Nonlinear dielectric response at the excess wing of glass-forming liquids

We present nonlinear dielectric measurements of glass-forming glycerol and propylene carbonate applying electrical fields up to 671 kV/cm. The measurements extend to sufficiently high frequencies to allow for the investigation of the nonlinear behavior in the regime of the so-far mysterious excess wing, showing up in the loss spectra of many glass formers as a second power law at high frequencies. Surprisingly, we find a complete lack of nonlinear behavior in the excess wing, in marked contrast to the alpha-relaxation where, in agreement with previous reports, a strong increase of dielectric constant and loss is found.Comment: 8 pages (including 3 pages Supplementary Information), 4 + 1 figures. Revised according to suggestions of referee

Nonlinear dielectric effect of dipolar fluids

The nonlinear dielectric effect for dipolar fluids is studied within the framework of the mean spherical approximation (MSA) of hard core dipolar Yukawa fluids. Based on earlier results for the electric field dependence of the polarization our analytical results show so-called normal saturation effects which are in good agreement with corresponding NVT ensemble Monte Carlo simulation data. The linear and the nonlinear dielectric permittivities obtained from MC simulations are determined from the fluctuations of the total dipole moment of the system in the absence of an applied electric field. We compare the MSA based theoretical results with the corresponding Langevin and Debye-Weiss behaviors.Comment: 10 pages including 4 figure

Evaluation of a risk-stratification strategy to improve primary care for low back pain: the MATCH cluster randomised trial protocol

Background Despite numerous options for treating back pain and the increasing healthcare resources devoted to this problem, the prevalence and impact of back pain-related disability has not improved. It is now recognized that psychosocial factors, as well as physical factors, are important predictors of poor outcomes for back pain. A promising new approach that matches treatments to the physical and psychosocial obstacles to recovery, the STarT Back risk stratification approach, improved patients’ physical function while reducing costs of care in the United Kingdom (UK). This trial evaluates implementation of this strategy in a United States (US) healthcare setting. Methods Six large primary care clinics in an integrated healthcare system in Washington State were block-randomized, three to receive an intensive quality improvement intervention for back pain and three to serve as controls for secular trends. The intervention included 6 one-hour training sessions for physicians, 5 days of training for physical therapists, individualized and group coaching of clinicians, and integration of the STarT Back tool into the electronic health record. This prognostic tool uses 9 questions to categorize patients at low, medium or high risk of persistent disabling pain with recommendations about evidence-based treatment options appropriate for each subgroup. Patients at least 18 years of age, receiving primary care for non-specific low back pain, were invited to provide data 1–3 weeks after their primary care visit and follow-up data 2 months and 6 months (primary endpoint) later. The primary outcomes are back-related physical function and pain severity. Using an intention to treat approach, intervention effects on patient outcomes will be estimated by comparing mean changes at the 2 and 6 month follow-up between the pre- and post-implementation periods. The inclusion of control clinics permits adjustment for secular trends. Differences in change scores by intervention group and time period will be estimated using linear mixed models with random effects. Secondary outcomes include healthcare utilization and adherence to clinical guidelines. Discussion This trial will provide the first randomized trial evidence of the clinical effectiveness of implementing risk stratification with matched treatment options for low back pain in a United States health care delivery system

La biréfringence magnétique et le point critique de dissolution

On a étudié la variation thermique de la biréfringence magnétique des solutions de nitrobenzène dans l'hexane et dans le tétrachlorure de carbone. On a trouvé que le coefficient thermique de la biréfringence magnétique croît avec la concentration et qu'il devient anormalement grand au voisinage du point critique de dissolution. L'auteur a essayé d'expliquer ces résultats, en supposant que l'augmentation d'intensité des champs moléculaires (association, fluctuations) provoque l'augmentation de l'anisotropie des molécules du nitrobenzène; qu'il s'agisse de l'anisotropie magnétique, de l'anisotropie optique, ou des deux à la fois

Sur la relation entre la constante diélectrique des émulsions, la concentration volumétrique de la phase dispersée et le degré de dispersion

1. La constante diélectrique de l'émulsion croît avec le degré de dispersion ; elle est par conséquent fonction, non seulement de ε1, ε2 et δ, mais aussi des diamètres μ des gouttes : ε = f (ε1, ε2, δ, μ). 2. La valeur de la constante diélectrique ε de l'émulsion (ε1, ε2 et μ étant constants) dépend beaucoup plus de sa concentration volumétrique δ que ne le fait prévoir la formule de Lorenz-Lorentz : les constantes diélectriques mesurées sont 2,05 fois plus grandes que les valeurs calculées. 3. En se basant sur ces résultats, on peut tirer des conclusions sur certains effets secondaires ayant lieu dans des émulsions liquides. Ainsi le fait que la relation entre Δε et δ pour ces émulsions n'est pas linéaire (fig. 3, courbe E) fait supposer l'existence d'un effet négatif Δεd qui diminue la constante diélectrique. Cet effet provient probablement de l'action directrice des charges des grains sur les dipôles du liquide et croit en même temps que la concentration et la dispersion. La charge électrique des grains joue un rôle dans les solutions colloïdales

Action du champ magnétique sur la perméabilité diélectrique des liquides polaires

Variations of the dielectric permittivity of nitrobenzene in an intense magnetic field were found with a circuit capable of detecting Δε ~ 10-6. The measurements were carried out by the method of magnetic field strength differences, in order to reduce heating of the condenser by eddy currents. The field was varied from H 0 = ca. 30 kOe to H0 + h = ca. 40 kOe. The results of measurements were converted to ΔεH corresponding to a rise in magnetic field strength from 0 to 40 kOe. This latter variation ΔεH, in the case of nitrobenzene, was found to represent an increase in dielectric permittivity of 3.9 x 10-6, in very good agreement with the theoretical calculations of A. Piekara and S. Kielich for a local electric field of the Onsager type.On a trouvé des variations de la perméabilité diélectrique du nitrobenzène soumis à l'influence d'un champ magnétique intense, en utilisant une méthode dont la sensibilité permettait de déceler Δε ~ 10-6. Afin de réduire l'échauffement du condensateur par les courants de Foucault, les mesures furent effectuées par un procédé consistant à faire croître le champ magnétique, en commençant par une valeur H0 (de 30 kOe environ) jusqu'à une valeur H 0 + h (de 40 kOe environ). Les résultats ont été recalculés pour obtenir la variation ΔεH correspondant à un accroissement du champ de 0 à 40 kOe. Cette variation ΔεH, dans le cas du nitrobenzène, se présente sous la forme d'une augmentation de la perméabilité diélectrique de 3,9 x 10-5, en très bon accord avec les calculs théoriques effectués par A. Piekara et S. Kielich, admettant l'existence d'un champ électrique interne du type d'Onsager