Ref HAL: hal-01053378_v1
Ref Arxiv: 1404.4778
DOI: 10.1209/0295-5075/107/28009
WoS: 000340779900036
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39 Citations
Résumé:

The shear rheology of dense colloidal and granular suspensions is strongly nonlinear, as these materials exhibit shear-thinning and shear-thickening, depending on multiple physical parameters. We numerically study the rheology of a simple model of soft repulsive particles at large densities, and show that nonlinear flow curves reminiscent of experiments on real suspensions can be obtained. By using dimensional analysis and basic elements of kinetic theory, we rationalize these multiple rheological regimes and disentangle the relative impact of thermal fluctuations, glass and jamming transitions, inertia and particle softness on the flow curves. We characterize more specifically the shear-thickening regime and show that both particle softness and the emergence of a yield stress at the jamming transition compete with the inertial effects responsible for the observed thickening behaviour. This allows us to construct a dynamic state diagram, which can be used to analyze experiments.

Commentaires: 6 pages, 3 figures

Ref HAL: hal-01053069_v1
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Dynamics of partially wetted colloids at the interface between two fluids is a crucial and timeliness topic: for instance, it governs the behavior of kinetically arrested colloidal gels, the formation of bacteria based biofilm and the cellular signaling via membrane proteins. Moreover, this subject is also challenging from a theoretical point of view because of the complexity of hydrodynamics at the interface and of the unexplored role of the contact line [1]. Despite this great interest, the behavior of a single particle at a fluid interface was never directly characterized. In this contribution, the dynamics of micrometric spherical silica particles trapped at a flat air-water interface is fully addressed. The particle contact angle θ is finely tuned in the range 30°-140° by surface treatments and measured in situ at 0.5° resolution by a homemade Vertical Scanning Interferometer [2]. The particle dynamics and translational diffusion coefficients are obtained by particle tracking. Counter-intuitively, and against all hydrodynamical models [3], the diffusion is much slower than expected; the drag exerted on the particle increases when the particle is less immersed in water. To explain this extra dissipation we devised a model considering the effect related to surface waves. Such waves are generated by the thermally activated triple line jumps between hydrophilic sites of the particle surface [4]. The total friction is thus coherent with the measured particle diffusion, and in particular with its slowing down at large contact angles. The jump lengths λ are consistent with the Cassie - Baxter model of an heterogeneous surface characterized by an intrinsic roughness and by hydrophobic and hydrophilic patches. [1] D. M. Kaz, R. McGorty, M. Mani, M. P. Brenner, V. N. Manoharan, Nature materials, 11.2, 138-142 (2011). [2] C. Blanc et al., Phys.Rev. Lett.,111, 058302 (2013). [3] T. M. Fischer, P. Dhar, P. Heinig, T, J.Fluid. Mech., 558, 451-475 (2006). [4] T. Blake, J.Colloid Interface Sci., 299, 1-13 (2006).

Ref HAL: hal-01052742_v1
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After a brief review of unusual features of the light front canonical formulations of field theories, I present the analysis of the canonical structure of the first order formulation of general relativity on a lightlike foliation. It appears to be quite different from the usual spacelike case leading, for instance, to the presence of tertiary constraints. Besides, I discuss the issue of the zero modes and argue that there might be some hidden correspondence with two-dimensional theories.

Ref HAL: hal-01052685_v1
Ref Arxiv: 1403.7959
DOI: 10.1021/ma500635h
WoS: 000339462900033
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8 Citations
Résumé:

We study the rotational dynamics of a flexible polymer initially wrapped around a rigid rod and unwinding from it. This dynamics is of interest in several problems in biology and constitutes a fundamental instance of polymer relaxation from a state of minimal entropy. We investigate the dynamics of several quantities such as the total and local winding angles and metric quantities. The results of simulations performed in two and three dimensions, with and without self-avoidance, are explained by a theory based on scaling arguments and on a balance between frictional and entropic forces. The early stage of the dynamics is particularly rich, being characterized by three coexisting phases.

Ref Arxiv: 1407.5756
DOI: 10.1140/epjc/s10052-014-3117-7
WoS: 000347169900001
Ref. & Cit.: NASA ADS
32 Citations
Résumé:

Additional jet activity in dijet events is measured using $pp$ collisions at ATLAS at a centre-of-mass energy of 7 TeV, for jets reconstructed using the anti-kt algorithm with radius parameter R=0.6. This is done using variables such as the fraction of dijet events without an additional jet in the rapidity interval bounded by the dijet subsystem and correlations between the azimuthal angles of the dijets. They are presented, both with and without a veto on additional jet activity in the rapidity interval, as a function of the mean transverse momentum of the dijets and of the rapidity interval size. The double differential dijet cross section is also measured as a function of the interval size and the azimuthal angle between the dijets. These variables probe differences in the approach to resummation of large logarithms when performing QCD calculations. The data are compared to POWHEG, interfaced to the PYTHIA 8 and HERWIG parton shower generators, as well as to HEJ with and without interfacing it to the ARIADNE parton shower generator. None of the theoretical predictions agree with the data across the full phase-space considered; however, POWHEG+PYTHIA 8 and HEJ+ARIADNE are found to provide the best agreement with the data.These measurements use the full data sample collected with the ATLAS detector in 7 TeV $pp$ collisions at the LHC and correspond to integrated luminosities of 36.1 pb$^-1$ and 4.5 fb$^-1$ for data collected during 2010 and 2011 respectively.

Commentaires: See paper for full list of authors - 15 pages + author list + cover pages (29 pages total). 8 figures, 0 tables, submitted to EPJC. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2012-17/

Ref Arxiv: 1407.4314
DOI: 10.1103/PhysRevD.90.112016
WoS: 000348871500004
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37 Citations
Résumé:

Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb$^{-1}$. Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.

Commentaires: See paper for full list of authors - 20 pages plus author list + cover pages (33 pages total), 10 figures, 6 tables, submitted to Phys. Rev. D, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2013-01/

Ref HAL: hal-01032182_v1
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During the past years, considerable interest has been focused on materials that exhibit potential applications as luminescent solar concentrators (LSCs). Indeed, the incorporation of trivalent lanthanide ion (Ln3+) complexes in organic-inorganic coatings enables the down-shifting of the UV radiationinto light that can be efficiently trapped inside the substrate and guided to the edges by total internal reflection where it can be efficiently collected by a solar cell [2]. In this communication, we will present two new precursors based on ethane tetracarboxamide and ethane dicarboxamide fragments P4 (Figure 1 (a)) and P2 (Figure 1 (b)), and their conversion into luminescent hybrid materials by the sol-gel process in the presence of Eu3+ salts (EuCl36H2O) and ligand 2-thienoyltrifluoracetone (tta) (Eu3+/tta molar ratio of 1:2). The organic-inorganic hybrid materials (P4(Eu3+/tta) and P2(Eu3+/tta)) were processed as free standing films by solvent casting. They display maximum emission quantum yield () values of 0.22 and 0.48, respectively, under excitation at 320 nm. In addition, P4(Eu3+/tta) was processed by spin-coating on glass substrates, yielding films (P4-F(Eu3+/tta) with controlled thickness of 75 nm. Under UV irradiation the films display a bright red emission ascribed to the intra-4f6 Eu3+ transitions, as illustrated in Figure 1(c) and a remarkable  value of 0.60 under excitation at 345 nm.