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

### A venir

Thursday, June 06, 11:00

**Julien Tailleur (MSC, Université Paris Diderot)**

From pressure to surface tension : the anomalous thermomechanics of active particles

Tuesday, June 11, 11:00

**Vishwas Vasisht**

In this talk I will address the question of residual stress states that are created in athermally sheared disordered materials. Up on flow cessation, even after a long time relaxation, internal stresses can be frozen in a system, which are termed as residual stresses. These stresses can adversely affect material’s performance and if controlled well, one can produce materials with improved mechanical properties. We use a combination of mesoscopic simulation and molecular simulations to understand the non-trivial relaxation dynamics towards a residual stress state. The frozen-in stresses depend on the initial driving rate and the deformation history. We analyze various mechanical and structural properties of these states. Further, subjecting these states to shear deformation we analyze their rheological response.

### Séminaires passés

**2019**

Thursday, January 10, 11:00

**Vivien Lecomte**

TBA

Thursday, January 24, 11:00

**Jean-Louis Barrat / LIPhy**

I will discuss the Kubo formulae for heat conductivity and thermal interfacial resistance, and give a few examples of applications for nanostructured or amorphous systems. I will also discuss the shortcomings associated with neglecting quantum effects in the motion of atomic nuclei, and present the content of the "heatflow" project just funded by ANR.

Thursday, February 14, 11:00

**Baoshuang Shang**

Amorphous materials have a rich relaxation spectrum which is usually described in terms of alpha, beta, and possibly more complex relaxation mechanism. In this work, we investigate the local dynamic modulus spectrum in a model glass just above glass transition temperature by performing a mechanical spectroscopy analysis using molecular dynamics. We find that the spectrum, at the local as well as on the global scale, can be well depicted by Cole-Davidson formula in the frequency range explored with simulations. Surprisingly, the Cole-Davidson stretching exponent does not change with the size of the local region that is probed. The local relaxation time displays a broad distribution, as expected based on dynamic heterogeneity concepts, but the stretching is obtained independently of this distribution. We find that the size dependence of local relaxation time and modulus can be well explained by the elastic shoving model.

Thursday, April 04, 14:00

**Matthieu Tissier**

Thursday, May 09, 11:00

**Camille Scalliet (Montpellier)**

Despite being widely used for various applications, the fundamental nature of amorphous solids, such as granular matter, foams, emulsions, dense colloidal pastes, molecular and atomic glasses, is still unclear. The goal is to describe theoretically how their macroscopic properties (transport, vibrational, mechanical, etc.) arise from their disordered microscopic structure. To that aim, an exact mean field theory for glasses was recently derived. The solution surprisingly revealed existence of a new amorphous phase of matter, called the Gardner phase. The mathematical description of this phase reflects a complex and fragile structure : the solid responds in a violently nonlinear way to very weak perturbations, it evolves in time (aging) and involves rearrangements that propagate through the whole system by successive avalanches. This behavior is the one we seek to describe. I will present analytical and numerical results obtained with a simple glass-forming model. By changing external parameters, we continuously explore physical regimes relevant to granular matter, foams, emulsions, hard and soft colloids, and molecular glasses. I will discuss under which type of physical conditions this new amorphous phase of matter is found, and its implications for the behavior of the solid. Our results suggest that Gardner phase should be observable for colloidal and non-Brownian particles near their jamming transition. In this regime, we numerically observe spectacular rejuvenation and memory effects. By contrast, we find that molecular glasses do not present marginally stable phases, but our study reveals instead the presence of localised excitations presumably relevant for mechanical and low-temperature properties of structural glasses.

Thursday, May 23, 14:00

**Cyril Falvo (ISMO, Paris Sud / LIPhy)**

Carbon clusters show a significant ability to hybridize in sp, sp2 or sp3 chemical bonds, reflecting at finite size the wide allotropy of bulk carbon matter. In the recent years the interest in pure and hydrogenated carbon clusters has been driven by the recent detection of C60 buckminsterfullerene and C70 fullerene in the interstellar medium (ISM) from their peculiar infrared emission bands [1]. These bands accompany the so-called aromatic infrared bands, which trace polycyclic aromatic aliphatic mixed hydrocarbons [2]. To understand better the formation mechanisms of cosmic fullerenes and to identify other possible forms of carbon clusters in the ISM, it is essential to characterize the possible structural diversity of carbon clusters and map these structures onto their spectroscopic signature.

It is known for carbon clusters that below about 15 atoms, one-dimensional chains and rings are the most stable isomers. Two-dimensional flakes then become the most stable form up to 20-30 atoms. Finally for the larger clusters, three-dimensional fullerenes are the lowest energy isomers [3]. In this work we use atomistic simulations to explore the structural diversity and the thermodynamics over a large range of temperature of carbon clusters in the size range where they undergo the flake-to-fullerene transition. We show that depending on size and temperature carbon clusters are present in different structural families, namely chains, rings, pretzels/branched, flakes and cages (fullerenes). We also explore the effect of hydrogenation on the structure of these carbon clusters.

[1] J. Cami, J. Bernard-Salas, E. Peeters, and S. Malek, Science 329, 1180 (2010).

[2] A. Léger and J. L. Puget, Astron. Astrophys. 137, L5 (1984).

[3] S. K. Lai, I. Setiyawati, T. W. Yen, and Y. H. Tang. Theor. Chem. Acc. 136, 20 (2016).

Tuesday, May 28, 11:00

**Holger Kantz (Max Planck Institute for the Physics of Complex Systems, Dresden, Germany)**

We present a dynamical mechanism for a scale dependent error growth rate, by the introduction of a class of hierarchical models. The coupling of timescales and length scales is motivated by atmospheric dynamics. This model class can be tuned to exhibit a scale dependent error growth rate in the form of a power law, which translates in power law error growth over time instead of exponential error growth as in conventional chaotic systems. The consequence is a strictly finite prediction horizon, since in the limit of infinitesimal errors of initial conditions, the error growth rate diverges and hence additional accuracy is not translated into longer prediction times. By re-analyzing data of the NCEP Global Forecast System published by Harlim et al. (PRL 94 228501 (2005)) we show that such a power law error growth is indeed present in numerical weather forecast models, and using our concepts we calculate a mamiximal prediction horizon of 15-16 days for these data.

Thursday, June 06, 11:00

**Julien Tailleur (MSC, Université Paris Diderot)**

From pressure to surface tension : the anomalous thermomechanics of active particles

**2018**

Thursday, July 05, 11:00

**Marc Joyeux**

Role of salt valency in the switch of H-NS proteins between DNA-bridging and DNA-stiffening modes

Thursday, July 12, 11:00

**Victor Purello (Mar del Plata and Bariloche)**

Interfaces are ubiquitous and can be seen in phenomena as large asforest fires, or as small as domain walls in ferromagnetic materials.In our work, we describe the latter by developing models and runninglarge-scale simulations using GPGPU to test our predictions. We havestudied interfaces by modeling them as driven elastic manifolds inquenched random media, where we analyzed the role of the elasticity,the disorder and different thermal effects. Now we are consideringinterfaces with mass, which could play the same role as the memoryeffects induced by the phase in spintronic models. The inertial termis already giving us exciting results as, for instance, a lowercritical force for the depinning transition, or the possible presenceof a critical mass.

Tuesday, July 17, 14:00

**Pinaki Chaudhuri, visitor of the PSM group this month**

Thermal and allied transport in glass-forming materials

Thursday, September 27, 11:00

**Takahiro Nemoto (ENS, Paris)**

Large deviations of non-equilibrium time-extensive quantities have been extensively studied in the last decade in systems ranging from (a)thermally fluctuating particles (Brownian particles, biological motors, Granular particles…), exactly solvable lattice gas models (ASEP, KPZ, KCMs...) as well as high-dimensional chaotic dynamics (FPU chain, climate model,…). By definition, studying large deviations is difficult since the fluctuations leading to their occurrence are hardly observed. In this seminar, I will present an algorithm which allows the observation of these rare events in numerical simulations. The algorithm is based on population dynamics (a.k.a. splitting or diffusion quantum Monte-Carlo method) [1] : an ensemble of copies of the system is simulated and the dynamics of the population includes a selection-mutation process. Namely, rare copies are multiplied (have descendants) but typical ones are killed (become extinct) to select atypical trajectories of interest. After introducing this algorithm in a pedagogical way, I will present recent applications of the algorithm to active Brownian particles, a model of self-propelled particles, which show unexpected dynamical phase transitions to flocking/jammed states in their rare events [2].

[1] Cristian Giardinà, Jorge Kurchan and Luca Peliti, Phys. Rev. Lett. 96, 120603 (2006).

[2] T.N., Étienne Fodor, Michael E. Cates, Robert L. Jack and Julien Tailleur, arXiv:1805.02887 (2018).

Thursday, October 11, 14:00

**Achim Wirth, LEGI / Univ. Grenoble-Alpes**

We establish the fluctuation-dissipation relation for a turbulent fluid layer (ocean) subject to frictional forcing by a superposed lighter fluid layer (atmosphere) in local models of air-sea dynamics. The fluctuation-dissipation relation reflects the fact that air-sea interaction not only injects energy in the ocean but also dissipates it.

Energy injection and dissipation must therefore be related.

The competition between the two processes determines the oceanic energy budget in the idealized dynamics considered here. When applying the fluctuation-dissipation relation to a two-dimensional two-layer Navier-Stokes model with turbulent dynamics, in the atmosphere and the ocean, coupled by a quadratic friction law, the friction parameter is estimated within 8\% of the true value, while the estimation of the mass ratio between the atmosphere and the ocean fails, as the forcing time-scale is not faster than the characteristic time-scale of the atmospheric dynamics.

Link to paper : https://hal.[archives-ouvertes.fr/hal-01769301](http://archives-ouvertes.fr/hal-01769301)

Thursday, October 25, 11:00

**Suman Dutta**

TBA

Thursday, November 08, 10:30

**Florent Calvo / LIPhy**

The quantum structure of anionic hydrogen clusters

Thursday, November 29, 11:00

**Cesare Nardini**

Active agents are able to extract non-thermal energy from the environment and dissipate it to self-propel. It is well known that large assemblies of purely repulsive ones, can undergo bulk liquid-vapor phase separation. In experiments and large-scale simulations, however, more complex steady states are often seen, comprising a dynamic population of dense clusters in a sea of vapor, or dilute bubbles in a liquid.

In this talk, we show that these microphase separated states should be expected generically in active matter, without any need to invoke system-specific details. We show this by extending the phi^4 field theory of passive phase separation to allow for all local currents that break detailed balance at leading order in a gradient expansion. Microphase separation is explained by the fact that the classical Ostwald process, that would normally drive bulk phase separation to completion, can be reversed.

We conclude the talk discussing the transition from bulk to microphase separation beyond mean-field level. One loop dynamical renormalization group analysis shows that it probably belongs to a new non-equilibrium universality class.

The talk is based on :

- E. Tjhung, C. Nardini, M.E. Cates, Phys. Rev. X **8**, 031080, 2018

- F. Caballero, C. Nardini, M.E. Cates, arXiv:1809.10433, 2018

Thursday, December 06, 11:00

**Etienne Fodor (postdoc in Cambridge)**

Active systems are made of interacting particles which extract energy from their environment to perform a directed motion. For instance, bacteria consume some nutrients to exert forces with their flagella, which allows them to move persistently by alternating running and tumbling periods. In contrast with equilibrium, active systems constantly dissipate energy : this offers the opportunity to build some autonomous engines extracting work at constant temperature. I will discuss how to design efficient active engines which operate either by exploiting the current of some asymmetric obstacles or by driving some confining walls with cyclic protocols. Moreover, active particles can undergo spontaneous transitions driven by collective effects. I will explore the role of dissipation in the emergence of such collective states, to shed light on transitions between a phase separation with purely repulsive interactions and a collective motion despite the lack of aligning interactions.

Tuesday, December 18, 11:00

**Bertrand Fourcade**

(TBA)