The effect of wall depletion on the rheology of microfibrillated cellulose water suspensions by optical coherence tomography

Rheology of microfibrillated cellulose (MFC) water suspensions was characterized with a rotational rheometer, augmented with optical coherence tomography (OCT). To the best of the authors’ knowledge, this is the first time the behavior of MFC in the rheometer gap was characterized by this real-time imaging method. Two concentrations, 0.5 and 1 wt% were used, the latter also with 10^?3 and 10^?2 M NaCl. The aim was to follow the structure of the suspensions in a rotational rheometer during the measurements and observe wall depletion and other factors that can interfere with the rheological results. The stepped flow measurements were performed using a transparent cylindrical measuring system and combining the optical information to rheological parameters. OCT allows imaging in radial direction from the outer geometry boundary to the inner geometry boundary making both the shear rate profile and the structure of the suspension visible through the rheometer gap. Yield stress and maximum wall stress were determined by start-up of steady shear and logarithmic stress ramp methods and they both reflected in the stepped flow measurements. Above yield stress, floc size was inversely proportional to shear rate. Below the yield stress, flocs adhered to each other and the observed apparent constant shear stress was controlled by flow in the depleted boundary layer. With higher ionic strength (10^?2 M NaCl), the combination of yield stress and wall depletion favored the formation of vertical, cylindrical, rotating floc structures (rollers) coupled with a thicker water layer originating at the suspension—inner cylinder boundary at low shear rates.     

The scaling behavior of a highly aggregated colloidal suspension microstructure and its change in shear flow

The nature of the network structure and the evolution of structural change in shear flow were investigated for metal particle dispersions in terms of fractal aggregation of colloidal particles. Polymer-stabilized metal particle inks were prepared via a polyvinyl chloride coating dispersed in solvent. The fractal dimension of 1.74 was calculated with the scaling model based on the power law relationship between the elastic modulus and volume fraction. This scaling behavior can be explained by considering the deformable network structure of soft materials. While the elastic property of the floc was dominant, the limit of linearity was found at the inter-floc link, which is relatively weak and brittle. The steady shear results reveal two mechanisms that contribute to the breakdown of the microstructure in metal particle inks at increasing shear rate. Scaling of steady shear viscosity shows that these mechanisms are related to both inter-floc interactions and the elasticity of the floc itself. Further, these results suggest that individual flocs deform with weak inter-floc interactions and rupture into smaller flocs or aggregates at high shear stress, which is associated with the increased shear rate.     

Effect of cationic polymethacrylates on the rheology and flocculation of microfibrillated cellulose

Using two cationic methacrylate polymers: poly([2-(methacryloyloxy)ethyl] trimethyl ammonium iodide) (PDMQ) and poly[(stearyl methacrylate)-stat-([2-(methacryloyloxy)ethyl] trimethyl ammonium iodide)] (PSMA13Q), we modified microfibrillated cellulose (MFC) water suspensions. The aim was to affect the flocculation and rheological behavior of the MFC suspension. PDMQ is a strongly cationic polymer while PSMA13Q, also a cationic polymer, contains hydrophobic segments. We studied the MFC/polymer suspension rheological properties with a rotational rheometer in oscillatory and flow measurements. To observe structural changes in suspensions at different shear rates, we measured flow curves with transparent outer geometry and photographed the sample with a digital camera. The oscillatory measurements showed that a small amount of the cationic PDMQ in the MFC suspension strengthened the gel, whereas a small amount of amphiphilic PSMA13Q weakened it. Increased amounts of either polymer increased the gel strength. PSMA13Q also changed the rheological character of the MFC suspension turning it more fluid-like. When we photographed the flow curve measurement, we saw a clear change in the floc structure. This floc structure rupture coincided with a transient region in the flow curve.     

Floc breakage in agitated suspensions: Theory and data processing strategy

Flow visualization of chemical flocs in a simple extensional flow field reveals two distinct mechanisms for their breakage: splitting into a relatively small number of daughter fragments whose sizes are comparable to the parent flocs, along with continual disintegration by erosion to produce extremely fine particles from the extremities of the parent floc along the axis of extension. In turbulent flow, these two mechanisms still occur, although the kinematics of flow are more complex. This work presents a formulation of the population balance equation that governs the floc size distribution in turbulent flow, incorporating both the splitting and erosion mechanisms discussed above. Experiments were conducted in which floc size distributions of dilute suspensions are measured by a combination of techniques, including computerized optical scanning of photographs and pulse height analysis of signals from a light blockage transducer. The experimentally determined size distributions are then fit to those computed from the population balance equation, using constrained nonlinear least squares. This yields best values of certain coefficients that appear in the governing equation, providing a strategy to obtain a data base to promote deeper theoretical analysis. The method is demonstrated by analyzing data for kaolin-Fe(OH)₃ flocs in aqueous suspensions.     

Effect of floc structure on the rate of shear coagulation

We derived a mathematical expression for the temporal evolution of the number of particles due to shear coagulation, covering the later stage by expanding the initial stage approximation to take into account the formation of floc structure. In the derivation, it is assumed that flocculation proceeds through binary collisions between identical fractal flocs. The capture efficiency between flocs is calculated on the basis of trajectory analysis, which is determined by viscous hydrodynamic interaction between flocs and van der Waals attractive forces between two primary particles located at colliding points of flocs. The validity of the derived equation was tested by a coagulation experiment using polystyrene sulfate latex particles under conditions of rapid coagulation. The experiment was carried out in a laminar Couette flow generated in the gap between two concentric cylinders. Careful and direct observation of flocculation under microscopy provided the data on the fractal dimension as well as the temporal evolution of number concentration of flocs. The measured rate of coagulation gradually increases in accordance with the formation of the fractal structure of flocs. This behavior agreed very well with the prediction based on the derived equation.     

Coagulation and ultrafiltration: Understanding of the key parameters of the hybrid process

A hybrid coagulation–ultrafiltration process has been investigated to understand membrane performance. Coagulation prior to ultrafiltration is suspected to reduce fouling by decreasing cake resistance, limiting pore blockage and increasing backwash efficiency. Coagulation followed by tangential ultrafiltration should gather the beneficial effects of particle growth and cross-flow velocity. Our study aims at determining the key parameters to improve membrane performance, by describing floc behaviour during the hollow fibre ultrafiltration process. Flocs encounter a wide range of shear stresses that are reproduced through the utilization of different coagulation reactors. Performing a Jar-test enables the formation of flocs under soft conditions, whereas Taylor-Couette reactors can create the same shear stresses occurring in the hollow fibres or in the pump. Synthetic raw water was made by adding bentonite into tap water. Five organic coagulants (cationic polyelectrolytes) and ferric chloride were selected. Floc growth was thoroughly monitored in the different reactors by laser granulometry. Coagulation–ultrafiltration experiments revealed different process performance. The effect on the permeate flux depended on the coagulant used: some coagulants have no influence on permeate flux, another enables a 20% increase in permeate flux whereas another coagulant leads to a decrease of 50%. Flocs formed with ferric chloride do not resist shear stress and consequently have no influence on permeate flux. These results show the necessity to create large flocs, but the size is not sufficient to explain membrane performance. Even if flocs show a good resistance to shear stress, a high compactness (D_f = 3) will lead to a dramatic decrease of permeate flux by increasing the mass transfer resistance of the cake. On the contrary, flocs less resistant to shear stress, then smaller and also more open have no effect on permeate flux. An optimum was quantified for large flocs, resistant enough to shear stress facilitating flow between aggregates.     

Kinetics of rennet casein gelation at different cooling rates

A mathematical model was developed to quantitatively analyze the rheological data of rennet casein gelation at different cooling rates. Kinetic parameters were estimated and correlated with the microstructure development of the protein network. The kinetic model identified structure development upon cooling to be first order, and the network forming energies were estimated for four protein concentrations cooled at four rates. A lower energy for network formation was observed for a slower cooling rate and a higher protein concentration. This observation resulted from the availability of more flocs at a slower cooling rate and a higher casein concentration, simplifying floc cross-linking. By analyzing the kinetics during the aging process of casein gels, no difference in the reaction mechanism was observed. This study illustrated that structure formation resulted from the addition of flocs into the protein network: not all flocs were part of the network at a defined gel point. The incubation period following cooling integrated idle flocs into the network, thereby strengthening the gel. By understanding the gelation mechanism during cooling of rennet casein gels, the structure and thus quality of dairy products, such as processed cheese, may be better controlled.     

Cooling effects on a model rennet casein gel system: part II. Permeability and microscopy

Microscopy and permeability studies were performed to further illustrate the cooling effects on rennet casein gel structure and help interpret the rheological observations in the first part of this paper. Samples of gels cooled from 80 to 5 degrees C at four rates (0.5, 0.1, 0.05, and 0.025 degrees C/min) were studied with a confocal laser scanning microscope. A larger number of smaller flocs were generated at slower cooling rates, creating more cross-links within a network and corresponding to a stronger gel. Formation of a larger number of smaller flocs was hypothesized to result from a greater degree of doublet formation because the system spent more time within the temperature region where doublet formation is favored when cooled at slower rates. The doublets represent sites available for floc growth, similar to nucleation sites for crystal growth. Microscopy results further substantiated that the cooling effects were different from the aging effects because cooling affected floc size, and aging enabled the addition of idle flocs into the casein network. The conclusions for the cooling effects on floc size were further supported by permeability tests. A smaller permeability coefficient resulted from smaller flocs obtained with a slower cooling schedule. This study showed the importance of controlling floc numbers to modulate the strength of a gel, and cooling rates provide an approach of modulating functional properties when the chemical composition of a system is fixed.     

Analysis of rheology and wall depletion of microfibrillated cellulose suspension using optical coherence tomography

A rheometric method based on velocity profiling by optical coherence tomography (OCT) was used in the analysis of rheological and boundary layer flow properties of a 0.5% microfibrillated cellulose (MFC) suspension. The suspension showed typical shear thinning behaviour of MFC in the interior part of the tube, but the measured shear viscosities followed interestingly two successive power laws with an identical flow index (exponent) and a different consistency index. This kind of viscous behaviour, which has not been reported earlier for MFC, is likely related to a sudden structural change of the suspension. The near-wall flow showed existence of a slip layer of 2–12 μm thickness depending on the flow rate. Both the velocity profile measurement and the amplitude data obtained with OCT indicated that the slip layer was related to a concentration gradient appearing near the tube wall. Close to the wall the fluid appeared nearly Newtonian with high shear rates, and the viscosity approached almost that of pure water with decreasing distance from the wall. The flow rates given by a simple model that included the measured yield stress, viscous behavior, and slip behavior, was found to give the measured flow rates with a good accuracy.     

Cooling effects on a model rennet casein gel system: part I. Rheological characterization

The gelation of a model rennet casein system was studied during cooling at different rates. During cooling, casein network structure development was proposed to evolve over a few steps at different length scales: molecules, particles, flocs, or network. Rennet casein flocs are fractal in nature, and fractal dimension and floc size are two variables affecting the rheology and microstructure of a rennet casein gel. Casein structure formation during cooling from 80 to 5 degrees C at four different rates (0.5, 0.1, 0.05, and 0.025 degrees C/min) was monitored by dynamic rheological tests, and a stronger gel developed at a slower cooling rate. During different cooling schedules, similar fractal dimensions were observed due to a lack of difference in the colloidal interactions. Differences among rheological data were possibly caused by variability in floc size, as observed in the second part of this paper. A larger number of smaller-sized flocs enabled gelation at a higher temperature and created a stronger network at a slower cooling rate. Controlling cooling schemes thus provides an approach for manipulating casein gelation and the microstructure for a system of fixed chemical compositions.     

Imaging c-PAM-induced flocculation of paper fibers

The flocculation of paper fibers by cationic polyacrylamides (c-PAM) was studied by imaging the fibers that remain free during flocculation. Studies with fibers of different lengths showed that the degree of flocculation increases with fiber length, with the best flocs being formed with mixtures of short and long fibers. Short fibers did not flocculate by themselves but were captured by flocs formed with longer fibers. The short fibers strengthen the floc and give it shear resistance. Shear had the expected effect of promoting flocculation at low Reynolds number but disrupting it at higher values. For a given polymer the maximum floc size for a mixture of fibers is dictated by the length distribution of the fibers. The polymer dose governs the rate of flocculation. The technique is especially useful in following the tail end of the flocculation process. At this stage a floc is almost fully grown and a small increase in its size would be very difficult to measure by conventional techniques. In contrast, the number of free fibers measured by single fiber imaging decreases rapidly at this point.     

Effect of floc structure on the rate of Brownian coagulation

A modified expression for the Smoluchowski solution for the temporal evolution of the number concentration of flocs subject to Brownian coagulation is proposed, taking into account the effect of the growth of floc structure. In the proposed equation, the effect is expressed as a decrease of free volume in the liquid phase due to the increase of effective floc volume in accordance with the progress of coagulation. The validity of the proposed equation was tested by coagulation experiments using polystyrene latex particles. Direct counting of the number of flocs under microscopy provided accurate data on the temporal evolution of the number concentration of flocs. The obtained rate gradually increases in accordance with the growth of floc structure. This behavior agreed exactly with the prediction based on the proposed equation.     

聚合方法对一种正离子聚丙烯酰胺结构与性能的影响

对相同进料比下,以过硫酸胺/亚硫酸氢钠为氧化还原引发剂,分别用溶液法和反相微乳液法合成的丙烯酰胺(AM)与2-甲基丙烯酰氧乙基三甲基氯化胺(MADQUAT)的共聚物P(AM-MADQUAT),根据单体竞聚率计算了两种共聚物的序列分布和组成分布.考察了两种聚合物结构对高岭土絮体尺寸、zeta电位降以及絮体压缩屈服应力的影响,初步建立了不同聚合方法合成的阳离子聚丙烯酰胺结构与絮凝性能之间的相关性.     

Comparison of stages in oil agglomeration process of quartz with sodium oleate in the presence of Ca(II) and Mg(II) ions

The oil agglomeration of quartz with sodium oleate in the presence of calcium and magnesium ions comprises three consecutive stages: adsorption of cations onto quartz surfaces, which leads to coagulation of the suspension, shear flocculation with sodium oleate and finally, agglomeration of flocs by kerosene. The effects of pH and cation concentration on these stages were investigated and the results were presented comparatively. It was found that all the stages of oil agglomeration of quartz exhibited sharp dependences on pH and cation concentration. That is, these stages generally took place in the pH and concentration ranges in which hydroxy complexes of the cations existed in the suspension. In the case of magnesium ion, the coagulation, shear flocculation and especially oil agglomeration of quartz improved after precipitation of hydroxide. These species of calcium and magnesium ions formed at high pH were adsorbed on the negatively charged surface of quartz, as a result of which the adsorption of sodium oleate became possible and thus the shear flocculation of the particles was achieved. Thereafter, the hydrophobic quartz flocs could be agglomerated by kerosene as bridging liquid. The increase in the shear flocculation efficiency depending on the increase of surface hydrophobicity enhanced the oil agglomeration of quartz with kerosene. The maximum recoveries for all the stages of the quartz were obtained in the presence of 10(-3) M magnesium and 5x10(-3) M calcium ions at pH 11. However, some differences in the behavior of shear flocculation and oil agglomeration of quartz suspension were observed above 10(-3) M concentration of magnesium ion.     

The fractal nature of Escherichia coli biological flocs

The fractal structures of Escherichia coli biological flocs were characterised in terms of fractal dimension, which is a measurement of how the bacteria in the flocs occupy space. The dimensional analysis methods, based on power law correlations between floc perimeter, projected area and maximum length, were used to determine the one- and two-dimensional fractal dimensions (D(1) and D(2)) of E. coli flocs formed by flocculation in chitosan solution with a concentration of 10.0 mg chitosan per g dry cell weight (DCW), giving D(1)=1.07+/-0.06 and D(2)=1.70+/-0.08 (+/-S.D.). The three-dimensional fractal dimension (D(3)) of the E. coli flocs was determined by the two-slopes method, using cumulative size distributions of floc length and solid volume, to be 1.99+/-0.08 (+/-S.D.), which is close to the value of D(3)=2.14+/-0.04 (+/-S.D.) measured by the small angle light scattering method. The results demonstrate that E. coli flocs flocculated with chitosan have a fractal nature, as their fractal dimensions D(1), D(2) and D(3) differ from the values of 1, 2 and 3 expected for the spherical Euclidean object, respectively.     

Advective flow in spherical floc

Numerous structural models of flocs, such as homogeneous model or radially-varying model, were proposed in literature for predicting the extent of advective flow on the intrafloc transport processes. This work probed the three-dimensional structure of original and chemically flocculated wastewater flocs using the fluorescence in situ hybridization (FISH) and the confocal laser scanning microscope (CLSM) techniques, from which the spherical mesh model on real floc structure was constructed. Simulation results revealed that if an average characteristic of sludge floc, such as porosity or drag force correction factor of sludge floc is of concern, both homogeneous or radially-varying models may be able to apply, particularly for those flocs that were closely compacted. However, the detailed flow patterns inside real floc are much more tortuous than those of the homogeneous or radially-varying models. If local hydrodynamic environment within the floc is of interest, then only the complicated structural model with real floc could be applicable.     

Effect of temperature and pH on droplet aggregation and phase separation characteristics of flocs formed in oil–water emulsions after coagulation

Coagulation process is used for destabilization of emulsions to promote aggregation of oil droplets on flocs which can be subsequently removed by sedimentation or flotation. The objectives of this study were to investigate the effect of temperature and pH on the effectiveness of destabilization of olive oil–water emulsions in relation to floc morphology and aggregation characteristics of oil droplets, and to quantify the ability of flocs to capture and separate oil. A cationic polyelectrolyte was used for the coagulation of oil droplets in edible olive oil–water emulsions using a jar test apparatus. The flocs formed in olive oil–water emulsions after coagulant addition were analyzed using microscopic image analysis techniques. Fractal dimension, radius of captured oil droplets on flocs, number of oil droplets aggregated on flocs, and floc size were used to quantitatively characterize and compared the effectiveness of the coagulation process at different conditions (pH and temperature) and the ability of flocs to remove oil from water. Analysis of microscopic images showed that floc size was not always the best measure of effectiveness of coagulation process in oil–water emulsions. The flocs forming at different pH levels and temperatures had significant morphological differences in their ability to aggregate different sizes and numbers of oil droplets, resulting in significant differences in their ability for separating oil. Fractal dimension did not correlate with the ability of flocs to aggregate oil droplets nor the total amount of oil captured on flocs. Temperature had a significant effect on droplet size and number of droplets captured on flocs. The differences in floc sizes at different temperatures were not significant. However, the flocs forming at 20 °C had fewer but larger droplets aggregating larger amounts of oil than flocs formed at 30 °C and 40 °C. The size of droplets at different pH levels was similar, however, there were significant differences in number of droplets aggregating on flocs and floc sizes. The amount of oil captured on flocs at pH 7 and pH 9 was significantly higher than those at pH 5 and pH 11. The calculated fractal dimensions of the flocs (all less than 1.8) indicated that the coagulation process was diffusion limited implying that there was no repulsion between the colliding particles (i.e., droplets and flocs); hence, each collision between flocs and droplets resulted in attachment.     

Effect of pre‐treatment of sugarcane bagasse on the cellulose solution and application for the cellulose hydrogel films

Sugarcane bagasse was used as a cellulose resource, and the transparent cellulose hydrogel films were obtained from the purified cellulose by phase inversion process without chemical cross‐linking, when the dissolved cellulose in lithium chloride/N,N‐dimethyl acetamide was transformed into the solid film. On these processes, bagasse was pre‐treated by 10 wt% sodium hydroxide in the absence and presence of bleaching of 10 vol% sodium hypochlorite (NaOCl) solution in order to obtain cellulose fibers. Here, the bleaching temperature was varied from 40 to 50°C. The effect of pre‐treatment conditions on the resultant cellulose solution and hydrogel films was investigated. It was seen that strong bleaching removed most of lignin component from the bagasse. However, viscosity and size exclusion chromatogram of the cellulose indicated that this operation decreased average molecular weight of the cellulose fibers from 2.1 × 10⁶ to 4.8 × 10⁵. These property changes of fibers also caused increase of water content and weakening of mechanical strength of the resultant hydrogels. In addition, scanning probe microscopy in wet state revealed that the porous fiber network structure in the hydrogel was greatly influenced by bleaching with NaOCl. The average pore size of fiber network was decreased from 8.1 to 5.9 nm as the NaOCl treatment was at 50°C, because of expanded fibers in the swollen hydrogel. Copyright © 2016 John Wiley & Sons, Ltd.     

Rheology and particle interaction of thickened latex

The pseudoplastic rheological properties of concentrated monodisperse polystyrene latexes with known sodium lauryl sulfate and methylcellulose surface coverages have been studied. It was assumed that the flow units of a concentrated thickened latex subjected to mechanical shear are “flocs” which comprise many particles with immobilized medium in the interstices. During shearing, the particle-particle bonds within the flocs undergo compression and stretching, sometimes breaking and reforming, causing the energy dissipation measured as the yield stress. A model was developed to calculate the average number of bonds per floc and this model was applied to an empirical modification of Firth and Hunter's elastic floc model to correlate the yield stress with the particle-particle separation pressure (defined as a measure of the interaction strength). It was found that the yield stress of a thickened latex is affected by the particle-particle interaction and the morphology of the particle flocs. The particle-particle interaction is affected by the surface coverage of thickener and emulsifier, and their concentrations in the aqueous phase, as well as other factors. The morphology of the particle flocs is affected by the particle interaction and the mechanical treatment. The adsorption of emulsifier and thickener, the rheology of the thickened latexes, the morphology of the particle flocs, and the particle-particle interactions, as well as their interrelationships, are described.     

The rheological properties of hydrogenated castor oil crystals

The present study focuses on the rheological performance of a surfactant-rich aqueous suspension containing hydrogenated castor oil (HCO) crystals. HCO can be typically crystallized in five distinct shapes: spherically shaped, irregularly shaped, star-shaped (also called rosettes), short needles, and thick or thin fibers. The effect of the differences in shape on the rheological performance is studied, and the rheological properties are compared to the behavior of other triacylglycerol’s (TAG) suspensions. A suspension of TAG crystals usually behaves as a colloidal gel wherein a colloidal gel is defined as a network of flocs, with each floc being an aggregate of smaller subunits. All of these surfactant-rich aqueous suspensions of HCO crystals behaved according to a colloidal gel in the transient regime, independent of the studied crystal shapes, except the long thin fibers at a concentration above 0.1 wt% HCO transitioning from a heterogeneous fractal rod network to a homogeneous rod network, shifting from a colloidal gel to a glass.