Paul F. Luckham

The colloidal and rheological properties of bentonite suspensions

The objective of this paper is to give an overview of the colloidal properties of clays and to describe the work performed until the present moment in the area of adsorption of non-ionic polymers onto clay and the rheology of clay suspensions. The review is particularly focused on Na+-montmorillonite clay of the smectite group (Bentonite), due to its high swelling capacity, the ability to adsorb significant amounts of polymer on its surfaces, and the formation of a gel-like structure with yield characteristics and viscoelastic properties at relatively low clay concentrations. A significant amount of research has been performed in these areas with applicability to various fields, such as soil science and drilling fluids, where interaction of organic polymers with the clay surface and the rheological response in the presence of electrolytes under different temperature and pressure conditions are of considerable relevance. In order to understand the interaction of these compounds with the clay surface and the rheological response of the resulting suspensions, it becomes essential to understand the structure of clay minerals, the interaction of clay particles in aqueous medium, the swelling behaviour and electrokinetic properties of Na+-montmorillonite clay. For this reason, the above topics are addressed, along with the general features of stabilisation of colloidal particles and the adsorption mechanism of non-ionic compounds on hydrophilic surfaces. A comprehensive review of the most relevant studies of the adsorption of non-ionic surfactants and polymers onto both silica and clay is presented. Finally, an overview of the rheological behaviour of concentrated suspensions and models used to describe the flow behaviour of clay suspensions is given, along with the effect of electrolytes, polymers, high pressure and high temperature on the flow behaviour of Na+-montmorillonite suspensions.

The effects of hydrogen bonding upon the viscosity of aqueous poly(vinyl alcohol) solutions

Abstract This paper describes the rheological properties of a number of aqueous solutions of a poly(vinyl alcohol) (PVA) as a function of the degree of polymer hydrolysis, ambient temperature, pressure and variations induced upon the addition of electrolytes to these solutions. Experimental results reported show that the rheological behaviour of these aqueous PVA solutions mainly depends upon the relative strength of the hydrogen bonding existing between the polymer chains and water molecules, compared with that of the inter and intra chain hydrogen bonding. Changes in temperature, pressure, degree of hydrolysis of the PVA, and the addition of electrolytes produce a corresponding effect in varying these two types of hydrogen bonding and thus the resulting rheological properties of the PVA solutions.

Imaging erythrocytes under physiological conditions by atomic force microscopy.

Since its invention in the mid 1980s atomic force microscopy has revolutionised the way in which surfaces can be imaged. Close to atomic resolution has been achieved for some materials and numerous images of molecules on surfaces have been recorded. Atomic force microscopy has also been of benefit to biology where protein molecules on surfaces have been studied and even whole cells have been investigated. Here we report a study of red blood cells which have been imaged in a physiological medium. At high resolution, the underlying cytoskeleton of the blood cell has been resolved and flaws in the cytoskeleton structure may be observed. Comparison of the normal doughnut shaped cells with swollen cells has been undertaken. Differences in both the global properties of the cells and in the local features in cytoskeleton structure have been observed.

The spreading of surfactant solutions on thin liquid films

The spreading of a surfactant solution on a water film at first glance seems a trivial problem. However, in the last 30 years or so this has been shown to be anything like the case. There have been numerous studies which show that Marongoni driven fingering of the spreading surfactant front exists. In this paper this work has been reviewed and an attempt has been made to rationalise the results. The paper concludes with some recent observations of ours concerning the spreading of sodium dodecyl sulfate over relatively thick water films, 200 microm or less.

Influence of non-ionic polymers on the rheological behaviour of Na+-montmorillonite clay suspensions—I Nonylphenol–polypropylene oxide–polyethylene oxide copolymers

Abstract The present experimental investigation relates the adsorption of non-ionic surfactants onto Na+-montmorillonite clay with the rheological behaviour of the concentrated clay suspensions. The non-ionic surfactants used were the commercially available nonylphenol(polypropyleneoxide)(polyethyleneoxide) surfactants of varying ethylene oxide chain length. Adsorption, electrokinetic, and phase analysis light scattering measurements gave evidence of the presence of surfactant micelles on the clay particles. Adsorption isotherms were of the Langmuir type; the maximum amount adsorbed and the affinity for the clay surface decreased with increasing number of ethylene oxide units. The zeta potential of the Na+-montmorillonite particles was calculated as a function of surfactant concentration. The surfactant layer thickness was estimated from the former measurements and was found to increase linearly with the number of ethylene oxide units. The rheological behaviour of the clay suspensions was obtained by shear stress–shear rate measurements within a wide range of shear rates (0.1–1000 s−1). The data was fitted to both the Bingham and Herschel-Bulkley models; from which the Bingham yield stress, plastic viscosity, Herschel–Bulkley yield stress, flow behaviour index and consistency coefficient were estimated. Dynamic oscillatory shear measurements enabled the determination of viscoelastic parameters; such as the complex, elastic and loss modulus. Results show that flocculation of the suspension occurs below maximum surfactant coverage of the clay particles, and occurs to a greater extent at approx. 50% surface coverage. Above this value, a reduction of the rheological properties takes place and a plateau value is reached as the maximum amount adsorbed is approached. Flocculation was attributed to the bridging of micelles between the particles, which promotes the formation of a strong gel-like network. Scaling of the elastic modulus with clay concentration gave a power law fit. The power exponent was related to the flocculation or stability of the Na+-montmorillonite suspensions, depending on the clay particle coverage.

The rheology of deformable and thermoresponsive microgel particles

Abstract This paper describes the preparation and characterization of two monodisperse, hydrophilic polymer latices, with different particle diameters, of poly-N-isopropylacrylamide crosslinked with N, N′- methylenebisacrylamide. The uncrosslinked polymer is water soluble and non-ionic and exhibits a lower consolute solution temperature of about 31–32°C. The latex was prepared via a surfactant-free dispersion polymerization route using water as a solvent at a polymerization temperature of 70°C. The physical properties of the resulting latex were strongly dependent on temperature. At high temperatures (65°C), the particle diameters were 23 and 50 nm, whilst at 25°C they were 130 and 470 nm. This five- to ten-fold increase in the particle diameter corresponds to a 100- to 1000-fold increase in volume. Consequently, the rheological properties, both continous and oscillatory shear, of these systems are strongly dependent on temperature. Volumetric and particle size changes observed by photon correlation spectroscopy and differential scanning calorimetry thermal analysis were in agreement, confirming that the volume phase transition is controlled and influenced by the same factors observed in the swelling/deswelling behaviour of microgels. The viscoelastic properties of these systems and the elasticity of the dispersion decreased as the temperature was increased and the fluid changed from a predominantly elastic to a viscous material. In addition, at lower temperatures, all the studies showed an increase in the storage modulus of the dispersion with increasing particle concentrations. The rheology of the dispersions as a function of the volume fraction of the particles was monitored and the viscosity and shear-thinning increased as volume fraction increased.

Interaction of binders with dispersant stabilised alumina suspensions

Abstract The rheological response of selected aqueous alumina suspensions, stabilised with a polyelectrolyte or with an organic polyvalent salt dispersant, and including poly(vinyl) alcohol (PVA) as a binder, are described in this study. The polymer dispersant was composed of an ammonium salt of poly(methacrylate) and the organic polyvalent compound was a sodium salt of an aromatic sulphate. The results show that the addition of PVA, without any included dispersant does not significantly influence the rheology of the system. However, in the presence of a dispersant the rheology is greatly affected. At a given concentration of the dispersant, the viscosity, storage and loss moduli all increase, as the PVA concentration is increased. Also, for a given concentration of the PVA, it is observed that the viscosity, storage and loss moduli values increase as the concentration of the dispersant is increased. It is argued that at low PVA concentrations, an excess concentration of the unadsorbed dispersant causes flocculation of the particles in the suspension by a reduction of the repulsive electrostatic (double layer) effect. In contrast, at higher concentrations of the PVA the flocculation of the suspension is promoted via a depletion mechanism.

Forces between proteins and model polypeptides adsorbed on mica surfaces

The forces of interaction between proteins adsorbed onto mica have been measured as a function of the distance of separation between the two mica surfaces in aqueous solutions. The results for three proteins, myelin basic protein, concanavalin A and cytochrome c, are presented together with the results for a model basic protein, poly(L-lysine). With the exception of cytochrome c at large separations, the forces of interaction are due to charges on the protein surfaces and may be fitted closely to theoretical predictions. For cytochrome c, however, no long-range electrical repulsion is observed, indicating that the negatively charged mica surface has been neutralised by the adsorption of the positively charged protein. At short surface separations, an attraction between the protein surfaces was noted. For concanavalin A, a weak attraction was observed in the presence of calcium and manganese ions only. For poly(L-lysine) and cytochrome c the attraction can be explained simply in terms of van der Waals interactions between the proteins. However, for myelin basic protein the observed attraction was an order of magnitude larger than that predicted by van der Waals theory. We believe that this additional attraction may be due to hydrophobic interactions between the adsorbed myelin basic protein molecules.

Depletion flocculation of particulate systems induced by hydroxyethylcellulose

Abstract The flocculation behaviour of three different aqueous colloidal dispersion systems is examined in the presence of soluble hydroxyethyl cellulose (HEC) molecules by means of rheology. One alumina and two polystyrene dispersions were used for comparison with particle solid contents varying from 20 to 50% (v/v). All dispersions were sterically stabilised by either chemical or physical adsorption of an appropriate polymeric dispersant. Addition of HEC (MW range 170u2008000–1u2008000u2008000) followed stabilisation within a concentration range of 0.2–1% (w/solvent volume). Both continuous and oscillatory shear measurements were carried out and the yield value and elastic modulus have been chosen as providing a good indication of the degree of aggregation. The variation of the above parameters is examined with respect to HEC and solid contents as well as to the polymer molecular weight and the flocculation mechanism and aggregation kinetics are discussed. It is suggested that particles aggregate as a result of a depletion flocculation mechanism. According to this mechanism, above a critical concentration (critical flocculation point), polymer is depleted from the interparticle space after a particle collision which gives rise to an osmotic pressure gradient with respect to the bulk of the solution and flocculation starts taking place. All systems seem to exhibit characteristics of a fractal based structure above a certain solid volume fraction The differences found in the aggregation kinetics of the ceramic dispersion compared to the two polystyrene dispersions were attributed to the significant degree of pre-shearing of the ceramic dispersion. In addition, some form of interaction between the dispersant and the polymer was suggested in the case of chemically grafted polystyrene dispersions. Generally, the aggregation process can be explained with existing depletion flocculation theories.

Rheological properties of poly(ethylene oxide) aqueous solutions

This article describes the rheological properties of certain poly(ethylene oxide)s dissolved in water-based solvents. The experimental results show that the rheological properties in aqueous solutions are significantly affected by the solvent properties, which have been changed by the use of ethanol–water mixtures and electrolyte solutions and by the variation of the ambient pressure and temperature. The variation of the temperature and pressure is seen to change the polymer chain configuration and also the interactions of polymer segments with the solvent molecules. This gives rise to distinctive and apparently unusual rheological properties for these systems with the variation of the ambient temperature and pressure. The study generally illustrates that the rheology of these systems are, to a large degree, influenced by the hydrogen bonding in the solvent and between the solvent as well as the polymer. At a first-order level, the increase of the pressure and the temperature and also the addition of electrolytes, and the inclusion of an aqueous diluent, produce comparable effects. In essence, these changes seem to disrupt the hydrogen bonding structure in the solutions and, hence, the solvent quality in a comparable fashion.