of alumina, mullite, and alumina–mullite mixtures was studied, focusing on the optimum concentration of deflocculant, the mixing procedure, and milling time. The sintered materials were characterised (scanning electron microscopy, dynamic Young's modulus, Vickers indentation hardness, and toughness) and compared with an alumina material with similar density and microstructure. Small mullite additions (5–15 vol.-%) allow desirable values of hardness and toughness of alumina to be maintained while reducing the Young's modulus below that of alumina, so that it may be expected that the thermal shock behaviour will be improved.

Rheological characteristics of alumina platelet–Hydroxyapatite composite suspensions

The rheological behaviour of aqueous suspensions of alumina platelet–hydroxyapatite mixtures for slip casting was investigated. The stabilisation of the suspensions requires the use of a dispersing agent and the breakdown of powder agglomerates. The addition of alumina platelets to the HAP powder does not modify significantly the behaviour of the suspensions which remains always quasi-Newtonian. Nevertheless, this behaviour becomes shear-thinning at low shear rates for high alumina contents when platelets of small size are used. The viscosity increases at low shear rates with the increase of small platelets content. These modifications are assumed to result from orientation phenomena of alumina disks under shear stress in the direction of flowing. The disk-shaped morphology of alumina is detrimental to the preparation of high density green composites. Suspensions containing between 50 and 70 wt% of powder are castable but the best rearrangement of solid particles during the casting process is reached for suspensions containing 65 wt% of powder.

Studies on the Rheological Behaviour of Aqueous Suspensions of Pre-Fired Terracotta Compositions in Relation to Newtonian Factor ‘n’

Utilization of common clay in admixtures with quartz and fly ash for fabrication of utility goods and art object is widely in practice. However, slip casting of slurries poses several problems in shaping of green bodies. In the present investigation, Newtonianity of the suspensions were evaluated and an attempt has been made to correlate Newtonian factor ‘n’ with the castability of a slip. Different factors affecting ‘n’ are analysed by rheological investigation of the suspensions. It was found that the suspensions were pseudoplastic in nature with a marked tendency towards Newtonian. The castability of the suspensions were explained in terms of Newtonianity (‘n’ values) of the suspensions. The results were utilized for further fabrication of green articles by slip casting.

Effects of particle classification on colloidal processing of mullite

Rheological behavior of aqueous suspensions of colloidal mullite, green densities and sintering behavior of consolidated powder compacts were studied on as-received and classified mullite powders of submicrometer sizes. Classification of large particles (> 1 μm) leads to an increase in the viscosity of the suspension and a decrease in the green density as compared with those of the unclassified powder. However, the classified powder is superior to the unclassified powder as for the formation of sinterable uniform green texture of narrow pore-size distribution and for the increase of driving force of sintering associated with the decrease of average particle size.