Breakup Of Aggregates Research Articles

00/00336 Solar thermochemical conversion of biomass

Turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters, and their respective aggregate break-up. This study examines the inter-relationships between floc characteristics over increasing turbidity (80–200 mg l−1) and turbulent shear (0–0.6 N m−2) environments, by the use of a laboratory flume within which a suspension can be sheared at a controlled rate, and with an unintrusive macro-lens miniature video camera mounted in a viewing port on the flume channel wall. The camera enables the direct simultaneous measurement of both floc size and settling velocity, from which accurate estimates of floc effective density and porosity can be made. Measurements were made 120 s after the induced turbulence has ceased. The instrument has an upper viewing turbidity limit of 210 mg l−1, and a lower resolution of 20 μm. The sediment was collected from the inter-tidal mudflats at Weir Quay on the Tamar Estuary in Devon, Southwest England. The results indicated that increasing turbidity at low shear levels encouraged floc growth, but the effect of the increasing turbulent shear (0.35 N m−2) together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process. At shears up to 0.35 N m−2, the largest size and settling velocity flocs were produced at high concentrations, whereas above 0.35 N m−2 disruption caused smaller flocs at higher concentrations. The use of algorithms which were based either on a single floc characteristic (i.e., size or settling velocity) or a mean fractal dimension, were seen not to accurately approximate the experimental data. A multiple regression analysis of the experimental data produced the following formula, based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions (referred to as max20size mean values): settling velocity=0.301−0.00337 rms of the gradient in turbulent velocity fluctuations−0.000606 SPM+0.00335 floc size, which proved to be the most accurate representation with an R2 value of 0.95. A similar formula was determined for the average value of the four fastest settling flocs within each sample-group (max4WS). This highlights the importance of modelling algorithms that are developed from data that take into account effective density variations (i.e., simultaneous size and settling velocity measurement).

Incorporation of Aggregate Breakup in the Simulation of Orthokinetic Coagulation

The agglomeration and breakup of floc aggregates formed in orthokinetic coagulation is examined. By considering local flow strain-rate, a breakup rate kernel is derived based on flow-induced normal stresses. The new breakup kernel is included in a population size class balance for floc aggregates. The resulting population balance was solved numerically over a wide range of parameters to obtain a variety of floc size distributions. Results indicate that the inclusion of a breakup kernel in orthokinetic coagulation modeling eliminates the computational growth to a maximum size class, producing more realistic distributions. The breakup kernel was rigorously compared to prior research and found to be consistent with the earlier theories of coagulation agglomeration and breakup.

A laboratory examination of floc characteristics with regard to turbulent shearing

Turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters, and their respective aggregate break-up. This study examines the inter-relationships between floc characteristics over increasing turbidity (80–200 mg l −1) and turbulent shear (0–0.6 N m −2) environments, by the use of a laboratory flume within which a suspension can be sheared at a controlled rate, and with an unintrusive macro-lens miniature video camera mounted in a viewing port on the flume channel wall. The camera enables the direct simultaneous measurement of both floc size and settling velocity, from which accurate estimates of floc effective density and porosity can be made. Measurements were made 120 s after the induced turbulence has ceased. The instrument has an upper viewing turbidity limit of 210 mg l −1, and a lower resolution of 20 μm. The sediment was collected from the inter-tidal mudflats at Weir Quay on the Tamar Estuary in Devon, Southwest England. The results indicated that increasing turbidity at low shear levels encouraged floc growth, but the effect of the increasing turbulent shear (0.35 N m −2) together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process. At shears up to 0.35 N m −2, the largest size and settling velocity flocs were produced at high concentrations, whereas above 0.35 N m −2 disruption caused smaller flocs at higher concentrations. The use of algorithms which were based either on a single floc characteristic (i.e., size or settling velocity) or a mean fractal dimension, were seen not to accurately approximate the experimental data. A multiple regression analysis of the experimental data produced the following formula, based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions (referred to as max20size mean values): settling velocity=0.301−0.00337 rms of the gradient in turbulent velocity fluctuations−0.000606 SPM+0.00335 floc size, which proved to be the most accurate representation with an R 2 value of 0.95. A similar formula was determined for the average value of the four fastest settling flocs within each sample-group (max4W S). This highlights the importance of modelling algorithms that are developed from data that take into account effective density variations (i.e., simultaneous size and settling velocity measurement).

Description of asphaltene polydispersity and precipitation by means of thermodynamic model of self-assembly

A thermodynamic approach describing the precipitation of asphaltene materials from petroleum fluids as a consequence of aggregate break-up is presented. The aggregation equilibrium for asphaltenes and resins is considered along with the bulk-phase equation of state (EOS) to account for the effects of pressure and solvents on the thermodynamic behavior of asphaltene dispersions. Polydispersity of asphaltene aggregates is taken into account. The effect of the resin molecular shape on the size distribution is considered. Relation between the shape factors and elastic constants of the resin shell of the aggregates is given. Precipitation curves for several real crude oil mixtures are calculated and structural characteristics of the aggregates are estimated. Comparison with the experimental data shows that the results are reasonable.

The kinetics and mechanism of Corynebacterium glutamicum aggregate breakup in bioreactors

Cultivation of bacteria requires high levels of agitation and aeration to satisfy the mass transfer requirements of the cells. Associated with these conditions are turbulent forces which may act on the surfaces of cells and be detrimental to their growth, metabolism and morphology. The kinetics and mechanism of hydrodynamic trauma has been investigated for the breakup of aggregates of Corynebacterium glutamicum ( ATCC 13032) in a stirred-tank reactor and a capillary flow loop system. The effect of forces associated with turbulent eddies in the impeller discharge zone of the stirred tank reactor has been compared with that of collapsing air bubbles at the air medium interface. A model is presented to describe the initial rate of aggregate breakup caused by fluid–aggregate interactions. It assumes that aggregate disruption is caused by the interaction of aggregates with similarly sized turbulent eddies. The extent of aggregate breakup is a function of the magnitude of the turbulent force as well as the total duration of the force event. The applicability of the model to animal cell systems has been investigated. Results showed that both the interaction of microbial cells with turbulent eddies in the viscous dissipation subrange in the impeller discharge zone, as well as with collapsing air bubbles at the air medium interface contributed to the total force acting on the cells.

The particle size characteristics of fine-grained channel deposits in the River Exe Basin, Devon, UK

Deposition and storage of fine-grained ( 95% 62·5 μm sized particles. Spatial variation in the effective particle size and aggregation parameters was of secondary importance relative to temporal variation. The effective particle size distribution of the FGBS was consistently coarser and more aggregated than the associated suspended sediment and there was evidence of aggregate break-up in samples of resuspended bed sediment. The implications of these findings for sediment transport modelling are considered. Copyright © 1999 John Wiley & Sons, Ltd.

Shear-induced structuring of particulate whey protein gels

The effects of steady shear on particulate whey protein isolate (WPI) gels, at pH 5.4, have been investigated by light microscopy (LM) and dynamic oscillatory measurements. The steady shear was performed on suspensions at constant rates between 0.5 and 126/s. The gel point under static conditions (Tg) was around 78°C and the shearing was performed during heating from 20 to 76 or to 82°C. The gel point was postponed by the shear up to 82°C. Steady shear up to 76°C, at rates less than ∼6/s, resulted in a weaker storage modulus (G′), less frequency dependence and a higher stress at fracture compared to the unsheared gel. Steady shear up to 82°C, at rates below ∼6/s, resulted in the formation of two different types of network structure. One structure was similar in appearance to the unsheared network, showing pores in the range of 50μm. The other structure was dense, composed of smaller particles than the unsheared network and with pores in the range of 10μm. The gels composed of two structures showed a lower G′ and stress at fracture compared to the unsheared gel. A shear rate above 24/s up to 76°C resulted in irregular networks, which were composed of two different types of structures. One was loose and open, similar in appearance to the unsheared network structure. The other structure was dense and compact, and was present as individual aggregates. These gels also showed a weaker G′ than the unsheared gel. A shearing up to 82°C at rates above 24/s resulted in a coarse, inhomogeneous network structure. The gels showed a weak G′, indicating aggregate break-up during the steady shear.

Shear-thinning predictions from particle motion modeling

A Stokesian dynamics model is presented to predict suspension viscosity and microstructure for concentrated suspensions. The influence of electrostatic repulsive forces, London–van der Waals attractive forces, and boundary interactions on the degree of shear-thinning is reported. Three different mechanisms for shear-thinning are described and quantified. The degree of shear-thinning caused by Brownian motion is well predicted by using random and layered structures. Shear-thinning for repulsive forces between particles is caused by the “melting” or breakup of an ordered or semicrystalline configuration as the shear rate increases. For suspensions flocculated into a secondary minimum, shear-thinning is produced by breakup of particle aggregates as the hydrodynamic forces dominate the interparticle forces. At high concentrations, the dispersed and flocculated suspensions can form slip planes that reduce suspension viscosity. Rough walls increase the viscosity predictions by an order of magnitude and cause the shear thinning prediction to compare well with experimental results from the literature.

The use of image analysis to characterize aggregates in a shear field

A means of morphological characterization of flocculated suspensions in a shear field is described. By use of image analysis combined with rheometry it was possible to measure the size and fractal dimension of solid aggregates in non-aqueous suspensions under variable shear stress. This enabled structural changes such as alignment and aggregate break-up to be observed and quantified. Studies of the aggregation kinetics were also performed by monitoring the size increase of aggregates, after the relaxation of an applied shear stress sufficient to disperse the aggregates. The technique is illustrated by measurements on lactose aggregates in 1,1,2-trichlorotrifluoroethane, a system relevant to the behaviour of inhalation aerosols. Aggregate size R was rapidly reduced by the application of shear stresses s up to 1 N m −2 followed by a more gradual decrease at greater stresses. The average aggregate size showed good agreement with an empirical law of the form R ∼ s − 1 3 , which enabled estimates of the aggregate binding energy to be obtained. Boundary fractal dimension showed only a slight reduction from 1.4 to 1.3 over the same range of shear stress, indicating that the rugged aggregate structure and self similarity were largely preserved during disruption. Measurement of the formation kinetics and fractal dimension of aggregates showed a correlation between the resultant aggregate morphology and the formation rate which was in agreement with existing theories of fractal aggregation kinetics.

Bacterial motility, collisions, and aggregation in a 3-μm-diameter capillary

Motility of Escherichia coli K-12 cells was studied in a capillary tube with an inside diameter of 3 mum, which is comparable to the size of the cells' body. The extreme restriction, imposed by the capillary on the bacterial motility, caused a series of phenomena such as cell aggregation, swimming as a cluster, and break-up of aggregates, which were observable for the first time, and which are reported here. (c) 1997 John Wiley & Sons, Inc.

O uptake rate measurements as a novel tool to study shear effects on suspended strawberry cells

Despite the same mean volumetric power input of 19 and 47 W/m 3 , the specific oxygen uptake rate (OUR S ) of strawberry ( Fragaria ananassa) cell suspensions was higher in bioreactors equipped with a Rushton Turbine (4.4 and 6.2 3 10 -5 mol O 2 /kg.s) than with an anchor stirrer (2.5 and 4.6 3 10 -5 mol O 2kg.s). The increase in OUR S was caused by stress-activated respiration and appeared to be correlated with the locally dissipated power input. OUR S was corrected for the increase in surface through aggregate break-up and reached a maximum of 6.0 3 10 -5 mol O 2kg.s when agitating with approximately 200 kW/m 3 locally dissipated power input.

Modelling the Aggregation and Break-up of Fractal Aggregates in a Shear Flow

A population balance model has been proposed to describe simultaneous coagulation and fragmentation of fractal aggregates during shear-flocculation induced by means of a Couette-flow system. Given enough time, a floc-size distribution achieves the steady state, which reflects the balance between coagulation and fragmentation forces. Experimental results obtained recently show that higher shear rate values shift the steady state to smaller aggregate sizes. Also, for a fixed value of the shear rate, when the volume fraction of the particles increases, the steady state size increases if the flow is laminar, and decreases if the flow is turbulent. In order to model the fragmentation of the aggregates, a power dependence between the breakage rate coefficient and the shear rate has been proposed. The model is adjusted by using two different parameters: the effective probability of success for collisions αeff, and the break up coefficient for shear fragmentation, B′. The dependence of these two parameters on the shear rate and the volume fraction of the particles is discussed.

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.

Fractal Structures in Carbon Black Reinforced Rubbers

Abstract This paper considers the fractal nature of primary and secondary carbon black aggregates in rubber and the implications for the mechanical and electrical properties of rubber goods. In particular, the effects of dispersion and primary aggregate breakup during mixing are investigated. Founded on recent studies of the disordered growth processes of colloids, a model concerning formation, structure and properties of tenuous secondary carbon black aggregates (clusters) in rubber is formulated. We distinguish between two different mechanisms of cluster growth at carbon black concentrations ϕ below and above the gel point ϕ** of the filler network. For ϕ≤ϕ**, the restricted mobility of the dispersed primary aggregates governs the cluster growth and neighboring clusters are seperated by a rubber-specific minimum distance d0. These gaps between neighboring clusters are joined together in a bond percolation model that determines the conductivity in a transition regime ϕ*≤ϕ≤ϕ** above the electrical percolation threshold ϕ*. The mechanical action of carbon black clusters below the gel point ϕ** is estimated by a hydrodynamical amplification factor that is related to a rigidy condition for the clusters. At sufficient large filler concentration for ϕ≥ϕ**, the restricted mobility of primary aggregates in the rubber matrix is insignificant for the cluster growth and a kinetic cluster by cluster aggregation (CCA) process is applied. The resulting fractal carbon black network corresponds to a spacefilling configuration of CCA-clusters. From this network structure, a scaling invariant power law arises for the small-strain modulus as function of carbon black concentration. The conductivity in the networking regime ϕ≥ϕ** shows a typical power law behavior that is implied by an anomalous diffusion of the charge carriers on the fractal clusters. For the frequency dependence of the conductivity a cross-over to a power law regime at large frequencies results. The predicted properties of carbon black filled rubbers are discussed in the framework of experimental results taken from different authors.

The breakup of the intermediate gold aggregates

The synthesis of nanometer scale gold particles is well known historically. Stable and highly colored suspensions result from the reduction of gold cations in solution. But the formation of the gold particles is not monotonic, that is, the mechanism is not simply nucleation and growth. Rather, the pronounced color changes in the evolving suspension indicate that the mechanism is nucleation, aggregation, deaggregation, and growth. Until recently, the structure of the intermediate aggregate had been assumed to be an aggregation of primary particles, each 4-7 nm in diameter. HRTEM studies have shown that the intermediates are continuous ramified structures (Figure 1) which undergo a desintering process to form the final particles (~24-26 nm diameter). In this paper, we discuss a model which explains the role of the surfactant on the gold intermediate in the breakup of the intermediate structure.Gold colloids were prepared by reducing AuIIICl3 (aq) with sodium citrate as in previous studies:

Sortable silt and fine sediment size/composition slicing: Parameters for palaeocurrent speed and palaeoceanography

Fine sediment size (<63 µm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 µm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10‐µm grain size, and noncohesive above that size. Thus silt coarser than 10 µm displays size sorting in response to hydrodynamic processes and its properties may be used to infer current speed. Silt that is finer than 10 µm behaves in the same way as clay (<2 µm). Useful parameters of the distribution are the 10–63 µm mean size and the percentage 10–63 µm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, KM), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface KE of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar Current, and Brazil/Falkland currents in the Argentine Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both coccoliths and foraminiferal fragments can be recognized and analyzed, with the boundary between them again at about 10 µm. The flux of less than 10 µm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.

Simple Relationships among Zeta Potential, Particle Size Distribution, and Cake Specific Resistance for Colloid Suspensions Coagulated with Ferric Chloride

Abstract Kaolinite and latex colloidal suspensions were coagulated with various doses of ferric chloride before being filtered on microfiltration membranes. Subsequent improvements of the filtration rate (7 to 25 times) were observed for small coagulant doses (10 to 20 mg Fe3+/g). The variations of three parameters, the cake specific resistance, the particle interaction energy calculated from zeta potential measurements, and the fraction of colloidal particles remaining in suspension, all showed a minimum at virtually the same concentration of ferric chloride. The fixation of iron ions, which brings positive charges onto the originally negative surfaces, resulted in charge destabilization and aggregation of fines. For the latex, however, overdosing caused the breakup of aggregates and an accordingly lower filtrability. The cake structure was found to be more open at optimum ferric chloride dosage as shown by electron microscopy.

Fragmentation of Colloidal Aggregates Induced by Polymer Adsorption

Using a particle counter technique, we investigated the fragmentation process of aggregates of polystyrene suffonate latices induced by the adsorption of polyvinylpyridine at the particle-liquid interface by determining the aggregate size distribution c( x, t), concentration of aggregates of size x, and the weight S( t) and number N ( t) average sizes at time t. The fragmentation of two typical aggregates were studied. The first aggregates obtained under conditions of perikinetic diffusion-limited aggregation (1 M NaCl and pH 3.0 medium) were transferred to aqueous polyelectrolyte solutions at different concentrations in the presence of 0.15 M NaCl and at pH 3.0. The second aggregates obtained under conditions of reaction-limited aggregation (0.15 M NaCl and pH 3.0 medium) were transferred to aqueous polyelectrolyte solutions in the presence of 0.0225 M NaCl and at pH 3.0. We determined that fragmentation is a very slow process. The size distribution of the fragments was self-similar and the variation in the number average size N( t) and the weight average size S( t) as a function of time followed dynamic scaling laws whose exponents depended on the mode of formation and the initial size of the aggregates, as well as on the concentration and the molecular weight of the polymer. Using Cheng and Redner's theory ( Phys. Rev. Lett. 60, 2450 (1988); J. Phys. A: Math Gen. 23, 1233 (1990)), we calculated the rate of aggregate breakup, which was found to be independent of the initial size of the aggregates and of the polyelectrolyte concentration. The mode of aggregate formation and the molecular weight of the polymer in the fragmenting medium were found to fix the rate of breakup and the fragment size distribution. The discrepancies between the rate of aggregate break-up and the rates of fragmentation determined from the variation of the average sizes were attributed to the existence of a concomitant aggregation during fragmentation.

Monitoring of the Dispersion of Aggregated Red Blood Cells by Flow/Hyperlayer Field-flow Fractionation

High-speed flow/hyperlayer field-flow fractionation (Fl/HyFFF) was used to characterize glutaraldehyde-fixed rabbit red blood cells (RBCs). A suspension of powdered cell sample in a buffered medium prepared by vortexing was found to be partially aggregated. This was then sonicated to disrupt the aggregates. A gradual breakup of cell aggregates with sonication time was evident from altered Fl/HyFFF elution profiles of the cell sample. It was found that all RBC aggregates were reduced to singlets after a 30-min sonication time. The presence of RBC triplets, doublets and singlets in the original suspension and their sequential elution in Fl/HyFFF was confirmed by electron microscopy.

Effect of fluid shear forces on plant cell suspensions

The sensitivity of plant cell culture to fluid forces, or fluid-mechanical sensitivity, in a stirred bioreactor is evaluated by shearing the cells in viscometers for short duration under a range of laminar and turbulent conditions. This short-term evaluation is compared with the experiments in a stirred bioreactor under chemostat mode at various agitation speeds. Fluid-mechanical sensitivity was quantified by measuring various biological parameters including regrowth ability, membrane integrity, mitochondrial activity, aggregate size and lysis. A critical hydrodynamic variable was derived based on a model whereby the biological activity of cells after a given period in a defined shearing environment was a cumulative effect of the total work done by fluid forces on the cells. This hydrodynamic variable, calculated as total energy dissipation on cells per unit volume, was used as a common basis to quantify the agitation-based shear forces under laminar and turbulent flow conditions. Fluid-mechanical sensitivity data for plant cell suspensions under different flow conditions was successfully correlated with this hydrodynamic variable. This correlation established a hierarchy of response, ranging from a subtle biological inhibitory effect on regrowth ability or sublytic effect, to the gross physical effect of aggregate breakup and lysis. Inhibition of growth rather than lysis was found to dictate the performance of plant cell culture in the bioreactor.