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Invited talksInvited speakers - Frédéric Blanc (Univ. Côte d'Azur, France) - Disorder-induced stress-flow misalignment in soft glassy materials revealed using multi directional shear __________________________________________________ Abstracts Frédéric Blanc (Univ. Côte d'Azur, France) - Disorder-induced stress-flow misalignment in soft glassy materials revealed using multi directional shear
Soft glassy materials, such as gels and emulsions, behave as elastic solids at rest but flow as liquids once a yield stress is exceeded. Using a homemade 2D shear rheometer, we show that after a controlled preshear and a change of shear direction, the force required to impose the flow is not aligned with the shear direction. This misalignment reveals a transient orthogonal stress and an anisotropic yield surface. The response originates from internal stress memories encoded at the microscopic scale. A mesoscopic elasto-plastic model confirms that local yield stress disorder governs these effects, providing a new way to probe microscopic disorder through macroscopic rheology.
Céline Cohen (Univ. Côte d'Azur, France) - Swimming of plant pathogens zoospores
Phytophthora species cause diseases in a wide range of plants and represent a major agricultural threat, leading to multibillion-dollar losses every year. Infection occurs when these biflagellate, unicellular zoospores move through the soil at their characteristic high speed and reach the roots of a host plant. The question then arises as to how they find the roots and how they manage to swim toward them in the porous environments of moist soils. To address this question, we focus on their swimming mechanisms as well as their chemotaxis — that is, their ability to orient themselves in response to a chemical stimulus. In this seminar, I will present their specific swimming mechanism and discuss the influence of the external environment on their mobility (in the presence of chemical gradients or under confinement). Ramin Golestanian (Oxford Univ., UK & MPI, Germany) - Non-reciprocal active matter
Non-reciprocal interactions (NRIs) are quite natural among higher organisms including humans, as we all know that the way two humans act towards each other does not derive from a mutual translationally symmetric ``interaction potential'' that gives rise to action=-reaction. It is, however, a remarkably surprising that NRIs can exist at the microscopic scale among brain-less particles, in particular enzymes, when they are catalytically or metabolically active, i.e. under non-equilibrium conditions. In this this talk, the topic of how experimentally realizable microscopic NRIs can lead to complexity will be introduced, following the developments in the field over the last decade. In particular, we discuss how non-reciprocal active matter can provide a paradigm within which we can understand how it may have been possible to form self-organized metabolic cycles at the early stages of life formation in a fast and robust manner. Elie Hachem (Mines Paris PSL, France) - Complex Blood Flow Prediction Using Space–Time Graph Neural Network
Intracranial aneurysms (IAs) involve highly complex blood flow dynamics governed by nonlinear interactions between vascular geometry, wall mechanics, and pulsatile forces. We present, in collaboration with CHU Nice, a physics-constrained digital twin that couples high-fidelity fluid simulations with space–time graph neural networks (GNNs) to predict full 3D hemodynamic fields across the cardiac cycle. Trained on a hybrid dataset combining synthetic benchmark geometries and patient-specific aneurysms, the model captures transient flow structures and reproduces key biomarkers such as wall shear stress and oscillatory shear index with near real-time accuracy. This integration of computational mechanics and AI offers a powerful step toward fast, physics-grounded vascular risk assessment and digital-twin-enabled clinical support.
Florence Marcotte (Univ. Côte d'Azur, France) - On nonlinear transitions, minimal seeds and "exact solutions" for the geodynamo
Nearly 50 years ago, Paul Roberts proposed that Earth's magnetic field, generated by turbulent convection in the liquid outer core, may arise from a subcritical dynamo instability operating in a so-called "strong-field" regime, i.e. balancing Coriolis, pressure and Lorentz forces. However, this dynamo, which could potentially operate below the threshold for hydrodynamic convection, remains elusive in direct numerical simulations. I will present a numerical exploration of subcritical convective dynamo action in a spherical shell, using techniques from optimal control and dynamical systems theory. This study represents a first step toward a new approach to uncover the nonlinear dynamics of geomagnetic field generation. Joint work with Calum Skene and Steve Tobias.
Suzie Protière (Sorbonne Univ., France) - Freezing a hydrogel
A hydrogel is a material consisting of a polymer matrix which can absorb up to 99% water. It is well known that water expands when it freezes. In this talk we discuss how the hydrogel’s matrix will affect the overall final shape of such a material during its freezing and how it interacts with the ice growth. We find that hydrogel drops expand solely vertically in the direction of the temperature gradient and propose a model to predict this shape change by assuming several conditions that we observe experimentally which differ from the freezing of a water drop. Moreover, we observe that for hydrogels with a low concentration of polymer, water is expelled from the gel onto its surface during the freezing process. We will discuss how this phenomenon may be related to the poroelastic nature of the material. Jacco Snoeijer (Twente Univ., the Netherlands) - Sticking without contact: Elastohydrodynamic adhesion
The adhesion between dry solid surfaces is typically governed by contact forces, involving surface forces and elasticity. For surfaces immersed in a fluid, out-of-contact adhesion arises due to the viscous resistance to the opening of the liquid gap. While the adhesion between dry solids is described by the classical Johnson-Kendall-Roberts (JKR) theory, there is no equivalent framework for wet adhesion in the case of very soft solids. Here, we describe a new type of viscous adhesion that emerges during the separation of a sphere from a soft elastic substrate. Unexpectedly, the elastic substrate closely follows the motion of the sphere, leading to a sticking without contact, which is followed by a violent snap-off of the adhesive contact. We quantify the sticking force and the snapping motion through similarity solutions, point out the relation with JKR theory, and discuss the relevance for applications involving viscous adhesion.
Lydie Staron (Sorbonne Univ., France) - Flowing Grains
Granular matter is a specific kind of yield stress material, in which plasticity and viscosity have long fought for the command of the flowing properties. Fortunately, the crafting of a viscosity based on the material internal friction has pacified the two conflictual descriptions, allowing at once to account for the shear thinning properties, while including at the same time the shear thickening ones. The talk recounts and illustrates this welcome truce through two challenging flow examples: the column collapse and the silo discharge, and concludes on the next step forward: cohesive matter.
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