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PubMed1999

Rheological Behavior of Suspensions Flocculated by Weak Bridging of Polymer Coils.

A polymer coil with weak affinity for the solid surface makes a flexible bridge between particles by reversible adsorption. When the coil size is comparable to particle diameter in solution, the suspensions flocculated by reversible bridging show shear-thickening flow in a narrow range of shear rates. The force generated by the rapid extension of bridges within the lifetime is responsible for the shear thickening. Although it is expected that the bridges become more flexible with increasing coil size, the polymers with high molecular weights do not necessarily give rise to shear thickening. The striking shear thickening is induced by polymers with molecular weights, Mw, of 2.5 x 10(5) and 4.5 x 10(5). The viscosity enhancement in the shear-thickening region is considerably suppressed for polymers with Mw = 7.5 x 10(5). From the sedimentation experiments, a large portion of the segments of a coil with Mw = 7.5 x 10(5) is adsorbed in train on the particle surface. The addition of small amounts of surfactant leads to an increase in the fraction of loops at the expense of trains and the bridges become more flexible. As a result, the flow of suspensions in the presence of surfactant becomes shear thickening. The rheology of suspensions flocculated by reversible bridging can be modified by a balance between loop and train fractions in a bridge. Copyright 1999 Academic Press.

Journal Article↗

PubMed2000

Characterization of a Polydisperse Depletion-Flocculated Emulsion.

The addition of nonadsorbing polymer to an alkane-in-water emulsion causes the droplets to flocculate into a space-spanning, stress-bearing network. We report rheological measurements of an emulsion of 1-bromohexadecane-in-water flocculated by hydroxy-ethylcellulose. Small-deformation oscillatory measurements allowed characterization of the structure during formation and an indication of the strength of the resulting network. Emulsions without polymer, and polymer solutions alone, showed essentially viscous behavior, with dominant viscous modulus over the whole frequency range (0.01-10 Hz). However, the emulsion containing polymer demonstrated a significant elastic modulus, dependent on the oil and polymer concentrations, attributable to interdroplet depletion interactions. Power-law relationships were observed between the elastic modulus, elastic strain limit, and oil volume fraction, but the indices were lower than those predicted by fractal models, giving unrealistic fractal dimensionalities. The modulus increased exponentially with polymer concentration, but the elastic strain limit was independent of added polymer. The rate of formation of the network was not consistent with diffusion-controlled aggregation. Copyright 2000 Academic Press.

Journal Article↗

PubMed2000

Kinetics of Cream Formation by the Mechanism of Consolidation in Flocculating Emulsions.

In this work we propose a theoretical description of the process of creaming of batch emulsions when a sharp boundary exists between a clear serum phase and the sedimenting drops. The creaming is represented as a continuous consolidation of partially aggregated network. The treatment reproduces correctly the trend for gradually decreasing rate of sedimentation as time goes on. The theoretical results are compared quantitatively with experimental measurements of the creaming rate. Oil-in-water systems, stabilized by proteins (beta-lactoglobulin (BLG), bovine serum albumin, and mixtures BLG+beta-casein) were investigated. Faster creaming is attributed to larger size of the sedimenting objects (flocs of emulsion droplets). In systems obeying the creaming mechanism with sharp boundary (SB) the flocs are smaller when the protein concentration is higher. This supports the hypothesis for the stabilizing role of the excess amount of protein (forming lumps and multilayers on the interface). Theoretical analysis demonstrates that the formation of flocs by gravitational coagulation is a much faster process than the consolidation of the cream. Hence, the dispersions first flocculate and then cream. With increasing beta-casein content in mixtures BLG+beta-casein the emulsions depart from the SB-type behavior and are characterized by the presence of small nonflocculated droplets, which do not sediment (the serum is turbid and the boundary with the concentrated dispersion is diffuse, DB behavior). This is connected with hindered flocculation, perhaps due to beta-casein's augmented ability to prevent droplet aggregation. Copyright 2000 Academic Press.

Journal Article↗

PubMed2002

Coagulation-flocculation of beech condensate: particles size distribution.

Beech wood (Fagus sylvatica L.) condensate from a steaming operation was studied. The objective of our work was to study the precipitation of these wood extracts in presence of calcium ion after autoxidation at basic pH (8). The autoxidation was carried out at 250 rpm for 30 min, and flocculation was followed up for 30 min. An investigation with a laser sizer Mastersizer of Malvern has been done in order to study the influence of the agitation on the state of aggregation of the condensate. A negative correlation was observed between the mean size of particles and the agitation rate. Without stirring, flocculation rapidly occurred and the mean size of particles was high. Calcium-induced aggregation of the condensate was also found to be reversible toward agitation.

Journal Article↗

PubMed1999

Construction of a flocculent Saccharomyces cerevisiae fermenting lactose.

A flocculent Saccharomyces cerevisiae strain with the ability to express both the LAC4 (coding for beta-galactosidase) and LAC12 (coding for lactose permease) genes of Kluyveromyces marxianus was constructed. This recombinant strain is not only able to grow on lactose, but it can also ferment this substrate. To our knowledge this is the first time that a recombinant S. cervisiae has been found to ferment lactose in a way comparable to that of the existing lactose-fermenting yeast strains. Moreover, the flocculating capacity of the strain used in this work gives the process several advantages. On the one hand, it allows for operation in a continuous mode at high cell concentration, thus increasing the system's overall productivity; on the other hand, the biomass concentration in the effluent is reduced, thus decreasing product separation/purification costs.

Kluyveromyces↗

PubMed1992

A protein homologous to glyceraldehyde-3-phosphate dehydrogenase is induced in the cell wall of a flocculent Kluyveromyces marxianus.

A protein with an apparent molecular weight of 37,000 (p37) is present in very large amounts in the cell wall of Kluyveromyces marxianus, after the induction of flocculation of the yeast. This protein was isolated by preparative gel electrophoresis and its purity checked by SDS-PAGE and reverse-phase HPLC. SDS-PAGE, endoglycosidase-H treatment and peptide sequencing indicated that p37 is a glycoprotein with a high identity to cytosolic glyceraldehyde-3-phosphate dehydrogenase from Saccharomyces cerevisiae. Polyclonal antibodies were used for Western blot analysis and immunocytochemistry, which showed that p37 is present in the cell wall of non-flocculent K. marxianus and, therefore, is a constitutive protein of the cell wall.

Amino Acid Sequence↗

PubMed2004

Critical flocculation density of dilute water-in-CO2 emulsions stabilized with block copolymers.

The critical flocculation density (CFD), that is, the CO(2) density below which flocculation occurs, was studied for dilute water-in-CO(2) (W/C) miniemulsions stabilized with poly(1,1-dihydroperfluorooctyl methacrylate)-b-poly(ethylene oxide) (PFOMA-b-PEO) surfactants. The CFD, which was measured by turbidimetry, decreased as the PFOMA molecular weight was increased, the average droplet size was decreased, the surfactant loading was increased, and the temperature was increased. A simple model, which addressed both the van der Waals attraction between droplets and osmotic solvent-tail interactions, was in good qualitative agreement with the experimentally observed trends for the CFD and predicted a decrease in emulsion stability as the CO(2) density was lowered toward the theta density for PFOMA in bulk CO(2).

Journal Article↗

PubMed1999

Influence of competitive adsorption on flocculation and rheology of high-pressure-treated milk protein-stabilized emulsions.

The effect of high-pressure treatment (HPT) on the droplet-size distribution and small-deformation rheology of oil-in-water emulsions containing beta-lactoglobulin and a nonionic surfactant or sodium caseinate has been investigated at neutral pH. Addition of Tween 20 (polyoxyethylenesorbitan monolaurate) to a beta-lactoglobulin-stabilized emulsion results in competitive displacement of the adsorbed globular protein film and, following HPT, the formation of a less flocculated emulsion. The age of the beta-lactoglobulin-stabilized emulsion prior to addition of sodium caseinate influences the competitive adsorption behavior. The strengthening of the beta-lactoglobulin layer with time makes it more resistant to disruption by sodium caseinate. The level of pressure-induced flocculation of beta-lactoglobulin-coated oil droplets depends on the intensity of processing conditions and on the degree of interfacial displacement. In contrast, beta-lactoglobulin added after emulsification appears to show little evidence of competitive adsorption behavior at the caseinate oil-water interface. Changes in the rheological properties of these latter systems following HPT can be attributed to pressure-induced denaturation and gelation of beta-lactoglobulin in the continuous phase of the emulsion.

Adsorption↗

PubMed2006

Flocculation-induced homolysis of hydrogen peroxide in aqueous colloid solution of titanium dioxide nanoparticles.

Thermal generation of oxygen and hydroxylated aromatic compounds by hydrogen peroxide, catalyzed by flocculation of titanium dioxide nanocrystallites aqueous suspension upon addition of hydrogen peroxide, is reported. The oxidation involves catalytic cleavage of a peroxide molecule followed by hydroxyl reaction with the organic solutes. The catalytic hydroxylation is associated with formation of TiO(2)-H(2)O(2) aggregates, which occurs within a specific range of [TiO(2)]/[H(2)O(2)] ratio. Comparison of the activation energy to literature values in the absence of nanoparticles indicates that flocculation induces an increase of the rate without decreasing the activation energy. This is, to the best of our knowledge, a unique case of nanoparticles catalysis driven by formation of a three-dimensional structure of the suspended particles.

Journal Article↗

PubMed2005

Calculation of flocculation and coalescence rates for concentrated dispersions using emulsion stability simulations.

A simple procedure for the quantification of flocculation (k(f)) and coalescence (k(c)) rates from emulsion stability simulations (ESS) of concentrated systems is presented. It is based on a simple analytical equation, which results from the sum of well-known formulas for the separate processes of flocculation and coalescence. The expression contains k(f) and k(c) as fitting parameters and is found to reproduce the behavior predicted by ESS spanning a wide range of volume fractions (1 < phi < 30%) and surfactant concentrations (1.2 x10(-5) < C < 1.2 x 10(-4) M). This procedure allows interpretation of ESS data in terms of the referred kinetic rates. Furthermore, it could also provide an additional mean for the direct comparison of the simulation data with experimental results.

Journal Article↗

PubMed1998

A transcriptional regulator of the LuxR-UhpA family, FlcA, controls flocculation and wheat root surface colonization by Azospirillum brasilense Sp7.

Genetic complementation of a spontaneous mutant, impaired in flocculation, Congo red binding, and colonization of root surface, led to the identification of a new regulatory gene in Azospirillum brasilense Sp7, designated flcA. The deduced amino acid sequence of flcA shared high similarity with a family of transcriptional activators of the LuxR-UphA family. The most significant match was with the AgmR protein, an activator for glycerol metabolism in Pseudomonas aeruginosa. Derivatives of Sp7 resulting from site-directed Tn5 mutagenesis in the flcA coding sequence were constructed by marker exchange. Characterization of the resulting mutant strains showed that flcA controls the production of capsular polysaccharides, the flocculation process in culture, and the colonization of the root surface of wheat. This study provides new information on the genetic control of the mechanism of plant root colonization by Azospirillum.

Amino Acid Sequence↗

PubMed2003

Interaction and flocculation of spherical colloids wetted by a surface-induced corona of paranematic order.

Particles dispersed in a liquid crystal above the nematic-isotropic phase transition are wetted by a surface-induced corona of paranematic order. Such coronas give rise to pronounced two-particle interactions. In this paper, we report details on the analytical and numerical study of these interactions published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially demonstrate how for large particle separations the asymptotic form of a Yukawa potential arises. We show that the Yukawa potential is a surprisingly good description for the two-particle interactions down to distances of the order of the nematic coherence length. Based on this fact, we extend earlier studies on a temperature induced flocculation transition in electrostatically stabilized colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa potential to establish a flocculation diagram for a much larger range of the electrostatic parameters, namely, the surface charge density and the Debye screening length. As a distinguished feature, a kinetically stabilized dispersion close to the nematic-isotropic phase transition is found.

Journal Article↗

PubMed2006

Characterization of a flocculation-like phenotype in Cryptococcus neoformans and its effects on pathogenesis.

We investigated the phenomenon of cell-cell aggregation (flocculation) in a serotype D strain of Cryptococcus neoformans (ATCC 24067, isolate RC-2). Cell aggregation into clumps of 5-40 cells (clump+ cells) occurred during the early log phase and disappeared in the beginning of the stationary phase (clump- cells). The cell aggregation phenomenon was medium dependent. Clump+ cells could be dispersed by either vortexing or proteinase K digestion. Most importantly, the transient change in cellular phenotype changed several important host-pathogen interactions. Adherence of clump+ cells to murine macrophage-like cells J774.16 was significantly (P < 0.001) enhanced compared with adherence of clump- cells. Furthermore, complement-mediated phagocytosis efficacy of dispersed clump+ cells was significantly higher (P < 0.001) compared with clump- cells. Similar findings were documented with an in vivo phagocytosis assay. Infection of mice with a low inoculum (10(4)) of clump+ cells resulted in lower fungal burden when compared with mice infected with clump- cells. Accordingly, mice infected with clump+ cells survived significantly longer than mice infected with clump- cells. These results indicate that the cellular phenotype undergoes significant changes that result in a transient flocculation-like phenotype. We hypothesize that this cell-cell aggregation is the result of changes in protein content in the polysaccharide capsule. We conclude from our data that the change in cellular phenotype has a dramatic effect on cell adherence, and on complement-mediated phagocytosis, both of which can affect the pathogenesis of the disease in the host. Our results underscore the complexity of studies that investigate host pathogen interactions and may explain differences and inconsistencies observed in in vitro and in vivo assays.

Animals↗

PubMed1985

Flocculation effects on bound water in sludges as measured by nuclear magnetic resonance spectroscopy.

Nuclear magnetic resonance relaxation times (T(1) and T(2)) were measured for flocculated and unflocculated samples of activated sludge. The weight of water and solids in the sludge samples was found and related to T(1) to find the relative percentage of bound water. The results suggest that the amount of bound water increases as the samples become more unflocculated. The values of T(1) and T(2) also indicate that unflocculated individual particles are characterized by loose packing of shorter molecules and that the addition of larger molecules may induce flocculation.

Journal Article↗

PubMed1995

A mutant of Azospirillum brasilense Sp7 impaired in flocculation with a modified colonization pattern and superior nitrogen fixation in association with wheat.

We report here significant phenotypic and genetic differences between Azospirillum brasilense Sp7 and spontaneous mutant Sp7-S and their related properties in association with wheat. In contrast to the wild-type strain of Sp7, colonies of Sp7-S stained weakly with Congo red when grown on agar media containing the dye and did not flocculate in the presence of fructose and nitrate. Scanning and transmission electron micrographs showed clearly that the Sp7-S strain lacked surface materials present as a thick layer on the surface of the wild-type Sp7 strain. Different patterns of colonization on wheat roots between Sp7 and Sp7-S, revealed by in situ studies using nifA-lacZ as a reporter gene, were related to a large increase in nitrogenase activity (acetylene reduction) with Sp7-S in association with normal and 2,4-dichlorophenoxyacetic acid-treated wheat for assays conducted under conditions in which the nitrogenase activity of free-living Azospirillum organisms was inhibited by an excess of oxygen. Randomly amplified polymorphic DNA analysis indicated the close genetic relationship of Sp7-S to several other sources of Sp7, by comparison to other recognized strains of A. brasilense. Genetic complementation of Sp7-S was achieved with a 9.4-kb fragment of DNA cloned from wild-type Sp7, restoring Congo red staining and flocculation.

2,4-Dichlorophenoxyacetic Acid↗

PubMed2006

Flocculation and percolation in reversible cluster-cluster aggregation.

Off-lattice dynamic Monte-Carlo simulations were done of reversible cluster-cluster aggregation for spheres that form rigid bonds at contact. The equilibrium properties were found to be determined by the life time of encounters between two particles (te). te is a function not only of the probability to form or break a bond, but also of the elementary step size of the Brownian motion of the particles. In the flocculation regime the fractal dimension of the clusters is df=2.0 and the size distribution has a power law decay with exponent tau=1.5. At larger values of te transient gels are formed. Close to the percolation threshold the clusters have a fractal dimension df=2.7 and the power law exponent of the size distribution is tau=2.1. The transition between flocculation and percolation occurs at a characteristic weight average aggregation number that decreases with increasing volume fraction.

Journal Article↗

PubMed1987

Ultrastructural and clinicopathological studies on the toxicity of cationic acrylamide-based flocculant to rainbow trout.

Rainbow trout (Salmo gairdneri) of two age groups were exposed to a cationic acrylamide-based flocculant at various concentrations in static bioassay chambers. At lethal concentrations the flocculant produced severe gill alterations in all fish. The principal alterations were necrosis and separation of the respiratory epithelial cells covering secondary lamellae. Many necrotic chloride cells were also seen, their apical plasma membrane was destroyed, and mitochondria were swollen with separated cristae. An influx of a large amount of fluid into the interstitial spaces caused partial or complete separation of subepithelial spaces from the covering epithelial cells and basement membranes of underlying blood vessels. Clinicopathological alterations included marked decreases in blood pH, partial pressure of oxygen, bicarbonate and plasma sodium, and chloride concentrations. Hematocrit, total protein, and blood glucose were increased. Fish exposed to sublethal concentrations had gill alterations characterized by hypercellularity and thickening of the secondary lamellae. These were due to undifferentiated cell proliferation and macrophage and lymphocyte infiltration between the covering epithelial cells and the underlying blood vessels. Macrophages and undifferentiated cells had large phagolysosomes containing cytoplasmic organelles, an indication of cell injury and increased turnover.

Acrylamides↗

PubMed1995

Utilization of fermented flocculated poultry sludge as a feed constituent for pigs.

Flocculated poultry sludge was mixed with 3% molasses and was flow-therm pasteurized for 5 min at a core temperature of 95 C. The sludge was subsequently cooled to between 20 and 25 C and fermented with Lactobacillus plantarum as starter culture. Three groups of eight 8- to 10-wk-old, individually housed fattening pigs (Hypor) were fed according to a fixed scheme correlated with age. One control group received a restricted ration of commercial compound feed (Group A). The other control group was provided "nearly ad libitum" access to the same commercial compound feed (Group C). The experimental group received the same amount of commercial compound feed as Group A, but the diet was supplemented with fermented sludge, at an inclusion rate of 19 to 28% of the total ration (DM basis). The pigs fed the sludge-containing diet and those receiving the compound pig feed "nearly ad libitum" showed comparable growth results. It was concluded that the net energy (NEpig) level of .68 g DM of sludge was comparable to the NEpig level of 1 g compound pig feed (88% DM). A decrease in colony counts of Enterobacteriaceae in the intestinal tract of the pigs, was regarded as positive, as it might lower the risk of disturbance of the gut flora by enteropathogenic bacteria such as Escherichia coli and Salmonella. No adverse effects on health and performance were observed as a result of the feeding of pasteurized and subsequently fermented flocculated poultry sludge to fattening pigs. It is concluded that this sludge can serve as a valuable feed constituent as long as it is processed properly.

Abattoirs↗