The distribution of "time of flight" in three dimensional stationary chaotic advection

The distributions of "time of flight" (time spent by a single fluid particle between two crossings of the Poincar\'e section) are investigated for five different 3D stationary chaotic mixers. Above all, we study the large tails of those distributions, and show that mainly two types of behaviors are encountered. In the case of slipping walls, as expected, we obtain an exponential decay, which, however, does not scale with the Lyapunov exponent. Using a simple model, we suggest that this decay is related to the negative eigenvalues of the fixed points of the flow. When no-slip walls are considered, as predicted by the model, the behavior is radically dfferent, with a very large tail following a power law with an exponent close to -3

Mélange de structures scalaires étirées ou en feuillets en advection chaotique

À l'aide de simulations numériques directes d'écoulements chaotiques tridimensionnels et périodiques en temps, nous étudions le mélange des structures iso-scalaires créées par ces écoulements dans le cas de chaos global (absence de régions régulières). Ces structures peuvent être en feuillets (deux exposants de Lyapunov positifs, un négatif) ou étirées en renversant l'écoulement (un exposant positif, les deux autres négatifs). Nous montrons que, contrairement à l'intuition qui voudrait que la diffusion soit gouvernée par l'exposant le plus négatif, le taux de décroissance de l'énergie scalaire c-2 est identique dans les deux cas

Residence time distributions for in-line chaotic mixers

We investigate the distributions of residence time for in-line chaotic mixers; in particular, we consider the Kenics, the F-mixer and the Multi-level laminating mixer, and also a synthetic model that mimics their behavior and allows exact mathematical calculations. We show that whatever the number of elements of mixer involved, the distribution possesses a $t^{-3}$ tail, so that its shape is always far from Gaussian. This $t^{-3}$ tail also invalidates the use of second-order moment and variance. As a measure for the width of the distribution, we consider the mean absolute deviation and show that, unlike the standard deviation, it converges in the limit of large sample size. Finally, we analyze the performances of the different in-line mixers from the residence-time point of view when varying the number of elements and the shape of the cross-section

Diffusiophoresis at the macroscale

Diffusiophoresis, a ubiquitous phenomenon that induces particle transport whenever solute concentration gradients are present, was recently observed in the context of microsystems and shown to strongly impact colloidal transport (patterning and mixing) at such scales. In the present work, we show experimentally that this nanoscale mechanism can induce changes in the macroscale mixing of colloids by chaotic advection. Rather than the decay of the standard deviation of concentration, which is a global parameter commonly employed in studies of mixing, we instead use multiscale tools adapted from studies of chaotic flows or intermittent turbulent mixing: concentration spectra and second and fourth moments of the probability density functions of scalar gradients. Not only can these tools be used in open flows, but they also allow for scale-by-scale analysis. Strikingly, diffusiophoresis is shown to affect all scales, although more particularly the small ones, resulting in a change of scalar intermittency and in an unusual scale bridging spanning more than seven orders of magnitude. By quantifying the averaged impact of diffusiophoresis on the macroscale mixing, we explain why the effects observed are consistent with the introduction of an effective P\'eclet number.Comment: 13 page

Comment mélanger en écoulement laminaire ?

National audienceDans cet article, nous souhaitons toucher du doigt la théorie du mélange dans le cadre des écoulements laminaires, qui sont instinctivement connus pour mal mélanger. Nous allons appliquer ici le principe du chaos, sur un exemple basé sur des écoulements paraboliques croisés pour illustrer qu’il est possible de mélanger malgré le caractère laminaire de l’écoulement. Cet article peut faire l’objet d’un petit exercice de programmation très accessible, à partir de n’importe quel outil de programmation (C, Fortran, ...), ou peut servir de point de départ pour un TIPE (Travaux d’initiative personnelle encadrés)

Phoresis in cellular flows: from enhanced dispersion to blockage

In this article, we study numerically the dispersion of colloids in a two-dimensional cellular flow in the presence of an imposed mean salt gradient. Owing to the additional scalar, the colloids do not follow exactly the Eulerian flow field, but have a (small) extra-velocity proportional to the salt gradient, $\mathbf{v}_\mathrm{dp}=\alpha\nabla S$, where $\alpha$ is the phoretic constant and $S$ the salt concentration. We study the demixing of an homogenous distribution of colloids and how their long-term mean velocity $\mathbf{V_m}$ and effective diffusivity $D_\mathrm{eff}$ are influenced by the phoretic drift. We observe two regimes of colloids dynamics depending on a blockage criterion $R=\alpha G L/\sqrt{4 D_cD_s}$, where $G$ is the mean salt gradient amplitude, $L$ the length scale of the flow and $D_c$ and $D_s$ the molecular diffusivities of colloids and salt. When $R<1$, the mean velocity is strongly enhanced with $V_m \propto \alpha G \sqrt{Pe_s}$, $Pe_s$ being the salt P\'eclet number. When $R> 1$, the compressibility effect due to the phoretic drift is so strong that a depletion of colloids occurs along the separatrices inhibiting cell-to-cell transport

Whole-genome sequencing of Aspergillus tubingensis G131 and overview of its secondary metabolism potential

Background : Black Aspergilli represent one of the most important fungal resources of primary and secondary metabolites for biotechnological industry. Having several black Aspergilli sequenced genomes should allow targeting the production of certain metabolites with bioactive properties. In this study, we report the draft genome of a black Aspergilli, A. tubingensis G131, isolated from a French Mediterranean vineyard. This 35 Mb genome includes 10,994 predicted genes. A genomic-based discovery identifies 80 secondary metabolites biosynthetic gene clusters. Genomic sequences of these clusters were blasted on 3 chosen black Aspergilli genomes: A. tubingensis CBS 134.48, A. niger CBS 513.88 and A. kawachii IFO 4308. This comparison highlights different levels of clusters conservation between the four strains. It also allows identifying seven unique clusters in A. tubingensis G131. Moreover, the putative secondary metabolites clusters for asperazine and naphtho-gamma-pyrones production were proposed based on this genomic analysis. Key biosynthetic genes required for the production of 2 mycotoxins, ochratoxin A and fumonisin, are absent from this draft genome. Even if intergenic sequences of these mycotoxins biosynthetic pathways are present, this could not lead to the production of those mycotoxins by A. tubingensis G131

Chaotic mixing in effective compressible flows

International audienceWe study numerically joint mixing of salt and colloids by chaotic advection and how salt inhomogeneities accelerate or delay colloid mixing by inducing a velocity drift V dp between colloids and fluid particles as proposed in recent experiments [J. Deseigne et al., Soft Matter 10, 4795 (2014)]. We demonstrate that because the drift velocity is no longer divergence free, small variations to the total velocity field drastically affect the evolution of colloid variance σ ^2 = − ^2. A consequence is that mixing strongly depends on the mutual coherence between colloid and salt concentration fields, the short time evolution of scalar variance being governed by a new variance production term P = − /2 when scalar gradients are not developed yet so that dissipation is weak. Depending on initial conditions, mixing is then delayed or enhanced, and it is possible to find examples for which the two regimes (fast mixing followed by slow mixing) are observed consecutively when the variance source term reverses its sign. This is indeed the case for localized patches modeled as Gaussian concentration profiles

Signature of salt-indiced diffusion of particules in a turbulent water jet

International audienceWe study particles dispersion in a turbulent water jet. We focus especially on salt-induced particle transport (diffusiophore-sis). A coarse graining operation is used on scalar fields to quantify mixing scale evolution. Preliminary results show changes in the particle transport, characterized by an enhanced (or reduced) diffusion coefficient

Fungal naphtho-γ-pyrones—secondary metabolites of industrial interest

Naphtho-γ-pyrones (NGPs) are secondary metabolites mainly produced by filamentous fungi (Fusarium sp., Aspergillus sp.) that should be considered by industrials. Indeed, these natural biomolecules show various biological activities: anti-oxidant, anti-microbial, anti-cancer, anti-HIV, anti-hyperuricuric, anti-tubercular, or mammalian triacylglycerol synthesis inhibition which could be useful for pharmaceutical, cosmetic, and/or food industries. In this review, we draw an overview on the interest in studying fungal NGPs by presenting their biological activities and their potential values for industrials, their biochemical properties, and what is currently known on their biosynthetic pathway. Finally, we will present what remains to be discovered about NGPs