Journey of an intruder through the fluidisation and jamming transitions of a dense granular media

We study experimentally the motion of an intruder dragged into an amorphous monolayer of horizontally vibrated grains at high packing fractions. This motion exhibits two transitions. The first transition separates a continuous motion regime at comparatively low packing fractions and large dragging force from an intermittent motion one at high packing fraction and low dragging force. Associated to these different motions, we observe a transition from a linear rheology to a stiffer response. We thereby call "fluidisation" this first transition. A second transition is observed within the intermittent regime, when the intruder's motion is made of intermittent bursts separated by long waiting times. We observe a peak in the relative fluctuations of the intruder's displacements and a critical scaling of the burst amplitudes distributions. This transition occurs at the jamming point characterized in a previous study and defined as the point where the static pressure (i.e. the pressure measured in the absence of vibration) vanishes. Investigating the motion of the surrounding grains, we show that below the fluidisation transition, there is a permanent wake of free volume behind the intruder. This transition is marked by the evolution of the reorganization patterns around the intruder, which evolve from compact aggregates in the flowing regime to long-range branched shapes in the intermittent regime, suggesting an increasing role of the stress fluctuations. Remarkably, the distributions of the kinetic energy of these reorganization patterns also exhibits a critical scaling at the jamming transition.Comment: 12 pages, 11 figure

Formation of H3−_3^- by radiative association of H2_2 and H−^- in the interstellar medium

We develop the theory of radiative association of an atom and a diatomic molecule within a close-coupling framework. We apply it to the formation of H$_3^-$ after the low energy collision (below 0.5 eV) of H$_2$ with H$^-$. Using recently obtained potential energy and permanent dipole moment surfaces of H$_3^-$, we calculate the lowest rovibrational levels of the H$_3^-$ electronic ground state, and the cross section for the formation of H$_3^-$ by radiative association between H$^-$ and ortho- and para-H$_2$. We discuss the possibility for the H$_3^-$ ion to be formed and observed in the cold and dense interstellar medium in an environment with a high ionization rate. Such an observation would be a probe for the presence of H$^-$ in the interstellar medium

On the choosability of HH-minor-free graphs

Given a graph $H$, let us denote by $f_\chi(H)$ and $f_\ell(H)$, respectively, the maximum chromatic number and the maximum list chromatic number of $H$-minor-free graphs. Hadwiger's famous coloring conjecture from 1943 states that $f_\chi(K_t)=t-1$ for every $t \ge 2$. In contrast, for list coloring it is known that $2t-o(t) \le f_\ell(K_t) \le O(t (\log \log t)^6)$ and thus, $f_\ell(K_t)$ is bounded away from the conjectured value $t-1$ for $f_\chi(K_t)$ by at least a constant factor. The so-called $H$-Hadwiger's conjecture, proposed by Seymour, asks to prove that $f_\chi(H)=\textsf{v}(H)-1$ for a given graph $H$ (which would be implied by Hadwiger's conjecture). In this paper, we prove several new lower bounds on $f_\ell(H)$, thus exploring the limits of a list coloring extension of $H$-Hadwiger's conjecture. Our main results are: For every $\varepsilon>0$ and all sufficiently large graphs $H$ we have $f_\ell(H)\ge (1-\varepsilon)(\textsf{v}(H)+\kappa(H))$, where $\kappa(H)$ denotes the vertex-connectivity of $H$. For every $\varepsilon>0$ there exists $C=C(\varepsilon)>0$ such that asymptotically almost every $n$-vertex graph $H$ with $\left\lceil C n\log n\right\rceil$ edges satisfies $f_\ell(H)\ge (2-\varepsilon)n$. The first result generalizes recent results on complete and complete bipartite graphs and shows that the list chromatic number of $H$-minor-free graphs is separated from the natural lower bound $(\textsf{v}(H)-1)$ by a constant factor for all large graphs $H$ of linear connectivity. The second result tells us that even when $H$ is a very sparse graph (with an average degree just logarithmic in its order), $f_\ell(H)$ can still be separated from $(\textsf{v}(H)-1)$ by a constant factor arbitrarily close to $2$. Conceptually these results indicate that the graphs $H$ for which $f_\ell(H)$ is close to $(\textsf{v}(H)-1)$ are typically rather sparse.Comment: 14 page

An experimental hut evaluation of PermaNet(®) 3.0, a deltamethrin-piperonyl butoxide combination net, against pyrethroid-resistant Anopheles gambiae and Culex quinquefasciatus mosquitoes in southern Benin.

PermaNet 3.0 is a long-lasting combination net with deltamethrin present on the sides and a mixture of deltamethrin and piperonyl butoxide (PBO), an oxidase synergist, on the top panel. An experimental hut trial comparing unwashed and 20 times washed PermaNet 3.0 and PermaNet 2.0, Olyset Net and a conventional deltamethrin-treated net washed three times was conducted in southern Benin. Anopheles gambiae and Culex quinquefasciatus from this area are highly resistant to pyrethroids through kdr and cytochrome P450 mechanisms. The unwashed PermaNet 3.0 killed slightly more A. gambiae (52%) than the unwashed PermaNet 2.0 (44%) (P=0.036), indicating only partial synergism of resistance. After washing there was significant loss of activity to a similar level, with PermaNet 3.0 killing 31%, PermaNet 2.0 killing 29% and the conventional net killing 26%. Blood-feeding rates were partially inhibited for unwashed PermaNet 3.0 and Olyset Net (27% inhibition). Personal protection against A. gambiae derived from PermaNet 3.0 was similar to that from PermaNet 2.0 before washing (50% vs. 47%), and after 20 washes it decreased to 30%. Against C. quinquefasciatus, no treatment killed >24% entering the huts. The synergism from unwashed PermaNet 3.0 was lower than expected, probably due to an unidentified resistance mechanism unaffected by PBO

Exoplanets imaging with a Phase-Induced Amplitude Apodization Coronagraph - I. Principle

Using 2 aspheric mirrors, it is possible to apodize a telescope beam without losing light or angular resolution: the output beam is produced by ``remapping'' the entrance beam to produce the desired light intensity distribution in a new pupil. We present the Phase-Induced Amplitude Apodization Coronagraph (PIAAC) concept, which uses this technique, and we show that it allows efficient direct imaging of extrasolar terrestrial planets with a small-size telescope in space. The suitability of the PIAAC for exoplanet imaging is due to a unique combination of achromaticity, small inner working angle (about 1.5 $\lambda/d$), high throughput, high angular resolution and large field of view. 3D geometrical raytracing is used to investigate the off-axis aberrations of PIAAC configurations, and show that a field of view of more than 100 $\lambda/d$ in radius is available thanks to the correcting optics of the PIAAC. Angular diameter of the star and tip-tilt errors can be compensated for by slightly increasing the size of the occulting mask in the focal plane, with minimal impact on the system performance. Earth-size planets at 10 pc can be detected in less than 30s with a 4m telescope. Wavefront quality requirements are similar to classical techniques.Comment: 35 pages, 16 figures, Accepted for publication in Ap