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Theses

Phase separation in active systems : non-equilibrium fingerprints

Abstract : Active matter is intrinsically out of equilibrium because energy is converted into systematic motion by its constituents, and exhibit fascinating collective phenomena. One of them is phase separation between dense and dilute regions which, unlike in equilibrium, can happen even in absence of attraction among particles.In this Thesis, we study phase separation in active systems. First, we adopt the coarse-grained point of view of Active Model B+, a field theory based on symmetry arguments and conservation laws. This predicts the emergence of novel types of phase separation, impossible in equilibrium: a microphase separated state and bubbly phase separation (the coexistence between this microphase separated state and the homogeneous phase). In the first part of the Thesis, we study the statistical properties of micro and bubbly phase separation, confirm their asymptotic existence and show how the time convergence to the steady state is system size dependent in the bulk phase separation.Secondly we study the concept of interfacial tension in active systems, and derive from first principles (and for the first time) the capillary wave tension, which determines the elasticity of active liquid-vapor interfaces. By doing so, we show that no unique definition of surface tension can exist in active systems, thus ending a long-dated debate. Discovering that the capillary interfacial tension can become negative because of activity, we also find new types of phase separation, among which a previously unknown `active foam' state.Finally, we introduce and study a minimal model for the dynamics of vapor bubbles in micro and bubbly phase separation. Thanks to this, we shed light on the statistical properties of these phases and on how they might be controlled in the future in particle-based models.
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Submitted on : Saturday, August 13, 2022 - 1:01:15 AM
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  • HAL Id : tel-03750918, version 1

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Giordano Fausti. Phase separation in active systems : non-equilibrium fingerprints. Soft Condensed Matter [cond-mat.soft]. Université Paris-Saclay, 2021. English. ⟨NNT : 2021UPASP141⟩. ⟨tel-03750918⟩

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