Polyelectrolyte Complexes from Oppositely Charged Filamentous Viruses

Here, we present an explorative study on a new type of polyelectrolyte complex made from chemically modified filamentous fd viruses. The fd virus is a semiflexible rod-shaped bacteriophage with a length of 880 nm and a diameter of 6.6 nm, which has been widely used as a well-defined model system of colloidal rods to investigate phase, flow, and other behavior. Here, chemically modified viruses have been prepared to obtain two types with opposite electrical charges in addition to a steric stabilization layer by poly(ethylene glycol) (PEG) grafting. The complex formation of stoichiometric mixtures of these oppositely charged viruses is studied as a function of virus and salt concentration. Furthermore, static light scattering measurements show a varying, strong increase in scattering intensity in some samples without visual macroscopic complex formation. Finally, the results of the complex formation are rationalized by comparing to model calculations on the pair interaction potential between oppositely charged viruses.

Silica Fouling in Reverse Osmosis Systems-Operando Small-Angle Neutron Scattering Studies

We present operando small-angle neutron scattering (SANS) experiments on silica fouling at two reverse osmose (RO) membranes under almost realistic conditions of practiced RO desalination technique. To its realization, two cells were designed for pressure fields and tangential feed cross-flows up to 50 bar and 36 L/h, one cell equipped with the membrane and the other one as an empty cell to measure the feed solution in parallel far from the membrane. We studied several aqueous silica dispersions combining the parameters of colloidal radius, volume fraction, and ionic strength. A relevant result is the observation of Bragg diffraction as part of the SANS scattering pattern, representing a crystalline cake layer of simple cubic lattice structure. Other relevant parameters are silica colloidal size and volume fraction far from and above the membrane, as well as the lattice parameter of the silica cake layer, its volume fraction, thickness, and porosity in comparison with the corresponding permeate flux. The experiments show that the formation of cake layer depends to a large extent on colloidal size, ionic strength and cross-flow. Cake layer formation proved to be a reversible process, which could be dissolved at larger cross-flow. Only in one case we observed an irreversible cake layer formation showing the characteristics of an unstable phase transition. We likewise observed enhanced silica concentration and/or cake formation above the membrane, giving indication of a first order liquid-solid phase transformation.

Self assembled monolayer of silica nanoparticles with improved order by drop casting

This paper reports on the formation of large area, self assembled, highly ordered monolayers of stearyl alcohol grafted silica nanospheres of ≈50 nm diameter on a silicon substrate based on the drop-casting method. Our novel approach to achieve improved order uses stearyl alcohol as an assistant by adding it to the colloidal NanoParticle (NP) dispersion from which the monolayers are formed. Additionally, a heat treatment step is added, to melt the stearyl alcohol in the monolayer and thereby give the particles more time to further self-assemble, leading to additional improvement in the monolayer quality. The formation of the monolayers is significantly affected by the concentration of the NPs and the stearyl alcohol, the volume of the drop as well as the time of the heat treatment. A high surface coverage and uniform monolayer film of SiO2 NPs is achieved by appropriate control of the above-mentioned preparation parameters. Structural characterization of the obtained SiO2 NP monolayer was done locally by Scanning Electron Microscopy (SEM), and globally by X-ray reflectivity (XRR) and grazing incidence small-angle X-ray scattering (GISAXS), where the data was reproduced by simulation within the Distorted Wave Born Approximation (DWBA). In conclusion, our modified drop-casting method is a simple, inexpensive method, which provides highly ordered self-assembled monolayers of silica particles, if combined with a compatible additive and a heat treatment step. This method might be more general and also applicable to different particles after finding an appropriate additive.

Thermophoresis of a Colloidal Rod: Contributions of Charge and Grafted Polymers

In this study, we investigated the thermodiffusion behavior of a colloidal model system as a function of the Debye length, λ_DH, which is controlled by the ionic strength. Our system consists of an fd-virus grafted with poly(ethylene glycol) (PEG) with a molecular mass of 5000 g mol^-1. The results are compared with recent measurements on a bare fd-virus and with results of PEG. The diffusion coefficients of both viruses are comparable and increase with the increasing Debye length. The thermal diffusion coefficient, D_T, of the bare virus increases strongly with the Debye length, whereas D_T of the grafted fd-virus shows only a very weak increase. The Debye length dependence of both systems can be described with an expression derived for charged rods using the surface charge density and an offset of D_T as adjustable parameters. It turns out that the ratio of the determined surface charges is inverse to the ratio of the surfaces of the two systems, which means that the total charge remains almost constant. The determined offset of the grafted fd-virus describing the chemical contributions is the sum of D_T of PEG and the offset of the bare fd-virus. At high λ_DH, corresponding to the low ionic strength, the S_T values of both colloidal model systems approach each other. This implies a contribution from the polymer layer, which is strong at short λ_DH and fades out for the longer Debye lengths, when the electric double layer reaches further than the polymer chains and therefore dominates interactions with the surrounding water.

Anisotropic hydrodynamic function of dense confined colloids

Dense colloidal dispersions exhibit complex wave-vector-dependent diffusion, which is controlled by both direct particle interactions and indirect nonadditive hydrodynamic interactions mediated by the solvent. In bulk the hydrodynamic interactions are probed routinely, but in confined geometries their studies have been hitherto hindered by additional complications due to confining walls. Here we solve this issue by combining high-energy x-ray photon correlation spectroscopy and small-angle x-ray-scattering experiments on colloid-filled microfluidic channels to yield the confined fluid's hydrodynamic function in the short-time limit. Most importantly, we find the confined fluid to exhibit a strongly anisotropic hydrodynamic function, similar to its anisotropic structure factor. This observation is important in order to guide future theoretical research.

Influence of temperature and charge effects on thermophoresis of polystyrene beads⋆

We study the thermodiffusion behavior of spherical polystyrene beads with a diameter of 25 nm by infrared thermal diffusion Forced Rayleigh Scattering (IR-TDFRS). Similar beads were used to investigate the radial dependence of the Soret coefficient by different authors. While Duhr and Braun (Proc. Natl. Acad. Sci. U.S.A. 104, 9346 (2007)) observed a quadratic radial dependence Braibanti et al. (Phys. Rev. Lett. 100, 108303 (2008)) found a linear radial dependence of the Soret coefficient. We demonstrated that special care needs to be taken to obtain reliable thermophoretic data, because the measurements are very sensitive to surface properties. The colloidal particles were characterized by transmission electron microscopy and dynamic light scattering (DLS) experiments were performed. We carried out systematic thermophoretic measurements as a function of temperature, buffer and surfactant concentration. The temperature dependence was analyzed using an empirical formula. To describe the Debye length dependence we used a theoretical model by Dhont. The resulting surface charge density is in agreement with previous literature results. Finally, we analyze the dependence of the Soret coefficient on the concentration of the anionic surfactant sodium dodecyl sulfate (SDS), applying an empirical thermodynamic approach accounting for chemical contributions.

Optimization of the All-D Peptide D3 for Aβ Oligomer Elimination

The aggregation of amyloid-β (Aβ) is postulated to be the crucial event in Alzheimer's disease (AD). In particular, small neurotoxic Aβ oligomers are considered to be responsible for the development and progression of AD. Therefore, elimination of thesis oligomers represents a potential causal therapy of AD. Starting from the well-characterized d-enantiomeric peptide D3, we identified D3 derivatives that bind monomeric Aβ. The underlying hypothesis is that ligands bind monomeric Aβ and stabilize these species within the various equilibria with Aβ assemblies, leading ultimately to the elimination of Aβ oligomers. One of the hereby identified d-peptides, DB3, and a head-to-tail tandem of DB3, DB3DB3, were studied in detail. Both peptides were found to: (i) inhibit the formation of Thioflavin T-positive fibrils; (ii) bind to Aβ monomers with micromolar affinities; (iii) eliminate Aβ oligomers; (iv) reduce Aβ-induced cytotoxicity; and (v) disassemble preformed Aβ aggregates. The beneficial effects of DB3 were improved by DB3DB3, which showed highly enhanced efficacy. Our approach yielded Aβ monomer-stabilizing ligands that can be investigated as a suitable therapeutic strategy against AD.

Anisotropic de Gennes Narrowing in Confined Fluids

The collective diffusion of dense fluids in spatial confinement is studied by combining high-energy (21 keV) x-ray photon correlation spectroscopy and small-angle x-ray scattering from colloid-filled microfluidic channels. We find the structural relaxation in confinement to be slower compared to the bulk. The collective dynamics is wave vector dependent, akin to the de Gennes narrowing typically observed in bulk fluids. However, in stark contrast to the bulk, the structure factor and de Gennes narrowing in confinement are anisotropic. These experimental observations are essential in order to develop a microscopic theoretical description of collective diffusion of dense fluids in confined geometries.

Structure and Dimensions of Core-Shell Nanoparticles Comparable to the Confocal Volume Studied by Means of Fluorescence Correlation Spectroscopy

In some applications the dye distribution within fluorescently labeled nanoparticles and its stability over long periods of time are important issues. In this article we study numerically and experimentally the applicability of fluorescence correlation spectroscopy (FCS) to resolve such questions. When the size of fluorescently labeled particles is comparable to or larger than the confocal volume, the effective confocal volume seen in FCS experiments is increasing. Such an effect has already been studied for uniformly labeled spherical particles. In this work we analyze the form of the FCS correlation functions (CFs) for core-labeled and shell-labeled core-shell particles. For shell-labeled particles an additional fast decay was found both in simulations and in experiments on custom-made surface-labeled particles. Universal scaling of FCS correlation times based on the squared ratio of the labeled part radius of gyration to the Gaussian radius of the beam profile was found. Recipes based on the analysis of simulated CFs, proposed for interpretation of experimental results, were successfully applied to the FCS results on suspensions of large core-labeled and surface-labeled particles.

Tuning the instrument resolution using chopper and time of flight at the small-angle neutron scattering diffractometer KWS-2

Using a double-disc chopper with a variable slit opening in concert with a velocity selector and the time-of-flight data acquisition mode, controlled variation of the wavelength spread Δλ/λ between 2 and 20% has become routinely possible at the KWS-2 SANS diffractometer of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center.

Following demand from the user community regarding the possibility of improving the experimental resolution, the dedicated high-intensity/extended Q-range SANS diffractometer KWS-2 of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center in Garching was equipped with a double-disc chopper with a variable opening slit window and time-of-flight (TOF) data acquisition option. The chopper used in concert with a dedicated high-intensity velocity selector enables the tuning at will of the wavelength resolution Δλ/λ within a broad range, from 20% (standard) down to 2%, in a convenient and safe manner following pre-planned or spontaneous decisions during the experiment. The new working mode is described in detail, and its efficiency is demonstrated on several standard samples with known properties and on a completely new crystallizable copolymer system, which were investigated using both the conventional (static) and TOF modes.

Size of Submicrometer Particles Measured by FCS: Correction of the Confocal Volume

When fluorescence correlation spectroscopy (FCS) in combination with a confocal microscope is used to determine the hydrodynamic radius a of particles comparable to or larger than the linear size σ of the confocal volume of the microscope, a correction must be used that depends on the a(2)/σ(2) ratio and the distribution of the dye within the particle. Here we present the experimental validation of the theoretically predicted approximate correction necessary for appropriate measurements of the size of uniformly fluorescently labeled spheres of radius comparable to the size of the confocal volume. We also test the approximate correction formula for different ranges of the a/σ ratio and propose a simple procedure to obtain the correct nanoparticle size from such a measurement.

Electrochemical artifacts originating from nanoparticle contamination by Ag/AgCl quasi-reference electrodes

Electrochemical techniques rely on the stability of a defined reference potential. Due to the need for miniaturization, electrochemical lab-on-a-chip platforms often employ Ag/AgCl quasi-reference electrodes for this purpose. Here, we report on electrochemical artifacts resulting from nanoparticle-electrode collisions originating from standard chlorinated silver wires.

Capturing dynamics with Eiger, a fast-framing X-ray detector

Eiger is the next-generation single-photon-counting pixel detector following the widely used Pilatus detector. Its smaller pixel size of 75 µm × 75 µm, higher frame rate of up to 22 kHz, and practically zero dead-time (~4 µs) between exposures will further various measurement methods at synchrotron sources. In this article Eiger's suitability for X-ray photon correlation spectroscopy (XPCS) is demonstrated. By exploiting its high frame rate, complementary small-angle X-ray scattering (SAXS) and XPCS data are collected in parallel to determine both the structure factor and collective diffusion coefficient of a nano-colloid suspension. For the first time, correlation times on the submillisecond time scale are accessible with a large-area pixel detector.

Electrophoresis of fd-virus particles: experiments and an analysis of the effect of finite rod lengths

The electrophoretic mobility of rodlike fd viruses is measured and compared to theory, with the theoretical calculations performed according to Stigter (Stigter, D. Charged Colloidal Cylinder with a Gouy Double-Layer. J. Colloid Interface Sci. 1975, 53, 296-306. Stigter, D. Electrophoresis of Highly Charged Colloidal Cylinders in Univalent Salt- Solutions. 1. Mobility in Transverse Field. J. Phys. Chem. 1978, 82, 1417-1423. Stigter, D. Electrophoresis of Highly Charged Colloidal Cylinders in Univalent Salt Solutions. 2. Random Orientation in External Field and Application to Polyelectrolytes. J. Phys. Chem. 1978, 82, 1424-1429. Stigter, D. Theory of Conductance of Colloidal Electrolytes in Univalent Salt Solutions. J. Phys. Chem. 1979, 83, 1663-1670), who describes the electrophoretic mobility of infinite cylinders including relaxation effects. Using the dissociation constants of the ionizable groups on the surfaces of the fd viruses, we can calculate the mobility without any adjustable parameter (apart from the possible Stern layer thickness). In addition, the approximation in the theoretical description of Stigter (and others) of using a model of infinitely long cylinders, which consequently is independent of the aspect ratio, is examined by performing more elaborate numerical calculations for finite cylinders. It is shown that, although the electrophoretic mobility of cylindrical particles in the limit of low ionic strength depends on the aspect ratio much more than "end effects", at moderate and high ionic strengths the finite and infinite cylinder models differ only to a degree that can be attributed to end effects. Furthermore, the range of validity of the Stokes regime is systematically calculated.

Anisotropic pair correlations and structure factors of confined hard-sphere fluids: an experimental and theoretical study

We address the fundamental question: how are pair correlations and structure factors of hard-sphere fluids affected by confinement between hard planar walls at close distance? For this purpose, we combine x-ray scattering from colloid-filled nanofluidic channel arrays and first-principles inhomogeneous liquid-state theory within the anisotropic Percus-Yevick approximation. The experimental and theoretical data are in remarkable agreement at the pair-correlation level, providing the first quantitative experimental verification of the theoretically predicted confinement-induced anisotropy of the pair-correlation functions for the fluid. The description of confined fluids at this level provides, in the general case, important insights into the mechanisms of particle-particle interactions in dense fluids under confinement.

Charge reversal of the rodlike colloidal fd virus through surface chemical modification

There is increasing interest in the use of viruses as model systems for fundamental research and as templates for nanomaterials. In this work, the rodlike fd virus was subjected to chemical modifications targeting different solvent-exposed functional groups in order to tune its surface properties, especially reversing the surface charge from negative to positive. The carboxyl groups of fd were coupled with different kinds of organic amines by carbodiimide chemistry, resulting in modified viruses that are positively charged over a wide range of pH. Care was taken to minimize intervirus cross linking, which often occurs because of such modifications. The surface amino groups were also grafted with poly(ethylene glycol) (PEG) end-functionalized with an active succinimidyl ester in order to introduce a steric stabilization effect. By combining charge reversal with PEG grafting, a reversible attraction between positively and negatively charged PEG-grafted fd viruses could be realized, which was tuned by the ionic strength of the solution. In addition, a charge-reversed fd virus forms only a pure nematic phase in contrast to the cholesteric phase of the wild type. These modified viruses might be used as model systems in soft condensed matter physics, for example, in the study of polyelectrolyte complexes or lyotropic liquid-crystalline phase behavior.

Experimental determination of resolution function parameters from small-angle neutron scattering data of a colloidal SiO2dispersion

A dilute dispersion of charge-stabilized, monodisperse silica nanoparticles has been used to determine the instrumental resolution function parameters of a conventional small-angle neutron scattering instrument in pinhole geometry over the entire accessible range of momentum transferq. Independent determination of the structure parameters of the colloidal silica dispersion by small-angle X-ray scattering enables refinement of both theq-independent geometric and theq-dependent (wavelength spread or polychromaticity of the neutron beam) contributions to the resolution function. The procedure described is appropriate if no further instrumental characterization is available. It is demonstrated that the sample used for refining the resolution function parameters has to exhibit sharp structural scattering features such as Bragg reflections, form factor maxima and minima, or an inter-particle correlation peak for each instrumental configuration at which the measurements have been performed to cover aqrange of two orders of magnitude (0.03 <q< 3.2 nm−1).

Grating-based holographic X-ray diffraction: theory and application to confined fluids

A grating-based holographic X-ray diffraction technique has been developed for reconstructing density profiles of nano-scale fluids confined in channel arrays. Within this approach, the reference wave is due to diffraction from the fabricated channel array, whereas the object wave is generated by the confinement-induced ordering of the fluid. The ensemble-averaged density profile of the fluid across the confining channel, which constitutes a weak phase object, is then determined in a model-independent manner from the interference between the reference and object waves by direct Fourier inversion. The validity of the linear holographic approach and its connection to the autocorrelation function, the inclusion of channel tapering, and volume-diffraction effects are discussed in detail.

Polymer depletion-driven cluster aggregation and initial phase separation in charged nanosized colloids

We study polymer depletion-driven cluster aggregation and initial phase separation in aqueous dispersions of charge-stabilized silica spheres, where the ionic strength and polymer (dextran) concentration are systematically varied, using dynamic light scattering and visual observation. Without polymers and for increasing salt and colloid content, the dispersions become increasingly unstable against irreversible cluster formation. By adding nonadsorbing polymers, a depletion-driven attraction is induced, which lowers the stabilizing Coulomb barrier and enhances the cluster growth rate. The initial growth rate increases with increasing polymer concentration and decreases with increasing polymer molar mass. These observations can be quantitatively understood by an irreversible dimer formation theory based on the classical Derjaguin, Landau, Verwey, and Overbeek pair potential, with the depletion attraction modeled by the Asakura-Oosawa-Vrij potential. At low colloid concentration, we observe an exponential cluster growth rate for all polymer concentrations considered, indicating a reaction-limited aggregation mechanism. At sufficiently high polymer and colloid concentrations, and lower salt content, a gas-liquidlike demixing is observed initially. Later on, the system separates into a gel and fluidlike phase. The experimental time-dependent state diagram is compared to the theoretical equilibrium phase diagram obtained from a generalized free-volume theory and is discussed in terms of an initial reversible phase separation process in combination with irreversible aggregation at later times.

Structure and phase diagram of an adhesive colloidal dispersion under high pressure: a small angle neutron scattering, diffusing wave spectroscopy, and light scattering study

We have applied small angle neutron scattering (SANS), diffusing wave spectroscopy (DWS), and dynamic light scattering (DLS) to investigate the phase diagram of a sterically stabilized colloidal system consisting of octadecyl grafted silica particles dispersed in toluene. This system is known to exhibit gas-liquid phase separation and percolation, depending on temperature T, pressure P, and concentration phi. We have determined by DLS the pressure dependence of the coexistence temperature and the spinodal temperature to be dP/dT=77 bar/K. The gel line or percolation limit was measured by DWS under high pressure using the condition that the system became nonergodic when crossing it and we determined the coexistence line at higher volume fractions from the DWS limit of turbid samples. From SANS measurements we determined the stickiness parameter tau(B)(P,T,phi) of the Baxter model, characterizing a polydisperse adhesive hard sphere, using a global fit routine on all curves in the homogenous regime at various temperatures, pressures, and concentrations. The phase coexistence and percolation line as predicted from tau(B)(P,T,phi) correspond with the determinations by DWS and were used to construct an experimental phase diagram for a polydisperse sticky hard sphere model system. A comparison with theory shows good agreement especially concerning the predictions for the percolation threshold. From the analysis of the forward scattering we find a critical scaling law for the susceptibility corresponding to mean field behavior. This finding is also supported by the critical scaling properties of the collective diffusion.

Structure of confined fluids by x-ray interferometry using diffraction gratings

We develop a novel method for structure determination of confined fluids using diffraction-grating-based x-ray interferometry.Within this approach, diffraction from a microfluidic array, which acts both as confinement and transmission diffraction grating, provides the reference wave, whereas the density modulations of the confined fluid, acting as a weak phase object, generate the object wave. The ensemble-averaged density profile of the fluid perpendicular to the confining channel is then unambiguously obtained from the interference between the reference and object waves by direct Fourier inversion.

Colloidal dispersions of octadecyl grafted silica spheres in toluene: a global analysis of small angle neutron scattering contrast variation and concentration dependence measurements

In this paper we report measurements of the form factor and the structure factor of a sterically stabilized colloidal dispersion consisting of silica spheres coated with octadecane in toluene by small angle neutron scattering (SANS). The phase diagram of this system shows the liquid-liquid coexistence line and also a jamming transition at higher concentrations, where the jamming line intersects the coexistence line roughly at the critical point. We have performed SANS experiments at a temperature well above the transition temperature and at various volume fractions phi, spanning from the very dilute regime (phi=0.2%) to the critical concentration (phi=16%) and the highly viscous regime (phi=39.2%). Except for the very dilute regime, we observe a structure factor S(q) in all other cases. We fitted our data over the whole concentration regime using a global fitting routine with a core-shell model for the form factor P(q), taking into account the structure factor, which we describe with the Robertus model for an adhesive polydisperse core-shell particle. At a volume fraction of phi=5% a SANS contrast variation experiment has been performed. From that the product of the volume of the shell and the amount of solvent within the corona of our core-shell particle could be determined. At the most probable shell thickness of 2.3 nm a solvent content of about 50% within the corona was found. Moreover we could conclude that the core is not interpenetrated by solvent molecules. From the contrast variation experiment followed that the structure factor at zero average contrast exhibits a strong q dependence, which is an effect of an inhomogeneous particle in combination with a size distribution.

Synthesis of poly(ethylene glycol) (PEG)-grafted colloidal silica particles with improved stability in aqueous solvents

The known grafting procedures of colloidal silica particles with poly(ethylene glycol) (PEG) lead to grafting layers that detach from the silica surface and dissolve in water within a few days. We present a new grafting procedure of PEG onto silica with a significant improvement of the stability of the grafting layers in aqueous solvents. Moreover, the procedure avoids any dry states or other circumstances leading to strong aggregation of the particles. To achieve the improved water stability, Stöber silica particles are first pre-coated with a silane coupling agent (3-aminopropyl)triethoxysilane (APS) to incorporate active amine groups. The water solubility of the pre-coating layer was minimized using a combination of APS with bis-(trimethoxysilylpropyl)amine (BTMOSPA) or bis-(triethoxysilyl)ethane (BTEOSE). These pre-coated particles were then reacted with N-succinimidyl ester of mono-methoxy poly(ethylene glycol) carboxylic acid to form PEG-grafted silica particles. The particles form stable dispersions in aqueous solutions as well as several organic solvents.

Thermal diffusion behavior of hard-sphere suspensions

We studied the thermal diffusion behavior of octadecyl coated silica particles (R(h)=27 nm) in toluene between 15.0 and 50.0 degrees C in a volume fraction range of 1%-30% by means of thermal diffusion forced Rayleigh scattering. The colloidal particles behave like hard spheres at high temperatures and as sticky spheres at low temperatures. With increasing temperature, the obtained Soret coefficient S(T) of the silica particles changed sign from negative to positive, which implies that the colloidal particles move to the warm side at low temperatures, whereas they move to the cold side at high temperatures. Additionally, we observed also a sign change of the Soret coefficient from positive to negative with increasing volume fraction. This is the first colloidal system for which a sign change with temperature and volume fraction has been observed. The concentration dependence of the thermal diffusion coefficient of the colloidal spheres is related to the colloid-colloid interactions, and will be compared with an existing theoretical description for interacting spherical particles. To characterize the particle-particle interaction parameters, we performed static and dynamic light scattering experiments. The temperature dependence of the thermal diffusion coefficient is predominantly determined by single colloidal particle properties, which are related to colloid-solvent molecule interactions.

Surface molecular view of colloidal gelation

We investigate the phase behavior of surface-functionalized silica colloids at both the molecular and macroscopic levels. This investigation allows us to relate collective properties such as aggregation, gelation, and aging directly to molecular interfacial behavior. By using surface-specific vibrational spectroscopy, we reveal dramatic changes in the conformation of alkyl chains terminating submicrometer silica particles. In fluid suspension at high temperatures, the interfacial molecules are in a liquid-like state of conformational disorder. As the temperature is lowered, the onset of gelation is identified by macroscopic phenomena, including changes in turbidity, heat release, and diverging viscosity. At the molecular level, the onset of this transition coincides with straightening of the carbon-carbon backbones of the interfacial molecules. In later stages, their intermolecular crystalline packing improves. It is the increased density of this ordered boundary layer that increases the van der Waals attraction between particles, causing the colloidal gas to aggregate. The approach presented here can provide insights into phase transitions that occur through surface modifications in a variety of colloidal systems.

Diffusion of spheres in isotropic and nematic suspensions of rods

Diffusion of a small tracer sphere (apoferritin) in isotropic and nematic networks [of fd virus] is discussed. For a tracer sphere that is smaller than the mesh size of the network, screened hydrodynamic interactions between the sphere and the network determine its diffusion coefficient. A theory is developed for such interactions as well as their relation to the long-time self-diffusion coefficient. Fluorescence correlation spectroscopy measurements on mixtures of apoferritin and fd virus are presented. The long-time self-diffusion coefficient of apoferritin is measured as a function of the fd-virus concentration, both in the isotropic and nematic state, in directions parallel and perpendicular to the nematic director. The hydrodynamic screening length of the fd-virus network as a function of fd concentration is obtained by combining these experimental data with the theory. Surprisingly, the screening length increases with increasing concentration in nematic networks. This is due to the increase in the degree of alignment, which apparently leads to a strong increase of the screening length. Hydrodynamic screening is thus strongly diminished by alignment. A self-consistent calculation of the screening length does not work at higher concentrations, probably due to the strong variation of the typical incident flow fields over the contour of a rod.

Crystallization kinetics of colloidal spheres under stationary shear flow

A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal growth rates. In addition, the nonequilibrium phase diagram as it relates to the melting line is measured. Shear flow is found to strongly affect induction times, crystal growth rates, and the location of the melting line. The main findings are that (1) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate; (2) contrary to the monotonic increase in the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems; and (3) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings are partly explained on a qualitative level.

Diffusion of spheres in crowded suspensions of rods

Translational tracer diffusion of spherical macromolecules in crowded suspensions of rodlike colloids is investigated. Experiments are done using several kinds of spherical tracers in fd-virus suspensions. A wide range of size ratios L/2a of the length L of the rods and the diameter 2a of the tracer sphere is covered by combining several experimental methods: fluorescence correlation spectroscopy for small tracer spheres, dynamic light scattering for intermediate sized spheres, and video microscopy for large spheres. Fluorescence correlation spectroscopy is shown to measure long-time diffusion only for relatively small tracer spheres. Scaling of diffusion coefficients with a/xi, predicted for static networks, is not found for our dynamical network of rods (with xi the mesh size of the network). Self-diffusion of tracer spheres in the dynamical network of freely suspended rods is thus fundamentally different as compared to cross-linked networks. A theory is developed for the rod-concentration dependence of the translational diffusion coefficient at low rod concentrations for freely suspended rods. The proposed theory is based on a variational solution of the appropriate Smoluchowski equation without hydrodynamic interactions. The theory can, in principle, be further developed to describe diffusion through dynamical networks at higher rod concentrations with the inclusion of hydrodynamic interactions. Quantitative agreement with the experiments is found for large tracer spheres, and qualitative agreement for smaller spheres. This is probably due to the increasing importance of hydrodynamic interactions as compared to direct interactions as the size of the tracer sphere decreases.

Recovery from acute whiplash: the role of coping styles

STUDY DESIGN

Prospective cohort study. Victims of car accidents who initiated compensation claim procedures at a Dutch insurance company and presented themselves with neck complaints were sent a questionnaire containing neck-related questions and questions regarding the coping styles used shortly after the accident. An additional two questionnaires were administered 6 and 12 months, respectively, after the accident.

OBJECTIVES

To examine the association between the coping styles used and the development of late whiplash syndrome.

SUMMARY OF BACKGROUND DATA

Previous research has indicated that neither personality traits nor psychopathologic symptoms can predict the outcome of whiplash. No studies have yet been conducted on the association between coping styles and the development of late whiplash syndrome.

METHODS

The coping styles were determined using the Utrecht Coping List. The duration of neck complaints was measured from the time of the accident and from the time of filling in the first questionnaire. Survival analysis was used to study the association between the duration of neck complaints and the explanatory variables.

RESULTS

Of the 363 eligible claimants, 278 (77%) responded to the questionnaire; 242 (67%) were included in the analysis. After 12 months, 40% of the male and 50% of the female participating claimants still had neck complaints. The duration of the neck complaints was associated with gender, palliative reaction, and the seeking social support coping style.

CONCLUSION

The coping style during the first few weeks after the accident and the gender are related to the duration of neck complaints (Cox regression: palliative handling relative risk = 0.91, P = 0.002; seeking social support relative risk = 1.06, P = 0.042; and gender relative risk = 1.50, P = 0.036). Thereafter the intensity of somatic complaints plays a role. Paying attention to the coping style could contribute to the prevention of the development of late whiplash syndrome.