Suspensions Of Large Particles Research Articles

Rheology of bidisperse non-Brownian suspensions.

We study the rheology of bidisperse non-Brownian suspensions using particle-based simulation, mapping the viscosity as a function of the size ratio of the species, their relative abundance, and the overall solid content. The variation of the viscosity with applied stress exhibits shear-thickening phenomenology irrespective of composition, though the stress-dependent limiting solids fraction governing the viscosity and its divergence point are nonmonotonic in the mixing ratio. Contact force data demonstrate an asymmetric exchange in the dominant stress contribution from large-large to small-small particle contacts as the mixing ratio of the species evolves. Combining a prior model for shear thickening with one for composition-dependent jamming, we obtain a full description of the rheology of bidisperse non-Brownian suspensions capable of predicting effects such as the viscosity reduction observed upon adding small particles to a suspension of large particles.

Experimental Measurement of Enhanced and Hindered Particle Settling in Turbulent Gas‐Particle Suspensions, and Geophysical Implications

AbstractThe dynamics of geophysical dilute turbulent gas‐particles mixtures depends to a large extent on particle concentration, which in turn depends predominantly on the particle settling velocity. We experimentally investigate air‐particle mixtures contained in a vertical pipe in which the velocity of an ascending air flux matches the settling velocity of glass particles. To obtain local particle concentrations in these mixtures, we use acoustic probing and air pressure measurements and show that these independent techniques yield similar results for a range of particle sizes and particle concentrations. Moreover, we find that in suspensions of small particles (78 μm) the settling velocity increases with the local particle concentration due to the formation of particle clusters. These clusters settle with a velocity that is four times faster than the terminal settling velocity of single particles, and they double settling speeds of the suspensions. In contrast, in suspensions of larger particles (467 μm) the settling velocity decreases with increasing particle concentration. Although particle clusters are still present in this case, the settling velocity is decreased by 30%, which is captured by a hindered settling model. These results suggest an interplay between hindered settling and cluster‐induced enhanced settling, which in our experiments occur respectively at Stokes number O(100) and O(1). We discuss implications for volcanic plumes and pyroclastic currents. Our study suggests that clustering and related enhanced or hindered particle settling velocities should be considered in models of volcanic phenomena and that drag law corrections are needed for reliable predictions and hazard assessment.

Colloidal coal in water suspensions

We discuss the possible clean coal applications of colloidal suspensions of coal in water (CCW) manufactured with a proprietary wet-comminution device. These suspensions are a new material with new properties. First, the colloidal fraction plus water is a pseudo fluid good for transport, handling and suspension of large particles. Second, the surface area per unit volume of coal available for chemical reaction and burning is greatly increased and finally, CCW may be milled with a third fluid, seeding the mixture with submicron coal.The colloidal nature of the majority of particles provides for very good features such as outstanding long-term stability, in contrast to regular coal water slurries (CWS) which rapidly sediment under storage. Moreover, the very small particles create an increased reactivity to combustion because small particles with large surface area react faster than large particles with the same volume.

Assemblies of poly(styrene/⇌-tert-butoxy-ω-vinyl-benzyl-poryglycidol) Microspheres with different diameters deposited on mica plates

Formation of poly(styrene/α-tert-butoxy-ω-vinyl-benzyl-polyglycidol) microsphere assemblies on mica plates modified with 3-aminopropyltriethoxysilane was investigated. Microsphere assemblies contained two types of particles similar with respect of their chemical structure but with different diameters (D n = 1000 and 350 nm). Methods of particle deposition included: deposition from water suspension of a mixture of small and large particles on mica plates placed at the bottom of suspension container, deposition of particles from a drop of ethanol suspension (containing large and small microspheres) placed on the mica substrate, deposition of microspheres on modified mica plates crossing the liquid-air interface-sequential deposition of large and small particles, and one-batch deposition from a mixed water suspension of large and small microspheres. Deposition from water suspension containing large and small microspheres on plates placed on the bottom of suspension container yielded assemblies with large particles randomly distributed among the small ones. Fraction of large particles in adsorbed particle assembly was smaller than fraction of large particles in suspension. Particle assemblies prepared by placement of ethanol suspension of large and small microspheres on mica were composed of quite regularly distributed large particles among the small ones. A two step process consisting of withdrawal of mica plate from water suspension of large particles and then on using this plate as substrate in a second step during which the plate was withdrawn from suspension of small particles yielded particle assemblies containing aggregates of large particles randomly distributed among the small ones. Withdrawal of mica plates from water suspension of large and small microspheres resulted in particle assemblies composed of regularly distributed stripes of large and small microspheres. Formation of the described above microsphere assemblies is possible only in case of reversible adsorption of particles.

Rheological characteristics of debris-flow material in South-Gargano watersheds

In recent years, debris flows have represented a severe natural hazard in South-Gargano watersheds (Puglia Region, Southern Italy). Hill slopes erosion, caused by the inadequate protection of the degraded forests, produces large amounts of soil and debris that are conveyed downstream during heavy rainstorms. The involved material is characterized by limestone fragments and blocks in a sandy-silt matrix. In this paper, the rheological properties of such debris-flow materials have been investigated. Eight specimens of particulated sediments of particle diameter of d ≤ 4 mm have been analysed using a rheometric tool for large particle suspensions, the ball measuring system (BMS). The influence of sediment concentration on the bulk rheological behaviour has been evaluated at concentrations by volume obtained in fully water-saturated conditions (volumetric sediment concentration C V ~ 0.8) and with fixed water content (C V = 0.42). The rheological data were fitted to the Bingham, Herschel–Bulkley, and O’Brien and Julien models to define viscosity and yield stress dependency on sediment concentration, to provide viscosity information for hazard mitigation as well as for comparison to other debris-flow events.

Suspension Structure Effects on Transmission Fluctuation Spectrometry with Autocorrelation: Theoretical Prediction, Numerical Simulation and Experimental Verification

AbstractThe spatial structure of particle suspensions changes gradually with the particle concentration or monolayer density, and hence, affects the statistical properties of the transmission fluctuations. The changes of the suspension structure can be traced back to the concentration dependent changes in the structure of particle monolayers by using the layer model.In the present paper, transmission fluctuation signals of radiation through a monolayer of monodisperse, opaque, hard spherical particles are investigated by the autocorrelation technique in the time domain, mathematically expressed in terms of the expectancy of the transmission product (ETP). By changing the autocorrelation time, a transmission fluctuation spectrum is obtained and can be applied to perform particle size analysis. Due to the fact that the ETP and the expectancy of the transmission (ET) are measurable directly from the experiments, the corresponding transmission fluctuation correlation properties are characterized by a transition function, H, which is defined as H = lnETP/lnET for practical purposes.By using the direct correlation function proposed in a previous paper, the theory of transmission fluctuation spectrometry with the autocorrelation technique (TFS‐AC) is extended to high particle concentrations or monolayer densities. In order to observe the effect of the monolayer density on the spatial structure of the monolayer, some numerical simulations are performed on the TFS‐AC. Measurements on a flowing suspension of large particles are carried out using a narrow focused beam of the He‐Ne laser. The experimental results are found to be in agreement with the theoretical prediction and numerical simulations on the transition function over a wide range of monolayer densities. All of these considerations are based to date on the assumptions of geometrical ray propagation and monodisperse, opaque, hard spheres.

Radiolytic Hydrogen Yields in Aqueous Suspensions of Gold Particles

The effect of high concentrations of large gold particles, in the hundreds of nanometer size regime, on the yields of molecular hydrogen, G(H(2)), produced in the radiolysis of several aqueous solutions was determined. In particular we look for direct effect of radiation absorbed by the solid particles on the yield of water products. These particles, however, are catalytically active in the conversion of reducing radicals to molecular hydrogen as well. A very small increase in G(H(2)) observed in bromide solutions upon addition of 50 wt % of gold particles indicates that the radiolysis of the solid particles does not affect the yields in the aqueous phase. Very little exchange of charge carriers or energy between the two phases occurs in these large particle suspensions. On the other hand, efficient catalytic conversion of (CH(3))(2)C(*)OH radicals to H(2) is shown to occur. The efficiency of the presently studied suspensions in the redox-catalytic process is similar to that of suspensions of small particles of similar total surface area. In the presence of radicals from hydrogen atom abstraction from tert-butyl alcohol the yield decreases significantly, again similar to the behavior in suspensions of small particles. We conclude that the redox catalysis does not depend on the size of the particles when their size exceeds a few nanometers.

Gel trapping of dense colloids

Phase density differences in sols, foams, or emulsions often lead to sedimentation or creaming, causing problems for materials where spatial uniformity over extended periods of time is essential. The problem may be addressed through the use of rheology modifiers in the continuous phase. Weak polymer gels have found use for this purpose in the food industry where they appear to be capable of trapping dispersoid particles in a three-dimensional matrix while displaying water-like viscosities at low shear. Attempts to predict sedimentation stability in terms of particle properties (size, shape, density difference) and gel yield stress have led to qualitative success for suspensions of large particles. The effect of particle size, however, in particular the case in which colloidal dimensions are approached, has not been investigated. The present work seeks to determine useful stability criteria for colloidal dispersions in terms of readily accessible viscoelastic descriptors. Results are reported for systems consisting of 12 μm poly(methyl methacrylate) (PMMA) spheres dispersed in aqueous gellan gum. Monovalent salt concentration is varied to control rheological properties, and sedimentation/centrifugation experiments are performed to determine dispersion stability. Necessary conditions for stability consist of a minimum yield stress together with a value of tan δ less than unity.

Bidisperse Electrorheological Fluids Using Hydrolyzed Styrene-Acrylonitrile Copolymer Particles: Synergistic Effect of Mixed Particle Size

Monodisperse micron-sized styrene-acrylonitrile copolymer (SAN) particles with three different sizes (about 5, 10, and 15 microm) were prepared by a two-step seeded polymerization and used for a study of bidisperse electrorheological (ER) suspensions. The effect of the particle size and the size-mixing fraction on ER properties was studied with varying the size of these monodisperse copolymer particles. When the two particle sizes were mixed, the suspension generally showed a decrease in the shear yield stress, reaching a minimum value. However, a bidisperse ER suspension of large particles containing a small fraction of fine particles showed an interesting synergy effect of size mixing on ER response, giving enhanced yield stresses over the other size-mixing fractions. This synergistic ER suspension also showed a great increase in the viscoelastic property. The current density of suspensions was maximum at the synergistic bidisperse suspension. This synergy effect in a particular bidisperse suspension was investigated in view of the structure model consideration and was concluded to be due to a close packing and a peculiar structural ordering at an optimum size ratio and mixing fraction.

Rheological Behavior of Fine and Large Particle Suspensions

A new rheometric system, the ball measuring system, is used to determine the rheological behavior of fluids with large particles. To assess the applicability of the new system for debris flow material, artificial fine particle suspensions are investigated with the ball measuring system and a conventional concentric cylinder system. The agreement between the flow curves obtained by both systems is good. In a second step, defined amounts of coarser grains are added stepwise to the fine particle suspension to obtain different large particle suspensions. There is an increase in data scattering with an increase in large particle concentration. Thus, the question is raised for which grain size distribution, sediment concentration, and which process the traditional rheological approach is still appropriate when describing the flow process of debris flow mixtures. The curve fitting for the different fine and large particle suspensions is described using the Herschel–Bulkley model. Finally, the range of application for the ball measuring system is discussed. Today it is a suitable new rheometric system for fluids with particle sizes up to 10 mm and for measurements in the laminar flow field. The application for this system could potentially be expanded, for example, to measure fluids with particles sizes up to 200 mm with a large scale device.

Thermal conductivity of suspension containing SiC particles

Among various heat transfer enhancement techniques, an innovative way of improving the thermal conductivities of working media is to suspend ultrafine metallic or nonmetallic solid powders in traditional fluids since the thermal conductivities of most solid materials are higher than those of liquids. Theoretical and experimental studies have reported that suspensions containing solid particles have significantly larger thermal conductivities than the base fluids without solid particles [1, 2]. Unfortunately, suspended particles of micrometer, or even nanometer, dimensions may cause some severe problems such as abrasion and clogging in small passages. Furthermore, for all the large particle suspensions, stabilities are very poor since the coarse grained solid particles settle out eventually. Extensive research conducted on nanosized powder preparation and processing technology in recent years may offer a solution to the problem. Nanoparticle suspensions are expected to exhibit excellent properties. The large surface area and enhanced heat capacity of nanocrystalline materials [3–5] could improve the heat transfer capacities of nanoparticle suspensions. Also, Brownian movement of nanoparticles in a suspension increases stability [6]. Furthermore, nanoparticles can act as a lubricating medium because of their small sizes [7]. Adding a small amount of metal or metal oxide nanosized particles to a fluid has increased the thermal conductivity of the suspension over that of the base fluid [8–10]. The enhancement is highly dependent on the surface area of the particles and the thermal conductivity of base fluid. A nanosized metal particle suspension used as a new coolant in a microchannel heat exchanger offered significant benefits [11]. In this study, three types of α-SiC powder were dispersed in water and ethylene glycol (EG), respectively. The properties of SiC powders are summarized in Table I. SiC particles were deagglomerated by intensive ultrasonication after being mixed with base fluid and then the suspensions were homogenized by magnetic force agitation. The pH values of all the aqueous suspensions were adjusted to 10.0 such that the suspensions were stable. The thermal conductivities of the suspensions were measured by the hot-wire method [12] at a temperature of 4 ◦C. The thermal conductivities (λ) of the suspensions with SiC powers dispersed in water and EG as a function of solid particle volume fraction are shown in Fig. 1a and b, respectively. Fig. 2a and b show the ratio of the thermal conductivity enhancement (δλ = λ − λ0)

The Correlation Functions of a Suspension of Large Particles in Amorphous Polybutadiene: A Molecular Dynamics Simulation Study

The correlation functions of a suspension of Lennard-Jones large particles in a model 1,4-cis-polybutadiene solvent have been investigated by molecular dynamics simulations. We present the effects of temperature, the degree of polymerization, and the solvent/large particle density on the effective interactions between the large particles and the solvent. It is found that as the temperature increases, the structure between solvent–solvent, solvent–large particle, and large–large particle decreases. Additionally, as the bulk density or the chain length is increased, the attractive part of the large–large interaction becomes weaker and small. We believe that in part, this is due to the polymer having collapsed onto itself and entangling the large particles and lessening their interaction until they are actually in contact. Increasing the length of the polymer also entangles the large particles. However, we believe that this may be a general feature that is a characteristic of a polymer solvent containing macroscopically large colloidal particles even though entanglement should be of less significance.

Measuring the viscosity of large-particle (and flocculated) suspensions — a note on the necessary gap size of rotational viscometers

A simple theory is presented to account for the viscometer walls effects encountered when measuring suspensions of large particles (or flocs). The often-recommended minimum ratio of gap-to-particle size ratio of 10:1 is shown to be appropriate for phase volumes up to 25%, but above this value, the ratio needs to be greater. The theory is supported by recent experimental evidence.

Human visual perception of haze and relationships with instrumental measurements of turbidity. Thresholds, magnitude estimation and sensory descriptive analysis of haze in model systems

Spherical polymer beads (0.769, 2.600 and 10.300 μm diameter) were suspended in clear, yellow and red liquids. The samples were measured by turbidimetry and assessed by panelists. Thresholds were determined by the Ascending Method of Limits and ranged from 0.384 to 0.815 NTU. The results were influenced by both particle size and solution color. Visual intensity (assessed by Magnitude Estimation) rose linearly with particle concentration until it reached a plateau. A regression model was developed that expressed visual haze intensity as a function of particle concentration and size, and liquid color ( R 2=0.949). A relationship between visual and instrumental responses was also developed ( R 2=0.870); when particle size was included, this improved to R 2=0.978. Turbidimeter response could be predicted from particle concentration and size ( R 2=0.986). Principal Components Analysis was applied to Descriptive Analysis results and showed that two factors accounted for 99% of the observed variation. Suspensions of large particles at intermediate concentrations appeared non-homogeneous.

Automated high-speed Mueller matrix scatterometer

A new scatterometer-polarimeter is described. It measures the angular distribution of intensity and of the complete Mueller matrix of light scattered by rough surfaces and particle suspensions. The measurement time is 1 s/scattering angle in the present configuration but can be reduced to a few milliseconds with modified electronics. The instrument uses polarization modulation and a Fourier analysis of four detected signals to obtain the 16 Mueller matrix elements. This method is particularly well suited to online, real time, industrial process control involving rough surfaces and large particle suspensions (an arithmetic roughness or particle diameter of >1 microm). Some results are given.

Calculation of optic fibres calibration curves for the measurement of solids volume fractions in multiphase flows

A model that calculates calibration curves (probe response functions) of reflection type optical probes for the measurement of solids volume fractions is presented. The reflection from a single surface is calculated from a relaxed inverse square law to allow the introduction of the effects of multiple reflections that occur in the cavities of rough surfaces, represented as an assembly of spherical particles. The probe response function is then calculated as the summation of the reflections of several successive surfaces with correction for obscuration. The probe response function obtained agrees reasonably well with experiments using aqueous suspensions of aluminium hydroxide particles, with a surface volume mean diameters of 75 and 20 μm. For 200 μm diameter sand particles good agreement was found for solids volume fractions greater than 5%. The difference between the predictions and experiments for low solids volume fraction suspensions of large particles was shown to be related to the large interparticle distances in comparison to the fibre diameter. When applied to a suspension of the same solids in air, the method gives a probe response function that is different from that obtained in water, especially at low solids volume fractions. The departure of the probe response function from the power law rule at low solids volume fraction, observed experimentall, is replicated by the model.

Random pulsations in a homogeneous coarsely dispersed gas suspension

Internal pulsations of both phases of a monodisperse suspension of large particles in a gas are considered under the assumption of statistical independence of particles and isotropy of their pulsations isotropy provided by particle collisions. Statistical characteristics of the pulsations are computed as functions of the mean concentration and physical parameters for macroscopically uniform states of the gas suspension, disregarding the energy dissipation in particle collisions.

Measurement of solid concentration in suspension using supersonic waves.

The relation between the solid volume fraction and the attenuation coefficient of supersonic waves in suspension of large particles was investigated. Experiments were performed on the gravity flow of two kinds of spherical polystylene pellets (0.406mm and 1.50mm) in a vertical pipe. The attenuation rate was measured for two frequencies of a supersonic wave (40kHz and 200kHz). The solid fraction was measured up to about 14%. It was found that the attenuation coefficient was proportional to the solid volume fraction in the range of measurements. In addition to the experiment, theoretical calculation was carried out, assuming the particles to be fixed rigid spheres. The calculated results compared well with the measurements.

Ultrasonic Attenuation in Suspension of large Particles

Ultrasonic Attenuation in Suspension of large Particles

FLOCCULATION IN SUSPENSIONS OF LARGE PARTICLES

Based upon an assumed mechanism of particle aggregation by shear-induced interparticle collisions, a new method of studying flocculation in suspensions of large particles is described, and representative results obtained with cellulose–fiber suspensions are presented. The method consists in subjecting the suspension to a reproducible condition of shear motion and analyzing the fluctuations in optical transmission coefficient electronically. An approximate statistical theory is presented on the basis of which a flocculation index (σ/N0) appropriate to the method is proposed. The index is a function of both the number and size of the aggregates. The flocculation index was found to decrease with increasing rate of shear and decreasing fiber length. The effect of concentration and of additives was investigated. Alternative experimental techniques are outlined and limitations to the present method are pointed out.