113 research outputs found
Questioning the relationship between the 4 susceptibility and the dynamical correlation length in a glass former
Clusters of fast and slow correlated particles, identified as dynamical
heterogeneities (DHs), con-stitute a central aspect of glassy dynamics. A key
ingredient of the glass transition scenario is asignificant increase of the
cluster size 4 as the transition is approached. In need of
easy-to-computetools to measure 4 , the dynamical susceptibility 4
was introduced recently, and used in various ex-perimental works to probe DHs.
Here, we investigate DHs in dense microgel suspensions using imagecorrelation
analysis, and compute both 4 and the four-point correlation function G4 .
The spatialdecrease of G4 provides a direct access to 4 , which is found
to grow significantly with increasingvolume fraction. However, this increase is
not captured by 4 . We show that the assumptions thatvalidate the
connection between 4 and 4 are not fulfilled in our experiments.Comment: The present version was accepted for publication in Soft Matter
(http://pubs.rsc.org/en/journals/journalissues/sm
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
Activity statistics in a colloidal glass former: experimental evidence for a dynamical transition
In a dense colloidal suspension at a volume fraction slightly lower than that
of its glass transition, we follow the trajectories of an assembly of tracers
over a large time window. We define a local activity, which quantifies the
local tendency of the system to rearrange. We determine the statistics of the
time and space integrated activity, and we argue that it develops a low
activity tail that comes on a par with the onset of glassy behavior and
heterogeneous dynamics. These rare events may be interpreted as the reflection
of an underlying dynamic phase transition.Comment: 20 pages, 16 figure
Spatially heterogeneous dynamics in a thermosensitive soft suspension before and after the glass transition
The microscopic dynamics and aging of a soft thermosensitive suspension was
investigated by looking at the thermal fluctuations of tracers in the
suspension. Below and above the glass transition, the dense microgel particles
suspension was found to develop an heterogeneous dynamics, featured by a non
Gaussian Probability Distribution Function (PDF) of the probes' displacements,
with an exponential tail. We show that non Gaussian shapes are a characteristic
of the ensemble-averaged PDF, while local PDF remain Gaussian. This shows that
the scenario behind the non Gaussian van Hove functions is a spatially
heterogeneous dynamics, characterized by a spatial distribution of locally
homogeneous dynamical environments through the sample, on the considered time
scales. We characterize these statistical distributions of dynamical
environments, in the liquid, supercooled, and glass states, and show that it
can explain the observed exponential tail of the van Hove functions observed in
the concentrated states. The intensity of spatial heterogeneities was found to
amplify with increasing volume fraction. In the aging regime, it tends to
increase as the glass gets more arrested.Comment: 19 pages, 10 figures, Soft Matter accepte
Simple and flexible mesh parameterization method
International audienceWe describe a method to compute conformal parameterizations with a natural boundary based on a simple differentiable expression measuring angles between edges by using complex numbers. The method can be adapted to preserve metric properties or map textures with con- strained positions. Some illustrations are shown to assess the efficiency of the algorithms
Note concernant le traitement des mesures effectuées avec une mini-sonde Seabird lors des campagnes NOE
Vent, courants et température à 0°, 4°W de février 1983 à octobre 1984
Dans ce document sont présentées les observations de vent, de courants et de température obtenues sur la ligne de mouillage déployée au point O°, 4°W (Golfe de Guinée), de février 1983 à octobre 1984. Ces mesures ont été effectuées dans le cadre du programme français FOCAL (Programme Français Océan Climat en zone équatoriale atlantique) mené en collaboration avec le programme américain SEQUAL (Seasonal Equatorial Atlantic experiment). (Résumé d'auteur
Programme NOE : résultats de courantométrie (mouillage de subsurface) au point 6°11'N - 51°01'W du 27 novembre 1990 au 15 septembre 1991
Programme NOE : résultats hydrologiques et courantologiques des campagnes NOE mai-juin et juillet 1989
Repeated co-option of HMG-box genes for sex determination in brown algae and animals
In many eukaryotes, genetic sex determination is not governed by XX/XY or ZW/ZZ systems but by a specialized region on the poorly studied U (female) or V (male) sex chromosomes. Previous studies have hinted at the existence of a dominant male-sex factor on the V chromosome in brown algae, a group of multicellular eukaryotes distantly related to animals and plants. The nature of this factor has remained elusive. Here, we demonstrate that an HMG-box gene acts as the male-determining factor in brown algae, mirroring the role HMG-box genes play in sex determination in animals. Over a billion-year evolutionary timeline, these lineages have independently co-opted the HMG box for male determination, representing a paradigm for evolution’s ability to recurrently use the same genetic “toolkit” to accomplish similar tasks.</p
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