Shear-induced transitions and instabilities in surfactant wormlike micelles

In this review, we report recent developments on the shear-induced transitions and instabilities found in surfactant wormlike micelles. The survey focuses on the non-linear shear rheology and covers a broad range of surfactant concentrations, from the dilute to the liquid-crystalline states and including the semi-dilute and concentrated regimes. Based on a systematic analysis of many surfactant systems, the present approach aims to identify the essential features of the transitions. It is suggested that these features define classes of behaviors. The review describes three types of transitions and/or instabilities : the shear-thickening found in the dilute regime, the shear-banding which is linked in some systems to the isotropic-to-nematic transition, and the flow-aligning and tumbling instabilities characteristic of nematic structures. In these three classes of behaviors, the shear-induced transitions are the result of a coupling between the internal structure of the fluid and the flow, resulting in a new mesoscopic organization under shear. This survey finally highlights the potential use of wormlike micelles as model systems for complex fluids and for applications.Comment: 64 pages, 31 figures, 2 table

Viscoelastic liquid curtains: Experimental results on the flow of a falling sheet of polymer solution

International audienceWe experimentally investigate the extensional flow of a sheet-or curtain-of viscoelastic liquid falling freely from a slot at constant flow rate under gravity. Extruded liquids are aqueous solutions of flexible polyethylene oxide (PEO) and of semi-rigid partially hydrolysed polyacrylamide (HPAM) with low shear viscosities. Velocimetry measurements reveal that the mean velocity field U(z), z being the distance from the slot exit, does not reduce to a free-fall. More precisely, we show that the liquid falls initially with sub-gravitational accelerations up to a distance from the slot which scales as gτ 2 f il , where g is gravity and τ f il is the extensional relaxation time of the liquid, beyond which the local acceleration reaches the asymptotic free-fall value g. The length of the sub-gravitational part of the curtain is shown to be much larger than the equivalent viscous length ((4η/ρ) 2 /g) 1/3 for Newtonian liquids of density ρ and dynamic viscosity η, which is usually small compared to the length of the curtain. The elastic length gτ 2 f il can indeed be surprisingly large when adding high molecular weight polymer molecules to a low-viscosity Newtonian solvent. By analogy with Newtonian curtains, we show that the velocity field U(z) rescales on a master curve. Besides, we show that the flow is only weakly affected by the history of polymer deformations in the die upstream of the curtain. Furthermore, investigations of the curtain stability reveal that polymer addition reduces the minimum flow rate required to maintain a continuous sheet of liquid

Des bandes de cisaillement à la turbulence viscoélastique

En rhéologie, les solutions de micelles géantes sont utilisées comme systèmes modèles depuis près de trente ans. Elles furent d'abord étudiées aux faibles déformations, pour lesquelles elles se comportent comme des fluides de Maxwell, l'exemple le plus simple de viscoélasticité. Par la suite, il fut découvert qu'aux plus larges déformations, l'écoulement se sépare en bandes de cisaillement. Au cours des vingt années écoulées depuis cette découverte, les écoulements en bandes de cisaillement ont été étudiés par de nombreuses techniques comme la rhéométrie, la vélocimétrie ou encore la biréfringence d'écoulement. Toutefois, un grand nombre de données collectées sur ces systèmes mirent en évidence de nombreuses fluctuations spatiotemporelles imprévues. Dans cette thèse, nous montrons comment la plupart de ces fluctuations peut être la conséquence d'écoulements secondaires jusque là ignorés. Nous montrons que les écoulements secondaires ont pour origine une instabilité purement viscoélastique qui n'avait jusque là été observée que dans les solutions de polymères. Dans un premier temps, l'instabilité génère des écoulements cohérents mais non-triviaux, et à plus hautes déformations, l’écoulement devient turbulent. Nous étudions en détail l'exemple de l'écoulement de Couette cylindrique. Nous adaptons le critère d'instabilité au cas des écoulements en bandes. Cela suggère l'existence de trois catégories d’écoulements en bandes, en fonction de leur stabilité, ce que nous confirmons à travers des expériences sur de nombreuses conditions différentes. En accord avec nos prédictions théoriques, une augmentation de la concentration en micelles ou de la courbure de la géométrie déstabilise l'écoulement, alors qu'une augmentation de la température le stabilise. Mais les expériences suggèrent aussi certains comportements allant au-delà du critère d'instabilité purement viscoélastique. Nous mettons en particulier l'accent sur l'importance potentielle des modes d'interface et l'influence du glissement

Transcriptomic analysis of Arabidopsis developing stems: a close-up on cell wall genes

<p>Abstract</p> <p>Background</p> <p>Different strategies (genetics, biochemistry, and proteomics) can be used to study proteins involved in cell biogenesis. The availability of the complete sequences of several plant genomes allowed the development of transcriptomic studies. Although the expression patterns of some <it>Arabidopsis thaliana </it>genes involved in cell wall biogenesis were identified at different physiological stages, detailed microarray analysis of plant cell wall genes has not been performed on any plant tissues. Using transcriptomic and bioinformatic tools, we studied the regulation of cell wall genes in <it>Arabidopsis </it>stems, <it>i.e. </it>genes encoding proteins involved in cell wall biogenesis and genes encoding secreted proteins.</p> <p>Results</p> <p>Transcriptomic analyses of stems were performed at three different developmental stages, <it>i.e.</it>, young stems, intermediate stage, and mature stems. Many genes involved in the synthesis of cell wall components such as polysaccharides and monolignols were identified. A total of 345 genes encoding predicted secreted proteins with moderate or high level of transcripts were analyzed in details. The encoded proteins were distributed into 8 classes, based on the presence of predicted functional domains. Proteins acting on carbohydrates and proteins of unknown function constituted the two most abundant classes. Other proteins were proteases, oxido-reductases, proteins with interacting domains, proteins involved in signalling, and structural proteins. Particularly high levels of expression were established for genes encoding pectin methylesterases, germin-like proteins, arabinogalactan proteins, fasciclin-like arabinogalactan proteins, and structural proteins. Finally, the results of this transcriptomic analyses were compared with those obtained through a cell wall proteomic analysis from the same material. Only a small proportion of genes identified by previous proteomic analyses were identified by transcriptomics. Conversely, only a few proteins encoded by genes having moderate or high level of transcripts were identified by proteomics.</p> <p>Conclusion</p> <p>Analysis of the genes predicted to encode cell wall proteins revealed that about 345 genes had moderate or high levels of transcripts. Among them, we identified many new genes possibly involved in cell wall biogenesis. The discrepancies observed between results of this transcriptomic study and a previous proteomic study on the same material revealed post-transcriptional mechanisms of regulation of expression of genes encoding cell wall proteins.</p

Transcriptomic analysis of Arabidopsis developing stems: a close-up on cell wall genes

<p>Abstract</p> <p>Background</p> <p>Different strategies (genetics, biochemistry, and proteomics) can be used to study proteins involved in cell biogenesis. The availability of the complete sequences of several plant genomes allowed the development of transcriptomic studies. Although the expression patterns of some <it>Arabidopsis thaliana </it>genes involved in cell wall biogenesis were identified at different physiological stages, detailed microarray analysis of plant cell wall genes has not been performed on any plant tissues. Using transcriptomic and bioinformatic tools, we studied the regulation of cell wall genes in <it>Arabidopsis </it>stems, <it>i.e. </it>genes encoding proteins involved in cell wall biogenesis and genes encoding secreted proteins.</p> <p>Results</p> <p>Transcriptomic analyses of stems were performed at three different developmental stages, <it>i.e.</it>, young stems, intermediate stage, and mature stems. Many genes involved in the synthesis of cell wall components such as polysaccharides and monolignols were identified. A total of 345 genes encoding predicted secreted proteins with moderate or high level of transcripts were analyzed in details. The encoded proteins were distributed into 8 classes, based on the presence of predicted functional domains. Proteins acting on carbohydrates and proteins of unknown function constituted the two most abundant classes. Other proteins were proteases, oxido-reductases, proteins with interacting domains, proteins involved in signalling, and structural proteins. Particularly high levels of expression were established for genes encoding pectin methylesterases, germin-like proteins, arabinogalactan proteins, fasciclin-like arabinogalactan proteins, and structural proteins. Finally, the results of this transcriptomic analyses were compared with those obtained through a cell wall proteomic analysis from the same material. Only a small proportion of genes identified by previous proteomic analyses were identified by transcriptomics. Conversely, only a few proteins encoded by genes having moderate or high level of transcripts were identified by proteomics.</p> <p>Conclusion</p> <p>Analysis of the genes predicted to encode cell wall proteins revealed that about 345 genes had moderate or high levels of transcripts. Among them, we identified many new genes possibly involved in cell wall biogenesis. The discrepancies observed between results of this transcriptomic study and a previous proteomic study on the same material revealed post-transcriptional mechanisms of regulation of expression of genes encoding cell wall proteins.</p

Transitions de phases induites par écoulement dans les systèmes de micelles géantes : étude optique et rhéologique

Not availbleNous nous sommes intéressés au comportement sous écoulement de solutions semi-diluées de micelles géantes flexibles qui présentent une transition de phase induite par le cisaillement au-delà d'un gradient de vitesse critique. La signature mécanique d'une telle transition se traduit par la présence, dans la courbe d'écoulement, d'un plateau de contrainte associé à l'apparition d'un écoulement hétérogène qui se structure en bandes de cisaillement. Afin d'étudier la cinétique de formation de ce type d'écoulement, nous avons mis en ?uvre différentes techniques expérimentales telles que la rhéologie, la biréfringence d'écoulement et la diffusion de la lumière aux petits angles. L'objectif principal de notre approche a été de corréler les réponses optiques et mécaniques à celle des propriétés microstructurales pour tenter d'apporter des informations sur les mécanismes à l'origine de la formation de la structure de bandes. Plusieurs échantillons ont été étudiés, tous préparés à partir du même agent tensioactif, le bromure de cetyltrimethylammonium auquel nous avons ajouté différents sels minéraux ou organiques. Au travers des résultats obtenus, nous avons montré que l'établissement de la structure de bandes est associé à une évolution temporelle générique des grandeurs macroscopiques (contrainte tangentielle, angle d'extinction et intensité de la birefringence) avec des effets plus ou moins marqués selon la composition de l'échantillon. Par contre, les visualisations directes de l'entrefer de la cellule de Couette révèlent que s'il existe une universalité du comportement macroscopique transitoire, il n'en est pas de même des propriétés au niveau microscopique, les structures de bandes observées en régime stationnaire étant elles-mêmes très variée

Transitions de phases induites par écoulement dans les systèmes de micelles géantes (étude optique et rhéologique)

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