[S1] Lateral line hair cells integrate mechanical and chemical cues to orient navigation. Laura Desban, Julian Roussel, Olivier Mirat, François-Xavier Lejeune, Ludovic Keiser, Nicolas Antoine Michalski & Claire Wyart bioRxiv
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Abstract: The lateral line is a superficial sensory system responding to environmental hydrodynamic changes to orient locomotion of aquatic vertebrate species. Whether this system also detects chemical cues is unknown. We find that zebrafish lateral line hair cells express numerous chemoreceptors, including ionotropic receptors for serotonin. We show that the serotonin enriched in skin neuroepithelial cells is released upon injury and that environmental serotonin activates lateral line hair cells. We show that larval zebrafish exposed to serotonin in their environment rely on the lateral line to swim fast and away. These results uncover the sensory versatility of lateral line hair cells and how these properties modulate navigation in response to environmental stimuli.
Published
[15] Embolism propagation in Adiantum leaves and in a biomimetic system with constrictions. Ludovic Keiser, Benjamin Dollet & Philippe Marmottant Journal of the Royal Society Interface, 21 (217): 20240103 (2024)
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Abstract: Drought poses a significant threat to forest survival worldwide by potentially generating air bubbles that obstruct sap transport within plants’ hydraulic systems. However, the detailed mechanism of air entry and propagation at the scale of the veins remains elusive. Building upon a biomimetic model of leaf which we developed, we propose a direct comparison of the air embolism propagation in Adiantum (maidenhair fern) leaves, presented in Brodribb et al. (Brodribb TJ, Bienaimé D, Marmottant P. 2016 Revealing catastrophic failure of leaf networks under stress. Proc. Natl Acad. Sci. USA 113, 4865–4869 (doi:10.1073/pnas.1522569113)) and in our biomimetic leaves. In particular, we evidence that the jerky dynamics of the embolism propagation observed in Adiantum leaves can be recovered through the introduction of micrometric constrictions in the section of our biomimetic veins, mimicking the nanopores present in the bordered pit membranes in real leaves. We show that the intermittency in the propagation can be retrieved by a simple model coupling the variations of pressure induced by the constrictions and the variations of the volume of the compliant microchannels. Our study marks a step with the design of a biomimetic leaf that reproduces particular aspects of embolism propagation in real leaves, using a minimal set of controllable and readily tunable components. This biomimetic leaf constitutes a promising physical analogue and sets the stage for future enhancements to fully embody the unique physical features of embolizing real leaves.
[14] Releasing long bubbles trapped in thin capillaries via tube centrifugation and inclination. Alice Marcotte, Pier Giuseppe Ledda, Valentin Buriasco, Paul Dené, François Gallaire, Ludovic Keiser Journal of Fluid Mechanics, 999: A9 (2024)
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Abstract: In confined systems, the entrapment of a gas volume with an equivalent spherical diameter greater than the dimension of the channel can form extended bubbles that obstruct fluid circuits and compromise performance. Notably, in sealed vertical tubes, buoyant long bubbles -- called Taylor bubbles -- cannot rise if the inner tube radius is below a critical value near the capillary length. This critical threshold for steady ascent is determined by geometric constraints related to matching the upper cap shape with the lubricating film in the elongated part of the bubble. Developing strategies to overcome this threshold and release stuck bubbles is essential for applications involving narrow liquid channels. Effective strategies involve modifying matching conditions with an external force field to facilitate bubble ascent. However, it's unclear how changes in acceleration conditions affect the motion onset of buoyancy-driven long bubbles. This study investigates the mobility of elongated bubbles in sealed tubes with an inner radius near the critical value inhibiting bubble motion in a vertical setting. Two strategies are explored to tune bubble motion, leveraging variations in axial and transversal accelerations: tube rotation around its axis and tube inclination relative to gravity. By revising the geometrical constraints of the simple vertical setting, the study predicts new thresholds based on rotational speed and tilt angle, respectively, providing forecasts for the bubble rising velocity under modified apparent gravity. Experimental measurements of motion threshold and rising velocity compare well with theoretical developments, thus suggesting practical approaches to control and tune bubble motion in confined environments.
[13] Breaking one into three: surface-tension-driven droplet breakup in T-junctions. Jiande Zhou, Yves-Marie Ducimetière, Daniel Migliozzi, Ludovic Keiser, Arnaud Bertsch, François Gallaire & Philippe Renaud Physical Review Fluids 8 (5): 054201 (2023)
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Abstract: Droplet breakup is an important phenomenon in the field of microfluidics to generate daughter droplets. In this work, a novel breakup regime in the widely studied T-junction geometry is reported, where the pinch-off occurs laterally in the two outlet channels, leading to the formation of three daughter droplets, rather than at the center of the junction for conventional T-junctions which leads to two daughter droplets. It is demonstrated that this new mechanism is driven by surface tension, and a design rule for the T-junction geometry is proposed. A model for low values of the capillary number Ca is developed to predict the formation and growth of an underlying carrier fluid pocket that accounts for this lateral breakup mechanism. At higher values of Ca, the conventional regime of central breakup becomes dominant again. The competition between the new and the conventional regime is explored. Altogether, this novel droplet formation method at T-junction provides the functionality of alternating droplet size and composition, which can be important for the design of new microfluidic tools.
[12] CSF-contacting neurons respond to Streptococcusem> pneumoniae and promote host survival during central nervous system infection. Andrew E Prendergast, Kin Ki Jim, Hugo Marnas, Laura Desban, Feng B Quan, Lydia Djenoune, Valerio Laghi, Agnès Hocquemiller, Elias T. Lunsford, Julian Roussel, Ludovic Keiser, et al. Current Biology 33 (5): 940-956.e10 (2023)
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Abstract: The pathogenic bacterium Streptococcus pneumoniae (S. pneumoniae) can invade the cerebrospinal fluid (CSF) and cause meningitis with devastating consequences. Whether and how sensory cells in the central nervous system (CNS) become activated during bacterial infection, as recently reported for the peripheral nervous system, is not known. We find that CSF infection by S. pneumoniae in larval zebrafish leads to changes in posture and behavior that are reminiscent of pneumococcal meningitis, including dorsal arching and epileptic-like seizures. We show that during infection, invasion of the CSF by S. pneumoniae massively activates in vivo sensory neurons contacting the CSF, referred to as “CSF-cNs” and previously shown to detect spinal curvature and to control posture, locomotion, and spine morphogenesis. We find that CSF-cNs express orphan bitter taste receptors and respond in vitro to bacterial supernatant and metabolites via massive calcium transients, similar to the ones observed in vivo during infection. Upon infection, CSF-cNs also upregulate the expression of numerous cytokines and complement components involved in innate immunity. Accordingly, we demonstrate, using cell-specific ablation and blockade of neurotransmission, that CSF-cN neurosecretion enhances survival of the host during S. pneumoniae infection. Finally, we show that CSF-cNs respond to various pathogenic bacteria causing meningitis in humans, as well as to the supernatant of cells infected by a neurotropic virus. Altogether, our work uncovers that central sensory neurons in the spinal cord, previously involved in postural control and morphogenesis, contribute as well to host survival by responding to the invasion of the CSF by pathogenic bacteria during meningitis.
[11] Whirling instability of an eccentric coated fibre. Shahab Eghbali, Ludovic Keiser, Edouard Boujo & François Gallaire Journal of Fluid Mechanics 952: A33 (2022)
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Abstract: We study a gravity-driven viscous flow coating a vertical cylindrical fibre. The destabilisation of a draining liquid column into a downward moving train of beads has been linked to the conjunction of the Rayleigh–Plateau and Kapitza instabilities in the limit of small Bond numbers Bo. Here, we focus on quasi-inertialess flows (large Ohnesorge number Oh) and conduct a linear stability analysis on a unidirectional flow along a rigid eccentric fibre for intermediate to large Bo. We show the existence of two unstable modes, namely pearl and whirl modes. The pearl mode depicts asymmetric beads, similar to that of the Rayleigh–Plateau instability, whereas a single helix forms along the axis in the whirl mode instability. The geometric and hydrodynamic thresholds of the whirl mode instability are investigated, and phase diagrams showing the transition thresholds between different regimes are presented. Additionally, an energy analysis is carried out to elucidate the whirl formation mechanism. This analysis reveals that despite the unfavourable capillary energy cost, the asymmetric interface shear distribution, caused by the fibre eccentricity, has the potential to sustain a whirling interface. In general, small fibre radius and large eccentricity tend to foster the whirl mode instability, while reducing Bo tends to favour the dominance of the pearl mode instability. Finally, we compare the predictions of our model with the results of some illustrative experiments, using highly viscous silicone oils flowing down fibres. Whirling structures are observed for the first time, and the measured wavenumbers match our stability analysis prediction.
[10] Intermittent air invasion in pervaporating compliant microchannels. Ludovic Keiser, Philippe Marmottant & Benjamin Dollet Journal of Fluid Mechanics 948: A52 (2022)
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Abstract: We explore air invasion in a dead-end compliant water-filled microchannel containing a constriction. The phenomenon is driven by the pervaporation of the liquid present in the channel through the surrounding medium. The penetration is intermittent, jerky, and characterised by a stop-and-go dynamics as the bubble escapes the constriction. We demonstrate that this sequence of arrest and jump of the bubble is due to an elasto-capillary coupling between the air–liquid interface and the elastic medium. When the interface enters the constriction, its curvature increases strongly, leading to a depressurisation within the liquid-filled channel that drives a compression of the channel. As the interface is forced to leave the constriction at a given threshold pressure, due to the ongoing loss of liquid content by pervaporation, the pressure is suddenly released, which gives rise to a rapid propagation of the air bubble away from the constriction and a restoration of the rest shape of the channel. Combining macroscopic observations and confocal imaging, we present a comprehensive experimental study of this phenomenon. In particular, the effect of the channel geometry on the time of arrest in the constriction and the jump length is investigated. Our novel microfluidic design succeeds in mimicking the role of inter-vessel pits in plants, which transiently stop the propagation of air embolisms during long and severe droughts. It is expected to serve as a building block for further biomimetic studies in more complex leaf-like architectures, in order to recover this universal phenomenon of intermittent propagation reported in real leaves.
[9] Automated analysis of cerebrospinal fluid flow and motile cilia properties in the central canal of zebrafish embryos. Olivier Thouvenin, Yasmine Cantaut-Belarif, Ludovic Keiser, François Gallaire & Claire Wyart Bio-protocol 11 (5): e3932 (2021)
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Abstract: Circulation of cerebrospinal fluid (CSF) plays an important role during development. In zebrafish embryos, the flow of CSF has been found to be bidirectional in the central canal of the spinal cord. To compare conditions and genetic mutants across each other, we recently automated the quantification of the velocity profile of exogenous fluorescent particles in the CSF. We demonstrated that the beating of motile and tilted cilia localized on the ventral side of the central canal was sufficient to generate such bidirectionality locally. Our approach can easily be extended to characterize CSF flow in various genetic mutants. We provide here a detailed protocol and a user interface program to quantify CSF dynamics. To interpret potential changes in CSF flow profiles, we provide additional tools to measure the central canal diameter, characterize cilia dynamics, and compare experimental data with our theoretical model to estimate the impact of cilia in generating a volume force in the central canal. Our approach can also be used for measuring particle velocity in vivo and modeling flow in diverse biological solutions.
[8] Environmental control of amyloid polymorphism by modulation of hydrodynamic stress. Jiangtao Zhou, Leonardo Venturelli, Ludovic Keiser, Sergey K Sekatskii, François Gallaire, et al. ACS Nano 15 (1): 944-953 (2021)
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Abstract: The phenomenon of amyloid polymorphism is a key feature of protein aggregation. Unraveling this phenomenon is of great significance for understanding the underlying molecular mechanisms associated with neurodegenerative diseases and for the development of amyloid-based functional biomaterials. However, the understanding of the molecular origins and the physicochemical factors modulating amyloid polymorphs remains challenging. Herein, we demonstrate an association between amyloid polymorphism and environmental stress in solution, induced by an air/water interface in motion. Our results reveal that low-stress environments produce heterogeneous amyloid polymorphs, including twisted, helical, and rod-like fibrils, whereas high-stress conditions generate only homogeneous rod-like fibrils. Moreover, high environmental stress converts twisted fibrils into rod-like fibrils both in-pathway and after the completion of mature amyloid formation. These results enrich our understanding of the environmental origin of polymorphism of pathological amyloids and shed light on the potential of environmentally controlled fabrication of homogeneous amyloid biomaterials for biotechnological applications.
[7] Origin and role of the cerebrospinal fluid bidirectional flow in the central canal. Olivier Thouvenin, Ludovic Keiser, Yasmine Cantaut-Belarif, Martin Carbo-Tano, et al. eLife 9: e47699 (2020)
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Abstract: Circulation of the cerebrospinal fluid (CSF) contributes to body axis formation and brain development. Here, we investigated the unexplained origins of the CSF flow bidirectionality in the central canal of the spinal cord of 30 hpf zebrafish embryos and its impact on development. Experiments combined with modeling and simulations demonstrate that the CSF flow is generated locally by caudally-polarized motile cilia along the ventral wall of the central canal. The closed geometry of the canal imposes the average flow rate to be null, explaining the reported bidirectionality. We also demonstrate that at this early stage, motile cilia ensure the proper formation of the central canal. Furthermore, we demonstrate that the bidirectional flow accelerates the transport of particles in the CSF via a coupled convective-diffusive transport process. Our study demonstrates that cilia activity combined with muscle contractions sustain the long-range transport of extracellular lipidic particles, enabling embryonic growth.
Abstract: We study the sedimentation of highly viscous droplets confined inside Hele-Shaw cells with textured walls of controlled topography. In contrast with common observations on superhydrophobic surfaces, roughness tends here to significantly increase viscous friction, thus substantially decreasing the droplets mobility. However, reducing confinement induces a jump in the velocity as droplets can slide on a lubricating layer of the surrounding fluid thicker than the roughness features. We demonstrate that increasing the viscosity of the surrounding liquid may counterintuitively enhance the mobility of a droplet sliding along a rough wall. Similarly, a sharp change of the droplet mobility is observed as the amplitude of the roughness is modified. These results illustrate the nontrivial friction processes at the scale of the roughness, and the coupling between viscous dissipation in the drop, in the front meniscus, and in the lubricating film. They could enable one to specifically control the speed of droplets or capsules in microchannels, based on their rheological properties.
[5] Marangoni bursting: Evaporation-induced emulsification of a two-component droplet. Guillaume Durey, Hoon Kwon, Quentin Magdelaine, Ludovic Keiser, et al. Physical Review Fluids 3 (10): 100501 (2018)
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Abstract: This paper is associated with a video winner of a 2017 APS/DFD Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available from the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2017.GFM.V0020
[4] Dynamics of non-wetting drops confined in a Hele-Shaw cell. Ludovic Keiser, Khalil Jaafar, José Bico & Étienne Reyssat Journal of Fluid Mechanics 845: 245-262 (2018)
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Abstract: We experimentally investigate the sedimentation of a non-wetting drop confined between two parallel walls. The whole system is immersed in a bath of liquid of low viscosity and a lubricating film may be dynamically formed between the drop and the walls of the cell. Depending on the thickness of the film and on the viscosity ratio between the droplet and the surrounding liquid, viscous dissipation localizes either in the lubrication layer or in the bulk of the drop. The velocity of the droplet is non-trivial as the thickness of the lubricating layer may depend on the interplay between interfacial tension and viscous dissipation. Interestingly, thin films whose nanometric thickness is set by long range intermolecular interactions may lubricate efficiently the motion of highly viscous droplets. We derive asymptotic models that successfully capture the settling velocity of the drop in the different regimes observed experimentally. The effect of partial wetting is finally illustrated by a sharp increase of the velocity of the drops that we attribute to a wetting transition.
[3] Marangoni bursting: Evaporation-induced emulsification of binary mixtures on a liquid layer. Ludovic Keiser, Hadrien Bense, Pierre Colinet, José Bico & Étienne Reyssat Physical Review Letters 118 (7): 074504 (2017)
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Abstract: Adjusting the wetting properties of water through the addition of a miscible liquid is commonly used in a wide variety of industrial processes involving interfaces. We investigate experimentally the evolution of a drop of water and volatile alcohol deposited on a bath of oil: The drop spreads and spontaneously fragments into a myriad of minute droplets whose size strongly depends on the initial concentration of alcohol. Marangoni flows induced by the evaporation of alcohol play a key role in the overall phenomenon. The intricate coupling of hydrodynamics, wetting, and evaporation is well captured by analytical scaling laws. Our scenario is confirmed by experiments involving other combinations of liquids that also lead to this fascinating phenomenon.
[2] Drop friction on liquid-infused materials. Armelle Keiser, Ludovic Keiser, Christophe Clanet & David Quéré Soft Matter 13 (39): 6981-6987 (2017)
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Abstract: We discuss in this paper the nature of the friction generated as a drop glides on a textured material infused by another liquid. Different regimes are found, depending on the viscosities of both liquids. While a viscous drop simply obeys a Stokes-type friction, the force opposing a drop moving on a viscous substrate becomes non-linear in velocity. A liquid on an infused material is surrounded by a meniscus, and this specific feature is proposed to be responsible for the special frictions observed on both adhesive and non-adhesive substrates.
[1] Washing wedges: Capillary instability in a gradient of confinement. Ludovic Keiser, Rémy Herbaut, José Bico & Étienne Reyssat Journal of Fluid Mechanics 790: 619-633 (2016)
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Abstract: We present experimental results on the extraction of oil trapped in the confined region of a wedge. Upon addition of a more wetting liquid, we observe that oil fingers develop into this extracting liquid. The fingers eventually pinch off and form droplets that are driven away from the apex of the wedge by surface tension along the gradient of confinement. During an experiment, we observe that the size of the expelled oil droplets decreases as the unstable front recedes towards the wedge. We show how this size can be predicted from a linear stability analysis reminiscent of the classical Saffman–Taylor instability. However, the standard balance of capillary and bulk viscous dissipation does not account for the dynamics found in our experiments, leaving as an open question the detailed theoretical description of the instability.
PhD thesis
Formation et déplacement de gouttes confinées : Instabilités et dynamiques. Ludovic Keiser, Université Sorbonne Paris Cité PhD thesis in French (2018/1/29)
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Abstract: Les écoulements biphasiques en milieux poreux sont généralement accompagnés par des phénomènes d'émulsification d'une phase dans l'autre. Les causes peuvent être nombreuses, de la digitation visqueuse aux instabilités purement capillaires. Cette thèse expérimentale a pour objet l'étude d'un mécanisme particulier d'émulsification de l'huile en milieu poreux, ainsi que le transport des gouttes produites dans des milieux confinés. Dans la première partie de cette thèse, l'instabilité gravito-capillaire de Rayleigh-Taylor est revisitée dans un coin formé entre deux plaques de verre centimétriques. La présence d'un gradient de confinement introduit une force capillaire supplémentaire à cette instabilité canonique, susceptible de stabiliser une couche de liquide suspendue au-dessus du vide. Le seuil de stabilité, les longueurs d'onde caractéristiques et les taux de croissance sont bien modélisés par une analyse de stabilité linéaire de l'interface. La caractérisation de cette force capillaire induite par le gradient de confinement nous amène par la suite à l'étude d'une instabilité purement capillaire se produisant lorsqu'un fluide en mouillage très favorable migre vers les régions les plus confinées d'un coin, occupées initialement par un fluide en mouillage moins favorable. Le gradient de confinement introduit alors une force déstabilisante, aboutissant à l'inversion de la position respective des deux phases. Le liquide le moins mouillant est complètement émulsifié et transporté vers les régions les moins confinées sous la forme de gouttelettes. Une analyse de stabilité linéaire de l'interface permet, là encore, de prédire cette sélection de taille. Les taux de croissance mesurés ne sont en revanche pas en accord avec la modélisation, basée sur la loi de Darcy. Leur valeur suggère une localisation de la dissipation visqueuse dans les lignes de contact déplacées durant le développement de l'instabilité, ainsi que dans les films de lubrification également déposés. Ces dynamiques "non-darciennes" nous ont amenés dans une seconde partie de la thèse à l'étude du transport de gouttes d'huile très visqueuses confinées dans de l'eau en mouillage total. Dans cette configuration, la présence de films de lubrification d'eau entre la goutte et le substrat assure la localisation de la dissipation dans les films peu visqueux, favorisant ainsi la mobilité des gouttes. Nous montrons également que la présence de rugosités sur les parois du confinement induit un ralentissement significatif de la vitesse des gouttes, lié à l'amincissement du film de lubrification par ces rugosités. L'interdépendance subtile entre friction visqueuse à l'avant de la goutte et dans son volume est notamment mise en lumière. Dans une dernière partie, nous étudions l'instabilité capillaire se produisant lorsqu'une goutte binaire d'eau et d'alcool est déposée à la surface d'un bain d'huile. L'évaporation majoritaire de l'alcool à la surface de la goutte induit des variations locales de la tension de surface. Des écoulements interfaciaux de Marangoni se produisent, et aboutissent à la déstabilisation spectaculaire de la goutte en étalement.
Other
Fragmentation de Marangoni : les gouttes qui s’éclatent. Ludovic Keiser, Hadrien Bense, Cyril Sturtz, Pierre Colinet, Benoît Roman, José Bico & Étienne Reyssat Les Reflets de la Physique 59: 32-35 (2018)
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Abstract: Une goutte d'eau déposée à la surface d'un bain d'huile de tournesol forme une lentille liquide flottante. Une goutte d'alcool, au contraire, s'étale à la surface de l'huile. Un mélange d'eau et d'alcool produit un spectaculaire phénomène d'étalement et de fragmentation spontanée.
Si la goutte contient suffisamment d'alcool, elle donne naissance en quelques secondes à une myriade de gouttelettes dont la taille dépend fortement de la composition initiale. La combinaison de l'hydrodynamique, du mouillage et de l'évaporation est à l'origine de cette instabilité originale.
Patent
Blood condition analyzing device, blood condition analyzing system, and blood condition analyzing program. Marc-Aurèle Brun, Yoshihito Hayashi & Ludovic Keiser US Patent 10527605 (2020)
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Abstract: There is provided a blood condition analyzing device including: an extraction unit configured to use temporal change data of an electrical characteristic of blood at an arbitrary frequency to extract a feature of the data; and a blood condition evaluation unit configured to evaluate a condition change of blood from a feature extracted in the extraction unit.
