Number Of Colloidal Particles Research Articles

Polydisperse Colloids Two-Moment Diffusion Model Through Irreversible Thermodynamics Considerations

Abstract This study deals with the problem of diffusion for polydisperse colloids. The resolution of this complex problem usually requires computationally expensive numerical models. By considering the number of colloidal particles and their mass as independent variables, the equations of state for a dilute polydisperse colloid are derived on a statistical mechanics basis. Irreversible thermodynamics is then applied to obtain a simple two-moment diffusion model. The validity of the model is illustrated by comparing its results with those obtained by a classical size spectrum approach, in a sedimentation equilibrium problem and in an unsteady one-dimensional diffusion problem in Stokes–Einstein regime, and under the hypothesis that the size spectrum distribution is stochastic. In the first problem, the two-moment diffusion problem allows to represent rigorously the vertical size segregation induced by gravity, while in the second one, it allows a convenient description of the diffusion of polydisperse colloids by using two coupled diffusion equations, with an accuracy comparable with that of the classical size spectrum approach. The contribution of our work lies primarily in the application of a non-equilibrium thermodynamics methodology to a challenging issue of colloid modeling, namely, polydispersity, by going from statistical mechanics to the derivation of phenomenological coefficients, with the two-moment approach as a guideline.

Intensive rearing of cod larvae (Gadus morhua) in recirculating aquaculture systems (RAS) implementing a membrane bioreactor (MBR) for enhanced colloidal particle and fine suspended solids removal

Intensive rearing of Atlantic cod larvae (Gadus morhua) was investigated in a conventional recirculating aquaculture system (cRAS) and a membrane modified RAS (mRAS). Cod larvae are sensitive to water quality, and beneficial effects on growth and survival from enhanced removal of colloidal particles, fine suspended solids and nutrient reduction were expected. Membrane bioreactors (MBR) are a potential technology for advanced water treatment in aquaculture. The aim of this project was to assess the effect of an MBR system for enhanced treatment in RAS. A cRAS and mRAS treatment train were operated in parallel. In the mRAS Scheme 8.5% of the recycle stream was filtered through the membrane at any time. The mRAS scheme demonstrated a significantly lower turbidity and number of colloidal particles as compared to the cRAS scheme, as well as significantly lower bacteria concentrations and more stability. Overall a 13% higher cod larvae growth (weight, %) at 40dph and 3.5% higher survival rate at 50dph was measured in the mRAS scheme. Results show there is a great potential of implementing a membrane filtration system in aquaculture recycling systems.

Numerical evaluation of the limit of concentration of colloidal samples for their study with digital lensless holographic microscopy

The number of colloidal particles per unit of volume that can be imaged correctly with digital lensless holographic microscopy (DLHM) is determined numerically. Typical in-line DLHM holograms with controlled concentration are modeled and reconstructed numerically. By quantifying the ratio of the retrieved particles from the reconstructed hologram to the number of the seeding particles in the modeled intensity, the limit of concentration of the colloidal suspensions up to which DLHM can operate successfully is found numerically. A new shadow density parameter for spherical illumination is defined. The limit of performance of DLHM is determined from a graph of the shadow density versus the efficiency of the microscope.

A study of the composition of polyacrylamide-polyaluminum chloride polymer-colloid complexes

The behavior of aqueous solutions of polymer-colloid complexes based on polyacrylamide (M ∼ 5.2 × 105) and colloid particles of polyaluminum chloride depending on the initial component ratio and the concentration of a low-molecular-mass electrolyte (NaCl) was studied by means of viscometry and turbidimetry. It was suggested that the composition of polymer-colloid complexes, ϕ (the number of colloid particles attached to one macromolecule), depends on the component ratio. At ϕ > 1, the polymer-colloid complexes take on the polyelectrolyte properties, namely, repulsion in the case of overlap of the diffuse layers of counterions of colloid particles bound to the polymer chain leads to unfolding of macromolecular coils. Correspondingly, as the concentration of a low-molecular-mass salt increases, the coil size diminishes and the solubility of the complex drops. At ϕ = 1, the concentration of low-molecular-mass salt has no effect on the solubility of the complex and the viscosity of its solution.

Kinetics of Colloidal Templating Using Emulsion Drop Consolidation

The emulsion templating of ordered colloidal microsphere assemblies by Manoharan et al. involves a consolidation process where dispersed phase fluid is transported from droplets into a continuous phase. Consolidation can be approximated as a diffusion process with moving boundaries. The kinetics of consolidation are investigated here by following droplet shrinkage with time as a prelude to understanding rate effects on assembly structure. Consolidation kinetics are influenced by liquid diffusivity, the number of colloidal particles in a droplet, and the surfactant concentration. While surfactant exhibits little effect well below its critical micelle concentration (CMC) value, it significantly slows consolidation above the CMC. For a specific continuous phase (i.e., silicone oil and fluorinated silicone oil), with proper scalings, the droplet size shrinks with time following a power law independent of droplet diameter, surfactant concentrations, and particle number concentration. The power law exponent varies from 1/2 to 2/3 with different continuous oil phases as a result of concentration and interfacial effects. This study leads to an improved understanding of colloidal microstructure development at interfaces that can be applied in novel materials synthesis and drug delivery areas.

The Effects of Intermittent Aeration on the Characteristics of Bio-Cake Layers in a Membrane Bioreactor

The effects of a sequencing variation for dissolved oxygen (DO) concentrations on the membrane permeability in a submerged membrane bioreactor (MBR) were studied. An MBR was continuously operated under alternating DO conditions, e.g., 36 h of an aerobic phase, followed by 36 h of an anoxic phase. The rate of increase in transmembrane pressure (TMP) in the anoxic phase was always steeper than that in the aerobic phase, indicating that the fouling rate was higher in the anoxic than in the aerobic condition. Regardless of the phases, the rate of TMP increase became steeper as the cycles were repeated. However, this trend became less important as the cycle numbers increased. Even in identical microbial communities, the number of colloidal particles and soluble extracellular polymeric substances (EPS) in the bulk solution were increased during the anoxic condition, which caused a reduction in the porosity of the bio-cake. During analysis of the bio-cake profile along the cake depth, the temporal variation of the bio-cake structure was attributed to the temporal change in the number of colloidal particles as well as the change in compression forces acting on the bio-cake. The influence of the latter was found to be more important than that of the former, which was verified by comparing the various structures of bio-cake formed in differing DO environments.

Partial Validation of Cross Flow Ultrafiltration by Atomic Force Microscopy

Atomic force microscopy was used to evaluate a cross flow ultrafiltration (CFUF) system. The CFUF system was used for the size fractionation of natural colloidal material from freshwaters. Analysis of the images of bulk water, permeates, and retentates shows the primary materials observed were near-spherical structures with height dimensions up to approximately 12 nm. The number of colloidal particles (per unit area) on the mica surfaces derived from the retentates increased by a factor of 2 between a concentration factor (cf) of 1 and of 20. Colloidal densities of nanoparticles were approximately 2 orders of magnitude lower in the permeate compared to the retentate, indicating a good size fractionation. As the cf value increased from 1 to 20, the percentage of <1-nm material decreased substantially and the percentage of >1-nm material increased substantially in the retentates. Line transects along a surface and surface roughness values show good agreement with the above results. Results suggest the size fractionation is good but not perfect and high cf values produce a better size fractionation, although some retention of small material is always observed. High cf values are recommended.

Model of aggregation of colloidal fine particles in the presence of supersized polymer

A simple model of the process of stabilization and destabilization of fine colloidal suspensions induced by supersized linear polymers has been tested by the direct simulation method. In the model, a single polymer molecule may bind a number of colloidal particles and thus form an aggregate. It is assumed that a simultaneous attachment of a few fine particles to one macromolecule does not necessarily destabilize the suspension. The destabilization of the system (occurring if aggregate sedimentation dominates its diffusion ability) takes place only when the number of the attached particles per macromolecule exceeds the critical value which depends on the polymer coil dimension in the dispersion medium. The model permits interpretation of several experimental observations of the behavior of colloidal sols upon introduction of very high molecularweight polymers. The simulation results have been compared with the experimental data on the effect of polyacrylamide on the stability of AgI sol.

On the collision efficiency in polymer flocculation of colloidal particles

The applicability of the conventional collision efficiency approach for the case where the collision of colloidal particles is not necessary for the rate-determining step is examined. On the basis of a dynamical treatment, we show that the fundamental assumption made in the conventional analysis may lead to a significant error. Depending upon the ratio of the number of sites on the colloidal surfaces to the number of polymer molecules, and the relative magnitudes of the desorption rate constant and the adsorption rate constant, the collision efficiency calculated by the conventional analysis may be either underestimated or overestimated. The present approach yields the temporal variation of the distribution of the fractional surface coverage. It can be shown that if the number of colloidal particles is large, using the mean fractional surface coverage instead of the exact distribution of the fractional surface coverage is sufficient for the evaluation of the collision efficiency.

Alternatives to Transfusion Therapy

Summary and Conclusion Attempts to develop a hemoglobin-based red-cell substitute have spanned many decades12,13 but have not yet resulted in a clinically useful product. The issues that have prevented clinical application have been primarily ones of safety and not efficacy.27 Numerous animal studies have documented the efficacy of stroma free hemoglobin.3,12 Though effective, there were limitations that were of concern. Oncotic considerations limit the concentration of the infusate SFH to 6 to 8 g/dL, or half-normal. Owing to the loss of organic phosphate modulators of P50 such as 2,3-DPG, the P50 of SFH is typically between 12 to 14 torr, which again is half the normal value. And finally, the intravascular half-life of SFH is too short, ranging from 2 to 6 hours.3 Polymerization provides a means of correcting these limitations of SFH. The high O2 affinity can be greatly diminished by covalent binding of pyridoxal-5′-phosphate to the N-terminal of the β-chains. COP exerted by a protein solution is proportional to the number of discrete colloid particles. By polymerization, the number of colloid particles are reduced, leading to a decrease in COP. Our data show that this can indeed be achieved in a reproducible fashion. The rate at which this diminution of COP is accomplished determines the yield of polymeric species, as well as their molecular weight distribution. We demonstrate that the polymerization can be reproducibly controlled to result in a yield of 75% to 85% polymers, with a molecular weight distribution of 128,000 to 400,000 daltons. The number average and the weight average molecular weights indicate that the large proportion of the polymers represent the crosslinking of two tetramers. The data that reflect the interaction of O2 with Poly SFH-P indicate that the O2-carrying function of hemoglobin has not been significantly altered by the chemical modifications. The binding coefficient of O2 is unchanged. As one would have anticipated, there is a loss of cooperativity (diminished Hill coefficient) between the hemoglobin chains, suggesting structural restrictions in the polymeric species owing to the crosslinking. One would expect to see a reduced alkaline Bohr effect, and our data confirm that. And finally, one would expect to see some increase in O2 affinity with polymerization. This is indeed the case, though the P50 of Poly SFH-P is comparable to banked blood (18 to 22 torr). A modified hemoglobin solution, to be clinically useful, would need to have a reasonable shelf-life. Our data demonstrate that polymerized hemoglobin can be stored in the cold (4°C to 8°C) for several months with minimum change in methemoglobin concentration. Furthermore, our HPLC and viscosity data clearly indicate that no significant alterations in the polymeric species occur during storage.

A model of flocculation by very-high-molecular-weight polymers

A simple model is proposed to describe flocculation of fine particles by high polymers. The model is based on the assumption that the number of colloid particles attached to a single polymer molecule is a random quantity, and that a minimum number of particles must be attached to a polymer molecule to trigger its removal from the system. The predictions of the model are confronted with experimental data concerning the stability of the system polyacrylamide-sol AgJ.

Number fluctuations of interacting particles

The mean-square fluctuation in the number of colloidal particles (radius approximately 45 nm) in a small volume element ( approximately 8 mu m3) of an aqueous dispersion was measured by photon-correlation laser light scattering. Both randomly distributed, non-interacting particles and those showing a 'liquid-like' spatial arrangement owing to long-range repulsive Coulombic interactions were studied. The magnitude of the reduced fluctuations in the latter case agreed with that predicted from the structure of the dispersion, which was determined independently from the angular dependence of the average scattered light intensity. This provides the first direct experimental verification of the fundamental Ornstein-Zernike relationship between numbers fluctuations and the pair distribution function g(r) in a system of interacting particles. Possible extensions of the experiment, including the measurement of four-particle correlations, are discussed briefly.