Rotational microrheology of Maxwell fluids using micron-sized wires

We demonstrate a simple method for rotational microrheology in complex fluids, using micrometric wires. The three-dimensional rotational Brownian motion of the wires suspended in Maxwell fluids is measured from their projection on the focal plane of a microscope. We analyze the mean-squared angular displacement of the wires of length between 1 and 40 microns. The viscoelastic properties of the suspending fluids are extracted from this analysis and found to be in good agreement with macrorheology data. Viscosities of simple and complex fluids between 0.01 and 30 Pa.s could be measured. As for the elastic modulus, values up to ~ 5 Pa could be determined. This simple technique, allowing for a broad range of probed length scales, opens new perspectives in microrheology of heterogeneous materials such as gels, glasses and cells.Comment: to appear in Soft Matte

A general kinetic model for the photothermal oxidation of polypropylene

A general kinetic model for the photothermal oxidation of polypropylene has been derived from the basic auto-oxidation mechanistic scheme in which the main sources of radicals are the thermolysis and photolysis of the most unstable species, i.e hydroperoxides. Thermolysis is a uni- or bi-molecular reaction whose rate constant obeys an Arrhenius law. In contrast, photolysis is exclusively a unimolecular reaction and its rate constant is independent of temperature. According to the quantum theory, this latter is proportional to the energy absorbed by photosensitive species and thus, accounts for the impact of UV-light intensity and wavelength on the global oxidation kinetics. The validity of this model has been checked on iPP films homogeneously oxidized in air over a wide range of temperatures and UV-light sources. It gives access to the concentration changes of: (i) primary (hydroperoxides) and secondary (carbonyls) oxidation products, (ii) double bonds, (iii) chain scissions and crosslinking nodes, but also to the subsequent changes in molecular masses. These calculations are in full agreement with the photolysis results reported by Carlsson and Wiles in the 70s [1–3]. However, the model seems to be only valid for UV-light energies equivalent to about 10 suns as upper boundary, presumably because of multiphotonic excitations or chromophores photosensitization (i.e. termolecular photo-physical reactions), both enhanced at high irradiances

The contribution of starspots to coronal structure

Significant progress has been made recently in our understanding of the structure of stellar magnetic fields, thanks to advances in detection methods such as Zeeman-Doppler Imaging. The extrapolation of this surface magnetic field into the corona has provided 3D models of the coronal magnetic field and plasma. This method is sensitive mainly to the magnetic field in the bright regions of the stellar surface. The dark (spotted) regions are censored because the Zeeman signature there is suppressed. By modelling the magnetic field that might have been contained in these spots, we have studied the effect that this loss of information might have on our understanding of the coronal structure. As examples, we have chosen two stars (V374 peg and AB Dor) that have very different magnetograms and patterns of spot coverage. We find that the effect of the spot field depends not only on the relative amount of flux in the spots, but also its distribution across the stellar surface. For a star such as AB Dor with a high spot coverage and a large polar spot, at its greatest effect the spot field may almost double the fraction of the flux that is open (hence decreasing the spindown time) while at the same time increasing the X-ray emission measure by two orders of magnitude and significantly affecting the X-ray rotational modulation.Comment: 10 pages, 10 figure

Non-linear oscillatory rheological properties of a generic continuum foam model: comparison with experiments and shear-banding predictions

The occurence of shear bands in a complex fluid is generally understood as resulting from a structural evolution of the material under shear, which leads (from a theoretical perspective) to a non-monotonic stationnary flow curve related to the coexistence of different states of the material under shear. In this paper we present a scenario for shear-banding in a particular class of complex fluids, namely foams and concentrated emulsions, which differs from other scenarii in two important ways. First, the appearance of shear bands is shown to be possible both without any intrinsic physical evolution of the material (e.g. via a parameter coupled to the flow such as concentration or entanglements) and without any finite critical shear rate below which the flow does not remain stationary and homogeneous. Secondly, the appearance of shear bands depends on the initial conditions, i.e., the preparation of the material. In other words, it is history dependent. This behaviour relies on the tensorial character of the underlying model (2D or 3D) and is triggered by an initially inhomogeneous strain distribution in the material. The shear rate displays a discontinuity at the band boundary, whose amplitude is history dependent and thus depends on the sample preparation.Comment: 18 pages - 17 figure

Equatorial currents system north of New-Guinea

Eight trans-equatorial cross sections giving zonal and meridional components of currents north of New Guinea were carried out during the cruises FOC 1 and FOC 2 (January-february and June-July 1971) of the R/V CORIOLIS of the centre ORSTOM of Noumea. The equatorial currents system is described during two contrased wind regimes and comments are made on space-time variations of the main water drifts. (Résumé d'auteur

Quand l’historien doit faire confiance à des faux : les chartes confirmatives de Conan iv, duc de Bretagne, aux Templiers et aux Hospitaliers

Les possessions des ordres religieux-militaires de l’Hôpital et du Temple dans le duché de Bretagne sont difficiles à recenser avec précision. Leur étude s’appuie principalement sur deux chartes, qui énumèrent de façon sommaire les biens des Hospitaliers et des Templiers en Bretagne, datées respectivement de1160 et1182, et attribuées au duc Conan iv. Elles ont été longtemps tenues pour authentiques, mais leur caractère apocryphe a été démontré en 1872 par Anatole de Barthélemy, qui plaçait leur rédaction dans la seconde moitié du xiiie siècle. Pourtant, l’examen des conditions dans lesquelles apparurent les premières copies de ces chartes suggère qu’elles ont probablement été réalisées toutes deux par les Hospitaliers dans la décennie 1310. La charte concernant les Hospitaliers aurait permis d’obtenir la sauvegarde royale en 1312, celle concernant les Templiers aurait servi peu après à répondre aux contestations auxquelles se heurtaient les Hospitaliers quand ils cherchaient à être investis des biens templiers. L’état du temporel des ordres militaires, tel qu’il est décrit dans ces chartes apocryphes, serait donc celui du début du xive siècle.Studying the importance and the location of the property belonging to the religious military orders of the Temple and the Hospital in the duchy of Brittany happens to be a difficult task. The main sources upon which such a study is based are two deeds, supposedly drawn up by Duke Conan iv in 1160 and 1182: those deeds enumerate, somewhat disorderly, the alms given to Knights Templars and Hospitallers in Brittany. In 1872, Anatole de Barthélemy showed that the two documents, which had long been considered as authentic, were in fact apocryphal; he argued that they might have been written in the second half of the thirteenth century. However, the most ancient copies of these titles, as well as the way they were used, suggest that the Hospitallers forged them both in the 1310 decade. The deed to the Hospitallers may have been essential to get royal protection in 1312, and the deed to the Templars may have been useful for the Hospitallers to bar any attempt to deprive them of the Temple’s estates in 1312 or soon after. The two false titles thus describe the Templar and Hospitaller property at the beginning of the fourteenth century