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

### A venir

Tuesday, February 18, 11:00

**Shivakumar Athani (PSM)**

## Granular drag forces under dynamic loadings

This work examines the mechanical response of large objects called intruders or anchors, which are embedded into a granular packing and subjected to dynamic uplift loadings. By using a numerical approach based on a discrete element method, the study focuses on a canonical test comprising a plate-shaped intruder being uplifted vertically. The research is articulated into three projects.

The first project considers steady and quasi-static loading conditions, whereby the intruder is uplifted at a constant velocity. Besides validating the numerical method against established models for the maximum drag force, this project quantified to what extent it is possible to downscale/upscale the size of the intruder relative to the grain size.

The second project considers the mobility response under cyclic loading, whereby the object is subjected to a cyclic uplift force. A series of numerical tests exploring a range of loading frequency and magnitude reveals the existence of three possible mobility responses. The object can either move up steadily, not move up at all or exhibit a creep trajectory. Furthermore, this study points out a phenomena of elasto-inertial resonance inducing a fluidisation of the packing even at low loading magnitudes.

The third project considers loading patterns including some acceleration of the object. This reveals a new contribution to the drag force, which we named “inertial drag”. We show that this contribution results from gradual mobilisation and acceleration of grains in the packing above the object. We further find that achieving a complete grain mobilisation takes a finite period of time, controlled by the elasto-inerital stress propagation from the object to the free surface. These three projects highlight fundamental differences between the drag force under quasistatic loading and dynamic loading conditions. A number of analytical models, built from identified micro-mechanical processes, are proposed to rationalise these effects.

Tuesday, March 10, 00:00

**Tatiana Morozova (PSM)**

TBD

### Séminaires passés

**2020**

Thursday, January 09, 14:00

**Charlie Duclut (MPI Dresden)**

## Collective dynamics of chemotactic cells

Understanding the self-organization of living systems is one of the biggest conceptual challenges of the present century. A generic mechanism that drives such organization is interaction among the individual elements — which may represent cells, bacteria, or even enzymes — via chemical signals. After deriving a minimal microscopic model for a single chemotactic particle, I will present a coarse-grained model to describe an assembly of such particles. I will then study the scaling properties of this model using a dynamical renormalization group approach. This analysis reveals exact dynamic scaling exponents that represent superdiffusive behavior of the particles. The number fluctuations within sub-regions of the system show either a hyperuniform structure or exhibit giant number fluctuations, depending on whether or not the noise is conserved.

Thursday, January 23, 14:00

**Eric Woillez**

## Surprising non-equilibrium features of active matter in activated processes and diffusion processes.

At the heart of our understanding of equilibrium activated processes lies the simple picture, first derived by Kramers, of the escape of a particle from a metastable state. I will show that the corresponding laws for activated processes are drastically different for active systems, and that surprising new phenomena can appear : to escape from a metastable state, an active particle may choose the higher barrier instead of the lower one. In other cases, an active particle can hop over a metastable state without "feeling" it. As a consequence of Kramer’s law, the exponential scaling of the mean escape time is responsible for anomalous diffusion of equilibrium particles in the presence of disorder. In the second part of the talk, I will also derive the long-time scaling for diffusion of active particles in the presence of various types of disorder. Those results emphasize once more the difference between passive and active systems at the fundamental level.

Tuesday, February 04, 11:00

**Lorenzo Dall’Amico (GIPSA Lab)**

## Spectral clustering in sparse and heterogeneous networks

Spectral clustering is one of the most popular, yet still incompletely understood, methods for community detection on graphs. This talk presents a spectral clustering algorithm based on the Bethe-Hessian matrix for sparse and heterogeneous graphs. The proposed algorithm is capable of retrieving communities in this setting and to accurately estimate the number of communities. Strong connections are presented with other commonly used spectral clustering techniques, from both statistical physics and mathematics perspectives.

**2019**

Thursday, April 04, 14:00

**Matthieu Tissier (LPTMC)**

Thursday, May 09, 11:00

**Camille Scalliet (Montpellier)**

## A new amorphous phase of matter

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)**

## Thermodynamics of carbon clusters : chains, rings, pretzels, flakes and fullerenes

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)**

## Power law error growth rates – a dynamical mechanism for a strictly finite prediction horizon in weather forecasts

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

Tuesday, June 11, 11:00

**Vishwas Vasisht**

## Residual stresses in athermal dense disordered systems

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.

Thursday, October 03, 11:30

**Achim Wirth (LEGI)**

## Evidence of a fluctuation theorem for the input of mechanical power to the ocean at the air-sea interface from satellite data

The ocean dynamics is predominantly driven by the shear between the atmospheric winds and the ocean currents at the sea surface. The ocean mostly receives energy, when the wind accelerates the current, but it can also lose energy, when the wind slows down the ocean currents. When the input of mechanical power to the ocean is considered we are interested in averages over time and space of varying extent. A Fluctuation Theorem (FT) holds when the logarithm of the ratio between the occurrence of positive and negative events of a certain magnitude of the power input is a linear function of the magnitude and the averaging period. FTs are widely discussed in statistical mechanics, but have so far not been applied to geophysical observations. Here we show that data for the input of mechanical power into the ocean shows evidence of a FT, for regions within the subtropical Gyre in the North Atlantic and the North Pacific, but not for the Gulf Stream and Kuroshio extension. The investigation is based on 24 years of global satellite data from different satellites collected every 6 hours. In the absence of an universal distribution for non-equilibrium processes, a fluctuation theorem puts a strong constraint on the temporal distribution of fluctuations of power input of varying magnitude. It connects variables obtained with different length of temporal averaging, guides the temporal down- and up-scaling of data and puts a constraint on the occurrence of extreme events.

Tuesday, October 15, 14:00

**Pinaki Chaudhuri (IMS Chennai)**

## Two dimensional glassy materials in external fields

The response of glass forming liquids to complex environments is of interest both in the domain of biological systems as well as applications. In that context, we study how a hard disk mixture evolves in the presence of an external spatially varying potential, and investigate how the onset of glassy dynamics depends on the interplay between the liquid’s density and the external potential’s variation. We have also done some preliminary study regarding how the properties of the mixture lead to the formation of glassy states in the presence of such external fields. Finally, we will discuss how the time variation of the external field, in the form of a turbulent flow, can lead to inhomogeneous self-assembly.

Thursday, October 24, 11:00

**Ezequiel Ferrero (Centro Atómico Bariloche)**

## Criticality in elastoplastic models of amorphous solids with stress-dependent yielding rates

We analyze the behavior of different elastoplastic models approaching the yielding transition. We propose two kind of rules for the local yielding events : yielding occurs above the local threshold either at a constant rate or with a rate that increases as the square root of the stress excess. We establish a family of ``static’’ universal critical exponents which do not depend on this dynamic detail of the model rules : in particular, the exponents for the avalanche size distribution $P(S)\sim S^

-\tau_Sf(S/L^d_f)$ and the exponents describing the density of sites at the verge of yielding, which we find to be of the form $P(x)\simeq P(0) + x^\theta$ with $P(0)\sim L^-a$ controlling the extremal statistics. On the other hand, we discuss ``dynamical’’ exponents that are sensitive to the local yielding rule details. We find that, apart form the dynamical exponent $z$ controlling the duration of avalanches, also the flowcurve’s (inverse) Herschel-Bulkley exponent $\beta$ ($\dot\gamma\sim(\sigma-\sigma_c)^\beta$) enters in this category, and is seen to differ in 1/2 between the two yielding rate cases. We give analytical support to this numerical observation by calculating the exponent variation in the Hébraud-Lequeux model and finding an identical shift. We further discuss an alternative mean-field approximation to yielding only based on the so-called Hurst exponent of the accumulated mechanical noise signal, which gives good predictions for the exponents extracted from simulations of fully spatial models.

Thursday, November 07, 11:00

**Joerg Rottler (University of British Columbia)**

## Nonlinear mechanics of physically crosslinked polymer elastomers and glasses

Polymers can be tough materials, but for very different reasons. While the tensile strength of elastomers is ultimately of entropic origin, polymer glasses exhibit strain hardening because of dissipation enforced by chain connectivity. The operation of a given mechanism depends on the competition between relaxation and deformation time scales. We first present molecular simulations of uniaxial deformation of a triblock copolymer elastomer that is physically crosslinked by microphase separation. Here, segmental relaxation times are independent of deformation rate and equilibrium statistical physics applies. This allows us to develop an entropic network model that accounts for the distinct stress contributions arising from chain crosslinks as well as entanglements by coupling analytical expressions for an entropic strain energy density directly with chain deformations obtained from the molecular dynamics simulations. Our theory quantitatively reproduces the macroscopic stress response of simulated linear [1] and star [2] block copolymer elastomers well into the nonlinear regime. The simulations reveal the evolution of entanglements and how the breakup of physical crosslinks contributes to additional strain hardening. In the glassy regime, however, simulations shows that segmental relaxation times decrease with increasing strain rate and tensile stress [3], and the hardening stress is now due to local plastic rearrangements as the chains are forced to deform in the glassy matrix. A phenomenological model for the acceleration of segmental mobility is developed that accounts for all relevant deformation variables.

[1] A. J. Parker and J. Rottler, Nonlinear Mechanics of triblock copolymer elastomers : from molecular simulations to network models, ACS Macro Letters

6, 786 (2017).[2] A. J. Parker and J. Rottler, Entropic network models for Star Block Copolymer Thermoplastic Elastomers, Macromolecules

51, 10021 (2018).[3] J. Rottler, Molecular mobility in driven monomeric and polymeric glasses, Phys. Rev. E.

98, 010501(R) (2018).

Tuesday, November 12, 11:00

**Suman Dutta (IMS Chennai)**

## Onset of Fluidization in Yield Stress Materials : Insights from Microscopic Simulations

Yield stress materials are responsive to applied stress. When impacted by the imposition of stress, they yield beyond a critical threshold. We take such solids of different preparation history to study the mechanical response using creep, implemented via the molecular dynamics simulations. We analyze the microscopic dynamics and investigate the onset of flow and the persistence of spatial heterogeneity at different imposed stresses. We identify distinct regions of fluidity via the local fluidization maps and show how increasing spatial fluctuations between the flowing and non-flowing regions lead to macro-scale flow.

Tuesday, November 19, 14:00

**Thibaud Maimbourg (LPTMS, U. Paris-Sud)**

## Some insights on structural glasses from mean-field theory

Glasses are amorphous solids which show intriguing experimental features. A prominent example is the wealth of phenomena that appear in the out-of-equilibrium dynamics (through e.g. a quench or an external drive) at low temperatures. Another occurs at even lower temperatures, where quantum effects start to play a role : thermodynamic quantities display an anomalous behaviour with respect to what one expects from standard (ordered) solids. I will first give a short overview of the mean-field theory of structural glasses, able to unify many physical situations (dense liquids, soft and hard colloids, granular materials, emulsions) and transitions in the same conceptual framework. Then I will review recent results deriving from these ideas to the above-mentioned examples.

Thursday, November 28, 11:00

**Fabien Brieuc**

Path integral simulations of molecules solvated by helium

**2018**