Video microscopy of colloidal suspensions and colloidal crystals

Colloidal suspensions are simple model systems for the study of phase transitions. Video microscopy is capable of directly imaging the structure and dynamics of colloidal suspensions in different phases. Recent results related to crystallization, glasses, and 2D systems complement and extend previous theoretical and experimental studies. Moreover, new techniques allow the details of interactions between individual colloidal particles to be carefully measured. Understanding these details will be crucial for designing novel colloidal phases and new materials, and for manipulating colloidal suspensions for industrial uses.     

Computer simulation and modeling of food colloids

Recent developments in theoretical modeling and computer simulations of colloidal systems of potential relevance to foods are reviewed. Attention has been focused on those areas where simulation studies have provided major new insight, complementary to experimental results. These include formation, structure and mechanical properties of colloidal gels; viscoelastic phase separation and transient networks; competitive displacement of protein layers from interfaces and the problem of disproportionation in foams and bubble dispersions. The usefulness of many conclusions emerging from the literature reviewed here points to an increased potential use of modeling and computer simulation methods in the study of food colloidal systems in the future.     

Depletion-induced phase separation in colloid-polymer mixtures

Phase separation can be induced in a colloidal dispersion by adding non-adsorbing polymers. Depletion of polymer around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. This communication reviews theoretical and experimental work carried out on the polymer-mediated attraction between spherical colloids and the resulting phase separation of the polymer-colloid mixture. Theoretical studies have mainly focused on the limits where polymers are small or large as compared to the colloidal size. Recently, however, theories are being developed that cover a wider colloid-polymer size ratio range. In practical systems, size polydispersity and polyelectrolytes (instead of neutral polymers) and/or charges on the colloidal surfaces play a role in polymer-colloid mixtures. The limited amount of theoretical work performed on this is also discussed. Finally, an overview is given on experimental investigations with respect to phase behavior and results obtained with techniques enabling measurement of the depletion-induced interaction potential, the structure factor, the depletion layer thickness and the interfacial tension between the demixed phases of a colloid-polymer mixture.     

The primary electroviscous effect in colloidal suspensions

Although a well-defined electrokinetic phenomenon, the primary electroviscous effect in dilute colloidal suspensions is still an unsolved problem. Most of the experimental tests of the different theories that we have studied have shown a lack of agreement. We have developed, during the last years, new theoretical approaches obtaining, finally, a much better agreement with the experimental results. The corrections are defined in two lines: first, it is accepted that ions present into the Stern layer, in which the fluid is stagnant, can tangentially move; second, it is accepted that the hydrodynamic interaction between colloidal particles exists although the suspensions are extremely diluted. The remarkable conclusion of our work is that the combination of both corrections should give correct theoretical results.     

Dynamic effects on colloidal electric interactions

Dynamic behaviors are abundant in field-responsive colloidal suspensions. Being beyond the usual point-dipole approximation, we develop a multiple image method of dipoles for two dynamic unequal colloidal dielectric spherical particles, which can be perfectly reduced to those for two static conducting particles. The method is applied to investigate colloidal electric interparticle forces under various conditions of dynamics. As a result, we find that the force can be enhanced, reduced, or even changed from attraction to repulsion, or vice versa. Some other interesting results are also reported. Our theoretical results are compared favorably with existing experimental observations. Therefore, it becomes possible to achieve desired colloidal structures by adjusting colloidal interactions by choosing appropriate dynamic phenomena.     

Recent explorations in electrostatic multilayer thin film assembly

New developments in the area of electrostatic layer-by-layer assembly are reviewed, with emphasis on work in the past two years. Advances in fundamental understanding of polyelectrolyte adsorption is addressed, including the use of new probes and experimental techniques which examine final structure, film interpenetration, and control of thickness. Both theoretical and experimental studies of adsorption of weak polyelectrolytes have been addressed. The role of secondary interactions such as hydrogen bonding or dispersion forces on these parameters is a more recent area of focus. Molecular scale order has been achieved in layered films to produce noncentrosymmetric films; further control of the ordering of molecular side groups in these systems could lead to new and interesting electrical and optical properties. Finally, it has been shown that polyelectrolyte multilayers may be templated onto a number of surfaces; these materials can be patterned onto surfaces to make three dimensional microstructures, or grown on a sacrificial colloidal template to form encapsulant membranes.     

Energy of interaction in colloids and its implications in rheological modeling

This work deals with the problem of deriving theoretical connections between rheology and interparticle forces in colloidal suspensions. The nature of interparticle forces determines the colloidal structure (crystalline order due to long range repulsive forces, flocculation due to attractive forces, etc.) and hence, the flow behavior of suspensions. The aim of this article is to discuss how these interactions enter the modeling of rheometric functions, in particular, the shear viscosity. In this sense, the main interactions commonly appearing in colloids are reviewed, as well as the role they play in phase transition behavior. Then, a series of approaches relating the interaction potential to viscosity is examined. The results of applying these models to experimental data are also discussed. Finally, examples of viscosity modeling for different interaction potentials are given, by using the structural model proposed previously by the authors. The possibility of relating the flow behavior of colloidal suspensions to the interaction between particles offers new perspectives for the study and technical applications of these systems.     

Specific ion effects in colloidal and biological systems

During the last ten years significant progress has been made in the understanding of specific ion effects. On the one hand new ideas about the origin of these effects came up, and on the other hand new experimental techniques were developed so that now even the ion concentration profile near surfaces can be measured with some confidence. In the present review some of the most important new progresses are summarised and critically discussed, especially in the context of colloidal and biological systems.     

Ion partitioning at the oil-water interface as a source of tunable electrostatic effects in emulsions with colloids

We present a combined experimental and theoretical investigation of the surprisingly strong electrostatic effects that can occur in mixtures of low- and high-polar liquids (e.g. oil-water emulsions), here in the presence of colloidal particles. For our experiments, we used confocal microscopy imaging, supplemented with electrophoresis and conductivity measurements. Theoretically, we studied our systems by means of a modified Poisson-Boltzmann theory, which takes into account image charge effects and the electrostatic self-energies of the micro-ions in the different dielectric media. Our results show that the unequal partitioning of micro-ions between the two liquid phases is the common driving force behind most of the observed electrostatic effects. The structural signatures of these effects typically develop on a time scale of hours to days and are qualitatively well-described by our theory. We demonstrate how the partitioning process and its associated phenomena can be controlled by shifting the balance of the interlocked ionic dissociation and partitioning equilibria. Moreover, we present strong experimental proof that the two-dimensional colloidal crystals at the oil-water interface are due to long-ranged Coulombic repulsion through the oil phase. The acquired insight in the role of electrostatics in oil-water emulsions is important for understanding the interactions in particle-stabilized ('Pickering') and charge-stabilized emulsions, emulsion production, encapsulation and self-assembly.     

Dynamic Mobility of Colloidal Particles with Thick Double Layers

The dynamic mobility spectra of several colloidal systems having a ratio of particle radius to double-layer thickness between 1 and 20 have been measured using the technique of electroacoustics. Good agreement is found between the experimental mobility spectra and the theoretical spectra generated by the computer program of Manglesdorf and White for spherical monodisperse suspensions with sizes in the neighborhood of 0.1 μm. Smaller and larger particles show some minor discrepancies which are more likely to be due to limitations of the model systems being used for the test than to any basic limitation of the theoretical analysis. Copyright 2000 Academic Press.     

Vapor detection enabled by self-assembled colloidal photonic crystals

Here we report the sensitive and reversible detection of vapors by using self-assembled colloidal photonic crystals. The condensation of various vapors in the interstitials of silica colloidal photonic crystals leads to red-shift and amplitude reduction of optical stop bands. A linear relationship between wavelength shift and vapor partial pressure has been observed for a variety of vapors including ethanol, water, and toluene. Importantly, the sensitivity of colloidal photonic crystal-based vapor detectors can be improved by nearly two orders of magnitude by using a new full-peak analysis technique that takes advantage of the manifest amplitude reduction of optical stop bands during vapor condensation. Optical simulation based on a scalar-wave approximation model shows that the predicted optical responses during vapor condensation in colloidal photonic crystals agree well with experimental results. The condensation of vapors between submicrometer-scale microspheres, a topic that has received little examination, has also been investigated by both experiments and theoretical calculations. Predictions based on a modified Kelvin equation match with the experiments for a wide range of vapor partial pressures.     

Simulations of the PDF functions for dilute colloidal suspensions of molecular particles flowing in mesopores with rough surface boundaries

Simulations have been carried out to analyze the dynamics of dilute colloidal suspensions of macromolecular particles in solutions flowing in pores, subject to hydrodynamic forces, Brownian motion and stochastic collisions at rough pore boundaries in a two-dimensional spatial frame. A theoretical model is developed and intensively analyzed for the treatment of the mechanical restitution of the particles due to dynamic collisions at these boundaries. In particular we are able to calculate the Probability distribution functions for the spatial positions and the orientations of rod-like particles inside the pores. The results are presented for different widths of pore channels referenced to the size of a rod-like particle. These simulations are general in the sense that they are developed for confining and open pore channels, rough at the nano scale. The simulations also permit calculating the nematic order parameters for colloidal suspensions; the model calculation is applied for dilute colloidal suspensions of carbon nano-tubes in an aqueous single-stranded DNA solution flowing inside pores. Our calculated nematic order results for dilute suspensions of particles of known lengths flowing inside porous systems should indicate, when coupled to birefringence and dichroism experimental results, the possibility to estimate the pore widths for these systems.     

Paradox of stability of nanoparticles at very low ionic strength

The scaling of electrical double layer interaction energy from a plate-plate system to a sphere-plate system was reexamined, and it was found that accurate scaling without resorting to the Derjaguin approximation theoretically predicts the destabilization of nanoparticles in water depleted of added electrolyte and, consequentially, a maximum stability at a moderate ionic strength. This theoretical feature re-emphasizes the dual-role nature of added electrolyte that was supported by experimental results of direct surface force measurement but not by those of colloidal stability of nanoparticle deposition/aggregation. Inconsistences between the theoretical prediction and the experimental observation and between experimental observations in different systems were discussed. Possible reasons leading to the inconsistences were explored, including the effect of curvature, the contribution from counterions, the mode of interaction, and the applicability of an equilibrium model to describe the colloidal interaction of a nanoparticle suspension.     

Interactions between colloidal particles in simple shear flow

Recent progress in experimental and theoretical developments dealing with colloidal. interactions between two spheres in shear flow is reviewed. A systematic comparison is made between spheres suspended in simple electrolyte and in cationic polyelectrolye solutions. Microrheological observations, performed with the traveling microtube, make an in-depth investigation possible of the colloidal forces and the mechanisms of polymer bridge formation. Reasons are discussed for the often-observed aging of colloidal aggregates. Finally, coagulation rates are presented for systems with weak and strong Brownian motion. It is shown that the often-used assumption of additivity of the ortho- and perikinetic coagulation rates is incorrect.     

Tailoring the interfacial assembly of colloidal particles by engineering the mechanical properties of the interface

Fluid interfaces can be used as a platform for promoting the direct and spontaneous self-assembly of colloidal particles, where the driving force is the reduction in interfacial energy. In addition, fluid interfaces allow fine-tuning of the particles ensemble by an external force, such as the presence of an imposed interfacial flow, or by engineering the interparticle interactions dictated by the interplay of interfacial forces. As a consequence, a wide-ranging set of interfacial structures can be achieved from liquid-like layers, which can flow under stress, to amorphous solids that are able to sustain static stress. Here, far from a comprehensive overview of the interfacial assembly of colloidal particles, different ways of tailoring it by rationally designing the rheological properties of the interface are provided, with a focus on experimental and theoretical methods and model systems that have been recently exploited. In particular, ligand-coated nanoparticles, with a strong emphasis on the effect of the ligands on the interfacial structure and the rheological properties, and soft nanogel particles, in which an environmental factor, such as the temperature, drives to different interfacial structures and mechanical responses will be further discussed.     

Importance of weak interactions in the formulation of organic phases for efficient liquid/liquid extraction of metals

Recent experimental studies demonstrate the need to take into account weak interactions in the understanding of solvent extraction processes. This well-established industrial technology now beneficiates of a supramolecular approach, complementary to the traditional analysis based on coordination chemistry. In this article, we focus on the integration of a colloidal approach in the analysis of solvent extraction systems: organic phases used are complex fluids, in which extracting molecules self-assemble into reverse aggregates. We detail the available analytical tools used towards characterization of these organic phases and emphasize the recent results in aggregation-driven extraction. All experimental data are discussed in light of theoretical approaches which propose adequate thermodynamic models and shed light on the importance of entropy on the phenomena. Diluent effects and synergism have been successfully rationalized, efficient new formulations based on a physicochemical analysis have been proposed and the door is now open for further development at industrial scale.     

Re-entrant kinetic arrest and elasticity of concentrated suspensions of spherical and nonspherical repulsive and attractive colloids

We have designed and studied a new experimental colloidal system to probe how the weak shape anisotropy of uniaxial particles and variable repulsive (Coulombic) and attractive (van der Waals) forces influence slow dynamics, shear elasticity, and kinetic vitrification in dense suspensions. The introduction of shape anisotropy dramatically delays kinetic vitrification and reduces the shear elastic modulus of colloidal diatomics relative to their chemically identical spherical analogs. Tuning the interparticle interaction from repulsive, to nearly hard, to attractive by increasing suspension ionic strength reveals a nonmonotonic re-entrant dynamical phase behavior (glass-fluid-gel) and a rich variation of the shear modulus. The experimental results are quantitatively confronted with recent predictions of ideal mode coupling and activated barrier hopping theories of kinetic arrest and elasticity, and good agreement is generally found with a couple of exceptions. The systems created may have interesting materials science applications such as flowable ultrahigh volume fraction suspensions, or responsive fluids that can be reversibly switched between a flowing liquid and a solid nonequilibrium state based on in situ modification of suspension ionic strength.     

Dynamics of interacting Brownian particles

A memory function formulation has been used for the dynamics of interacting Brownian particles in colloidal systems and polymer dispersions. By introducing approximations at the level of the force correlation function, the theoretical scattering functions agree well with the experimental photon correlation spectroscopy data.