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Articles
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Séminaire PMMH - Olivier Plé (LOCIE-CNRS)
4 décembre 2023Vendredi 5 avril de 11h00 à 12h00 - Salle réunion PMMH 1Rammed earth : A complex medium
Rammed earth is a complex material which is attractive nowadays in construction because of its small environmental footprint. However, this material has specific characteristics, especially : its sensitivity to water, its low mechanical static capacity but a fairly good seismic performance, its high thermal conductivity which cannot guarantee sufficient winter comfort, its high heterogeneity which makes it a size effect material….. In short, a complex, unsaturated heterogeneous material with numerous couplings.
In this presentation I will try to show the complexity of the material and how physics helps us to understand for a better use in construction. -
Séminaire PMMH - Sophie Marbach (Phenix, Sorbonne Université)
4 décembre 2023Vendredi 2 février de 11h00 à 12h00 - Salle réunion PMMH 1The Countoscope : Measuring Dynamics by Counting Particles in Boxes
Any imaging technique is limited by its field of view. As objects or particles move in and out of the observation field, tracking their motion, especially over long periods, becomes challenging. In addition, available analysis techniques face challenges reconstructing trajectories in dense and heterogeneous systems where accurately labelling particles is difficult. Here, we shift this paradigm by introducing a broadly-applicable technique that probes dynamics simply by counting particles in finite observation boxes.
Using colloidal experiments, simulations, and theory, we demonstrate that statistical properties of fluctuating counts can be used to determine self-diffusion coefficients. Our framework alleviates the hurdles associated with trajectory reconstruction. We subsequently show that by increasing the observation box size, counting naturally enables the study of collective dynamics.
We extend our technique to various suspensions, such as ions or active particles. The "Countoscope'' offers the unique possibility to systematically link individual and collective behavior, opening up broad soft matter and statistical physics perspectives.
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Séminaire PMMH - Santiago Benevides (Universidad Politécnica de Madrid)
4 décembre 2023Vendredi 12 janvier de 11h00 à 12h00 - Salle réunion PMMH 1Uncovering complex behavior in turbulence and sediment transport through simplified models
Although the dynamics of a turbulent fluid and of grains being transported downriver might not have much in common, it's the qualities they share that make them challenging subjects to study. Both contain many interacting components and nonlinearities, producing a chaotic, multi-scale, out-of-equilibrium system whose statistical properties are difficult to predict in a rigorous way. Furthermore, the range of temporal and spatial scales that they possess make direct numerical simulations of large systems (e.g. large Reynolds number flows or an alluvial channel) challenging and limited in scope. Because of this, an understanding of the system must be achieved through sensible approximations and simplifications of the full system. In this talk, I will introduce two open questions (1) the existence of sharp transitions of energy cascade directions in geophysical turbulence, and (2) the origins of intermittent statistics in sediment transport, and demonstrate the power that simplified models have in helping us uncover previously unattainable detail and understanding.
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Séminaire PMMH - Emanuela Del Gado (Georgetown University)
1er décembre 2023Vendredi 1er décembre 2023 de 11h00 à 12h00 - Salle réunion PMMH 1Memory of shear flow in soft jammed materials
Cessation of flow in simple yield stress fluids results in a complex stress relaxation process that depends on the preceding flow conditions and leads to finite residual stresses. By complementing experiments with numerical simulations we gain new insight into the microscopic origin of these phenomena for jammed suspensions of soft repulsive particles. Our spatio-temporal analysis of microscopic particle motion and local particle configurations highlights that flow in yield stress fluids can be seen as a training process during which the material stores information of the flowing state through the development of domains of correlated particle displacements and the reorganization of particle packings optimized to sustain the flow. This encoded memory can then be retrieved in flow cessation experiments.
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Séminaire PMMH - François Peaudecerf, ETH Zürich
24 novembre 2023Vendredi 24 novembre 2023 de 11h00 à 12h00 - Salle réunion PMMH 1Wall entrapment enhances bacterial chemotactic response to deposited aerosols in the microlayer
The sea surface microlayer is the thin layer of water separating the atmosphere from marine waters below. This typically half-millimeter-deep laminar layer mediates all gas exchange and receives all material deposited from the atmosphere, such as aerosol particles, before any transfer to deeper water can occur. The microlayer is a harsh environment characterized by large temperature and salinity fluctuations and strong ultraviolet radiation. Yet field sampling suggests a microlayer bacterial community does indeed exist, distinct from that in deeper waters. We hypothesize that motile microlayer and near-surface bacteria can successfully exploit the transient nutrient patches produced by surface-deposited aerosols using chemotaxis-driven foraging strategies, thus obtaining privileged access to rare resources. We developed a novel millifluidic device to image a static air-water interface with falling aerosol particles and swimming bacteria, enabling tracking of individual particles and cells. We observed that marine bacteria swam to and accumulated at the surface when exposed to environmentally relevant fluxes of deposited chemoattractant aerosols. These accumulations formed within seconds to minutes in extraordinarily thin films (< 0.1 mm) and were approximately an order of magnitude higher than expected from previously established chemotactic accumulations observed in bulk. Using a novel theoretical model of bacterial behavior, we demonstrated that this strong accumulation can indeed be explained by the coupling of chemotaxis to dissolved aerosol-borne attractants and ‘wall entrapment' at the air-water interface – a physical mechanism by which motile bacteria reside near boundaries for longer times. Our results highlight how interfaces in the environment modify the behavior of microorganisms, and in particular demonstrate that the microlayer is an environment where motile bacteria, through both active and passive mechanisms, quickly respond to aerosol deposition and accumulate in high-nutrient regions, thus resulting in a competitive advantage for motile cells.