Les séminaires du groupe se déroulent habituellement dans la salle de lecture du LIPhy

### A venir

Tuesday, March 08, 11:00

**Aurélien Grabsch (LPTMC, Paris)**

## Generalised Density Profiles in Single-File Systems

Single-file transport, where particles diffuse in narrow channels while not overtaking each other, is a fundamental model for the tracer subdiffusion observed in confined systems, such as zeolites or carbon nanotubes. This anomalous behavior originates from strong bath-tracer correlations in 1D, which we characterise in this talk through Generalised Density Profiles (GDPs). These GDPs have however remained elusive, because they involve an infinite hierarchy of equations. Here, for the Symmetric Exclusion Process, a paradigmatic model of single-file diffusion, we break the hierarchy and unveil a closed equation satisfied by these correlations, which we solve. Beyond quantifying the correlations, the central role of this equation as a novel tool for interacting particle systems will be further demonstrated by showing that it applies to out-of equilibrium situations, other observables and other representative single-file systems.

### Séminaires passés

**2022**

**2021**

Thursday, January 21, 11:00

**Sara Dal Cengio (University of Barcelona)**

## Generalized Green-Kubo relations for active fluids

We address the question of how interacting active systems in a non-equilibrium steady-state respond to an external perturbation. We consider two paradigmatic models of interacting self- propelled particles, namely Active Brownian Particles (ABP) and Active Ornstein- Uhlenbeck Particles (AOUP) and analyze the non-equilibrium character of these systems. We derive corresponding extended Green-Kubo relations for them, clarifying which features of these active model systems are genuinely non-equilibrium.

Tuesday, February 02, 11:00

**Nicolas Levernier (IUSTI, Aix-Marseille Université)**

## First-passage time of non markovian random walks

In this talk, I will present part of the results I got during my PhD, and part of those obtained during my first postdoc

I) The computation of the encounter time of particles is a key question in many contexts, as this time quantifies the reactivity rate for diffusion-limited processes. In the case of markovian random walks, such as brownian motion, some analytic results can be obtained. But in the case of non-markovian processes, much fewer results do exist, although "non-markov is the rule and markov is the exception" (Van Kampen). In this talk I will present a formalism we have developed to deal with non-markovian random walks and show its application to Fractional Brownian Motion, a paradigmatic example of highly-correlated process. If time allows, I will briefly present how aging of the dynamics can deeply modify the encounter time statistics.

II) In the second part of my talk, I will briefly present results I got during my first postdoc. I will show how chaotic motion can arise in an extended active gel layer, typically describing cortical cytoskeleton, where polymerization and contraction due to molecular motors are combined. This result questions the usual description of the cortex as a thin layer, as such a description cannot describe this instability. I will also briefly present recent experimental evidences of this predicted phenomenon.

Tuesday, March 09, 11:00

**Sara Dal Cengio**

Surface charge discontinuities as a sensing tool

Tuesday, March 16, 11:00

**Johannes Richardi (Sorbonne Université, Laboratoire de Chimie Théorique)**

## Simulation Study of Metallic Nanocrystals using Reactive Force Fields

The use of reactive force fields allows to improve the molecular dynamics simulations of nanoparticles. In particular, it enables the study of the formation of

stapleson metallic nanocrystal surfaces.Staples(CH3S-Au-SCH3) appear when two ligand molecules extract metallic atoms from the surface. We will first explain the method to generate reactive force field from quantum chemical data. A new ReaxFF for silver-thiol is presented [1]. Then, new simulation results for silver and gold nanocrystals are shown. They are markedly different from previous ones obtained with less accurate force fields [2].[1] Dulong, C. ; Madebene, B. ; Monti, S. ; Richardi, J. Optimization of a New Reactive Force Field for Silver-Based Materials. J. Chem. Theory Comput. 2020, 16, 7089–7099.

[2] Djebaili, T. ; Abel, S. ; Marchi, M. ; Richardi, J. Influence of Force-Field Parameters on the Atomistic Simulations of Metallic Surfaces and Nanoparticles, J. Phys. Chem. C 2017, 121, 27758−27765.

Tuesday, April 27, 11:00

**Stefano Mossa**

## Vibrational excitations and elastic heterogeneities in disordered solids

In a series of papers [1-4] we have reported on extensive numerical work focusing on elastic heterogeneity in disordered solids. The idea is quite simple : glasses and even (complex) crystals with defects show inhomogeneous mechanical response at the nanoscale, which does not conform to the macroscopic limit predictions. This property can strongly influence vibrational and thermal properties of the materials, and could ultimately lie at the bottom of puzzling anomalous features like boson peak, Raleigh-like strong scattering, or temperature dependence of thermal conductivity.

We have investigated the above issues systematically and in great details by classical Molecular Dynamics simulation. By employing toy models like monodisperse Lennard Jones nanoparticles and soft spheres binary mixtures, we have generated multiple phases with variable amounts of disorder, ranging from perfect crystals, through crystals with defects, to glassy configurations. In all cases, we have monitored independently mechanical response, collective excitations, and heat transfer features, establishing precise correlations among the heterogeneous local mechanical response, the nature of the vibrational states, and the variation of thermal conductivity.

In the talk I will concisely review our findings and put them in perspective, also with reference to advanced spectroscopy measurements with X-Rays. If time allows, I will also show how information of this nature can help to engineer interfaces, devising metamaterials with tailored functions[5].

—

1. H. Mizuno, G. Ruocco, and SM, Sound damping in glasses : interplay between anharmonicities and elastic heterogeneities [10.1103/PhysRevB.101.174206]2. H. Mizuno, SM, and J.-L. Barrat, Relation of vibrational excitations and thermal conductivity to elastic heterogeneities in disordered solids [10.1103/PhysRevB.94.144303]

3. H. Mizuno, SM, and J.-L. Barrat, Acoustic excitations and elastic heterogeneities in disordered solids [10.1073/pnas.1409490111]

4. H. Mizuno, SM, and J.-L. Barrat, Elastic heterogeneity, vibrational states, and thermal conductivity across an amorphisation transition [10.1209/0295-5075/104/56001]

5. H. Mizuno, SM, and J.-L. Barrat, Beating the amorphous limit in thermal conductivity by superlattices design [10.1038/srep14116]

Tuesday, July 20, 11:00

**Félix Helluin**

## Rolling motion of noodles on a wet cloth

I experimentally investigate the slow rolling motion of non-gelatinized pastas on a wet cloth at ambient temperature and relate the period of rotation of spaghetti noodles with the cloth moisture. I base this relation on an analogy with nylon fiber’s behavior in a thermal gradient (DOI : 10.1038/s41563-018-0062-0), and show it could be extended to other types of pastas.

Tuesday, September 14, 11:00

**Alejandro Kolton (CNEA, Bariloche, Argentina)**

## Curvature-driven AC-assisted creep dynamics of magnetic domain walls

The dynamics of micrometer-sized magnetic domains in ultra-thin ferromagnetic films is so dramatically slowed down by quenched disorder that the spontaneous elastic tension collapse becomes unobservable at ambient temperature. By magneto-optical imaging we show that a weak zero-bias AC magnetic field can assist such curvature-driven collapse, making the area of a bubble to reduce at a measurable rate, in spite of the negligible effect that the same curvature has on the average creep motion driven by a comparable DC field. An analytical model explains this phenomenon quantitatively.

Tuesday, October 12, 11:00

**Jiting Tian (Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics)**

## An introduction of my life and research in China

Having joined the PSM group for 8 months, I’m happy to give a relaxed and enjoyable talk about my life and research in China. In the first part, I’d like to introduce my hometown (including history, culture, scenery spots and foods) and Peking University where I got my BS and PhD degrees. In the second part, I’ll talk about my research on computer simulations of radiation damage in metals, especially energetic particles induced displacement cascades on the surface or in the bulk. This talk would not be a serious academic one, but I hope I can provide some information interesting and useful for the group members.

Tuesday, October 19, 11:00

**Laura Guislain (LIPhy, Grenoble)**

## Oscillating versus ferromagnetic order in a driven stochastic mean-field model

Out-of-equilibrium systems made of a large number of entities in interaction often present a rich phenomenology such as periodic macroscopic oscillations. To explore such a phenomenology in a minimal model, we consider a driven stochastic mean-field model with two types of microscopic variables : spins and fields. We find in this model two phase transitions from a disordered phase, either to an oscillating phase or to a ferromagnetic phase. We describe these phase transitions using standard tools like order parameters and susceptibilities as for equilibrium phase transition, and we further characterize the out-of-equilibrium aspects in terms of entropy production.

Tuesday, November 16, 11:00

**David Richard**

## Using soft excitations to predict and understand plastic flow in disordered solids

Imposing an external driving, amorphous solids can flow via a succession of plastic rearrangement of localized particles. Numerous numerical and experimental studies have shown that loci of plastic instability in glasses are triggered by spatially localized soft spots in direct analogy with dislocations present in crystalline solids, although the population and microscopic structure of the former being significantly different from the latter. The detection and nature of such “amorphous defects” have received a lot of attention, one of the goals being to predict from the microscopic structure itself which regions are likely to undergo a rearrangement upon deformation. In this context, I will discuss novel indicators that are constructed from an analysis of the potential energy landscape and show how one can construct anisotropic descriptors that consider the tensorial nature of the coupling of a soft spot with a particular loading geometry. These methods will be illustrated in the context of strain localization and fracture.

Tuesday, November 30, 11:00

**Romain Mari (LIPhy, Grenoble)**

## Transient flows and migration in granular suspensions : key role of Reynolds-like dilatancy

Dense non-Brownian suspensions are mixtures of micrometric particles and fluid, mixed in roughly equal proportions. When homogeneous, they have a non-Newtonian rheology, albeit a simple one, as stresses are linear in the shear rate. However in many relevant applications, these suspensions do not remain homogeneous, as dilation and migration phenomena affect the distribution of suspended particles. These transient phenomena are still poorly understood. In this work we simulate dilation in dense suspensions with Discrete Element Method under a varying imposed pressure on the particle phase. We show that a two-phase model incorporating a Reynolds-like dilatancy law, which prescribes the dilation rate of the suspension over a typical strain scale, quantitatively captures the suspension dilation/compaction over the whole range of parameters investigated. Together with the Darcy flow induced by the pore pressure gradient during dilation or compaction, this Reynolds-like dilatancy implies that the early stress response of the suspension is nonlocal, with a nonlocal length scale L which scales with the particle size and diverges algebraically at jamming. In regions affected by L, the stress level is fixed, not by the steady-state rheology, but by the Darcy fluid pressure gradient resulting from the dilation/compaction rate. Our results extend the validity of the Reynolds-like dilatancy flow rule, initially proposed for jammed suspensions, to flowing suspension below jamming, thereby providing a unified framework to describe dilation and shear-induced migration. They pave the way for understanding more complex unsteady flows of dense suspensions, such as impacts, transient avalanches or the impulsive response of shear-thickening suspensions.

Tuesday, December 14, 11:00

**Alan Sam (LIPhy, Grenoble)**

## Molecular dynamics simulations of water flow in nanopores

Nanofluidics, the study of fluid flow in systems with nanometric sizes, offers great potential to transform the state of the art water purification, energy storage, and biomedical technologies. Numerous studies have focused on the transport of water through carbon nanotubes (CNTs) ; a system that is particularly interesting for its extremely high flow rates. We investigated the effect of the tube flexibility on the water flow in CNTs using molecular dynamics (MD ) simulations. We have compared nonequilibrium molecular dynamics (NEMD) simulations with a thermostat applied to the confining wall atoms, or to the fluid while keeping wall atoms rigid. Streaming velocity profiles and mass flow rates are found to depend on the thermostatting approach, with flow rates up to 20% larger for systems in which the wall atoms are flexible. The larger flow rates in flexible CNTs are explained by a lower friction coefficient between water and flexible wall atoms. Further, we have shown that transport of water in CNTs depends strongly on the type of CNT, with the flow rate in the armchair tube significantly higher than in chiral and zigzag tubes of similar diameter. The difference in flow rates with varying tube structure can be primarily attributed to the alteration in the potential energy landscape felt by the water molecules, leading to changes in the friction coefficient at the fluid-solid interface. We have also proposed an analytical method to calculate the slip length in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations. Our results provide insights into the dynamics of water flow through CNTs which could further improve the potential for CNT based nanofluidic system designs.

**2020**

**2019**

**2018**