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.

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.

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 and short-time dynamics in suspensions of charged silica spheres in the entire fluid regime.

We present an experimental study of short-time diffusion properties in fluidlike suspensions of monodisperse charge-stabilized silica spheres suspended in dimethylformamide. The static structure factor S(q), the short-time diffusion function D(q), and the hydrodynamic function H(q) have been probed by combining x-ray photon correlation spectroscopy experiments with static small-angle x-ray scattering. Our experiments cover the full liquid-state part of the phase diagram, including de-ionized systems right at the liquid-solid phase boundary. We show that the dynamic data can be consistently described by the renormalized density fluctuation expansion theory of Beenakker and Mazur over a wide range of concentrations and ionic strengths. In accordance with this theory and Stokesian dynamics computer simulations, the measured short-time properties cross over monotonically, with increasing salt content, from the bounding values of salt-free suspensions to those of neutral hard spheres. Moreover, we discuss an upper bound for the hydrodynamic function peak height of fluid systems based on the Hansen-Verlet freezing criterion.

Work disability after whiplash: a prospective cohort study.

STUDY DESIGN: Prospective cohort study. OBJECTIVE: To investigate the consequences of neck pain after motor vehicle accidents in terms of disability for work and the relationship this has with symptom and work-related factors. SUMMARY OF BACKGROUND DATA: Previous studies on work disability related to whiplash are very heterogeneous, are often limited in sample size and show a wide variability in terms of results. A relationship has been suggested between poor recovery from or persistent work disability after whiplash and female gender, older age, marital status, heavy manual work, self-employment, prior psychological problems, subjective complaints of poor concentration, pain catastrophizing, and kinesiophobia. METHODS: Individuals with neck complaints after involvement in traffic accidents, who initiated compensation claim procedures with a Dutch insurance company (n = 879), were sent questionnaires (Q1) concerning the accident, the injuries that they had sustained, their complaints at that time, and questions regarding work and disability. The course of complaints and work disability was monitored at 6 (Q2) and 12 months (Q3) after the accident. RESULTS: A total of 58.8% of the population with neck complaints studied was work-disabled after the accident. Age and impaired concentration complaints after 1 month were found to be related to work disability at 1 year, independent of physical complaints and work characteristics. CONCLUSION: Age and concentration complaints were important independent predictors of long-lasting work disability, whereas no evidence emerged to indicate that the degree of manual labor (blue or white collar work) or educational level was involved in persistent work disability in postwhiplash syndrome. The current results suggest that work disability could benefit most from interventions related to recovery from cognitive complaints and less from physically related interventions.

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.

Catastrophizing and causal beliefs in whiplash.

STUDY DESIGN: Prospective cohort study. OBJECTIVE: This study investigates the role of pain catastrophizing and causal beliefs with regard to severity and persistence of neck complaints after motor vehicle accidents. SUMMARY OF BACKGROUND DATA.: In previous research on low back pain, somatoform disorders and chronic fatigue syndrome, pain catastrophizing and causal beliefs were found to be related to perceived disability and prognosis. Furthermore, it has been argued with respect to whiplash that culturally dependent symptom expectations are responsible for a chronic course. METHODS: Individuals involved in traffic accidents who initiated compensation claim procedures with a Dutch insurance company were sent questionnaires (Q1) containing the Neck Disability Index, the Pain Catastrophizing Scale, and the Causal Beliefs Questionnaire-Whiplash. Of 1252 questionnaires dispatched, 747 (59.7%) were returned. Only car occupants with neck complaints were included in this study (n = 140). Complaints were monitored using additional questionnaires administered 6 (Q2) and 12 months (Q3) after the accident. RESULTS: Pain catastrophizing and causal beliefs were related to the severity of concurrent whiplash disability. The severity of initial complaints was related to the severity and persistence of whiplash complaints. Attributing initial neck complaints to whiplash was found to predict the persistence of disability at 6 and 12 months follow-up, over and above the severity of the initial complaints. CONCLUSION: The results suggest that causal beliefs may play a major role in the perceived disability and course of neck complaints after motor vehicle accidents, whereas pain catastrophizing is predominantly related to concurrent disability.The current findings are consistent with the view that an early conviction that neck complaints are caused by the medico-cultural entity whiplash has a detrimental effect on the course of symptoms.

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.