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International audience; The rheology of dense colloidal suspensions, which may undergo discontinuous shear thickening or shear jamming, is particularly difficult to analyze with conventional rheometers. Here, we develop a rheometer adapted to colloidal suspensions: the "capillarytron," which uses the air-suspension capillary interface to impose particle (or osmotic) pressure during shear. The main virtues of this new device are that (i) it gives direct access to the suspension friction coefficient, (ii) it operates for very dense suspensions up to jamming, and, most importantly, (iii) it decouples the stresses developed within the suspension from the applied shear rate. We can, thus, smoothly move through the different frictional states of the system, precisely in the range of volume fractions where discontinuous shear thickening or shear jamming occur under volume-imposed conditions. Our results obtained with the capillarytron provide the first complete characterization of the dual frictional behavior of a model shear-thickening suspension, in agreement with the recently proposed frictional transition scenario. Based on a new concept in rheometry, the capillarytron unlocks the path to pressure-imposed rheology on colloidal and Brownian suspensions. Moreover, its fine control of the particle pressure via the soft capillary interface opens the possibility to explore the flow of "fragile" particles close to jamming, such as Brownian colloids, active particles, and living cells.

We investigate experimentally and theoretically the stability of a shear-thickening suspension flowing down an inclined plane. In a previous paper (Darbois Texier et al., Commun. Phys., vol. 3, 2020), we have shown that for particle volume fractions $\phi$ above the discontinuous shear-thickening fraction $\phi _{DST}$ , long surface waves grow spontaneously at a flow Reynolds number much below 1. This motivated a simplified analysis based on a purely inertialess mechanism, called the ‘Oobleck waves’ mechanism, which couples the negatively sloped rheology of the suspension with the free-surface deflection and captures well the experimental instability threshold and the wave speed, for $\phi >\phi _{DST}$ . However, neglecting inertia does not allow us to describe the inertial Kapitza regime observed for $\phi , nor does it allow us to discriminate between Oobleck waves and other inertial instabilities expected above $\phi _{DST}$ . This paper fills this gap by extending our previous analysis, based on a depth-averaged approach and the Wyart–Cates constitutive shear-thickening rheology, to account for inertia. The extended analysis recovers quantitatively the experimental instability threshold in the Kapitza regime, below $\phi _{DST}$ , and in the Oobleck waves regime, above $\phi _{DST}$ . By providing additional measurements of the wave growth rate and investigating theoretically the effect of a strain delay in the rheology, it also confirms that the instability observed above $\phi _{DST}$ stems from the non-inertial Oobleck wave mechanism, which is specific to free-surface flows and dominates modes of inertial origin. These results emphasize the variety of instability mechanisms for shear-thickening suspensions and might be relevant to free-surface flows of other complex fluids displaying velocity-weakening rheology.

Granular materials whether dry or immersed in fluid show dilation or compaction depending upon the initial conditions, solid fraction and normal stress. Here we probe the transient response of a dense granular suspension subjected to change of applied normal stress under simple shear. In this aim, normal-stress-imposed discrete element particle simulations are developed considering the contributions arising from the drag induced on the particles by fluid phase. These pressure-imposed simulations show transient behaviors of dense granular suspensions such as dilation or compaction before reaching a steady state following the µ(J) rheology. Less expectedly, the transient behavior, in particular the height of the system as a function of applied strain, can also be described by assuming that the system follows the steady µ(J) rheology at all times.

Gravity is a major cue for the proper growth and development of plants. The response of plants to gravity implies starch-filled plastids, the statoliths, which sediments at the bottom of the gravisensing cells, the statocytes. Statoliths are assumed to modify the transport of the growth hormone, auxin, by acting on specific auxin transporters, PIN proteins. However, the complete gravitropic signaling pathway from the intracellular signal associated to statoliths to the plant bending is still not well understood. In this article, we build on recent experimental results showing that statoliths do not act as gravitational force sensor, but as position sensor, to develop a bottom-up theory of plant gravitropism. The main hypothesis of the model is that the presence of statoliths modifies PIN trafficking close to the cell membrane. This basic assumption, coupled with auxin transport and growth in an idealized tissue made of a one-dimensional array of cells, recovers several major features of the gravitropic response of plants. First, the model provides a new interpretation for the response of a plant to a steady stimulus, the so-called sine-law of plant gravitropism. Second, it predicts the existence of a gravity-independent memory process as observed recently in experiments studying the response to transient stimulus. The model suggests that the timescale of this process is associated to PIN turnover, calling for new experimental studies.

Recent simulations and experiments have shown that shear-thickening of dense particle suspensions corresponds to a frictional transition. Based on this understanding, non-monotonic rheological laws have been proposed and successfully tested in rheometers. These recent advances offer a unique opportunity for moving beyond rheometry and tackling quantitatively hydrodynamic flows of shear-thickening suspensions. Here, we investigate the flow of a shear-thickening suspension down an inclined plane and show that, at large volume fractions, surface kinematic waves can spontaneously emerge. Curiously, the instability develops at low Reynolds numbers, and therefore does not fit into the classical framework of Kapitza or ‘roll-waves’ instabilities based on inertia. We show that this instability, that we call ‘Oobleck waves’, arises from the sole coupling between the non-monotonic (S-shape) rheological laws of shear-thickening suspensions and the flow free surface. The way interactions at the microscopic scale influence emerging flow properties in complex fluids at the macroscopic scale is one of the core problems in soft matter physics. This work provides experimental evidence together with a theoretical explanation for ‘Oobleck waves’, an instability arising from the coupling between the flow free surface and the non-monotonic rheological laws of shear-thickening suspensions.

We study the impact between a plate and a drop of non-colloidal solid particles suspended in a Newtonian liquid, with a specific attention to the case when the particle volume fraction, $\phi$, is close to - or even exceeds - the critical volume fraction, $\phi_c$, at which the steady effective viscosity of the suspension diverges. We use a specific concentration protocol together with an accurate determination of $\phi$ for each drop and we measure the deformation $\beta$ for different liquid viscosities, impact velocities and particle sizes. At low volume fractions, $\beta$ is found to follow closely an effective Newtonian behavior, which we determine by documenting the low deformation limit for a highly viscous Newtonian drop and characterizing the effective shear viscosity of our suspensions. By contrast, whereas the effective Newtonian approach predicts that $\beta$ vanishes at $\phi_c$, a finite deformation is observed for $\phi>\phi_c$. This finite deformation remains controlled by the suspending liquid viscosity and increases with increasing particle size, which suggests that the dilatancy of the particle phase is a key factor of the dissipation process close to and above $\phi_c$., Comment: Submitted to JFM

Experiments on shoot gravitropism at the plant and cell scale reveal the existence of a memory process and provide a unifying framework for predicting the bending response of shoots to arbitrary gravi-stimulations., Gravity is a major abiotic cue for plant growth. However, little is known about the responses of plants to various patterns of gravi-stimulation, with apparent contradictions being observed between the dose-like responses recorded under transient stimuli in microgravity environments and the responses under steady-state inclinations recorded on earth. Of particular importance is how the gravitropic response of an organ is affected by the temporal dynamics of downstream processes in the signalling pathway, such as statolith motion in statocytes or the redistribution of auxin transporters. Here, we used a combination of experiments on the whole-plant scale and live-cell imaging techniques on wheat coleoptiles in centrifuge devices to investigate both the kinematics of shoot-bending induced by transient inclination, and the motion of the statoliths in response to cell inclination. Unlike previous observations in microgravity, the response of shoots to transient inclinations appears to be independent of the level of gravity, with a response time much longer than the duration of statolith sedimentation. This reveals the existence of a memory process in the gravitropic signalling pathway, independent of statolith dynamics. By combining this memory process with statolith motion, a mathematical model is built that unifies the different laws found in the literature and that predicts the early bending response of shoots to arbitrary gravi-stimulations.

International audience; Flows of granular media in air or in a liquid have been a research field for physicists for several decades. Sometimes solid, sometimes liquid, these particulate materials exhibit peculiar behaviors, which have motivated many studies at the frontiers between nonlinear physics, soft matter physics and fluid mechanics. This paper presents a summary of the recent advances in the field, with a focus on the development of continuous approaches, which make it possible to treat granular media as a complex fluid and to develop a granular hydrodynamics. We also discuss how the better understanding of granular flows we have today may help to address more complex materials, such as colloidal suspensions or some biological systems. © 2018 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).; Les écoulements de milieux formés de grains dans l'air ou dans un liquide intéressent les physiciens depuis plusieurs décennies. Tantôt solides, tantôt liquides, ces matériaux divisés ont des comportements singuliers qui sont au coeur de nombreuses études à la frontière entre la physique non linéaire, la physique de la matière molle et la mécanique des fluides. Cet article se propose de faire un point sur les avancées récentes dans le domaine, en se concentrant sur le développement d'approches continues qui permettent de traiter le milieu comme un fluide complexe et de développer une hydrodynamique granulaire. Nous discutons également en quoi la compréhension plus fine des écoulements granulaires que nous avons aujourd'hui permet de mieux appréhender les matériaux plus complexes comme les suspensions colloïdales, voire certains milieux biologiques.

International audience; In order to better understand the mechanism governing segregation in granular flows, the force experienced by a large particle embedded in a granular flow made of small particles is studied using discrete numerical simulations. An empirical expression of the segregation force is proposed as a function of the stress distribution.

In order to better understand the mechanism governing segregation in dense granular flows, the force experienced by a large particle embedded in a granular flow made of small particles is studied using discrete numerical simulations. Accurate force measurements have been obtained in a large range of flow parameters by trapping the large particle in a harmonic potential well to mimic an optical tweezer. Results show that positive or negative segregation lift forces (perpendicular to the shear) exist depending on the stress inhomogeneity. An empirical expression of the segregation force is proposed as a sum of a term proportional to the gradient of pressure and a term proportional to the gradient of shear stress, which both depend on the local friction and particle size ratio.

International audience; An elastic strip is transversely clamped in a curved frame. The induced curvature decreases as the strip opens and connects to its flat natural shape. Various ribbon profiles are measured and the scaling law for the opening length validates a description where the in-plane stretching gradually relaxes the bending stress. An analytical model of the strip profile is proposed and a quantitative agreement is found with both experiments and simulations of the plates equations. This result provides a unique illustration of smooth nondevelopable solutions in thin sheets. Geometry-induced rigidity is a fundamental feature of thin structures [1], which has long been used in engineering and architecture to design stiff fuselages, hulls, roofs, and deployable structures [2–4]. It is also widely encountered in living structures, such as in plant leaves where curvature can prevent the collapse of the leaves under their own weight [5]. A simple illustration of this rigidity induced by curvature is given by a strip of paper held at one end. When the strip is flat, it is unable to sustain its own weight and bends downward under gravity. However, if the end of the paper is slightly curved transversely, the strip straightens up and becomes much stiffer. Rigidity in these systems arises because bending in one direction is coupled to the transverse curvature and cannot occur without stretching the sheet—a costly mode of deformation in thin plates in terms of elastic energy [6]. Knowing the distance over which an induced curvature spreads is thus an important issue for predicting the rigidity of thin plates and shells. In this Letter, we address the question of the persistence length of curvature in thin sheets on a minimal system: a flat elastic ribbon of thickness t, width W, and length L W, which is clamped at one end over a cylinder of radius R [Fig. 1(a)]. After what distance from the clamp does the ribbon unfold and recover its flat natural shape? This deceptively simple problem is actually not straightforward as, to unfold, the ribbon has to stretch—a forbidden mode of deformation in the inextensible limit. In thin sheets, this constraint is usually resolved by focusing the stretch in elastic defects or singularities, such as the ridges and peaks of a crumpled paper, the rest of the surface being fully developable (i.e. free of stretching) [7–14]. However, another way to obtain the stretching of a thin sheet is to consider that the curvature variation on large distances is associated with a regular stretching, i.e., without defects. Surprisingly, the first insights of this approach are found in the studies of defects such as ridges [15] and pinches [16], where both the focused-stress and the diffuse-stress are present [17]. In each of these situations, regular developable solutions exist away from the defect but they are not observed as the bending energy can be progressively released by a small in-plane stretching (see also [18,19]). The stretching over large distances is also involved in the shape of drapes [20] and curtains [21], the tearing of sheets [22], or the dynamics of curved ribbons [23,24]. Our prototypal system provides a reduced model to probe these situations and the transition between smooth and singular solutions in strained sheets. A first observation of the opening is displayed in Fig. 1(b), using an acetate elastic sheet (t ¼ 110 m, W ¼ 4.5 cm) clamped in a circular frame of radius R ¼ 2.5 cm. The strip is positioned vertically to limit the out-of-plane deflection caused by its weight. Away from the clamping, the strip opens and its curvature decreases and connects to the flat stress-free region over a finite length L p. The opening also results in a small deflection of the strip corresponding to a tilt angle θ of the centerline. The persistence length of curvature L p can be estimated from a balance between the stretching and bending elastic energies using scaling arguments similar to [15]. On one hand, the bending energy of the ribbon scales as E b ∼ EtWL p ðt=RÞ 2 , where E is the Young modulus of the medium. On the other hand, the opening of the ribbon requires the stretching of the edges of the ribbon over a length L p. This is associated with a stretching energy E s ∼ EtWL p 2 , where ∼ Z 2 =2L 2 p is the typical in-plane strain and Z ∼ W 2 =8R is the out-of-plane deflection of the ribbon at the clamp [see Fig. 1(b) inset]. The trade off between the two energies E s and E b gives L p ∼ W 2 = ffiffiffiffiffi tR p or equivalently [15] L p ∼ W ffiffiffiffiffiffiffiffi Z=t p. The persistence length of the curved region is independent of the Young modulus and increases when the thickness is reduced.

National audience; Dépourvues de muscles, certaines plantes mettent en œuvre des mouvements dont la fulgurance est comparable à celle des animaux. Nous montrons dans cet article que beaucoup de ces mouvements, nécessités par la reproduction ou la nutrition, ont la même base physique : une instabilité mécanique qui libère de l’énergie élastique stockée. Deux grands types d’instabilités mécaniques sont utilisés par les plantes pour amplifier la vitesse de leur mouvement : les ruptures solides ou liquides (cavitation) pour la propulsion des graines ou des spores de fougères, et les instabilités de flambage élastique pour les pièges des plantes carnivores, telles que la Dionée ou l’utriculaire.

International audience; The ability of plants to move is central to many physiological processes from development to tropisms, from nutrition to reproduction. The movement of plants or plant parts occurs over a wide range of sizes and time scales. This review summarizes the main physical mechanisms plants use to achieve motility, highlighting recent work at the frontier of biology and physics on rapid movements. Emphasis is given to presenting in a single framework pioneering biological studies of water transport and growth with more recent physics research on poroelasticity and mechanical instabili-ties. First, the basic osmotic and hydration/dehydration motors are described that contribute to movement by growth and reversible swelling/shrinking of cells and tissues. The speeds of these water-driven movements are shown to be ultimately limited by the transport of water through the plant body. Some plant structures overcome this hydraulic limit to achieve much faster movement by using a mechanical instability. The principle is to impose an 'energy barrier' to the system, which can originate from geometrical constraint or matter cohesion, allowing elastic potential energy to be stored until the barrier is overcome, then rapidly transformed into kinetic energy. Three of these rapid motion mechanisms have been elucidated recently and are described here: the snapping traps of two carnivorous plants, the Venus flytrap and Utricularia, and the catapult of fern sporangia. Finally, movement mechanisms are reconsidered in the context of the timescale of important physiological processes at the cellular and molecular level.

International audience; Although they lack muscle, plants have evolved a remarkable range of mechanisms to create motion, from the slow growth of shoots to the rapid snapping of carnivorous plants and the explosive rupture of seed pods. Here we review the key fluid mechanics principles used by plants to achieve movements, summarizing current knowledge and recent discoveries. We begin with a brief overview of water transport and material properties in plants and emphasize that the poroelastic timescale of water diffusion through soft plant tissue imposes constraints on the possible mechanisms for motion. We then discuss movements that rely only on the transport of water, from irreversible growth to reversible swelling/shrinking due to osmotic or humidity gradients. We next show how plants use mechanical instabilities—snap buckling, cavitation, and fracture—to speed up their movements beyond the limits imposed by simple hydraulic mechanisms. Finally, we briefly discuss alternative schemes, involving capillarity or complex fluids.

International audience; – NousétudionsNousétudions expérimentalement la dynamique de moussesélastiquesmoussesélastiques (mousse ouverte flexible en polyuréthane) plongées dans un fluide visqueux. La mousse est initialement comprimée dans une direction puis brutalement relâchée. Le champ de vitesse de la mousse et la pression du fluide interstitiel (pression de pore) sont mesurés au cours de la décompaction du système. Pour des petites compactions initiales, on observe que la mousse relaxe exponentiellement vers sonétatsonétat d'´ equilibre, avec une dynamique diffusive en accord quantitatif avec les théories classiques de consolidation des sols. En revanche, pour des grandes compactions initiales, la dynamique de relaxation est inhomogène et se déroulè a travers un front de décompaction. Un modèle diphasique simple permet de prédire semi-quantitativement les observations et relie l'existence du front au caractère fortement non-linéaire de la réponse mécanique de la mousse. Mots clés : Poroélasticité / matériaux cellulaires / front Abstract – On the dynamics of poroelastic foams. We experimentally investigate the dynamics of soft polyurethane open foams immersed in viscous fluids. The elastic foam is initially compressed in one direction and then suddenly released. The velocity field of the foam and the dynamic pressure of the interstitial fluid (pore pressure) are measured during the swelling of the foam. For small initial compactions, the foams relax exponentially to equilibrium with a diffusive dynamics in quantitative agreements with classical theories of soil consolidation. On the other hand, for large initial compactions, the dynamics is strongly inhomogeneous and occurs through a localized decompaction front. A simple two-fluids model semi-quantitatively predicts the observations and links the existence of a localized front to the highly non-linear mechanical response of the foam.

International audience; From a biomechanical perspective, plants offer a fascinat- ing example of living systems capable of producing non- muscular movements (Skotheim and Mahadevan 2005; Dumais and Forterre, to be published). Although most of these movements are slow, some compete in speed with those observed in the animal kingdom and are involved in essential functions such as seed/pollen dispersal, defence and nutrition. Of these spectacular examples that have long fascinated scientists, the Venus flytrap (Figure 1(a)), for which the leaves snap together in a fraction of second to capture insects, has long been a paradigm for study. Recently, we have shown that this motion involves a snap- buckling instability due to the shell-like geometry of the leaves of the trap (Forterre et al. 2005). However, the origin of the active movement used by the plant to cross the instability threshold remains unknown. More generally, the physical mechanisms involved in rapid plant movements remain poorly understood, especially at the cell and tissue scale. Two main assumptions are found in the literature: (i) a rapid flow between the cells due to changes in osmotic pressure (Hill and Findlay 1981), (ii) a rapid cell expansion due to mechanical modifications (softening) in the cell wall (Williams and Bennett 1982). In both cases, the high-water pressure inside the plant cells is believed to play a central role. Our aim in this study was to measure in real time in-vivo mechanical and hydraulic properties of Venus flytrap cells (cell wall elasticity, cell membrane per- meability and cell water pressure) and to investigate the possible mechanisms for movements at the cell scale. To this end, we use a microfluidic pressure probe device, which applies a water flux inside a single cell while measuring the cell water pressure simultaneously (Steudle 1993).

We study how a shear band in a granular medium dramatically changes the mechanical behavior of the material further in the non sheared region. To this end, we carry out a microrheology experiment, where a constant force $F$ is applied to a small rod immersed outside the shear band. In the absence of a shear band, a critical force ${F}_{c}$ is necessary to move the intruder. When a shear band exists, the intruder moves even for a force $F$ less than the critical force ${F}_{c}$. We systematically study how the creep velocity ${V}_{\mathrm{creep}}$ of the rod varies with ${F}_{c}\ensuremath{-}F$ and with the distance to the shear band, and show that the behavior can be described by an Eyring-like activated process.

International audience; A non-local theory is proposed to model dense granular flows. The idea is to describe the rearrangements occurring when a granular material is sheared as a self-activated process. A rearrangement at one position is triggered by the stress fluctuations induced by rearrangements elsewhere in the material. Within this framework, the constitutive law, which gives the relation between the shear rate and the stress distribution, is written as an integral over the entire flow. Taking into account the finite time of local rearrangements, the model is applicable from the quasi-static regime up to the inertial regime. We have checked the prediction of the model in two different configurations, namely granular flows down inclined planes and plane shear under gravity, and we show that many of the experimental observations are predicted within the self-activated model.

The normal impact of a drop of yield-stress fluid on a flat rigid surface is investigated experimentally. Using different model fluids (polymer microgels, clay suspensions) and impacted surfaces (partially wettable, super-hydrophobic), we find a rich variety of impact regimes from irreversible viscoplastic coating to giant elastic spreading and recoil. A minimal model of inertial spreading, taking into account an elasto-viscoplastic rheology, allows explaining in a single framework the different regimes and scaling laws. In addition, semi-quantitative predictions for the spread factor are obtained when the measured rheological parameters of the fluid (elasticity, yield stress, viscosity) are injected into the model. Our study offers a means to probe the short-time rheology of yield-stress fluids and highlights the role of elasticity on the unsteady hydrodynamics of these complex fluids. Movies are available with the online version of the paper (go to journals.cambridge.org/flm).

International audience; Recent experiments and numerical simulations of dry granular flows suggest that a simple rheological description in terms of a friction coefficient varying both with shear rate and pressure through a dimensionless inertial number may be sufficient to capture the major properties of granular flows. In this paper we first present the empirical constitutive laws and their interpretation using dimensional analysis, before analysing the prediction for different flow configurations. The successes and limits of the approach are discussed based on comparison with recent studies.

A continuum description of granular flows would be of considerable help in predicting natural geophysical hazards or in designing industrial processes. However, the constitutive equations for dry granular flows, which govern how the material moves under shear, are still a matter of debate. One difficulty is that grains can behave like a solid (in a sand pile), a liquid (when poured from a silo) or a gas (when strongly agitated). For the two extreme regimes, constitutive equations have been proposed based on kinetic theory for collisional rapid flows, and soil mechanics for slow plastic flows. However, the intermediate dense regime, where the granular material flows like a liquid, still lacks a unified view and has motivated many studies over the past decade. The main characteristics of granular liquids are: a yield criterion (a critical shear stress below which flow is not possible) and a complex dependence on shear rate when flowing. In this sense, granular matter shares similarities with classical visco-plastic fluids such as Bingham fluids. Here we propose a new constitutive relation for dense granular flows, inspired by this analogy and recent numerical and experimental work. We then test our three-dimensional (3D) model through experiments on granular flows on a pile between rough sidewalls, in which a complex 3D flow pattern develops. We show that, without any fitting parameter, the model gives quantitative predictions for the flow shape and velocity profiles. Our results support the idea that a simple visco-plastic approach can quantitatively capture granular flow properties, and could serve as a basic tool for modelling more complex flows in geophysical or industrial applications., Comment: http://www.nature.com/nature/journal/v441/n7094/abs/nature04801.html

In this paper we study the steady uniform flows that develop when granular material is released from a hopper on top of a static pile in a channel. We more specifically focus on the role of side walls by carrying out experiments in setup of different widths, from narrow channels 20 particle diameters wide to channels 600 particle diameters wide. Results show that steady flows on pile are entirely controlled by side wall effects. A theoretical model, taking into account the wall friction and based on a simple local constitutive law recently proposed for other granular flow configurations (GDR MiDi 2004), gives predictions in quantitative agreement with the measurements. This result gives new insights in our understanding of free surface granular flows and strongly supports the relevance of the constitutive law proposed., a forgotten square root in Appendix B (Eq B4), and corrected coefficients in Appendix C; 25 pages, 17 figures, published in J. Fluid Mech

Shear thickening is a spectacular phenomenon which takes place in some dense suspensions. It manifests itself by a brutal increase of the suspension's viscosity above a certain critical stress. The most iconic example of shear-thickening suspensions is cornstarch and water mixes. Shear thickening long remained a mystery, until recent theoretical and numerical works which proposed a consistent microscopic model. This model explains the shear thickening transition as a frictional one due to the presence of a repulsive force between the grains.During my PhD, I provided one of the first direct experimental proofs of this mechanism. Inspired by granular physics, I showed that shear-thickening suspensions possess a frictionless flowing state at low granular pressure, which is consistent with the proposed model. I then evidenced the frictional transition with controlled experiments using suspensions of silica beads in ionic solutions. To do this, I developed new rheological techniques enabling pressure imposed measurements. Indeed, standard rheological tools do not allow access to the frictional properties of suspensions. The only rheometer that does that is not adapted to the suspensions we study here, which are colloidal. This work thus paves the way for the development of a new generation of pressure imposed rheometers, giving access to colloidal suspensions friction, which is a major challenge in complex fluids rheology., Le rhéoépaississement est un phénomène spectaculaire apparaissant dans certaines suspensions concentrées en particules. Il se manifeste par l’augmentation brutale de la viscosité de la suspension au delà d’une contrainte critique. L’exemple emblématique de suspension présentant ce type de comportement est le mélange d’amidon de maïs et d’eau. Le rhéoépaississement est longtemps resté une énigme, jusqu’à des travaux théoriques et numériques récents proposant un modèle microscopique cohérent. Selon celui-ci, le rhéoépaississement provient d’une transition frictionnelle due à la présence d’une force répulsive entre les grains. Au cours de ma thèse, j’ai réalisé une des premières démonstrations expérimentales directe de ce mécanisme. En m'inspirant de travaux venant du domaine des milieux granulaires, j'ai montré qu'en accord avec le modèle de transition frictionnelle, une suspension rhéoépaississante possède à faible pression granulaire un état non frottant. J'ai ensuite mis en évidence la transition en elle-même dans des suspensions contrôlées de billes de silice dans des solutions salines. Pour cela, j'ai dû développer de nouvelles méthodes de rhéologie à pression imposée. En effet, les rhéomètres standard ne permettent pas d'accéder aux propriétés de frottement des suspensions. Le seul rhéomètre qui le permet n'est pas adapté aux suspensions étudiées ici, qui sont constituées de particules colloïdales. Ce travail ouvre donc la voie au développement d’une nouvelle génération de rhéomètres permettant de mesurer le frottement dans les suspensions colloïdales, un enjeu majeur pour la rhéologie des fluides complexes.

Plants are constantly subjected to external mechanical loads such as wind or touch and respond to these stimuli by modifying their growth and development. A fascinating feature of this mechanical-induced-growth response is that it is not only local, but also non-local: bending locally a stem or a branch can induce a very rapid modification of the growth far away from the stimulated area, suggesting the existence of a signal that propagates across the whole plant. The nature and origin of this signal is still not understood, but it has been suggested recently that it could be purely mechanical and originate from the coupling between the local deformation of the tissues and the water pressure in the vascular system. The objective of this work is to understand the origin of this hydro/mechanical coupling using a biomimetic approach. Artificial microfluidic branches have been developed, that incorporate the mechanical and hydraulic key features of natural ones. We show that the bending of these branches generates a steady overpressure in the whole system, which varies quadratically with the bending deformation. A simple model based on a mechanism analogue to tube ovalization enables us to predict this non-linear poroelastic response, and identify the key physical parameter at play, namely the elastic bulk modulus of the branch. Further experiments conducted on natural tree branches reveal the same phenomenology. Once rescaled by the model prediction, both the biomimetic and natural branches falls on the same master curve, showing the universality of the identified mechanism for the generation of hydraulic signals in plants.; Dans la nature les plantes sont sans cesse soumises à des sollicitations mécaniques qui affectent et modifient leur croissance. Un aspect remarquable de cette réponse est qu’elle n’est pas seulement locale mais non-locale : la flexion d’une tige ou d’une branche inhibe rapidement la croissance loin de la zone sollicitée. Cette observation suggère l'existence d'un signal pouvant se propager à travers toute la plante. Parmi les différentes hypothèses, il a été suggéré que ce signal pouvait être purement mécanique, et provenir d’un couplage hydro/mécanique entre la déformation du tissu et la pression de l’eau contenue dans le système vasculaire de la plante. L’objectif de cette thèse est de comprendre l’origine physique de ce couplage par une approche biomimétique. Pour cela, nous avons développé des branches artificielles micro-fluidiques possédant des caractéristiques mécaniques et hydrauliques similaires à celles d'une branche d'arbre. Nous avons montré que la flexion de ces branches génère une surpression globale non-nulle dans le système, qui varie comme le carré de la déformation longitudinale. Un modèle simple basé sur un mécanisme analogue à l’ovalisation des tubes permet de prédire cette réponse poroélastique non-linéaire et d’identifier le paramètre physique clé pilotant cette réponse en pression : le module de compressibilité de la branche. A la lumière de ces résultats, des expériences sur des branches d'arbre ont ensuite été conduites et des signaux similaires sont obtenus et comparés au modèle théorique. La similitude suggère le caractère générique du mécanisme physique identifié pour la génération de signaux hydraulique dans les plantes.

This thesis presents an experimental study on the drop impact of yield-stress fluids. Beyond applications (solid ink-jet printing, lab modelling of high-speed collision of solids), this study offers a mean to probe the role of the elasticity on the short-time behaviour of these complex fluids. We have first studied drop impacts on solid rigid surfaces. Using different model yield-stress fluids (clay suspensions, Carbopol micro-gel) and impacted surfaces (partially wettable, super-hydrophobic), we have observed a rich variety of behaviours ranging from irreversible viscoplastic coating to giant elastic spreading and recoil. A minimal model of inertial spreading, including an elasto/viscoplastic rheology, allows explaining in a single framework the different regimes and scaling laws. In this study, we identified a specific phenomenon with Carbopol: for large impact velocities, the drop spreads much more on rough hydrophobic surfaces than on smooth surfaces. This apparent reduction of the basal friction is discussed in terms of slip length and splash instability. Endly, we investigated the impact of a drop onto a pool of the same fluid, using a transparent yield-stress fluid (Carbopol). The combination of scaling laws, micro-gravity experiments and local deformation measurements shows that the transient crater is dominated by elasticity, even beyond the flow threshold. These results could have implications for impact cratering in Planetary Sciences.; Cette thèse présente une étude expérimentale de l'impact de gouttes de fluides à seuil. Au-delà des applications (impression à jet d'encre solide, modélisation d'impact solide à grandes vitesses), cette étude permet de sonder le rôle de l'élasticité sur le comportement à temps court de ces fluides complexes. D'abord, nous nous sommes intéressés aux impacts sur une surface rigide. L'utilisation de fluides à seuil modèles (solutions concentrées d'argiles, micro-gel de Carbopol) et de surfaces d'impact variées (partiellement mouillante ou super-hydrophobe), révèle une grande variété de comportements, allant de l'étalement viscoplastique irréversible jusqu'à des déformations élastiques géantes. Un modèle minimal d'étalement inertiel, incluant une rhéologie élasto/viscoplastique, permet de décrire dans un cadre unique les principaux régimes observés. Au cours de cette étude, nous avons mis en évidence un phénomène spécifique avec le Carbopol : pour des grandes vitesses d'impact, on observe un étalement beaucoup plus grand sur des surfaces rugueuses hydrophobes que sur des surfaces lisses. Cette réduction apparente du frottement basal est discutée en termes de longueur de glissement et d'instabilité de " splash ". Enfin, nous avons étudié l'impact d'une goutte de fluide sur un sol constitué du même fluide, en utilisant un fluide à seuil transparent (Carbopol). La combinaison de lois d'échelle, d'expériences en " micro-gravité " et de mesures locales du champ de déformation montre que la dynamique du cratère transitoire est dominée par l'élasticité, même au-delà du seuil d'écoulement. Ces résultats pourraient avoir des implications dans le contexte des impacts de météorites en astrophysique.