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Couder Y.,CNRS Complex Systems and Materials Laboratory
Journal of Fluid Mechanics | Year: 2012

By what process can droplets be extracted out of the sea? This is an old problem, well-documented by precise field measurements of the size distribution of the spray aerosols. Lhuissier & Villermaux (J. Fluid Mech., this issue, vol. 696, 2012, pp. 5-44) study and characterize the bursting of an emerging bubble. They show that this single type of event can, by itself, generate the droplet size distributions in a sea spray. This is a remarkable result showing how, in a complex system, a statistical distribution can be entirely produced by the dynamics of one dominant phenomenon. © 2012 Cambridge University Press.

Couturier E.,CNRS Complex Systems and Materials Laboratory
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2016

Thin vegetal shells have recently been a significant source of inspiration for the design of smart materials and soft actuators. Herein is presented a novel analytical family of isometric deformations with a family of θ-folds crossing a family of parallel z-folds; it contains the isometric deformations of a banana-shaped surface inspired by a seedpod, which converts a vertical closing into either an horizontal closing or an opening depending on the location of the fold. Similarly to the seedpod, optimum shapes for opening ease are the most elongated ones. © 2016 The Author(s) Published by the Royal Society. All rights reserved.

Reffet E.,CNRS Laboratory for Space Studies and Astrophysical Instrumentation | du Pont S.C.,CNRS Complex Systems and Materials Laboratory | Hersen P.,CNRS Complex Systems and Materials Laboratory | Douady S.,CNRS Complex Systems and Materials Laboratory
Geology | Year: 2010

The shape of dunes depends on the history of wind regimes and sand availability. In deserts exposed to winds from two different directions but with comparable magnitude, dunes are found to be linear ridges, which are either perpendicular or parallel to the mean wind direction, depending on the angle between the two wind directions. These dunes, respectively observed for small and large angles between winds, are called transverse and longitudinal dunes. In both cases, their large width (hundreds of meters) and evolution time scale (years) strongly limit the investigation of their dynamics and thus our understanding of such structures. Here we show that, under water, similar structures can be obtained but at much smaller space and time scales. Performing controlled experiments together with numerical simulations, we highlight the physical mechanisms at play in the formation and long-term evolution of these structures. We show in particular that, while longitudinal dunes are stable and extend in time, transverse dunes are unstable. They evolve into wavy ridges and eventually break into barchans if the sand supply is too low. This fundamental difference is understood through the study of single sand piles and bars exposed to two winds. In the case of a large angle between winds, a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune. Such an elongation does not occur for a small angle where a sand pile evolves into a barchan. These results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes, while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field. © 2010 Geological Society of America.

Chapel J.-P.,University of Bordeaux 1 | Berret J.-F.,CNRS Complex Systems and Materials Laboratory
Current Opinion in Colloid and Interface Science | Year: 2012

Engineered nanoparticles made from noble metals, rare-earth oxides or semiconductors are emerging as the central constituents of future nanotech developments. In this review, a survey of the complexing strategies between nanoparticles and oppositely charged polyelectrolytes developed during the last three years and based on electrostatic interactions is presented. These strategies include the one-step synthesis of stable and functionalized nanoparticles, the one- and multilayer coating of individual nano-objects, the controlled clustering of particles and the generation of capsules and thin films with superior functionalities. Among the formulation processes reported, three main classes are identified: the direct mixing route, the desalting transition pathway and the well-known layer by layer method. Finally, some latter developments, trends and applications of electrostatic assemblies in materials science and nanomedicine are highlighted. © 2011 Elsevier Ltd.

Snoeijer J.H.,University of Twente | Brunet P.,CNRS Complex Systems and Materials Laboratory
American Journal of Physics | Year: 2012

A water drop that is gently deposited on a very cold surface freezes into a pointy ice-drop with a very sharp tip. The formation of this singular shape originates from the reduction of mass density during the freezing process and can be explained using a simplified model for which the universal structure of the singularity is revealed in full detail. The combination of a relatively simple, static experiment, and the accessible asymptotic analysis makes this system an ideal introduction to the topic of singularities. © 2012 American Association of Physics Teachers.

Mauroy B.,CNRS Complex Systems and Materials Laboratory | Bokov P.,CNRS Complex Systems and Materials Laboratory
Physical Biology | Year: 2010

The asymmetry of the bronchial tree has been reported on numerous occasions, and bronchi in the lung bifurcate most of the time into a major and a minor daughter. Asymmetry is most probably bound to play a role on the hydrodynamic resistance and volume occupation of the bronchial tree. Thus, in this work, we search for an optimal asymmetric airway tree crossed by Poiseuille flow that would be a good candidate to model the distal conductive part of the lung. The geometry is controlled by major and minor diameter reduction factors that depend on the generation. We show that the optimal asymmetric tree has diameter reduction factors that are adimensional from the second level of bifurcation and that they are highly dependent on the asymmetric ratio that defines the relative sizes of the major and minor branches in a bifurcation. This optimization also gives access to a cost function whose particularity is to be asymmetric around its minimum. Thus, the cliff-edge hypothesis predicts that if the system suffers variability, then the best tree is shifted from the optimal. We apply a recent theoretical model of cliff-edge in order to measure the role of variability on the determination of the best asymmetric tree. Then, we compare our results with lung data of the literature. In particular, we are able to quantify the variability needed to fit the data and to give hypothesis that could explain, at least partially, the shift found between the optimal tree and the measures in the case of asymmetric bronchial trees. Finally, our model predicts that, even if the population is adapted at best, there always exist individuals whose bronchial trees are associated with larger costs comparatively to the average and who ought to be more sensitive to geometrical remodeling. © 2010 IOP Publishing Ltd.

Courtois J.,CNRS Complex Systems and Materials Laboratory | Berret J.-F.,CNRS Complex Systems and Materials Laboratory
Langmuir | Year: 2010

The complexation between charge-neutral block copolymers and oppositely charged surfactants was investigated by light scattering experiments and by isothermal titration calorimetry (ITC). The copolymer was poly(sodium acrylate)-bpoly(acrylamide) and the surfactant dodecyltrimethylammonium bromide (DTAB). In a previous report, we had shown that the copolymers and the surfactants coassembled spontaneously into colloidal complexes. Depending on the charge ratio, the complexes were either single surfactant micelles decorated by copolymers or core-shell hierarchical structures. ITC was performed in order to investigate the thermodynamics of the complex formation. Titrations of copolymers by surfactants and of surfactants by copolymers revealed that the electrostatic coassembly was an endothermic reaction, suggesting a process dominated by the entropy of the counterions. Here, we found that the thermodynamic quantities associated with the reaction depended on the mixing order. When surfactants were added stepwise to copolymers, the titration was associated with the formation of single micelles decorated by a unique polymer. Above a critical charge ratio, the micelles rearranged themselves into 100 nm colloidal complexes in a collective process which displayed the following features: (i) the process was very slow as compared to the time scale of Brownian diffusion, (ii) the thermodynamic signature was a endothermic peak, and (iii) the stoichiometry between the positive and negative charges was modified from n=0.48 (single micelles) to 0.75 (core-shell complexes). When copolymers were added stepwise to surfactants, the titration resulted in the formation of the core-shell aggregates only. In both experiments, the amount of polyelectrolytes needed for complex formation exceeded the number required to compensate the net micellar charge, confirming the evidence of overcharging in the complex formation. © 2010 American Chemical Society.

Berret J.-F.,CNRS Complex Systems and Materials Laboratory
Advances in Colloid and Interface Science | Year: 2011

In this review, we address the issue of the electrostatic complexation between charged-neutral diblock copolymers and oppositely charged nanocolloids. We show that nanocolloids such as surfactant micelles and iron oxide magnetic nanoparticles share similar properties when mixed with charged-neutral diblocks. Above a critical charge ratio, core-shell hierarchical structures form spontaneously under direct mixing conditions. The core-shell structures are identified by a combination of small-angle scattering techniques and transmission electron microscopy. The formation of multi-level objects is driven by the electrostatic attraction between opposite charges and by the release of the condensed counterions. Alternative mixing processes inspired from molecular biology are also described. The protocols applied here consist in screening the electrostatic interactions of the mixed dispersions, and then removing the salt progressively as an example by dialysis. With these techniques, the oppositely charged species are intimately mixed before they can interact, and their association is monitored by the desalting kinetics. As a result, sphere- and wire-like aggregates with remarkable superparamagnetic and stability properties are obtained. These findings are discussed in the light of a new paradigm which deals with the possibility to use inorganic nanoparticles as building blocks for the design and fabrication of supracolloidal assemblies with enhanced functionalities. © 2011 Elsevier B.V. All rights reserved.

Berret J.-F.,CNRS Complex Systems and Materials Laboratory
Nature Communications | Year: 2016

When submitted to a magnetic field, micron-size wires with superparamagnetic properties behave as embedded rheometers and represent interesting sensors for microrheology. Here we use rotational magnetic spectroscopy to measure the shear viscosity of the cytoplasm of living cells. We address the question of whether the cytoplasm is a viscoelastic liquid or an elastic gel. The main result of the study is the observation of a rotational instability between a synchronous and an asynchronous regime of rotation, found for murine fibroblasts and human cancer cells. For wires of susceptibility 3.6, the transition occurs in the range 0.01-1 rad s-1. The determination of the shear viscosity (10-100Pas) and elastic modulus (5-20Pa) confirms the viscoelastic character of the cytoplasm. In contrast to earlier studies, it is concluded that the interior of living cells can be described as a viscoelastic liquid, and not as an elastic gel. © 2016, Nature Publishing Group. All rights reserved.

Brunet P.,CNRS Complex Systems and Materials Laboratory | Brunet P.,CNRS Institute of Electronics, Microelectronics and Nanotechnology
Soft Matter | Year: 2012

We study the size and shape of the final deposit obtained when a drop with colloidal particles has dried on a ultra-hydrophobic surface made of micro-posts. As expected, most of the particles lie inside a circular area, whose radius roughly corresponds to the Laplace pressure threshold for liquid impalement inside the structure (Cassie-Wenzel transition), inducing a coffee-stain deposit due to contact-line pinning. Less expected is the observation of tiny deposits on top of posts in the area external to the main ring, despite the low macroscopic liquid/solid friction. Experiments are carried out varying the concentration in particles and initial volume of drops, in order to determine the influence of these parameters on the size distribution of deposits. A microscopic insight of the tiny deposits is proposed, based on recent experiments of non-volatile liquid sliding drops. © 2012 The Royal Society of Chemistry.

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