On silver-thallous iodide suspensions

Summary In order to obtain general information on the colloid stability of a mixed system, silver-thallous iodide systems were preliminarily examined. In all the systems observed, the molar fractions of each metal, f_Ag and f_Tl were continuously changed by 5% so that the silver-thallous iodide systems consisted of 21 different samples. Tyndallometry was used as a simple and sufficiently accurate method for determination of colloid stability of investigated systems. For the control of the substance change in the suspensions X-ray diffraction analysis was employed. The results obtained show that at the higher percentage of silver or thallium in the systems, resulting stabilities are dependent on the stability of the main component, while in the case of equal amounts of metals two stability maxima were attained. Maxima were pointed out in discussion as the prominent points of the system.

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Zusammenfassung Um allgemeine Information über die kolloide Stabilität eines Mischsystems zu erhalten, untersuchten wir das System Silber-Thallium-Jodid. Die Molfraktionen der Metallef _Ag undf _Tl wurden kontinuierlich um 5% geändert, so daß insgesamt 21 Silber-Thallium-JodidProben untersucht wurden. Zur Bestimmung der kolloidalen Stabilität der untersuchten Systeme wurde als eine einfache und genügend genaue Methode dieTyndallometrie angewendet. Die Ag-Tl-J-Niederschläge wurden röntgenographisch untersucht. Die Ergebnisse zeigen, daß bei hohen Silberoder Thalliumgehalten die Stabilität durch die Hauptsubstanz bestimmt wird, während im Mittelbereich zwei Maxima in der Stabilität auftreten.

     

On the theory of structuring magnetic suspensions

The results of the three-dimensional computer and analytical simulation are presented for the kinetics of chain-shaped aggregate growth in suspensions of magnetizable non-Brownian particles. The results of the computer experiment show that, when the volume fraction of particles is no larger than 2–3%, chain-shaped aggregates are formed in the suspensions under the action of a field. The dependence of average number <n> of particles in a chain on time t is adequately described by the power law <n> = Ct ^k . The experiment indicates that, in contract to the common power approximations, in which exponent k is considered to be a universal constant parameter, it depends on the concentration of particles and their interactions with walls bounding a suspension. At concentrations noticeably exceeding 2–3%, dense bulk aggregates are formed in suspensions. The kinetics of their growth depends on the sizes of a suspension-containing vessel.     

Electrorheological suspensions

The objective of this article is to give a review of electrorheological (ER) suspensions whose rheological properties can abruptly change under an external electric field. Attention is given to the physical backgrounds behind ER phenomena reported recently. The criteria on how to design a high performance ER fluid and mechanisms explaining how an ER suspension displays the ER effect are focused upon. We begin with a brief historic introduction, ER materials, followed by positive ER effect, negative ER effect and photo-ER effect discussions. The physical parameters that can substantially affect the ER effect are discussed thereafter, and physical processes occurring in ER suspensions under an electric field are reviewed. The mechanisms of the ER effect proposed before are summarized. A future outlook on the ER material development and ER fluid applications is given.     

Ultrafiltration of suspensions

Suspensions of colloids and fine particles have been ultrafiltered with and without stirring. For the two colloids tested, the finer colloid gives the lower flux in the absence of stirring, but with stirring it gives the higher flux. This can be explained by assuming that under stirred conditions the polarisation of the colloids is more effectively controlled by diffusive back-transport. As particle size increases from 25 nm to 20 μm the flux passes through a minimum as the polarisation control changes from diffusive (decreasing with particle size) to non-diffusive (increasing with particle size). In the presence of a macrosolute the stirred ultrafiltration with the finer colloid is lowest. Interactions between the polarised species, such as the hindering of diffusive back-transport, can be inferred. With the larger particles flux can be enhanced provided the loading is high enough.     

On the influence of a patterned substrate on crystallization in suspensions of hard spheres

We present a computer simulation study on crystal nucleation and growth in supersaturated suspensions of mono-disperse hard spheres induced by a triangular lattice substrate. The main result is that compressed substrates are wet by the crystalline phase (the crystalline phase directly appears without any induction time), while for stretched substrates we observe heterogeneous nucleation. The shapes of the nucleated crystallites fluctuate strongly. In the case of homogeneous nucleation amorphous precursors have been observed [T. Schilling et al., Phys. Rev. Lett. 105(2), 025701 (2010)]. For heterogeneous nucleation we do not find such precursors. The fluid is directly transformed into highly ordered crystallites.     

On the behavior of the electrostatic colloidal interaction in the membrane filtration of latex suspensions

In order to investigate effects of the colloidal interaction in the membrane filtrations, the dead-end ultrafiltration of latex colloids was conducted with fully retentive membranes. Experimental results concerning the permeate flux during the filtration indicate that the void fraction of cake layer increased with the decrease of the ionic strength, due to the expanded Debye double layer thickness around the particles. The concentration dependence of the gradient diffusion coefficient of colloidal particles has been examined as a function of solution ionic strength. The NVT Monte Carlo simulation was applied on the bulk suspension so as to determine the thermodynamic coefficient, and the hydrodynamic coefficient was evaluated from the previously developed relation for an ordered system. The long-range electrostatic interactions between the particles are determined by using a singularity method, which provides accurate solutions to the linearized electrostatic field. The predictions on the variation of concentration polarization layer have been presented, from which we found that both the permeate flux and the particle diffusion are related to determine the concentration distribution above the cake layer.     

Rheological peculiarities of concentrated suspensions

The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.     

Viscosity of hard sphere suspensions

A simple functional representation of the concentration dependence of the low-shear viscosity eta of hard sphere suspensions is proposed. The representation, which agrees with published literature at all volume fractions phi, has a hitherto-unremarked transition in its functional form at phi approximately 0.42 identical with phi(t). phi(t) is definitely less than the volume fraction 0.49 of the hard sphere melting transition.     

Nonaqueous suspensions of surface-modified kaolin

A range of different stabilizers have been used to render natural kaolin clay particles hydrophobic and dispersible in nonpolar solvents such as heptane. Both silanol and aluminol groups are known to be present at the kaolin surface. Use of a Hammett indicator showed that silanes would not neutralize the acidic aluminol sites, whereas amines would neutralize these sites. Both types of stabilizer adsorbed chemically onto the clay. In addition, a combined silane + amine treatment and a polyisobutylene-based stabilizer with a succinimide/amine head group (SAP230) were also considered. Both would neutralize the acid sites. The final sediment density after settling under gravity was used to gauge suspension stability, which varied with the kaolin surface treatment as silanes < amines < silane + amine < SAP230. This behavior was very similar for suspensions in heptane and in a higher molecular weight branched alkane, polydecene. This trend of increasing stability correlated very well with an increase in surface coverage of the stabilizing moieties, a decrease in particle size found using small-angle light scattering, and a decrease in Bingham yield stress obtained by fitting rheological data.     

Optical anisotropy of nanotube suspensions

A semimacroscopic model of an optically anisotropic nanotube suspension is derived perturbatively from Maxwell's equations in a dielectric medium. We calculate leading-order expressions, valid in the dilute and semidilute limits, for the intrinsic and form contributions to the complex dielectric tensor in terms of the volume fraction, mean orientation, aspect ratio, optical anisotropy, and optical contrast of the nanotubes. The birefringence and dichroism are derived explicitly to leading order in fluctuations, and the connection with depolarized light scattering is established. The results are generalized to include tube deformation.     

Yield stress of magnetic suspensions

The theories available for the yield stress of magnetic suspensions imply that their transition from a quasi-elastic to a fluid behavior is related to the disruption of chainlike or bulk columnlike aggregates connecting opposite boundaries of a region containing a suspension. It is commonly assumed that aggregates are rigidly bonded to bounding walls. However, a slip of aggregates on the walls is frequently observed. In this work, the transition from an elastic shear strain of a magnetic suspension to its viscous flow due to the slip of aggregates on channel walls is theoretically studied. The value of the corresponding yield stress is estimated.     

On the dielectric relaxation of biological cell suspensions: the effect of the membrane electrical conductivity

Due to the mismatch of the electrical parameters (the permittivity ?' and the electrical conductivity σ) of the membrane of a biological cell with the ones of the cytosol and the extracellular medium, biological cell suspensions are the site, under the influence of an external electric field, of large dielectric relaxations in the radiowave frequency range. However, a point still remains controversial, i.e., whether or not the value of membrane conductivity σ(s) might be extracted from the de-convolution of the dielectric spectra or otherwise if it would be more reasonable to assign to the membrane conductivity a value equal to zero. This point is not to be considered with superficiality since it concerns an a priori choice which ultimately influences the values of the electrical parameters deduced from this technique. As far as this point is concerned, the opinion of the researchers in this field diverges. We believe that, at least within certain limits, the membrane conductivity can be deduced from the shape of the relaxation spectra. We substantiate this thesis with two different examples concerning the first a suspension of human normal erythrocyte cells and the second a suspension of human lymphocyte cells. In both cases, by means of an accurate fitting procedure based on the Levenberg-Marquardt method for complex functions, we can evaluate the membrane conductivity σ(s) with its associated uncertainty. The knowledge of the membrane electrical conductivity will favor the investigation of different ion transport mechanisms across the cell membrane.     

The viscosity of suspensions of spheres

Particle size distribution, often suggested as a factor influencing the viscosity of suspensions, is considered quantitatively. By an analysis of previously reported data, the viscosity of suspensions of spheres from the lowest to the highest concentration is shown to be described in terms of the viscosity of the pure liquid, the volume fraction of solids, the packed sediment volume of solids, and the geometric standard deviation for the solids.     

Detonability of aqueous suspensions of explosives

The detonability of aqueous suspensions of solid explosives (pentaerythritol tetranitrate, cyclotrimethylenetrinitramine, cyclotetramethylenetetranitramine) was used as a basis for evaluating the explosion safety of these substances in production and transportation. The dependence of the critical detonation parameters of the suspensions on the content and dispersity of the explosives was determined. A new common method for calculating the critical detonation diameters of suspensions of crystalline explosives in organic media and in water was developed. A novel procedure for preparing uniform sedimentation-resistant aqueous suspensions of crystalline explosives was suggested and developed.     

Spark-induced agglomeration of aqueous polymeric suspensions

Discharging a high-energy spark underwater creates a shock wave that dissipates through an acoustic field. Colloidal acrylate polymers suspended in water containing dissolved calcium are agglomerated as a result. The degree of agglomeration increases with the number of sparks applied and with increasing calcium content. The calcium decreases the charge on the colloidal particles and thereby increases their propensity to be agglomerated by the acoustic wave. These observations are confirmed in full-scale trials in paper recycling mills where sparking improves the efficiency of centrifugal cleaners by increasing the particle size of the contaminants.