Drop impact on a flexible fiber

When droplets impact fibrous media, the liquid can be captured by the fibers or contact then break away. Previous studies have shown that the efficiency of drop capture by a rigid fiber depends on the impact velocity and defined a threshold velocity below which the drop is captured. However, it is necessary to consider the coupling of elastic and capillary effects to achieve a greater understanding of the capture process for soft substrates. Here, we study experimentally the dynamics of a single drop impacting on a thin flexible fiber. Our results demonstrate that the threshold capture velocity depends on the flexibility of fibers in a non-monotonic way. We conclude that tuning the mechanical properties of fibers can optimize the efficiency of droplet capture.Comment: Soft Matter (2015

Damping of liquid sloshing by foams: from everyday observations to liquid transport

We perform experiments on the sloshing dynamics of liquids in a rectangular container submitted to an impulse. We show that when foam is placed on top of the liquid the oscillations of the free interface are significantly damped. The ability to reduce sloshing and associated splashing could find applications in numerous industrial processes involving liquid transport.Comment: Accepted for publication in Journal of Visualizatio

Wetting morphologies on an array of fibers of different radii

We investigate the equilibrium morphology of a finite volume of liquid placed on two parallel rigid fibers of different radii. As observed for identical radii fibers, the liquid is either in a column morphology or adopts a drop shape depending on the inter-fiber distance. However the cross-sectional area and the critical inter-fiber distance at which the transition occurs are both modified by the polydispersity of the fibers. Using energy considerations, we analytically predict the critical inter-fiber distance corresponding to the transition between the column and the drop morphologies occurs. This distance depends both on the radii of the fibers and on the contact angle of the liquid. We perform experiments using a perfectly wetting liquid on two parallel nylon fibers: the results are in good agreement with our analytical model. The morphology of the capillary bridges between fibers of different radii is relevant to the modeling of large arrays of polydisperse fibers

Wetting morphologies on randomly oriented fibers

We characterize the different morphologies adopted by a drop of liquid placed on two randomly oriented fibers, which is a first step toward understanding the wetting of fibrous networks. The present work reviews previous modeling for parallel and touching crossed fibers and extends it to an arbitrary orientation of the fibers characterized by the tilting angle and the minimum spacing distance. Depending on the volume of liquid, the spacing distance between fibers and the angle between the fibers, we highlight that the liquid can adopt three different equilibrium morphologies: (1) a column morphology in which the liquid spreads between the fibers, (2) a mixed morphology where a drop grows at one end of the column or (3) a single drop located at the node. We capture the different morphologies observed using an analytical model that predicts the equilibrium configuration of the liquid based on the geometry of the fibers and the volume of liquid

Dip-coating of suspensions

Withdrawing a plate from a suspension leads to the entrainment of a coating layer of fluid and particles on the solid surface. In this article, we study the Landau-Levich problem in the case of a suspension of non-Brownian particles at moderate volume fraction $10\% < \phi < 41\%$. We observe different regimes depending on the withdrawal velocity $U$, the volume fraction of the suspension $\phi$, and the diameter of the particles $2\,a$. Our results exhibit three coating regimes. (i) At small enough capillary number $Ca$, no particles are entrained, and only a liquid film coats the plate. (ii) At large capillary number, we observe that the thickness of the entrained film of suspension is captured by the Landau-Levich law using the effective viscosity of the suspension $\eta(\phi)$. (iii) At intermediate capillary numbers, the situation becomes more complicated with a heterogeneous coating on the substrate. We rationalize our experimental findings by providing the domain of existence of these three regimes as a function of the fluid and particles properties

Capillary Sorting of Particles by Dip Coating

In this letter, we describe the capillary sorting of particles by size based on dip coating. A substrate withdrawn from a liquid bath entrains a coating whose thickness depends on the withdrawal speed and the liquid properties. If the coating material contains particles, they will only be entrained when the viscous force pulling them with the substrate overcomes the opposing capillary force at the deformable meniscus. This force threshold occurs at different liquid thicknesses for particles of different sizes. Here, we show that this difference can be used to separate small particles from a mixed suspension through capillary filtration. In a bidisperse suspension, we observe three distinct filtration regimes. At low capillary numbers, Ca, no particles are entrained in the liquid coating. At high Ca, all particle sizes are entrained. For a range of capillary numbers between these two extremes, only the smallest particles are entrained while the larger ones remain in the reservoir. We explain how this technique can be applied to polydisperse suspension. We also provide an estimate of the range of capillary number to separate particles of given sizes. The combination of this technique with the scalability and robustness of dip coating makes it a promising candidate for high-throughput separation or purification of industrial and biomedical suspensions

Clogging by sieving in microchannels: Application to the detection of contaminants in colloidal suspensions

We report on a microfluidic method that allows measurement of a small concentration of large contaminants in suspensions of solid micrometer-scale particles. To perform the measurement, we flow the colloidal suspension through a series of constrictions, i.e. a microchannel of varying cross-section. We show and quantify the role of large contaminants in the formation of clogs at a constriction and the growth of the resulting filter cake. By measuring the time interval between two clogging events in an array of parallel microchannels, we are able to estimate the concentration of contaminants whose size is selected by the geometry of the microfluidic device. This technique for characterizing colloidal suspensions offers a versatile and rapid tool to explore the role of contaminants on the properties of the suspensions

Comprendre l’adaptation de Lactococcus lactis par une\ud approche de biologie intégrative à l’échelle du génome

L’adaptation de Lactococcus lactis à différentes conditions de culture a été appréhendée grâce à une\ud démarche de biologie intégrative. Cette approche intègre les données issues de différents niveaux de\ud régulation et combine diverses techniques de mesure à l’échelle globale (transcriptome, protéome,\ud stabilité des ARN messagers) et locale (suivi des paramètres de culture). Plusieurs outils\ud mathématiques de modélisation (tels que la modélisation numérique et la modélisation statistique)\ud ont étés développés pour intégrer l’ensemble de ces données hétérogènes.\ud Une culture continue de L. lactis à différents taux de dilution a permis d’étudier l’influence du taux\ud de croissance sur la physiologie de la bactérie, un paramètre qui n’est jamais distingué de la réponse\ud au stress lors des études dynamiques de l’adaptation. La réponse à la variation du taux de croissance\ud implique majoritairement les fonctions associées à la biogenèse mais demeure extrêmement étendue\ud puisqu’elle affecte l’expression de 30 % des gènes de L. lactis. Cette réponse concerne les niveaux\ud d’ARN messagers et de protéines mais aussi les processus cellulaires majeurs que sont la\ud traduction, la dilution et la dégradation. Il a été montré, par une approche de modélisation, que les\ud efficacités de traduction et les vitesses de dégradation des protéines étaient en effet inversement\ud proportionnelles au taux de croissance. Au final, l’influence des différents processus cellulaires a pu\ud être quantifiée par des calculs de coefficients de contrôle.\ud L’imposition progressive d’une carence en isoleucine lors d’une culture discontinue en batch a\ud permis de caractériser la réponse, encore peu étudiée, de L. lactis à une carence en acide aminé.\ud L’adaptation à ce stress nutritionnel entraîne une vaste réorganisation de la physiologie cellulaire\ud qui se divise en trois types de réponses : une répression globale des principales fonctions\ud biologiques associées à la croissance, une réponse propre au stress imposé visant à lutter\ud spécifiquement contre la carence en isoleucine, ainsi qu’une activation inexpliquée de mécanismes\ud en lien avec le stress oxydatif. L’implication de différents mécanismes (réponse stringente,\ud mécanisme lié au taux de croissance, régulations par CodY, GlnR et CcpA) dans la régulation de\ud cette réponse a été évaluée par transcriptomique comparative.\ud Les déterminants majeurs des concentrations en protéines au sein de la cellule ont été recherchés\ud mathématiquement grâce à un algorithme de sélection de modèles de covariances. Le biais de\ud codons (CAI) s’est avéré être un paramètre majeur, plus important que les concentrations en ARN\ud messagers, suggérant l’existence d’un contrôle génétique prépondérant sur l’adaptation\ud transcriptionnelle. Enfin, il a pu être démontré que le degré d’implication des différents\ud déterminants varie en fonction du mode d’adaptation.\ud L’approche de biologie intégrative suivie au cours de cette thèse a permis une meilleure\ud compréhension des mécanismes d’adaptation de L. lactis et est aujourd’hui entièrement\ud généralisable à d’autres processus comme à d’autres microorganismes. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- A systems biology approach was implemented to study Lactococcus lactis adaptation to\ud various growing conditions. This method combines growth parameter monitoring and\ud genome-wide measurement technologies (transcriptome, proteome, messenger RNA\ud stability). Data from these diverse regulation levels were integrated thanks to mathematical\ud tools developed on purpose.\ud Growth rate influence on L. lactis physiology, which is never dissociated from stress\ud responses when studying dynamic adaptation processes, was analysed through continuous\ud culture at various growth rates. This widespread response mainly involves biogenesis-related\ud functions and affects the expression of 30 % of L. lactis genes. Both messenger RNA and\ud protein levels are modified but cellular processes such as translation, dilution and degradation\ud are also concerned. As a matter of fact, translation efficiency and protein degradation rates\ud were proved to be inversely proportional to growth rate by a modelling approach. Control\ud coefficient calculations enabled the quantification of cellular processes influences.\ud The dynamic response of L. lactis to isoleucine starvation was studied by the progressive\ud consumption of this amino-acid in a discontinuous batch fermentation. This poorly\ud characterized adaptation process triggers a wide reorganization of cellular physiology that\ud could be divided in three parts: a global repression of the main biological functions related to\ud growth, a response more specific to the encountered stress to struggle against isoleucine\ud starvation and an unexplained activation of oxidative stress-related cellular functions.\ud Comparative transcriptomics allowed the implication of various mechanisms to be quantified\ud in the regulation of this adaptation response (stringent response, growth rate adaptation\ud mechanism, CodY, GlnR and CcpA regulation).\ud The major biological determinants of protein intracellular concentration were mathematically\ud investigated thanks to a covariance model selection algorithm. Codons bias (CAI) was found\ud to be the most influent parameter, even more than mRNA concentrations, which suggests that\ud genetic control is stronger than transcriptional adaptation. The weight of the different\ud determinants was also found to depend on adaptation modes.\ud The systems biology approach applied in this work enabled a better understanding of L. lactis\ud adaptation mechanisms and will be entirely transposable to other cellular processes as well as\ud other microorganism