1,948 research outputs found
Dynamic clustering in active colloidal suspensions with chemical signaling
In this paper, we explore experimentally the phase behavior of a dense active
suspension of self- propelled colloids. In addition to a solid-like and a
gas-like phase observed for high and low densities, a novel cluster phase is
reported at intermediate densities. This takes the form of a stationary
assembly of dense aggregates, with an average size which grows with activity as
a linear function of the self-propelling velocity. While different possible
scenarii can be considered to account for these observations - such as a
generic velocity weakening instability recently put forward -, we show that the
experimental results are reproduced by a chemotactic aggregation mechanism,
originally introduced to account for bacterial aggregation, and accounting here
for diffusiophoretic chemical interaction between colloidal swimmers.Comment: supplementary video :http://
www-lpmcn.univ-lyon1.fr/~lbocquet/Movie-Theurkauff-SI.av
Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal,
with a single conical band, located at the center of the Brillouin zone. The
cone's lack of protection by the lattice symmetry immediately sparked vast
discussions about the size and topological/trivial nature of a possible gap
opening. Here we report on a combined optical and transport study of ZrTe5,
which reveals an alternative view of electronic bands in this material. We
conclude that the dispersion is approximately linear only in the a-c plane,
while remaining relatively flat and parabolic in the third direction (along the
b axis). Therefore, the electronic states in ZrTe5 cannot be described using
the model of 3D Dirac massless electrons, even when staying at energies well
above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment
Bouncing or sticky droplets: impalement transitions on superhydrophobic micropatterned surfaces
When a liquid drops impinges a hydrophobic rough surface it can either bounce
off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel
droplets). The analysis of drop impact and quasi static ''loading'' experiments
on model microfabricated surfaces allows to clearly identify the forces
hindering the impalement transitions. A simple semi-quantitative model is
proposed to account for the observed relation between the surface topography
and the robustness of fakir non-wetting states. Motivated by potential
applications in microfluidics and in the fabrication of self cleaning surfaces,
we finally propose some guidelines to design robust superhydrophobic surfaces.Comment: 7 pages, 5 figure
Dynamic surface decoupling in a sheared polymer melt
We propose that several mechanisms contribute to friction in a polymer melt
adsorbed at a structured surface. The first one is the well known
disentanglement of bulk polymer chains from the surface layer. However, if the
surface is ideal at the atomic scale, the adsorbed parts of polymer chains can
move along the equipotential lines of the surface potential. This gives rise to
a strong slippage of the melt. For high shear rates chains partially desorb.
However, the friction force on adsorbed chains increases, resulting in
quasi-stick boundary conditions. We propose that the adsorbed layers can be
efficiently used to adjust the friction force between the polymer melt and the
surface
Low Friction Flows of Liquids at Nanopatterned Interfaces
With the recent important development of microfluidic systems,
miniaturization of flow devices has become a real challenge. Microchannels,
however, are characterized by a large surface to volume ratio, so that surface
properties strongly affect flow resistance in submicrometric devices. We
present here results showing that the concerted effect of wetting . properties
and surface roughness may considerably reduce friction of the fluid past the
boundaries. The slippage of the fluid at the channel boundaries is shown to be
drastically increased by using surfaces that are patterned at the nanometer
scale. This effect occurs in the regime where the surface pattern is partially
dewetted, in the spirit of the 'superhydrophobic' effects that have been
recently discovered at the macroscopic scales. Our results show for the first
time that, in contrast to the common belief, surface friction may be reduced by
surface roughness. They also open the possibility of a controlled realization
of the 'nanobubbles' that have long been suspected to play a role in
interfacial slippag
The potential for climate-driven bathymetric range shifts: sustained temperature and pressure exposures on a marine ectotherm, Palaemonetes varians
Range shifts are of great importance as a response for species facing climate change. In the light of current ocean-surface warming, many studies have focused on the capacity of marine ectotherms to shift their ranges latitudinally. Bathymetric range shifts offer an important alternative, and may be the sole option for species already at high latitudes or those within enclosed seas; yet relevant data are scant. Hydrostatic pressure (HP) and temperature have wide ranging effects on physiology, importantly acting in synergy thermodynamically, and therefore represent key environmental constraints to bathymetric migration. We present data on transcriptional regulation in a shallow-water marine crustacean (Palaemonetes varians) at atmospheric and high HP following 168-h exposures at three temperatures across the organisms' thermal scope, to establish the potential physiological limit to bathymetric migration by neritic fauna. We observe changes in gene expression indicative of cellular macromolecular damage, disturbances in metabolic pathways and a lack of acclimation after prolonged exposure to high HP. Importantly, these effects are ameliorated (less deleterious) at higher temperatures, and exacerbated at lower temperatures. These data, alongside previously published behavioural and heat-shock analyses, have important implications for our understanding of the potential for climate-driven bathymetric range shift
Transcript profiling in Candida albicans reveals new cellular functions for the transcriptional repressors CaTup1, CaMig1 and CaNrg1.
The pathogenic fungus, Candida albicans contains homologues of the transcriptional repressors ScTup1, ScMig1 and ScNrg1 found in budding yeast. In Saccharomyces cerevisiae, ScMig1 targets the ScTup1/ScSsn6 complex to the promoters of glucose repressed genes to repress their transcription. ScNrg1 is thought to act in a similar manner at other promoters. We have examined the roles of their homologues in C. albicans by transcript profiling with an array containing 2002 genes, representing about one quarter of the predicted number of open reading frames (ORFs) in C. albicans. The data revealed that CaNrg1 and CaTup1 regulate a different set of C. albicans genes from CaMig1 and CaTup1. This is consistent with the idea that CaMig1 and CaNrg1 target the CaTup1 repressor to specific subsets of C. albicans genes. However, CaMig1 and CaNrg1 repress other C. albicans genes in a CaTup1-independent fashion. The targets of CaMig1 and CaNrg1 repression, and phenotypic analyses of nrg1/nrg1 and mig1/mig1 mutants, indicate that these factors play differential roles in the regulation of metabolism, cellular morphogenesis and stress responses. Hence, the data provide important information both about the modes of action of these transcriptional regulators and their cellular roles. The transcript profiling data are available at http://www.pasteur.fr/recherche/unites/RIF/transcriptdata/
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