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Silica photonic colloidal crystals on a flat glass substrate have been grown using the depletion attraction induced by nonadsorbing polymers. The dried samples have been characterised with high-resolution synchrotron small-angle X-ray diffraction and their surface topography has been studied by atomic force microscopy. The presence of long-range ordered crystals with a thickness as large as 90 planes is demonstrated. However, on some places the films are polycrystalline and contain disordered regions.

Recently, Grassberger [1997, Phys. Rev. E, 56, 3682] has presented a new algorithm ('PERM') for simulating flexible polymer chains. This algorithm has been shown to have a good efficiency and has been used in a wide class of systems. A drawback of this algorithm is that it is static: it is therefore not suited for Markov-chain Monte Carlo simulations. Here, we present a dynamic generalization of the PERM algorithm. For a specific example, we compare the efficiency of DPERM to that of other Monte Carlo algorithms. In the case studied, we find that DPERM is only marginally more efficient. However, this result may depend on the details of the implementation.

Suspensions of sterically stabilized colloidal gibbsite platelets have recently been found to exhibit both an isotropic-nematic (I-N) and a nematic-columnar (N-C) phase transition. In the present paper we show that depletion attraction, brought about by the addition of nonadsorbing polymer, enriches the phase behavior of these platelet suspensions even further. Pronounced broadening of the isotropic-nematic gap occurs, with purely nematic samples re-entering the biphasic state by the addition of nonadsorbing polymer. At the same time, depletion attraction enhances size fractionation between coexisting phases, which actually provides an effective means for reducing the polydispersity of the suspensions. An additional isotropic phase emerges, which leads to the appearance of several three-phase equilibria and even a four-phase equilibrium. We explain the observed topology of the phase diagram by the interplay between depletion attraction and the platelets' polydispersity.

Assemblies of Polyoxometalates Polyoxometalates, or POMs for short, come in a variety of shapes and sizes. Some of them are among the largest inorganic molecules known today. These molecules can be highly symmetrical and, as the name already implies, consist of (mainly) metal (molybdenum, tungsten, vanadium, iron, etc) and oxygen atoms in the form of metal oxide polyhedrons. Although individual POM molecules are in itself already a very challenging field of research, assemblies of POMs have great challenges of their own. Take for instance the solution behavior of a subclass of POMs known as Keplerates. Keplerates are highly symmetric, hollow, spherically shaped POMs. In solution they spontaneously assemble into large, hollow, spherical superstructures or POM-shells. These POM-shells have an average radius in water of several tens of nanometer and are composed of a monolayer of more than a thousand of individual POMs. Although the underlying principles behind this spontaneous assembly are still not precisely known, much can already be understood about this curious phenomenon. For instance, the equilibrium size of POM-shells of Keplerate type POMs is inversely proportional to the relative dielectric constant of the medium in which they are dispersed. This behavior is in line with predictions made by a stabilization mechanism based on Coulomb repulsion combined with charge regulation. This model also explains a new structural instability of the POM-shells. Besides the colloidal instability, i.e. the formation of aggregates that consist of many single layered shells, these systems also display an instability on a structural scale within the shell-like assemblies when the ionic strength reaches a critical value. Not only the ionic strength has an influence on the properties of POM-shells in solution. It has been claimed that the number of charges on the POMs themselves plays an important roll in their formation and stabilization as well. For instance, for the Keplerate {Mo72Fe30} experiments show that the interaction energy between the POMs in the shell becomes stronger when the charge density on the POMs increases. This interaction energy can also be estimated from analysis based on the charge regulation model in combination with a model for defects on a sphere. The charge regulation model therefore gives much insight into the behavior of the POM-shells. Up till now the POM-shells have been considered thermodynamically stable. Our experimental results however, challenge this view. It is shown that POM-shells change over time and that the preparation route of solutions of POMs affects the resulting species in solution. Moreover, in concentrated samples, a transition from spherical objects to elongated agglomerates was observed. The elongated objects subsequently grow into large, crystalline, needle-like structures. From these observations it is concluded that POMs follow an unusual nucleation route in which the POM-shells are in fact meta-stable intermediates. The term assembly can also be taken in a more general way. For instance polyoxomolybdates can be used to create extended structures. By using elementary molybdenum oxide building blocks and urea in a one-pot directed synthesis, an inorganic-organic hybrid extended structure, consisting of right- and left-handed helical units can be obtained.

The impact of food industry on societal aspects grows dynamically, thereby posing new challenges for food developers to comply to consumer demands. Consumers expect new and more sophisticated products that are tuned to their very specific needs, like health and life-style. This means that food products must be fully programmable to meet these market demands. The taste, smell, and texture perception of food are the most important properties that influence the consumer’s experience. To provide control over these features the physics behind them during oral manipulation and structure breakdown must be understood. Most desirably, the physical properties can be tightly linked to attributes of oral perception. In this thesis the texture of semi-solid and liquid (model) food is studied using Quantitative Descriptive Analysis. Despite the quantification of sensory scores, it remains difficult to translate these into food physical characteristics in an absolute mode. For that reason, in parallel a physical study is performed using mainly tribology to provide quantitative description of the mechanical processes in the oral environment. Therefore, the experimental setup is adapted such to mimic oral conditions by employing soft surfaces. Next to more complex commercial products, like milk, the samples studied include aqueous dispersions of protein aggregates or polysaccharides, and emulsion filled gels. These components may contribute to fat-feel in low or no-fat dairy products and provide a major uncertainty in predicting the sensorial restraints of the product. In this thesis we investigate how various factors influence the friction of above-mentioned systems and how this friction can be understood at a more fundamental level. In addition, the surface effects are studied in detail to determine how sensitive the choice of the oral surface analog is on the friction and what the most important surface properties are that are relevant to better predict sensory attributes. In this thesis we present an overview of correlations obtained between different sensory attributes and frictional data. We show that oral perception can be translated into physical quantities using friction. The best correlations are obtained for physical conditions that resemble most the oral environment, i.e. speed below 50 mm/s, shear rate of about 50 s-1 and soft, rough surface. For instance a low friction coefficient correlates well with the creamy perception of the consumer. This finding may contribute to develop healthy foods with a full fat creamy taste. Moreover, samples with low friction coefficient can be perceived as soft and velvety, while high friction results in rough texture perception. Some attributes, like filmy or slimy, correlate better with viscosity, as their perception is determined by the bulk properties of a sample rather than interaction with oral surfaces. In a few cases the correlation with friction coefficient could not be established, e.g. for sticky or powdery attributes. The latter one, however, shows a good correlation with particles size if they are present in the solution. This thesis presents a comprehensive picture of the relation between physical quantities and sensory attributes and provides a step forward towards a better control over oral perception of food.

We study colloidal gels formed upon centrifugation of dilute suspensions of spherical colloids (radius 446 nm) that interact through a long-range electrostatic repulsion (Debye length ≈ 850 nm) and a short-range depletion attraction (∼12.5 nm), by means of confocal scanning laser microscopy (CSLM). In these systems, at low colloid densities, colloidal clusters are stable. Upon increasing the density by centrifugation, at different stages of cluster formation, we show that colloidal gels are formed that significantly differ in structure. While significant single-particle displacements do not occur on the hour time scale, the different gels slowly evolve within several weeks to a similar structure that is at least stable for over a year. Furthermore, while reference systems without long-range repulsion collapse into dense glassy states, the repulsive colloidal gels are able to support external stress in the form of a centrifugal field of at least 9g.

Biaxial nematic and biaxial smectic phases were found in a colloidal model system of goethite ( -FeOOH) particles with a simple boardlike shape and short-range repulsive interaction. The macroscopic domains were oriented by a magnetic field and their structure was revealed by small angle x-ray scattering. In accordance with theoretical predictions, biaxiality appears in a system with particles that have a shape almost exactly in between rodlike and platelike. Our results suggest that biaxial phases can be readily obtained by a proper choice of the particle shape.

We report an analytical ultracentrifugation study on sedimentation in dilute stable dispersions of uniform, magnetic iron oxide (Fe3O4) colloids. On increase of the dipolar coupling constant, tuned by the average particle size, the linear concentration dependence of the sedimentation velocity shows an abrupt transition from the hindered sedimentation expected for hard spheres to a marked acceleration already for weak dipolar interactions. This transition is not reproduced by sedimentation theory derived from an effective, isotropic pair correlation function of the type proposed by De Gennes and Pincus, for reasons which are made clear. Accelerated settling, instead, follows a scaling based on the mass action law for dimer formation with dipoles in head-to-tail configuration. Our work illustrates that orientational averaging of dipole interactions of ferromagnetic colloids in the dilute regime is inapplicable, with an obvious implication for the possible existence of isotropic gas-liquid criticality for such colloids.

Understanding the formation processes of zeolites is essential if a more “rational” design principle, leading to more catalytically useful materials, is to be achieved. Optically clear “model” solutions allow for significant insight, while systematic variation of selected experimental conditions can reveal additional detail of these complex processes. In this work we have studied the synthesis of zeolite A from clear solutions and compared the effect of isomorphous germanium substitution on this process. To follow the entire process, a combination of in situ time-resolved small-angle X-ray scattering, wide-angle X-ray scattering, and dynamic light scattering was used. These techniques indicate a mechanism similar to that observed for silicalite-1 with the formation of amorphous nanoparticles that become increasingly stable until growth continues by addition to aggregates. Additionally, by comparison between the unsubstituted and germanium-substituted materials, we have identified the effect of metal substitution and have shown how it has a significant influence over the entire formation process, from the formation and stabilization of the initial nanoparticles to the type and rate of crystallization and the final crystal morphology. This study therefore demonstrates the importance of metal substitution effects for controlling the rational design of zeolites and indicates that a greater understanding of these effects is required to fully understand these systems.

A magnetorheological fluid consisting of colloidal silica spheres suspended in an organic ferrofluid is described. Its linear viscoelastic behavior as a function of frequency, magnetic field strength, and silica volume fraction was investigated with a specially designed magnetorheometer. The storage modulus G' is at least an order of magnitude larger than the loss modulus G" at all magnetic field strengths investigated. G' does depend only weakly on frequency, and linearly on volume fraction. A model is presented for the high frequency limit of the storage modulus G'(infinity). In the model our system is treated as a collection of single noninteracting chains of particles. Assuming a dipolar magnetic interaction, theory and experiment show reasonable agreement at high frequencies.

This thesis reports results from a study of particle dynamics in colloidal glasses in the absence and presence of a gravitational field. It also investigates the reentrant glass transition. All the experimental results are obtained after a real space analysis using a confocal scanning laser microscopy as experimental technique that significantly widens the time window over which colloidal systems can be studied. A fluorescent recovery of bleached regions in concentrated suspensions of fluorescent colloidal hard spheres was followed in real space. This method provided data for mean squared particle displacements up to time scales that are three orders of magnitude beyond those available by present experimental techniques like dynamic or static light scattering. It was shown that, above the (hard sphere) glass transition density, particles move over distances on the order of their own diameter on time scales of 106 to 108 Brownian times. Moreover, the mean squared displacement, , showed power-law behavior over seven time (?) decades: ??^ (0.30?0.05). This behavior is different from earlier observations by dynamic light scattering. It was argued that these differences are caused by gravity effects. Further on, the influence of gravity on the long-time behavior of the mean squared displacement in glasses of colloidal hard spheres was studied. For the first time, a significant influence of gravity on the mean squared displacements of the particles was presented. In particular, the systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) showed anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. It was shown that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explained the observation that colloidal hard sphere systems which are a glass on earth rapidly crystallize in space. A quantitative analysis of the structure and dynamics of concentrated suspensions of colloids in which the magnitude of the short range attractive potential was increased by adding non-adsorbing polymers was done. These systems undergo a reentrant glass transition upon increasing polymer concentration. The melting of the glass is accompanied by significant changes in the displacement distribution and its moments. However, no significant variations were detected in the shapes of the displacement distributions. Moreover, structural correlation functions and the magnitude of local density fluctuations did not vary significantly between the glass states and the fluid. The influence of gravity on long-time diffusion in glasses was also studied using a new experimental setup that allowed a real space determination of particle displacements in directions parallel and perpendicular to the gravity field. Dispersions of particles were prepared in different solvent combinations that provide different gravitational lengths. It was shown that the direction of the external gravitational field couples to the particle dynamics and influences the values of mean square particle displacements.

This thesis is made up of several studies of boundaries occurring in colloidal hard sphere crystals and phase separated colloid-polymer mixtures. These boundaries can be studied on the particle level, in real space and in real time by confocal microscopy. A general introduction on the experimental systems and on confocal microscopy is given in Chapter 1. This thesis consists of four main parts. The first part deals with stacking disorder in hard sphere crystals. The second part deals with geometrically frustrated hard sphere crystals. The third part describes the statistics of Brownian interface fluctuations in phase-separated colloid-polymer mixtures. The final part of this thesis deals with the transport of particles through such an interface. In the first part of this thesis we study stacking disorder in hard sphere crystals. Due to the small difference in free energy between the hexagonal close packed (HCP) and face centered cubic (FCC) crystal configurations, both structures can coexist within these crystals. As a result the crystals are often made up of a random combination of ABA and ABC type stacking sequences of hexagonal layers. Chapters 2 and 3 describe in-plane stacking disorder in hard sphere crystals. Here, we show that stacking can change as well within a hexagonal layer, both via abrupt transitions through line-defects (Chapter 2) and via continuous transitions through lattice deformations (Chapter 3). Therefore the hexagonal layers are made up of multiple islands of FCC or HCP stacking type. In Chapter 3 the relation between lattice vacancies and stacking transitions is investigated as well. Due to the osmotic pressure imbalance, particles are pushed slightly toward the vacancy. Furthermore, in areas where lattice deformations occur due to stacking transitions, the vacancy concentration is much higher, allowing the accommodation of lattice deformations. Chapter 4 describes the relation between grain size and typical stacking island dimension. Using methods to distinguish both the stacking type and the stacking direction, we are able to identify the typical island dimension. The fraction of FCC type particles determines the relative size of FCC and HCP type islands, which we can relate to lateral islands with an A, B or C type lateral position through simulations. The second part of this thesis describes how geometry affects hard sphere crystals and crystallization. Chapter 5 describes the crystallization of hard spheres near large, nearly immovable objects: much larger spheres, which are in a way the simplest conceivable impurities or dopants. Crystal nucleation, the initial formation of sufficiently large crystals may be facilitated if the curvature of the dopant is sufficiently low. The subsequent crystal growth of these small crystals is retarded by the presence of impurities, but due to their high curvature, small impurities slow the crystal growth much more down than larger ones. Chapter 6 deals with the obtained crystal structure after crystallization has completed. We introduce a frustration length which quantifies the extent of lattice distortion depending on the size of the dopants. If the dopants are sufficiently close, we show that grain boundaries may form in between directly, which is a result of the delayed growth process in between the impurities. Whether the frustration lengths persist in the sample or slowly anneal out on time depends on the impurity spacing. Chapter 7 compares lattice frustration by two methods: 1) the insertion of impurities and 2) a polygonal particle shape. Both sources of lattice frustration induce polycrystallinity. The third part of this thesis concerns the liquid-liquid interface of phase separated colloid polymer mixtures. Such interfaces are characterised by large interface fluctuations due to the low interfacial tension. Experiments and theory on the residence time of a certain fluctuation above a certain height and the related waiting time in between such heights are presented in Chapter 8. In the final part of this thesis the transport of rigid spheres through such a fluctuating interface is presented. In Chapter 9 we compare the approach of droplets and spheres to a deformable interface. The similarity of the problems shows that the approach of the sphere already captures much of the physics of the more complicated approach of a droplet. The transport of rigid spheres through interfaces is the subject of Chapters 10 and 11. We distinguish between low Bond numbers (Chapter 10) and high bond numbers (Chapter 11), which respectively represent the limits of dominant interfacial tension and gravity. The low Bond number transport configuration is characterized by a draining film in between the sphere and the interface. In the meantime the interface deforms up to maximally about half the sphere's diameter. When the sphere is sufficiently close, it is wetted by the interface and dragged through it, before it starts sedimenting away again. The high Bond number transport configuration is characterized by a thin film of gas which persists around the sphere, while the interface deforms more than half a sphere's diameter. Depending on the Bond number, two scenarios may be observed. At moderate Bond numbers the sphere leaves a V-shaped interface behind. At even higher Bond numbers, the sphere drags a column of material of the phase it was originally in behind it that eventually breaks up through a Rayleigh-Plateau instability.

This thesis deals with the random packing of colloids and granular matter. A random packing is a stable disordered collection of touching particles, without long-range positional and orientational order. Experimental random packings of particles with the same shape but made of different materials show large similarities, independent of particle size, indicating that particle shape is an important factor for their packing. The particle shapes that have been investigated are spheres, spherocylinders, spheroids and cut spheres. It was found that the packing density increases upon slightly deviating from spherical shape and for high aspect ratio the density is inversely proportional to the aspect ratio of the particles. The random contact equation for thin rod packings has been reproduced with a newly developed simulation method and contact numbers calculated agree well with experimental results. Packings of cut spheres were investigated to study the influence of a flat face on the packing structure. At aspect ratio two the flat face becomes important and order is induced by the flat face for higher aspect ratios resulting in columnar structures. Furthermore, the jamming of random packings was studied with the so-called caging number. The caging number is the average minimum number of neighbouring particles that need to be placed at random in fixed contact with a central particle such that all translations and rotations of the particle are blocked. The caging number was calculated analytically for 2-dimensional disks with varying radii and it was numerically calculated for spheres and spherocylinders. Contact numbers were used to construct geometric ensemble average of clusters of spheres to model random loose and random close packings of spheres. A lower bound on volume fraction for random loose packing was found that is surprisingly close to the freezing volume fraction for hard spheres. The ensemble analysis also highlights the importance of collective and global effects in random sphere packings by comparing clusters generated via local rules to random sphere packings and clusters that do include collective effects. Finally, some preliminary results are presented that show the possible relevance of random rod packings to the motility of certain biological cells.

In this paper we describe the use of columnar liquid crystals of silica coated colloidal gibbsite platelets as templates for the generation of ordered silica structures. The colloidal liquid crystal phases can be formed in a few hours by modest centrifugation forces (100–200g). Base catalysed hydrolysis of tetraethoxysilane (TEOS) is used to deposit silica in the space between the silica coated gibbsite particles. The size of the gibbsite platelets is varied between 200 and 500 nm and the thickness of the silica coating between 5 and 15 nm making the system a very versatile templating material. Acid leaching is used to selectively remove the gibbsite material.

Rheological, optical and structural properties of colloidal suspensions of charge-stabilized gibbsite platelets across the sol–gel transition region are investigated. In this work we focus on samples with a low salt content (10−4 M). While at a gibbsite concentration of 300 g l−1, a nematic–columnar phase separation is observed, an arrested state with a nematic signature and highly elastic response has been observed for a concentration of 400 g l−1. A temporal evolution of the structure of the arrested state, which leads to stronger interparticle correlations, has been observed on a timescale of 20 months. The results suggest that the arrested state develops into a glass with a columnar nematic structure.

The effect of fractionation on the phase behavior of colloidal goethite dispersions with different polydispersities (17%, 35%, and 55% in length) has been studied by small angle x-ray scattering and transmission electron microscopy. All systems show at least nematic and smectic phases. The occurrence of the latter phase at such a high polydispersity is remarkable. It is shown that in the highly polydisperse systems strong fractionation occurs, which is able to reduce the local length polydispersity up to a factor of 2. A columnar phase was only found in the 35% and 55% polydisperse systems. It seems that the columnar phase accommodates the particles that do not fit into the smectic layers and, thus, reduces the length polydispersity within the smectic phase even further. The fact that a columnar phase was not found in the system of lowest polydispersity indicates that the smectic phase is the most stable phase at higher concentrations.

Glycerol is an attractive renewable building block for the synthesis of di- and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. In this work, the selective etherification of glycerol to di- and triglycerol was studied in the presence of alkaline earth metal oxides and the data are compared with those obtained with Na2CO3 as a homogeneous catalyst. It was found that glycerol conversion increased with increasing catalyst basicity; that is, the conversion increases in the order: MgO90 % at 60 % conversion) are obtained over CaO, SrO, and BaO. For these catalysts no substantial acrolein formation was observed. Furthermore, at the start of the reaction mainly linear diglycerol was produced, whereas at higher conversion degrees branched diglycerol started to form. In another series of experiments different types of CaO materials were prepared. It was found that these CaO-based materials not only differed in their surface area and number of basic sites, but also in their Lewis acid strength. Within this series the CaO material possessing the strongest Lewis acid sites had the highest catalytic activity, comparable to that of BaO, pointing towards the important role of Lewis acidity for this etherification reaction. Based on these observations a plausible alternative reaction scheme for glycerol etherification is presented, which considers the facilitation of the hydroxyl leaving process. Finally, the stability of the catalytic solids under study was investigated and it was found that colloidal CaO particles of about 50-100 nm can be spontaneously generated during reaction. Catalytic testing of these CaO colloids, after isolation from the reaction medium, revealed a very high etherification activity. Understanding the nature of these Ca-based colloids opens new opportunities for investigating supported colloidal particle catalysts to take advantage of both their hetero- and homogeneous nature.

The behaviour of the different liquid crystalline phases observed in colloidal dispersions of goethite has been studied by small angle x-ray scattering. The particles align parallel to a small magnetic field and perpendicular to a large magnetic field, which was observed in the isotropic and nematic phase. The smectic phase also aligned parallel to a small magnetic field but it formed an aligned columnar phase with a distorted hexagonal structure in higher magnetic fields. The columnar phase mainly aligned in higher magnetic fields. Goethite particles were successfully modified with chromium. The Cr-modified goethite particles show similar phase behaviour as normal goethite particles but with different magnetic properties. The relative importance of the permanent and induced magnetic moment was found to have changed.

We report on the residence times of capillary waves above a given height h and on the typical waiting time in between such fluctuations. The measurements were made on phase-separated colloid-polymer systems by laser scanning confocal microscopy. Due to the Brownian character of the process, the stochastics vary with the chosen measurement interval Δt. In experiments, the discrete scanning times are a practical cutoff and we are able to measure the waiting time as a function of this cutoff. The measurement interval dependence of the observed waiting and residence times turns out to be solely determined by the time-dependent height-height correlation function g(t). We find excellent agreement with the theory presented here along with the experiments.

Mixtures of colloids and polymers display a rich phase behavior, involving colloidal gas (rich in polymer, poor in colloid), colloidal liquid (poor in polymer, rich in colloid) and colloidal crystal phases (poor in polymer, highly ordered colloids). Recently, the colloidal gas-colloidal liquid interface received considerable attention as well. Due to the colloidal length scale the interfacial tension is much lower than in the atomic or molecular analog (nN/m instead of mN/m). This ultra-low interfacial tension has pronounced effects on the kinetics of phase separation, the colloidal gas-liquid profile near a single wall and the thermally induced fluctuations of the interface. The amplitudes of these thermally excited capillary waves are restrained by the interfacial tension and are for that reason of the order of the particle diameter. Therefore, in molecular systems, the capillary waves can only be seen indirectly in scattering experiments. In colloidal systems, however, the wave amplitudes are on a (sub) micrometer scale. This fact enables the direct observation of capillary waves in both real space and real time using confocal scanning laser microscopy. Moreover, the real space technique enables us to demonstrate the strong influence of interface fluctuations on droplet coalescence and droplet break up.

A sol-gel-derived microporous ceramic membrane with an exceptionally low permeability for CO2 from gaseous streams was developed and characterized. The sols were prepared from a mixture of niobium and silicon alkoxide precursors by acid-catalyzed synthesis. Microporous films were formed by coating asymmetric -alumina disks with the polymeric sol (Si/Nb=3:1), followed by calcination at 500 °C. The membrane consists of a 150-nm-thick layer with a Si/Nb atomic ratio of about 1.5. The single-gas permeance of small gas molecules such as H2, CH4, N2, and SF6 decreases steadily with kinetic diameter. Hydrogen, helium, and carbon dioxide follow an activated transport mechanism through the membrane. The permeance of CO2 in this membrane is much lower than that in pure silica, and its behavior deviates strongly from the general trend observed with the other gases. This is attributed to a relatively strong interaction between CO2 and adsorption sites in the niobia-silica membrane.

Stokes drag on the (sub)micrometre scale plays a key role in phenomena ranging from Brownian motion to the rheology of particulate suspensions. We report the first measurement of the direction dependent Stokes drag in a nematic liquid crystal of colloidal rods, where the viscous forces are of equal importance to the elastic forces. By tracking a sedimenting sphere with combined fluorescence confocal microscopy and polarization microscopy we find that the Stokes drag for motion along the director is two times larger than for motion perpendicular to the director. This brings the unique viscoelastic properties of a colloidal liquid crystal into focus.

Recently, we have shown how to measure thermodynamic properties of colloidal hard sphere suspensions by microscopy [Dullens et al. (2006) PNAS 103, 529]. Here, we give full experimental details on how to acquire three dimensional snapshots of a colloidal hard sphere suspension over a wide range of densities by means of confocal laser scanning microscopy. Furthermore, we elaborate on the analysis of the data sets, in which we measure the available volume to insert an additional sphere and the surface area of that volume. These geometrical properties are related to key thermodynamic quantities by statistical geometry. This enables us to measure in a direct and non-interfering way the pressure, the chemical potential and the free energy density of a hard sphere suspension, which are in good agreement with theoretical predictions.

In this work, we show strong experimental evidence in favor of a proposed incorporation mechanism of hydrophobic semiconductor nanocrystals (or quantum dots, QDs) in monodisperse silica spheres (diameter ∼35 nm) by a water-in-oil (W/O) reverse microemulsion synthesis. Fluorescence spectroscopy is used to investigate the rapid ligand exchange that takes place at the QD surface upon addition of the various synthesis reactants. It is found that hydrolyzed TEOS has a high affinity for the QD surface and replaces the hydrophobic amine ligands, which enables the transfer of the QDs to the hydrophilic interior of the micelles where silica growth takes place. By hindering the ligand exchange using stronger binding thiol ligands, the position of the incorporated QDs can be controlled from centered to off-center and eventually to the surface of the silica spheres. The proposed incorporation mechanism explains how we can have high control over the incorporation of single QDs exactly in the middle of silica spheres. It is likely that the proposed mechanism also applies to the incorporation of other hydrophobic nanocrystals in silica using the same method. In conjunction with our findings, we were able to make QD/silica particles with an unprecedented quantum efficiency of 35%.

The controlled fabrication of submicrometer phase-separated morphologies of semiconducting organic materials is attracting considerable interest, for example, in emerging thin-film optoelectronic device applications. For thin films of spin-coated blends of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and cationic cyanine dyes, we used atomic force microscopy scans to infer the structure formation mechanism: The solutions separate into transient bilayers, which further spinodally destabilize because of long-range molecular interactions. A thin layer ruptures earlier than a thick layer, and the earlier instability determines the morphology. Consequently, the resulting morphology type mainly depends on the ratio of the layer thicknesses, whereas the periodicity of structures is determined by the absolute film thickness. These findings allow control of the feature sizes, and nodular domains with diameters well below 50 nm were produced. Films prepared with dyes possessing a mobile counterion were always unstable. To rationalize the findings, we developed a thermodynamic model showing that electrostatic forces induced by the mobile counterions act as destabilizing pressure.

The polyoxometalates or POMs (clusters comprising at least two metal and many oxygen atoms) have recently gained significant interest owing to their versatile architecture and especially their catalytic activities. Due to their high catalytic activity but low surface area, there is always a demand for making high surface area POMs. This work demonstrates the attachment of the anionic {Mo72Fe30} POMs to gibbsite nanoplatelets with a residual positive charge to form large surface area composites. The resulting composite reported here has been characterized using cryo-TEM imaging, EDX/STEM (elemental) analysis, ATR-IR spectroscopy, SAXS, electrophoretic mobility determination and XRD. The composite reported here could find application in catalysis.

We report on a study of the influence of a magnetic field on the nematic phase and the isotropic-to-nematic (IN) phase transition of hard colloidal gibbsite platelets. We find direct visual evidence of a shift of the IN transition to lower concentrations due to the magnetic field. The nematic phase readily aligns when contained between two parallel flat glass walls, with homeotropic orientation. This well-defined surface anchoring enables a measurement of the bend Frederiks transition, yielding the bend elastic constant of a nematic phase of hard disks as K3=7×10−14 N. By applying a rotating magnetic field on the nematic phase, we observe the bend-splay Frederiks transition, visible as a spatially periodic transient pattern with a wavelength that depends on sample thickness and field strength. Following a linear stability analysis we are able to account for this dependence in a qualitative way. Moreover, the rotating magnetic field enables us to render the nematic phase single domain, with well-defined director orientation.

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting.

Although several models have been proposed to account for how cytoskeleton polymerization drives protrusion in cell motility, the precise mechanism remains controversial. Here, we show that, in addition to force exerted directly against the membrane by growing filaments, the way elongating filaments pack also contributes to protrusion by generating an expansion of the cytoskeleton gel. Tomography shows that filament packing in the major sperm protein (MSP) -based nematode sperm-motility machinery resembles that observed with rigid rods. Maximum rod-packing density decreases dramatically as the rods lengthen. Therefore, as filaments elongate, the cytoskeleton gel expands to accommodate their packing less densely. This volume expansion combines with polymerization to drive protrusion. Consistent with this hypothesis, an engineered MSP mutant that generates shorter filaments shows higher filament-packing density and slower movement.

On-lattice Monte Carlo simulations of colloidal random-stacking hard-sphere colloidal crystals are presented. The model yields close-packed crystals with random-stacking hexagonal structure. We find a significant amount of in-plane stacking disorder, which slowly anneals in the course of the simulation. The in-plane stacking disorder leads to lateral broadening of the stacking-disorder-induced Bragg rods. It is found that not only the scattering intensity, but also the width is modulated along the Bragg rods.

We report a method to determine the particle size distribution of small colloidal silica spheres via analytical ultracentrifugation and show that the average particle size, variance, standard deviation, and relative polydispersity can be obtained from a single sedimentation velocity (SV) analytical ultracentrifugation (AUC) experiment. The particle size distribution (psd) from the enhanced van Holde-Weischet (vHW) analysis accounts for the dynamic light scattering results quite well. In addition, the vHW psd equals the psd from a continuous distribution of sedimentation coefficients analysis where whole sedimentation velocity boundaries are fitted. The SV AUC interference optical data also yield the specific particle volume such that distributions of sedimentation coefficients for colloidal spheres can be converted directly to particle size distributions. Our results show that SV AUC experiments may yield a quantitative particle size distribution without a priori knowledge of the particle size and the shape of the size distribution.

We report the results of a comprehensive study of the rheological properties of a series of mixed colloid systems where the shape of one of the components has been varied systematically. Specifically we have measured the oscillatory, transient (creep) and continuous steady shear flow behaviour of a 2.5 wt% dispersion in water of a well-characterised hectorite clay modified by the addition of a series of aluminasol colloidal particles whose shape varies systematically from rod (boehmite) to platelet (gibbsite) to sphere (alumina-coated silica), all having essentially the same smallest dimension, which is similar to that of the hectorite. The particle characterisation and rheological properties of the pure components have recently been reported in Part I of this series (Soft Matter, 2007, 3, 1145). The mixtures show the same general behaviour as the pure systems, displaying a complex yield space transition from an elastoviscous gel at low applied stresses to a viscous, weakly elastic, shear-thinning liquid at high stresses. The unifying theme of this work is that the addition of 0.25 wt% of the minor component in all cases results in dramatic enhancements to the dispersion rheological properties. At the same time the magnitude of this effect depends on the shape of the particles. Shear moduli, low stress viscosities and effective yield stresses all increase in the additive order rods

The dynamics of ordering and relaxation processes in magnetic colloids has been studied by means of stroboscopic small angle neutron scattering techniques in an oscillating magnetic field. Surfactant stabilized ferrofluids (FFs) of Fe3O4 and Co nanoparticles have been investigated as a function of temperature and frequency and compared to a solid Cu alloy with 0.8 vol % Co precipitates. This technique allowed elucidating the dynamical nature of the locally ordered domains in both ferrofluids as “living objects” becoming arrested below the freezing temperature of the solvent. The time-dependent intensities have been analyzed in terms of Langevin statistics including dynamical interparticle structure factors, which scale with the square of the Langevin function. The local ordering is mainly determined by the effective dipole-dipole interaction, which is enhanced by the partial alignment of the particle moments in an external magnetic field. Starting from the frozen state, the amount of freely rotating particle moments increases continuously with increasing temperature. The dynamical structure factors describing the hexagonal (Fe3O4-FF) or chainlike (Co-FF) ordering reach a maximum around the melting temperature. The alignment of particle moments along the applied field is governed by the fast Brownian rotation of individual nanoparticles and small aggregates, while the magnetic relaxation of longer dipolar chains and local hexagonal domains is much slower. In contrast, no field-induced interparticle correlations occur in the diluted solid CuCo alloy where the moment relaxation is purely of fast Néel type, which—due to a low anisotropy constant—follows the oscillating field at all temperatures.

The interplay of various mechanical forces leads to characteristic shapes of torn adhesive films. Analysis of these shapes provides potential for new approaches to material characterization.

A sensitive balanced differential transformer was built to measure complex initial parallel magnetic susceptibility spectra in the 0.01–1000 Hz range. The alternating magnetic field can be chosen sufficiently weak that the magnetic structure of the samples is only slightly perturbed and the low frequencies make it possible to study the rotational dynamics of large magnetic colloidal particles or aggregates dispersed in a liquid. The distinguishing features of the setup are the novel multilayered cylindrical coils with a large sample volume and a large number of secondary turns (55 000) to measure induced voltages with a good signal-to-noise ratio, the use of a dual channel function generator to provide an ac current to the primary coils and an amplitude- and phase-adjusted compensation voltage to the dual phase differential lock-in amplifier, and the measurement of several vector quantities at each frequency. We present the electrical impedance characteristics of the coils, and we demonstrate the performance of the setup by measurement on magnetic colloidal dispersions covering a wide range of characteristic relaxation frequencies and magnetic susceptibilities, from X=−10−5 for pure water to X>1 for concentrated ferrofluids.

Colloidal platelets of hydrotalcite, a layered double hydroxide, have been prepared by coprecipitation at pH 11−12 of magnesium nitrate and aluminum nitrate at two different magnesium to aluminum ratios. Changing the temperature and ionic strength during hydrothermal treatment, the platelets were tailored to different sizes and aspect ratios. Amino-modified polyisobutylene molecules were grafted onto the platelets following a convenient new route involving freeze-drying. Organic dispersions in toluene were prepared of the particles with the largest size and highest aspect ratio. The colloidal dispersions prepared in this way showed isotropic−nematic phase transitions above a limiting concentration in a matter of days. The number density at the transition and the width of the biphasic region were determined and compared to theory. The orientation of the platelets in nematic droplets (tactoids) and at the isotropic−nematic interface were analyzed by polarization microscopy. It was observed that sedimentation induces a nematic layer in samples that are below the limiting concentration for isotropic−nematic phase separation. No nematic phase was observed in the initial aqueous suspensions of the ungrafted particles.

The details of the arrangement of mixtures of semiconducting materials in thin-films have a major influence on the performance of organic heterojunction solar cells. Here, we exploit the phenomenon of spinodal dewetting during spin coating of blends of PCBM and a cyanine dye for the design of phase separated morphologies with increased interfacial area. AFM snapshots of as-prepared films and after selective dissolution suggest that the solution separates into transient bilayers, which destabilize due to long-range intermolecular interactions. We propose that film destabilization is effectively driven by electrostatic forces that build up due to mobile ions that cross the junction and dissolve partially in PCBM. The resulting morphology type is mainly dependent on the ratio between the layer thicknesses, whereas the dominant wavelengths formed are determined by the absolute film thickness. Solar cells were fabricated from films with known structure and a power conversion efficiency of η = 0.29 % was measured for a vertically segregated film consisting of a cyanine layer covering the anode and an upper phase composed of dewetted PCBM domains. We explain the merits of this structure in contrast to a lateral separated blend morphology where the efficiency was 3 times smaller.

Theories and simulations have demonstrated that field-induced dipolar chains affect the static magnetic properties of ferrofluids. Experimental verification, however, has been complicated by the high polydispersity of the available ferrofluids, and the morphology of the dipolar chains was left to the imagination. We now present the concentration- and field-dependent magnetization of particularly well-defined ferrofluids, with a low polydispersity, three different average particle sizes, and with dipolar chains that were imaged with and without magnetic field using cryogenic transmission electron microscopy. At low concentrations, the magnetization curves obey the Langevin equation for noninteracting dipoles. Magnetization curves for the largest particles strongly deviate from the Langevin equation but quantitatively agree with a recently developed mean-field model that incorporates the field-dependent formation and alignment of flexible dipolar chains. The combination of magnetic results and in situ electron microscopy images provides original new evidence for the effect of dipolar chains on the field-dependent magnetization of ferrofluids.

Magnetic order in two-dimensional islands of spherical 21 nm cobalt-ferrite (CoFe2O4) nanoparticles is studied by magnetic force microscopy and spectroscopy. Images obtained at a temperature of 105 K clearly reveal the presence of repulsive and attractive areas on top of the islands. Monte Carlo simulations on hexagonal arrangements of nanoparticles have been carried out and are consistent with the experimental findings. The simulations show that the magnetization patterns are determined by a competition between interactions among the nanoparticle magnetic dipole moments and alignment of the individual moments along the easy axes of magnetization. At an elevated temperature of 500 K, it will be shown that dipole-dipole interactions lead to short-range ferromagnetic order, which fluctuates rapidly in time due to thermal excitations. Upon reduction of the temperature to 105 K, the dipole moments are forced toward the easy axes of magnetization, thereby increasing the amount of disorder.

We report here a simple one-pot directed synthesis of an oxomolybdate urea composite in which elementary molybdenum oxide building blocks are linked together with the aid of urea. This type of directed material design resulted in large rod-like crystals of an inorganic-organic hybrid extended structure of {MoO3(NH2-CO-NH2)}∞ consisting of right- and left-handed helical units. In the crystal structure urea acts both as a glue that links the inorganic molybdenum units into a helix and as a supramolecular linker for the stabilization of the crystal structure as a whole. This type of molecular topology resulted in an unexpectedly high thermal stability.

Silica particles as a nanoparticulate carrier material for contrast agents have received considerable attention the past few years, since the material holds great promise for biomedical applications. A key feature for successful application of this material in vivo is biocompatibility, which may be significantly improved by appropriate surface modification. In this study, we report a novel strategy to coat silica particles with a dense monolayer of paramagnetic and PEGylated lipids. The silica nanoparticles carry a quantum dot in their center and are made target-specific by the conjugation of multiple αvβ3-integrin-specific RGD-peptides. We demonstrate their specific uptake by endothelial cells in vitro using fluorescence microscopy, quantitative fluorescence imaging, and magnetic resonance imaging. The lipid-coated silica particles introduced here represent a new platform for nanoparticulate multimodality contrast agents

We study the coalescence of a drop with its bulk phase in fluid–fluid demixing colloid–polymer mixtures. Such mixtures show behaviour analogous to molecular fluid–fluid systems, but the interfacial tension is between 105 to 107 times smaller than in the molecular case. Such an ultralow interfacial tension has several important consequences and offers significant advantages in the study of droplet coalescence. The coalescence process can be divided into three consecutive stages: (i) drainage of the continuous film between droplet and bulk phase, (ii) rupture of the film, and (iii) growth of the connection. These stages can be studied within a single experiment by optical microscopy thanks to the ultralow interfacial tension in colloid–polymer mixtures, which significantly changes the relevant characteristic length and time scales. The first stage is compared with existing theories on drainage, where we show several limiting theoretical cases. The experimental drainage curves of different colloid–polymer mixtures can be scaled and then show very similar behaviour. We observe that drainage becomes very slow and eventually the breakup of the film is induced by thermal capillary waves. The time it takes for a certain height fluctuation of the interface to occur, which turns out to be an important parameter for the kinetics of the process, can be directly obtained from experiment. During the third stage we observe that the radius of the connecting neck grows linearly with time both for gas bubbles and liquid droplets with an order of magnitude that is in good agreement with the capillary velocity. Finally, partially bleaching the fluorescent dye inside the liquid droplet reveals how the surface energy is transformed into kinetic energy upon coalescence. This opens the way for a more complete understanding of the hydrodynamics involved.

We present the phase diagram of a colloid-polymer mixture in which the radius a of the colloidal spheres is approximately the same as the radius R of a polymer coil (q=R/a~1). A three-phase coexistence region is experimentally observed, previously only reported for colloid-polymer mixtures with smaller polymer chains (q

Field-induced structure formation in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment was studied with small-angle neutron scattering (SANS) as a function of the magnetic interactions. The interactions were tuned by adjusting the size of the well-defined, single-magnetic-domain magnetite (Fe3O4) particles and by applying an external magnetic field. For decreasing particle dipole moments, the data show a progressive distortion of the hexagonal symmetry, resulting from the formation of magnetic sheets. The SANS data show qualitative agreement with recent cryogenic transmission electron microscopy results obtained in 2D [Klokkenburg et al., Phys. Rev. Lett. 97, 185702 (2006)] on the same ferrofluids.

A simple model is presented that allows calculation of the small-angle X-ray diffraction patterns of perfect colloidal crystals. The model is based on the Wentzel–Kramers–Brillouin approximation and permits a straightforward evaluation of multibeam interactions. Results are illustrated by several examples.

Thiol-functionalization is described for silica surfaces from diverging origin, including commercial silica nanoparticles and St¨ober silica as well as silica structures provided by porous glasses and novel polymer-templated silica membranes. The functionalization allows in all cases for the irreversible binding of metal(oxide) particles from a solution. Examples are the adsorption of CoFe2O4 particles for the preparation of magnetizable silica colloids and silica structures, and gold nanoparticles that directly nucleate and grow on thiol-functionalized silica in a solution of HAuCl4 upon addition of a reducing agent.

We demonstrate that highly polydisperse colloidal gibbsite platelets easily form an opal-like columnar crystal with striking iridescent Bragg reflections. The formation process can be accelerated by orders of magnitude under a centrifugation force of 900g without arresting the system in a disordered glassy phase. Using transmission electron microscopy and small-angle X-ray scattering techniques, we find that the forced sedimentation is accompanied by particle size fractionation, leading to inversion of the iridescent colors. The relatively easy self-organization of the polydisperse colloidal particles into opal-like crystals may be explained on the basis of the observed particle fractionation and possibly also on hexatic-like ordering.

The flow behaviour and rheology of colloidal dispersions are of considerable interest in many applications, for example colloidal clay particles find applications in oilfield and constructiondrilling fluids. The rheological properties of such fluids can be enhanced significantly by adding colloidal particles of different size and shape. To gain insight into the mechanism of this phenomenon, we have studied model mineral-colloid systems whose shape changes systematically from a plate-like aluminasol (gibbsite), through a lath-like smectite clay (hectorite), to a rod-like aluminasol (boehmite). The paper presents the results of a systematic and comprehensive multi-technique study (oscillatory, transient and steady shear) of the rheology of dispersions of these model systems. This gives a detailed account of the ‘yield space’ that characterises the complex transition of these soft materials from elastoviscous gels to viscoelastic liquids, and of the effect of particle size and shape on this behaviour. The observed phenomena are underpinned by two competing flowmediated microstructural rearrangements that have significantly different timescales. A physical model invoking flow-mediated building and disruption of fluid structure is described to rationalise the observed behaviour. The study also forms the baseline to a companion study (part II), which investigates the rheological behaviour of mixed anisometric colloid systems based on these pure components.

The magnetic properties of dispersions of magnetic silica microspheres have been investigated by measuring the magnetization curves and the complex magnetic susceptibility as a function of frequency and field amplitude. The silica spheres appear to have a net permanent magnetic dipole moment, even in zero field, which is increased significantly after a temporary exposure of the silica colloids to a saturating magnetic field. The magnetic properties of the microparticles in zero field are discussed in terms of the number and the orientations of the embedded nanoparticle dipoles along an easy axis of magnetization in the absence of an external field.