Microréversibilité

Résumé

La microréversibilité est une propriété de certains systèmes physiques de particules ou quasi-particules élémentaires décrits par des équations symétriques par rapport au temps. Cette propriété permet d'exhiber un lien entre une propriété du système macroscopique parcouru dans un sens et celle correspondante au sens inverse. Renversement du temps La symétrie des équations décrivant un système par rapport au temps permet une analyse de ceux-ci en mécanique classique ou quantique. En mécanique classique Dans le cas où le lagrangien est un polynôme du second degré des vitesses seulement alors \textstyle L(\mathbf{q},\dot{\mathbf{q}})=L(\mathbf{q},-\dot{\mathbf{q}}) où q représente la position. Un système de particules isolées sans spin possède cette propriété qui est préservée même en présence d'un champ statique externe. Dans ce dernier cas l'hamiltonien du système s'écrit : :H(\mathbf{r},\mathbf{p})=\frac{p^2}{2m}+V(\mathbf{r})=H(\mathbf{r},-\

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https://fr.wikipedia.org/wiki/Micror%C3%A9versibilit%C3%A9

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Loopy Levy flights enhance tracer diffusion in active suspensions

Andrea Cairoli, Tomohiko Sano

A theoretical framework describing the hydrodynamic interactions between a passive particle and an active medium in out-of-equilibrium systems predicts long-range Levy flights for the diffusing particle driven by the density of the active component. Brownian motion is widely used as a model of diffusion in equilibrium media throughout the physical, chemical and biological sciences. However, many real-world systems are intrinsically out of equilibrium owing to energy-dissipating active processes underlying their mechanical and dynamical features(1). The diffusion process followed by a passive tracer in prototypical active media, such as suspensions of active colloids or swimming microorganisms(2), differs considerably from Brownian motion, as revealed by a greatly enhanced diffusion coefficient(3-10) and non-Gaussian statistics of the tracer displacements(6,9,10). Although these characteristic features have been extensively observed experimentally, there is so far no comprehensive theory explaining how they emerge from the microscopic dynamics of the system. Here we develop a theoretical framework to model the hydrodynamic interactions between the tracer and the active swimmers, which shows that the tracer follows a non-Markovian coloured Poisson process that accounts for all empirical observations. The theory predicts a long-lived Levy flight regime(11) of the loopy tracer motion with a non-monotonic crossover between two different power-law exponents. The duration of this regime can be tuned by the swimmer density, suggesting that the optimal foraging strategy of swimming microorganisms might depend crucially on their density in order to exploit the Levy flights of nutrients(12). Our framework can be applied to address important theoretical questions, such as the thermodynamics of active systems(13), and practical ones, such as the interaction of swimming microorganisms with nutrients and other small particles(14) (for example, degraded plastic) and the design of artificial nanoscale machines(15).

2020

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Water significantly influences the mechanical behaviour of all granular materials but none as much as hygroscopic amorphous particles. With sufficiently high water content, particles can swell, agglomerate and their mechanical properties can be reduced, having direct effects on the macroscopic response of the material. In the food and pharmaceutical industry this can cause loss of product functionality. Despite their relevance, very little is known about the microscopic processes that induce these phenomena. Previous studies focused on single particle behaviour, the strength of agglomerated particles and the material flowability, leaving unexplored the link connecting the particle behaviour and the bulk response. This experimental study aims to investigate this aspect with quantitative measurements at both particle and macroscopic scales. A sample of fine couscous is exposed to a high relative humidity (RH) air flow, while being subjected to oedometric conditions, in order to reproduce the storage-silo conditions. In the meantime, X-ray tomographies are acquired continuously and the resulting images are analysed. The designed spatial resolution allows each particle of the sample to be identified and tracked, allowing volumetric evolution to be compared to the properties of the whole sample. The analysis reveals a dilation-compaction macroscopic behaviour, a result of the competition between the particle swelling and the higher deformability as the water content increases. The number, orientations and inter-particle contacts are computed. Their area is related to the applied boundary conditions, and is found to be consistent with the particle swelling and dependent on the applied stress direction.

ROYAL SOC CHEMISTRY2022

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Partial slip at fluid-solid boundaries by multiparticle collision dynamics simulations

Maxim Belushkin, Giuseppe Foffi, Samuel Hanot

The understanding of the exact boundary conditions at the interface between a solid and a fluid is becoming increasingly important, as the limitations of the no-slip boundary condition are becoming apparent, especially in micro- and nanofluidics applications. We present a systematic study of controlled partial-slip boundary conditions in multiparticle collision dynamics simulations. By studying a plane Poiseuille flow, we demonstrate that, in general, the slip length of the fluid strongly depends on the microscopic dynamics in the vicinity of the solid surface. We empirically derive the dependence of the slip length on the properties of the MPC fluid and of the wall, and demonstrate that the wall slip in this coarse-grained simulation approach is linearly correlated with the flux of momentum across the fluid-solid interface.

Royal Soc Chemistry2013

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Loopy Levy flights enhance tracer diffusion in active suspensions

Andrea Cairoli, Tomohiko Sano

2020