The Electroviscous Behavior of Aqueous Dispersions of Amorphous Silica (Ludox)

The electroviscous effects in very dilute aqueous dispersions of amorphous silica (Ludox) were investigated at various levels of salt, pH, and volume fraction

Response to Comments by M. Kosmulski on “The Electroviscous Behavior of Dispersions of Amorphous Silica (Ludox) in KCl Solutions” by J. Laven and H. N. Stein

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Equilibrium phase diagram of suspensions of electrically stabilized colloidal particles

\u3cp\u3eA second-order perturbation theory based on statistical thermodynamics is employed to calculate the phase diagram of suspensions of charge-stabilized, monodisperse, spherical, colloidal particles that are assumed to interact by a standard DLVO potential. Taking into account van der Waals forces, the phase diagram consists of an ordered colloidal crystal, a disordered fluid phase, and a liquid phase that is formed by particles flocculated into the secondary minimum.\u3c/p\u3

The impact of non-DLVO forces on the onset of shear thickening of concentrated electrically stabilized suspensions

This paper exposes an extension of an activation model previously published by the authors. When particles arranged along the compression axis of a sheared suspension, they may overcome the electrostatic repulsion and form force chains associated with shear thickening. A percolation-based consideration allows an estimation of the impact of the force chains on a flowing suspension. It suggests that similar to mode coupling models, the suspension becomes unstable before the critical stress evaluated from the activation model is reached. The percolated force chains lead to discontinuous shear thickening. The model predictions are compared with results from two experimental studies on aqueous suspensions of inorganic oxides; in one of them, hydration repulsion and in the other hydrophobic attraction can be expected. It is shown that the incorporation of non-Derjaguin-Landau-Verwey-Overbeek forces greatly improve predictions of the shear thickening instability. © Springer-Verlag 2009

Should the Gibbs analysis be revised?

Recently, some arguments were published that cast doubt on the validity of the Gibbs adsorption isotherm. The doubt was on whether the often visible linearly declining part in the surface tension versus logarithm of concentration plot of a surfactant solution, just before the critical micelle concentration, really represents a situation of constant adsorption. Those published arguments are partly of a conceptual nature and partly based on experimental evidence. The conceptual arguments appear to be based on a misunderstanding of the theory, while the arguments based on experimental evidence stem from an inaccurate treatment of these data. Our conclusion is that none of the relevant arguments put forward are valid. The experimental evidence, if properly treated, is in line with the Gibbs theory

Silver nanoparticles prepared by using poly(2-acrylamido-2-methylpropane sulphonic acid) as a surfactant

Silver nanoparticles were synthesised successfully using poly(2-acrylamido-2-methylpropane sulphonic acid) (PAMPS) as a surfactant. Silver nanoparticles prepared through this approach possess high purity and narrow size distribution. The size distribution result shows that the diameters ranging from 78.82 to 105.709 nm can account for 76.41% of nanoparticles. UV–vis spectra were used to record the formation of silver nanoparticles in detail. It is found that PAMPS can play an important role in the formation and the colloidal stabilisation of silver nanoparticles because of the high affinity of sulphonic and amide with silver ions. The formation is summarised in detail. In addition, a potential application in nanocomposites of this method is explored. The Ag/polyaniline (PANI) nanocomposite prepared using this method shows higher electrical conductivity than that prepared using other methods. This method is novel, convenient, efficient and environmental-friendly, especially suiting those wastewaters treatment facilities containing silver ions. At the same time, it is also promising to prepare Ag/PANI or other metals/polymer composites via in-situ polymerisation

Forces operative during film formation from latex dispersions

In this paper, the different forces operative on the latex particles during film formation are examined and estimates are given of the contribution of the forces to the deformation of these particles. The forces examined are gravitational forces, Van der Waals forces, electrostatic repulsion forces due to the overlap of diffuse double layers, capillary forces due to the receding water/air interface, and capillary forces due to liquid bridges between the latex particles. The magnitude of these forces is compared to the force needed to obtain sufficient deformation, i.e. the closure of the voids between the particles. Calculations show that both capillary forces are from the same order of magnitude, 1–3 10-7 N. The Van der Waals contribution is smaller by a factor of 20 than the contribution due to the capillary forces. However, for deformation the Van der Waals forces may be of considerable importance since the Van der Waals forces diverge for very small distances. A sound incorporation of the Van der Waals forces can be achieved by using the JKR equations. Under the assumption of constant potential, the electrostatic repulsion forces are approximately a factor of 1000 smaller than the capillary forces. The gravitational forces, 1×10-16 N, are negligible. The force needed for successful deformation amounts to 10-7 N, assuming that the Hertz theory is applicable in the description of polymer particle deformation. Furthermore, an equation for the capillary force due to the receding water/air interface is derived which is applicable for a wider range of degrees of deformation than is the Mason-equation. Three descriptions of the particle's response to deformation are examined: (i) the Hertz theory for purely elastic spheres, (ii) the JKR-theory for purely elastic spheres in the presence of Van der Waals forces, and (iii) the Yang-theory for linear visco-elastic spheres. These descriptions are combined with both capillary forces resulting in criterions determining successful deformatio

Time-dependent behavior and wall slip in concentrated shear thickening dispersions

The viscosity of concentrated shear thickening dispersions was measured as a function of shear rate, Couette cylinder size, and time. The level of the low shear rate viscosity, which was found to be independent of system size and time, could be predicted by the equation of Frankel and Acrivos. At shear rates above the critical shear rate for shear thickening in highly concentrated (¿¿0.57) dispersions of monodisperse particles strong viscosity instabilities were detected, together with a dependence on cylinder size. The instabilities are attributed to reversible order–disorder transitions, e.g., from strings to clusters. This dependence on cylinder size is due to wall slip, slipping planes in the dispersion, and even plug flow in the gap. With less concentrated or polydisperse dispersions the effects are much less severe but there is thixotropy, probably due to a reordering of the dispersion