R. Hidalgo-Álvarez

Magnetorheological fluids: a review

Magnetorheological (MR) materials are a kind of smart materials whose mechanical properties can be altered in a controlled fashion by an external magnetic field. They traditionally include fluids, elastomers and foams. In this review paper we revisit the most outstanding advances on the rheological performance of MR fluids. Special emphasis is paid to the understanding of their yielding, flow and viscoelastic behaviour under shearing flows.

Electrokinetic properties, colloidal stability and aggregation kinetics of polymer colloids

Abstract The purpose of this article is to present some important advances in the electrokinetic and colloidal characterization of polymer colloids. Special attention is paid to the new electrokinetic techniques: diffusiophoresis, dielectric dispersion and electro-acoustic. Also the most recent theoretical approaches are reviewed with respect to the electrokinetic properties of polymer colloids. Recently there has been intense discussion concerning electrokinetic processes and the theories used for data interpretation. Several concerns have been raised relating to the inability of the different processes and theories to yield the same electrokinetic potential. The most important explanations (shear plane expansion, preferential ion adsorption, osmotic swelling, crossing of the mobility/ξ-potential and anomalous surface conductance) to the electrokinetic behaviour of polymer colloids are discussed and analyzed. Also the effect of heat treatment on the electrokinetic properties of different types of polymer colloids is extensively considered. With regard to the colloidal stability of polymer colloids, three- and two-dimensional aggregations are presented. First, the stability factor W is introduced using the classical theory DLVO and the values obtained of Hamaker constant compared with the theoretical values estimated from the Lifshitz theory. The differences usually found by several authors are explained as due to the hydrodynamic interaction. Special attention is paid to the extended DLVO theory for studying homocoagulation of polymer colloids in three dimensions and to the new expressions for the van der Waals, electrostatic and structural forces that must be deduced to study the colloidal stability of polymer colloids in two dimensions. Also, the heterocoagulation of polymer colloids with different sign of surface charge density and particle size is reviewed, and a new definition of the heterocoagulation stability factor is given. The aggregation kinetics of polymer colloids in three dimensions is analyzed using the Smoluchowski theory (in terms of the reaction kernels k ij ) in the cases where the Smoluchowskis equation is analytically solvable (constant, sum, product kernel and linear combinations thereof). The dynamic scaling in aggregation phenomena with polymer colloids is studied using the classification scheme for homogeneous kernels due to Van Dongen and Ernst based on the relative probabilities of large clusters sticking to large clusters, and small clusters sticking to large clusters. The techniques (multiparticle and single particle detection) enabling us to provide cluster-size distribution of aggregating polymer colloids are also presented. Finally, the aggregation kinetics of two dimensional aggregation of polymer colloids is studied on the basis of the fractal dimension of the aggregates. The different scaling theories for two-dimensional aggregation are also considered.

Gel swelling theories: the classical formalism and recent approaches

In this work, the classical theory of polymer/polyelectrolyte gel swelling is reviewed. This formalism is easy to understand and has been widely applied to gels and microgel particles. Nevertheless, its limitations and obscure aspects should be known before use. The case of temperature-sensitive gels is discussed in some detail because it deserves particular clarification. The application to experimental swelling data (of both gels and microgels) is also reviewed. In this way, strengths and weaknesses of this approach can be elucidated. Moreover, other formalisms are also outlined. Many of them are inspired by the classical one. Their improvements are briefly commented in this case. Others are based on different grounds.

Cationic polymer nanoparticles and nanogels: from synthesis to biotechnological applications.

Biotechnological Applications Jose Ramos,† Jacqueline Forcada,*,† and Roque Hidalgo-Alvarez*,‡ †POLYMAT, Bionanoparticles Group, Departamento de Química Aplicada, UFI 11/56, Facultad de Ciencias Químicas, Universidad del País Vasco UPV/EHU, Apdo. 1072, 20080 Donostia-San Sebastiań, Spain ‡Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain

Stability of binary colloids: kinetic and structural aspects of heteroaggregation processes

This review reports on recent advances in our knowledge about the stability of binary colloids. We focus not only on experimental results but also discuss theoretical and simulation studies regarding kinetic and structural aspects of heteroaggregation processes arising in such systems. In the first part of this work, heteroaggregation of oppositely charged particles is reviewed. When the interactions are short ranged, binary diffusion-limited cluster-cluster aggregation takes place. In this case, the short time behavior of the system follows the Hogg, Healy and Fuerstenau (HHF) theory. At long times, however, stable aggregates may form and remain in the system. Furthermore, cluster discrimination is observed, clusters that differ only by one constituent particle were found to behave quite differently. When the range of the interactions is increased, the latter effects become more pronounced. The fractal dimension of heteroaggregates is, in general, smaller than the values reported for fast and slow homoaggregation processes. In some cases, even values close to unity were obtained. This means that heteroaggregates have an open branched structure that may approach a chain-like morphology. In the second part of this work, we briefly discuss similar effects arising in heteroaggregation phenomena due to differences in particle size and chemical composition. The third part of this review tackles recent developments in the field of equilibrium phase diagrams of binary colloids. In the last section, the relatively small number of papers about heteroaggregation processes in two-dimensional systems is also discussed.

Dynamic rheology of sphere- and rod-based magnetorheological fluids

The effect of particle shape in the small amplitude oscillatory shear behavior of magnetorheological (MR) fluids is investigated from zero magnetic field strengths up to 800 kA/m. Two types of MR fluids are studied: the first system is prepared with spherical particles and a second system is prepared with rodlike particles. Both types of particles are fabricated following practically the same precipitation technique and have the same intrinsic magnetic and crystallographic properties. Furthermore, the distribution of sphere diameters is very similar to that of rod thicknesses. Rod-based MR fluids show an enhanced MR performance under oscillatory shear in the viscoelastic linear regime. A lower magnetic field strength is needed for the structuration of the colloid and, once saturation is fully achieved, a larger storage modulus is observed. Existing sphere- and rod-based models usually underestimate experimental results regarding the magnetic field strength and particle volume fraction dependences of both storage modulus and yield stress. A simple model is proposed here to explain the behavior of microrod-based MR fluids at low, medium and saturating magnetic fields in the viscoelastic linear regime in terms of magnetic interaction forces between particles. These results are further completed with rheomicroscopic and dynamic yield stress observations.

Effect of particle shape in magnetorheology

Magnetorheological (MR) properties were investigated for sphere, plate, and rod-like iron particles in suspension under the presence of magnetic fields to ascertain the effect of particle shape in MR performance. A novel two-step synthesis route for micrometer sized iron particles with different morphologies is described in detail. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Finite element method calculations were performed to explain the effect of particle shape in the magnetic field-induced yield stress. Compared to their sphere and plate counterparts, rod-like particle based MR fluids present a larger storage modulus and yield stress. The effect of particle shape is found to be negligible at large particle content and/or magnetic field strengths.

A comparative study between the adsorption of IgY and IgG on latex particles.

The use of egg yolk antibodies (IgY) instead of IgG from mammalian species may present several advantages in the development of routine diagnostic immunoassays. On the one hand, the animal suffering is reduced, as antibodies are obtained directly from the egg. On the other hand, the use of IgY avoids the rheumatoid factor interference. The rheumatoid factor interacts with IgG molecules in many immunoassays causing false positive results. Despite these advantages, IgY antibodies are scarcely used. As part of an aim to develop a diagnostic test based on IgY-latex agglutination, a preliminary study on some characteristics of the IgY-latex complexes is carried out. In this work, protein adsorption and desorption, isoelectric point, electrokinetic mobility, and colloidal stability are analysed. Results are compared to those obtained by IgG. Interesting differences are observed (which mainly arise from the difference in molecular structure between IgY and IgG), suggesting that IgY is a more hydrophobic molecule than IgG. In addition, colloidal dispersions of IgY-covered latex particles are more stable (at pH 8) than those sensitized by IgG.

Contact angle measurements on two (wood and stone) non-ideal surfaces

Abstract Surface properties of wood and stones are very important in influencing the bonding and finishing of wood and for road construction. Two important surface properties are wettability and surface free energy. Contact angle measurement is a simple, useful and sensitive tool for quantifying the wettability and the surface energy of different materials in contact with pure water and/or aqueous surfactant solutions. Nevertheless, a sessile drop on a real surface shows different contact angle values due to its lack of symmetry or to the loss of volume by capillary action. For this reason alternative techniques which supply an average contact angle value are required. In view of this fact, we applied axisymmetric drop shape analysis-diameter (ADSA-D) using a top view of a sessile drop and axisymmetric drop shape analysis-profile (ADSA-P) with a side view of a captive bubble. Both techniques calculate the contact angle by solving the Young–Laplace equation although with different algorithms, the former needs the maximum drop diameter while the last makes use of the complete bubble profile. ADSA-D was applied to study two wood species (eucalyptus and pine) and ADSA-P with polished rocks of two different compositions (silicate and calcite). The maximum contour of the drop was fitted to an ellipse after being detected and extracted by means of a semi-automatic procedure of image processing. Thus, two different contact angles were found out. The captive bubble method in conjunction with the current ADSA-P technique allows to obtain comfortable, automatic and reproducible measurements of contact angle on porous stones.

Simulation of electric double layers with multivalent counterions: Ion size effect

In this paper, the structure of the electric double layer in the presence of (mostly) multivalent counterions is investigated through Monte Carlo simulations. Unlike previous similar studies addressing this matter, the difference of this study lies in the use of realistic hydrated ion sizes. Additionally, two different methods for calculating energies in the Metropolis algorithm are applied. The obtained results show that the conclusions of preceding papers must be revised. In particular, our simulations suggest the existence of certain ion layering effects at high surface charge densities, which are not accounted for by integral equation theories in the case of divalent counterions. These layering effects could justify why the overcharging phenomena due to ion size correlations are hardly observable in real colloids with divalent counterions. The existence of charge inversion due to ion size correlations (and without requiring specific counterion adsorption) is probed for trivalent counterions. Moreover, the hypernetted-chain/mean-spherical-approximation is tested under conditions not studied yet.