Table of contents

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In this paper we present a study of pattern formation in bidimensional systems with competing short-range attractive and long-range repulsive interactions. The interaction parameters are chosen in such a way as to allow us to analyse two different situations: the spontaneous pattern formation due to the presence of strong competing interactions on different length scales and the pattern formation as a response to an external modulating potential when the system is close to its Lifshitz point. We compare different Monte Carlo techniques showing that the parallel tempering technique represents a promising approach for the study of such systems and we present detailed results for the specific heat and the structural properties. We also present random phase approximation predictions concerning spontaneous pattern formation (or microphase separation), as well as linear response theory predictions concerning the induced pattern formation due to the presence of an external modulating field. In particular we observe that the response of our systems to external fields is much stronger than the response of a Lennard-Jones fluid.

We study thermodynamic and dynamic properties of model colloidal systems interacting with a hard core repulsion and a short-range attraction, and provide an overall picture of their phase diagrams which shows a very rich phenomenology. We focus on the slow dynamic properties of this model, investigating in detail the glass transition lines (both repulsive and attractive), the glass–glass transitions and the location of the higher order singularities. We discuss the relative location of the glass lines and of the metastable liquid–gas binodal, an issue relevant for the understanding of low density arrested states of matter.

By using Monte Carlo simulations in the grand canonical ensemble we investigate the bulk phase behaviour of a model colloid–polymer mixture, the so-called Asakura–Oosawa model. In this model the colloids and polymers are considered as spheres with a hard-sphere colloid–colloid and colloid–polymer interaction and a zero interaction between polymers. In order to circumvent the problem of low acceptance rates for colloid insertions, we introduce a cluster move where a cluster of polymers is replaced by a colloid. We consider the transition from a colloid-poor to colloid-rich phase which is analogous to the gas–liquid transition in simple liquids. Successive umbrella sampling, recently introduced by Virnau and Müller (2003 Preprint cond-mat/0306678), is used to access the phase-separated regime. We calculate the demixing binodal and the interfacial tension, also in the region close to the critical point. Finite size scaling techniques are used to accurately locate the critical point. Also investigated are the colloid density profiles in the phase-separated regime. We extract the interfacial thickness w from the latter profiles and demonstrate that the interfaces are subject to spatial fluctuations that can be understood by capillary wave theory. In particular, we find that, as predicted by capillary wave theory, w² diverges logarithmically with the size of the system parallel to the interface.

Kinetically frozen micelles formed by the amphiphilic diblock copolymer poly(ethylene-alt-propylene)–poly(ethylene oxide) (PEP–PEO) are proposed as a new model system for soft colloids. In this context soft is used with a twofold meaning: the intraparticle softness, i.e. the molecular architecture of an individual micelle, as well as the interparticle softness, i.e. the effective potential for micellar interactions. Both contributions can be precisely adjusted from hard sphere-like to ultrasoft (star-like) by changing the diblock copolymer composition and/or interfacial tension, as shown by small angle neutron scattering in combination with contrast variation techniques.

Depending on the degree of softness, PEP–PEO micellar solutions respond variably to the application of external shear fields. In particular, in the star-like regime solutions are already extremely sensitive at low shear rates. Therefore these micelles are an excellent starting point for a comprehensive study on the relation between softness and non-equilibrium phase behaviour in colloidal systems.

The dynamic behaviour of the molecular alignment strongly affects the rheological properties of nematic liquid crystals. The closed nonlinear relaxation equation for the five components of the alignment tensor which was derived within the framework of irreversible thermodynamics and also inferred from a generalized Fokker–Planck equation led to the prediction (Rienäcker et al 2002 Phys. Rev. E 66 040702(R); Physica A 315 537) that the rather complex orientational behaviour of tumbling nematics can even be chaotic in a certain range of the relevant control variables. Here the rheological consequences, in particular the shear stress and the normal stress differences, as well as the underlying dynamics of the alignment tensor are computed and discussed. For selected state points, long time averages are evaluated for imposed shear rates. Orientational and rheological properties are presented. The transitions between different dynamic states are detected and discussed. Representative examples of alignment orbits and rheological phase portraits give insight into the dynamic behaviour.

Shear-induced structures of concentrated temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel suspensions have been studied employing small angle neutron scattering (rheo-SANS). The interaction potential of swollen PNiPAM microgels could be varied from repulsive at temperatures below the lower critical solution temperature to attractive at temperatures above the lower critical solution temperature. In contrast to the case for suspensions of rigid spheres, the effective volume fraction could be changed by means of temperature while the mass concentration and particle number density were kept constant. Thus, aqueous PNiPAM microgels are interesting model systems with unique colloidal properties. Complementary information about shear-induced changes of both the internal particle structure and the overall microstructural phenomena were obtained from rheo-SANS experiments with PNiPAM microgels with different particle sizes. The shear-induced particle arrangements strongly depended on the particle–particle interaction potential. When the interaction potential was repulsive at temperatures below the lower critical solution temperature, no significant deformation of the swollen PNiPAM particles was observed even at high shear rates. Shear-induced ordering was found at high shear rates resulting in the formation of two-dimensional hexagonal close packed layers that aligned along the flow direction giving rise to shear thinning. The formation of sliding hexagonal close packed layers under shear flow is therefore proposed to be a general property of colloidal dispersion independent of the internal structure of the particle. At temperatures near the lower critical solution temperature, when the particle interaction potential is not yet strongly attractive, shear flow induces the collapse of an individual particle in concentrated suspension at high shear rates. A so-called butterfly scattering pattern indicates the shear-induced enhancement of concentration fluctuations along the flow direction leading to solvent being squeezed out of the particles until phase separation occurs finally.

We study the effect of shear flow on homogeneous crystal nucleation, using Brownian dynamics simulations in combination with an umbrella sampling-like technique. The symmetry breaking due to shear results in anisotropic radial distribution functions. The homogeneous shear rate suppresses crystal nucleation and leads to an increase of the size of the critical nucleus. These observations can be described by a simple, phenomenological extension of classical nucleation theory. In addition, we find that nuclei have a preferential orientation with respect to the direction of shear. On average the longest dimension of a nucleus is along the vorticity direction, while the shortest dimension is preferably perpendicular to that and slightly tilted with respect to the gradient direction.

Large, oriented single crystals may be obtained from shear melts of colloidal particles after nucleation at the container walls. We are here interested in the processes occurring during the initial phase of their formation. Using different microscopic and scattering techniques we here studied highly charged suspensions of spherical particles, dispersed in low salt or deionized water, in single and double wall confinement, during and after cessation of shear. While the equilibrium phase of our colloidal solids is body centred cubic, the shear induced precursors of heterogeneous nuclei consist of wall based, oriented, registered or freely sliding layers with in plane hexagonal symmetry. Cessation of shear initiates a complex heterogeneous nucleation process. If the layer structures are space filling, they register to form a meta-stable randomly stacked close packed hexagonal crystal. In double wall confinement the transformation to the equilibrium body centred cubic structure occurs on long timescales via nucleation and subsequent lateral growth. For non-space filling, wall based layer structures we find indications of competition between the decay of the layers in favour of the shear melt and their stabilization through registering and subsequent coverage by an epitaxially growing wall crystal. From quantitative growth curve measurements we obtain the initial wall crystal thickness d₀, which may serve as a lower bound to the extension of the layer structures under shear. We observe a pronounced dependence of d₀ on both former shear conditions and meta-stability of the melt.

We present a progress report on our work on lattice Boltzmann methods for colloidal suspensions. We focus on the treatment of colloidal particles in binary solvents and on the inclusion of thermal noise. For a benchmark problem of colloids sedimenting and becoming trapped by capillary forces at a horizontal interface between two fluids, we discuss the criteria for parameter selection, and address the inevitable compromise between computational resources and simulation accuracy.

We report on novel possibilities for studying colloidal suspensions in a steady shear field in real space. Fluorescence confocal microscopy is combined with the use of a counter-rotating cone–plate shear cell. This allows imaging of individual particles in the bulk of a sheared suspension in a stationary plane. Moreover, this plane of zero velocity can be moved in the velocity gradient direction while keeping the shear rate constant. The colloidal system under study consists of rhodamine labelled PMMA spheres in a nearly density and refractive index matched mixture of cyclohexylbromide and cis-decalin. We show measured flow profiles in both the fluid and the crystalline phase and find indications for shear banding in the case of a sheared crystal. Furthermore, we show that, thanks to the counter-rotating principle of the cone–plate shear cell, a layer of particles in the bulk of a sheared crystalline suspension can be imaged for a prolonged time, with the result that their positions can be tracked.

We present the non-equilibrium phase diagram of rod-like colloids (fd-viruses) under shear flow. The shear-induced displacement of the isotropic–nematic binodal is obtained from time resolved rheology measurements. Vorticity banding is observed within the biphasic region, as bounded by the binodal. Here, in the stationary state, regular, millimetre sized bands with mutually differing orientational order are stacked along the vorticity direction. For the fully nematic phase we determine the location of transition lines from tumbling to either wagging or flow aligning, depending on the concentration. The location of these dynamical transition lines agree with theory for hard rods, when scaling to the orientational order parameter in equilibrium.

The effect of the hydrodynamic interaction on the dynamics of flexible and rod-like polymers in solution is investigated. The solvent is simulated by the multi-particle-collision dynamics (MPCD) algorithm, a mesoscale simulation technique. The dynamics of the solvent is studied and the self-diffusion coefficient is calculated as a function of the mean free path of a particle. At small mean free paths, the hydrodynamic interaction strongly influences the dynamics of the fluid particles. This solvent model is then coupled to a molecular dynamics simulation algorithm. We obtain excellent agreement between our simulation results for a flexible polymer and the predictions of Zimm theory. The study of the translational diffusion coefficient of rod-like polymers confirms the predicted chain-length dependence. In addition, we study the influence of shear on the structural properties of rod-like polymers. For shear rates exceeding the rotational relaxation time, the rod-like molecule aligns with the shear flow, leading to an orientational symmetry breaking transverse to the flow direction. The comparison of the obtained shear rate dependencies with theoretical predictions exhibits significant deviations. The properties of the orientational tensor and the rotational velocity are discussed in detail as a function of shear rate.

We have studied some of the rheological properties of suspensions of hard-sphere colloids with particular reference to behaviour near the concentration of the glass transition. First we monitored the strain on the samples during and after a transient step stress. We find that, at all values of applied step stress, colloidal glasses show a rapid, apparently elastic, recovery of strain after the stress is removed. This recovery is found even in samples which have flowed significantly during stressing. We attribute this behaviour to 'cage elasticity', the recovery of the stress-induced distorted environment of any particle to a more isotropic state when the stress is removed. Second, we monitored the stress as the strain rate of flowing samples was slowly decreased. Suspensions which are glassy at rest show a stress which becomes independent of as . This limiting stress can be interpreted as the yield stress of the glass and agrees well both with the yield stress deduced from the step stress and recovery measurements and that predicted by a recent mode coupling theory of sheared suspensions. Thus, the behaviours under steady shearing and transient step stress both support the idea that colloidal glasses have a finite yield stress. We note however that the samples do exhibit a slow accumulation of strain due to creep at stresses below the yield stress.

The merging process of two amphiphilic cylindrical micelles has been simulated using a coarse grained model in which amphiphiles are represented as chains of one head particle and four tail particles. In our set-up with twisted boundary conditions, a ring-shaped worm is effectively entangled with itself. Upon stretching the box, putting the worms under tension, a fusion into an H-like structure is observed, which eventually transforms into an almost tensionless structure with two freely gliding Y-junctions. The tensions on the worms never reach the point where scission becomes an alternative to fusion. We end with a short discussion of the possible implications of these observations.

Suspensions of solid colloidal particles presenting a large mismatch of magnetic permeability or dielectric permittivity with the suspending fluid are interesting systems to correlate structure and rheology by tuning attractive dipolar forces with the help of an external field. The beginning of phase separation with formation of chainlike structure is experimentally studied in a suspension of silica particles using light scattering and in a suspension of magnetic colloidal polystyrene by means of light transmission. For a silica volume fraction of 10% we find that the mean distance between particles inside chains drops off suddenly with the increase of field. An analysis of evolution of size distribution with the field is attempted but does not show evidence for a strong increase of average sizes of chains with field. We determine the phase diagram of the magnetic suspension by measuring light transmission and show that the experimental critical volume fraction is much lower than that predicted with a theory based on transition in an isotropic medium.

In this work we investigate the experimentally observed electrohydrodynamic instabilities in a confined suspension of lambda-DNA under strong electric fields. We model the underlying stochastic motion of the DNA coils on a coarse grained level, using continuous functions to describe the charge density of the system in space and time. We find, within our approach, that in contrast to previous results there are no long ranged violations of electroneutrality around aggregates of lambda-DNA. We also show that although the corresponding Debye layer is small on the surface of a given aggregate, the electric field can induce a flow field of the solvent which in turn results in a stable pattern of aggregates, which is in agreement with the experimental observations.

We derive and describe in detail a recently proposed method for obtaining Coulomb interactions as the potential of mean force between charges which are dynamically coupled to a local electromagnetic field. We focus on the molecular dynamics version of the method and show that it is intimately related to the Car–Parrinello approach, while being equivalent to solving Maxwell's equations with a freely adjustable speed of light. Unphysical self-energies arise as a result of the lattice interpolation of charges, and are corrected by a subtraction scheme based on the exact lattice Green function. The method can be straightforwardly parallelized using standard domain decomposition. Some preliminary benchmark results are presented.

The electro-hydrodynamic coupling of electrolyte ions and surface-dissociated counterions, i.e., microions, to the motion of a suspended colloidal macroion leads to an additional contribution to the colloidal friction coefficient. On the basis of the primitive model and the generalized Smoluchowski diffusion equation, a simplified mode–mode coupling scheme (MCS) is developed for quantifying the effect of electrolyte friction on the tracer diffusion of a macroion. In this scheme, far-field hydrodynamic interactions between all ionic species are considered. The influence of the finite size of the microions is accounted for by using mean spherical approximation expressions of static pair correlation functions for unequal sizes. The present paper extends earlier work of one of the authors to include the effects of finite-sized and hydrodynamically interacting microions. Our theoretical results are used to test the relevance of finite size effects in suspensions of nano-sized particles such as charged globular micelles. Significant finite size effects are only observed for macroion–microion size ratios typically smaller than 10.

We report on comprehensive measurements of the electrophoretic mobility μ of a highly charged spherical colloid in deionized or low salt aqueous suspensions, where fluid and crystalline order develops with increased packing fraction Φ. We propose the existence of a 'universal' shape of the μ(Φ) showing three distinct regimes: a logarithmic increase, a plateau and a logarithmic decrease. The position and the height of the plateau are found to be influenced by the particle surface properties and the electrolyte concentration. In particular, it starts once the counter-ion concentration becomes equal to the concentration of background electrolyte. This coincides only loosely with the range of Φ where fluid order is developing. Also the better defined first order freezing transition is observed to be uncorrelated to the shape of the μ(Φ) curve.

The Donnan membrane equilibrium, applied to the sedimentation–diffusion (SD) equilibrium for charged colloids in gravity or an ultracentrifuge, yields limiting laws for the SD density profile and the recently predicted macroscopic electric field (van Roij 2003 J. Phys.: Condens. Matter 15 S3569) which necessarily accompanies the colloid density gradient as is also illustrated by the Donnan model. For SD equilibrium in an ultracentrifuge the radial 'Donnan' electric field accounts for the significant apparent mass reduction in thelow-salt limit already discussed by Tiselius (1926 Z. Phys. Chem. 124 449). Several other aspects are discussed, such as the striking analogy between the macroscopic Donnan condenser and the usual colloidal diffuse double layer.

We study the mobility of a charged colloidal particle in a constant homogeneous electric field by means of computer simulations. The simulation method combines a lattice Boltzmann scheme for the fluid with standard Langevin dynamics for the colloidal particle, which is built up from a net of bonded particles forming the surface of the colloid. The coupling between the two subsystems is introduced via friction forces. In addition, explicit counterions, also coupled to the fluid, are present. We observe a non-monotonic dependence of the electrophoretic mobility on the bare colloidal charge. At low surface charge density we observe a linear increase of the mobility with bare charge, whereas at higher charges, where more than half of the ions are co-moving with the colloid, the mobility decreases with increasing bare charge.

Single-file diffusion (SFD), prevalent in many chemical and biological processes, refers to the one-dimensional motion of interacting particles in pores which are so narrow that the mutual passage of particles is excluded. Since the sequence of particles in such a situation remains unaffected over time t, this leads to strong deviations from normal diffusion, e.g. an increase of the particle mean-square-displacement as the square root of t. We present experimental results of the diffusive behaviour of colloidal particles in one-dimensional channels with varying particle density. The channels are realized by means of a scanning optical tweezers. Based on a new analytical approach (Kollmann 2003 Phys. Rev. Lett.90 180602) for SFD, we can predict quantitatively the long-time, diffusive behaviour from the short time density fluctuations in our systems.

We model a system of particles suspended in a viscous fluid circle in optical vortices generated by holographic optical tweezer techniques (Curtis and Grier 2003 Phys. Rev. Lett.90 133901) and show that hydrodynamic interactions between the circling particles determine their collective motion. We perform a linear-stability analysis to investigate the stability of regular particle clusters and illustrate the limit cycle to which the unstable modes converge. We clarify that drafting of particle doublets is essential for the understanding of the limit cycle.

The crystal structure and melting transition of two-dimensional colloids interacting via an anisotropic magnetic dipole–dipole potential are studied. Anisotropy is achieved by tilting the external magnetic field inducing the dipole moments of the colloidal particles away from the direction perpendicular to the particle plane. We find a centred rectangular lattice and a two-step melting similar to the phase transitions of the corresponding isotropic crystals via a quasi-hexatic phase. The latter is broadened compared to the hexatic phase for isotropic interaction potential due to strengthening of orientational order.

Two-dimensional super-paramagnetic suspensions that are confined to a planar liquid–gas interface and exposed to an inhomogeneous external magnetic field directed perpendicular to the interface are studied by extensive Monte Carlo computer simulations. The external field is a superposition of a homogeneous field and a localized inhomogeneity, modelled by a Gaussian function. The inhomogeneity causes two combined effects that compete against each other: it provides an external potential, modifying at the same time the mutual interparticle repulsion. If the inhomogeneity enhances the strength of the homogeneous profile, the inhomogeneous field is a 'magnetic tweezer' for low particle densities. At higher densities, on the other hand, there is a small accumulation in the centre of the inhomogeneous field, which leads to a depletion zone outside the inhomogeneity due to the mutual interparticle repulsion. Very large inhomogeneities produce local crystallites surrounded by a depletion ring. If the inhomogeneity reduces the total field strength, particles are repelled from the inhomogeneity and voids are generated in the suspension. Our predictions are of relevance to the direct transport of magnetic particles and can be verified in real-space experiments of super-paramagnetic suspensions.

The nature of the melting transition for a system of hard discs with translational degrees of freedom in two spatial dimensions has been analysed by a combination of computer simulation methods and a finite size scaling technique. The behaviour of the system is consistent with the predictions of the Kosterlitz–Thouless–Halperin–Nelson–Young (KTHNY) theory.

The structural and elastic properties of binary colloidal mixtures in two and three spatial dimensions are discussed as well as those of colloidal systems with quenched point impurities.

Hard and soft discs in external periodic (light-) fields show rich phase diagrams including freezing and melting transitions when the density of the system is varied. Monte Carlo simulations for detailed finite size scaling analysis of various thermodynamic quantities like the order parameter, its cumulants, etc, have been used in order to map the phase diagram of the system for various values of the density and the amplitude of the external potential. For hard discs we find clear indication of a reentrant liquid phase over a significant region of the parameter space. The simulations therefore show that the system of hard discs behaves in a fashion similar to charge stabilized colloids which are known to undergo an initial freezing, followed by a remelting transition as the amplitude of the imposed modulating field produced by crossed laser beams is steadily increased. Detailed analysis of the simulation data shows several features consistent with a recent dislocation unbinding theory of laser induced melting. The differences and similarities of systems with soft potentials (DLVO, 1/r¹², 1/r⁶) and the relation to experimental data is analysed.

We report on the preparation and characterization of highly birefringent, monodisperse colloidal particles with sizes between 100 nm and some micrometres made by emulsification of a reactive acrylate monomer in aqueous solution. Photopolymerization of the emulsion droplets in the liquid crystalline state results in particles with frozen orientational order. Particles that had not been polymerized have a higher effective birefringence than the polymerized particles at room temperature, as shown by measurements of the depolarized scattering intensity using quasi-elastic light scattering (QELS). We also present preliminary results showing that larger particles can be made to rotate with optical tweezers with circular polarization.

Charge-stabilized colloidal spheres dispersed in weak 1:1 electrolytes are supposed to repel each other. Consequently, experimental evidence for anomalous long-ranged like-charged attractions induced by geometric confinement inspired a burst of activity. This has largely subsided because of nagging doubts regarding the experiments' reliability and interpretation. We describe a new class of thermodynamically self-consistent colloidal interaction measurements that confirm the appearance of pairwise attractions among colloidal spheres confined by one or two bounding walls. In addition to supporting previous claims for this as-yet unexplained effect, these measurements also cast new light on its mechanism.

Using density functional theory and Monte Carlo simulations we investigate the Asakura–Oosawa–Vrij mixture of hard sphere colloids and non-adsorbing ideal polymers under selective confinement of the colloids to a planar slab geometry. This is a model for confinement of colloid–polymer mixtures by either two parallel walls with a semi-permeable polymer coating or through the use of laser tweezers. We find that such a pore favours the colloidal gas over the colloidal liquid phase and induces capillary evaporation. A treatment based on the Kelvin equation gives a good account of the location of the capillary binodal for large slit widths. The colloid density profile is found to exhibit a minimum (maximum) at contact with the wall for large (small) slit widths.

We consider the Asakura–Oosawa–Vrij model of hard sphere colloids and ideal polymer coils in contact with a planar hard wall at (colloidal) liquid–gas coexistence. Using extensive numerical density functional calculations, the liquid–gas, wall–liquid and wall–gas interfacial free energies are calculated. The results are inserted into Young's equation to obtain the contact angle between the liquid–gas interface and the wall. As a function of polymer fugacity this angle exhibits discontinuities of slope ('kinks') upon crossing first-order surface phase transitions located on the gas branch of the bulk binodal. Each kink corresponds to a transition from n−1 to n colloid layers adsorbed at the wall, referred to as the nth layering transition. The corresponding adsorption spinodal points from n−1 to n layers upon reducing the polymer fugacity along the bulk binodal were found in a previous study (Brader et al 2002 J. Phys.: Condens. Matter. 14 L1; Brader et al 2003 Mol. Phys. 101 3349). Remarkably, we find desorption spinodal points from n to n−1 layers to be absent upon increasing polymer fugacity at bulk coexistence, and many branches (containing up to seven colloid layers) remain metastable. Results for the first layering binodal and both spinodal branches off bulk coexistence hint at a topology of the surface phase diagram consistent with these findings. Both the order of the transition to complete wetting and whether it is preceded by a finite or an infinite number of layering transitions remain open questions. We compare the locations of the first layering binodal line and of the second layering binodal point at bulk coexistence with recent computer simulation results by Dijkstra and van Roij (2002 Phys. Rev. Lett. 89 208303) and discuss our results for the contact angle in the light of recent experiments.

After a brief review of the theory of sedimentation equilibria of colloidal systems, we consider the specific case of binary systems of hard sphere colloids and non-interacting polymer coils, the latter of vanishing buoyancy mass. The density profiles of the two components are calculated within density functional theory and using Monte Carlo simulations. Under appropriate conditions the profiles exhibit discontinuities or steeply varying regions associated with the interface separating colloid-rich and colloid-poor phases. The position of the interface is shown to be very sensitive to the strength of the gravitational field and, more surprisingly, to the total height L of the suspension. Phase coexistence in the absence of gravity is shown to be entirely suppressed beyond a critical ratio of the height L over the gravitational length of the colloids.

In the presence of a thermal gradient, macromolecular solutes or dispersed particles drift to the cold or to the hot side: this effect is known as thermophoresis, and is the counterpart of particle suspensions of the Soret effect (or thermal diffusion) in simple fluid mixtures. Here I review recent experimental results on colloid thermophoresis and present new data suggesting a universal nature for the temperature dependence of thermophoresis in aqueous systems. There are strong analogies between thermophoresis in liquids and other thermally induced flow processes like gas thermal creep and membrane thermo-osmosis; starting from these, I present some guidelines for a general model of thermophoresis in disperse systems, accounting both for single-particle and collective effects.

We have studied the effect of small amounts of liquid on the dynamic behaviour of a granular system consisting of spherical glass beads under vertical agitation. The critical acceleration Γ_crit at which fluidization sets in is found to increase strongly when liquid is added. It furthermore depends upon sample parameters such as the bead size and the shaking frequency. A simple model is proposed which accounts quantitatively for our results and suggests that fluidization by vertical agitation is mainly a surface phenomenon.

The sedimentation dynamics of extremely low polydispersity, non-colloidal, particles are studied in a liquid fluidized bed at low Reynolds number, . When fluidized, the system reaches a steady state, defined where the local average volume fraction does not vary in time. In the steady state, the velocity fluctuations and the particle concentrations are found to strongly depend on height in the particle column. Using our results, we test a recently developed stability model (Segrè 2002 Phys. Rev. Lett. 89 254503) for steady state sedimentation. The model describes the data well, and shows that in the steady state there is a balancing of particle fluxes due to the fluctuations and the concentration gradient. Some results are also presented for the dependence of the concentration gradient in fluidized beds on particle size; the gradients become smaller as the particles become larger and fewer in number.

We study gas–liquid phase separating colloid–polymer mixtures using a horizontally placed confocal scanning laser microscope. The phase separation proceeds via spinodal decomposition; first images immediately show sharp interfaces, which is explained in terms of the colloid diffusion time. The diffusion in both the liquid and gas phase is measured in a real space fluorescence recovery after a photo-bleaching experiment. The coarsening rate of the characteristic length in the system can be understood in terms of the capillary velocity. We observe that the spinodal structure collapses due to gravity at a typical size of the order of the capillary length, which is obtained from the static gas–liquid profile near a single wall and is accurately described by the interplay between hydrostatic and Laplace pressure. The present technique allows for precise contact angle measurements and the system shows complete wetting for all statepoints measured. Finally, we study the possibility of capillary condensation in colloid–polymer mixtures and show first indicative experimental results. The observed Kelvin length is surprisingly large, possibly because the system is not yet in complete equilibrium.