Common to liquids, solids and substances in between the former two is that if a stress is applied to them, they will strain. Stress may be visualized by placing a small amount of fluid between two parallel plates. When one plate slides over the other, forces act on the fluid dependent upon the rate of the plate movement. This causes a shear stress on the liquid. Recall laminar flow of fluids through a tubular vessel. Strain is the response to the stress. If solids are elastic, they deform and return to their original shape. Since fluids are not elastic and, hence, viscous, their deformation is irreversible.

Introduction to rheology

This contribution critically reviews the state of knowledge on interactions of natural colloids and engineered nanoparticles with natural dissolved organic materials (DOM). These interactions determine the behavior and impact of colloids in natural system. Humic substances, polysaccharides, and proteins present in natural waters adsorb onto the surface of most colloids. We outline major adsorption mechanisms and structures of adsorption layers reported in the literature and discuss their generality on the basis of particle type, DOM type, and media composition. Advanced characterization methods of both DOM and colloids are needed to address insufficiently understood aspects as DOM fractionation upon adsorption, adsorption reversibility, and effect of capping agent. Precise knowledge on adsorption layer helps in predicting the colloidal stability of the sorbent. While humic substances tend to decrease aggregation and deposition through electrostatic and steric effects, bridging-flocculation can occur in th...

Interactions of Dissolved Organic Matter with Natural and Engineered Inorganic Colloids: A Review

The points of zero charge (PZC) and isoelectric points (IEP) from the recent literature are discussed. This study is an update of the previous compilation [M. Kosmulski, Surface Charging and Points of Zero Charge, CRC, Boca Raton, FL, 2009] and of its previous update [J. Colloid Interface Sci. 337 (2009) 439]. In several recent publications, the terms PZC/IEP have been used outside their usual meaning. Only the PZC/IEP obtained according to the methods recommended by the present author are reported in this paper, and the other results are ignored. PZC/IEP of albite, sepiolite, and sericite, which have not been studied before, became available over the past 2 years.

pH-dependent surface charging and points of zero charge II. Update.

Colloidal behavior of aqueous montmorillonite suspensions: The specific role of pH in the presence of indifferent electrolytes

Aggregation and disaggregation of iron oxide nanoparticles: Influence of particle concentration, pH and natural organic matter.

Interfacial and rheological properties of humic acid/hematite suspensions.

Investigation of alumina/(+)-catechin system properties. Part I: a study of the system by FTIR-UV–Vis spectroscopy

We describe the rheology of moderately concentrated suspensions of elongated goethite (β-FeOOH) particles with axial ratio around 8, both in the absence and presence of high-strength DC electric fields (up to 4.3 kV/mm). The selected liquid medium was a silicone oil with 1 Pa s nominal viscosity. The aim of this work is the evaluation of the electrorheological (ER) effect of suspensions containing highly anisotropic particles and the comparison with that exhibited by samples made of less anisotropic particles of similar chemical composition (hematite, α-Fe 2 O 3 ). Under the application of large electric fields, goethite suspensions changed their rheological behavior, as expected, from Newtonian – at zero field – to shear thinning, thus displaying electrorheological response. A well defined yield stress ( σ y ) was observed in the electrified suspensions, that increases with both the field strength and particle concentration ϕ , although following different trends to those predicted by the classical chain models. σ y was found to depend on ϕ in a parabolic fashion, as a consequence of the fact that field-induced structures in the suspensions do not consist of individual chains as the classical models consider, but of much more complex particle aggregates. The yield stress was found to be almost linearly dependent on the field strength, contrary to the predictions of the polarization model ( σ y  ∝  E 2 ). The field-induced enhancement of the conductivity of the host oil, leading to saturation of the electrical forces among polarized particles, is required to explain this deviation. The goethite suspensions were also analyzed under oscillating shear stresses for investigation of their viscoelastic properties as well. The results indicate that the ER effect was only noticeable for sufficiently high field strength and particle concentrations, typically >1 kV/mm and >4% in volume fraction, respectively. In such conditions the elastic modulus G ′ was independent of the shear frequency as corresponds to an elastic solid-like structure. Suspensions of goethite particles display an ER effect with the same characteristics as hematite dispersions (same tendencies of σ y with both E and ϕ 2 ), indicating that the physical mechanism responsible of such effects is the same in both cases. However, suspensions containing elongated particles produce a more efficient response to the electric field than those made of irregularly shaped solids, since the former give rise to higher yield stress for the same field strength, and exhibit a lower viscosity in absence of external excitation.

Electrorheology of suspensions of elongated goethite particles

In this work we analyze the rheological behavior of Na-montmorillonite (NaMt) suspensions in the presence of humic acid. The analysis starts from the fact that the electric charges on plate and edge surfaces of clay particles largely determine the formation of three-dimensional structures in suspensions. Zeta potential data of NaMt as a function of humic acid (HA) concentration suggest that its adsorption takes place preferentially on the edges of the particles. A good correlation between the parameters describing the viscosity-shear rate or viscosity-shear stress dependencies and the product of the ζ potentials of edges and faces, ζedge×ζface, is demonstrated. It indicates that the rheological behavior of NaMt suspensions is largely controlled by electrostatic interactions. Adsorption of HA also changes the viscoelastic properties of the suspensions significantly as determined by both oscillometric and creep-recovery tests. A trend of NaMt suspensions from approximately solid like when no HA is adsorbed,...

Effect of humic acid adsorption on the rheological properties of sodium montmorillonite suspensions

An investigation is described on the electric-field-induced structures in colloidal dispersions. Both rheological determinations and direct microscopic observations are used with that aim. The starting point of this study is the so-called electrorheological (ER) effect, consisting of the mechanical reinforcing of a fluid or suspension due to formation of chains of molecules or particles after being polarized by the action of the field. One macroscopic manifestation of this phenomenon is the transformation of the fluid from a typically Newtonian behavior to a viscoelastic material, with finite yield stress and high elastic modulus. The systems investigated were suspensions of elongated goethite (β-FeOOH) particles in silicone oils with varying amounts of silica nanoparticles. The results showed the rather unusual behavior known as "negative ER effect", which can be best described by saying that the application of an electric field reduces the yield stress and the elastic modulus, that is, produces destruction of structures rather than their build up. The negative behavior is also found for suspensions of other inorganic powders, including hematite and quartz. On the contrary, the usual positive ER response is found for suspensions of cellulose and montmorillonite clay. The same happens if goethite suspensions are prepared in high volume fractions, high-viscosity fluids, or both. All of the results found are compatible with the so-called interfacial model of electrorheology: the reduction of the yield stress of goethite suspensions when the applied field is high enough is the consequence of particle migration toward the electrodes because of charge injection and subsequent electrophoresis. The migration leaves the gap between the electrodes devoid of particles and explains the decrease in yield stress. The addition of silica nanoparticles contributes to reduce the strength of this effect by hindering the charging and making it necessary to increase the field strength to observe the negative effect. The model appears to also be applicable to cellulose, although the positive response found for such particles is explained by their large size: larger diameters bring about larger attraction forces between particles, leading to a tendency to produce strong aggregates. This is likely to occur in suspensions of colloids which, because of their relatively high electrical conductivity, tend to acquire charge even in such nonpolar liquids as silicone oils.

M.M. Ramos-Tejada

Papers

Interfacial and rheological properties of humic acid/hematite suspensions.

Investigation of alumina/(+)-catechin system properties. Part I: a study of the system by FTIR-UV–Vis spectroscopy

Electrorheology of suspensions of elongated goethite particles

Effect of humic acid adsorption on the rheological properties of sodium montmorillonite suspensions

Negative Electrorheological Behavior in Suspensions of Inorganic Particles