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

### A venir

Thursday, 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 staples on 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.

### Séminaires passés

**2021**

Thursday, January 14, 11:00

**Karel Proesmans (U. of Luxembourg)**

## Finite-time Landauer principle

We use optimal transport theory to minimize the thermodynamic cost associated with the erasure of a bit. In this way, we generalise Landauer’s principle to erasure over a finite amount of time. Furthermore, we discuss its generalisation to erasure with a finite erasure error and we will look at special protocols such as majority-logic decoding.

Thursday, January 21, 11:00

**Sara del 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.

Thursday, February 04, 11:00

**Nicolas Levernier (IUSTI, Aix-Marseille Univ.)**

## 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.

**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.

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, 11:00

**Tatiana Morozova (PSM)**

## Fabrication of Polymer Colloids through Rapid Solvent Exchange

Tatiana Morozova[1], and Arash Nikoubashman[2]

[1] Univ. Grenoble Alpes, CNRS, LIPhy, 38000 Grenoble, France

[2] Institute of Physics, Johannes Gutenberg University Mainz, Mainz,Germany

Polymer nanoparticles (NPs) are promising candidates for a wide range of applications such as colloidal self-assembly and targeted therapeutics. Flash nanoprecipitation (FNP) is a scalable technique for fabricating monodisperse polymer NPs through rapid micromixing of a polymer solution (e.g. polystyrene in tetrahydrofuran, THF) with a miscible poor solvent (water). To better understand the underlying self-assembly mechanisms at the level of a single chain, we first conduct molecular dynamics (MD) simulations on the united atom level of polystyrene chain of different lengths in various THF-water mixtures. In particular, we study the coil-globule transition of the polymers in the THF-water mixtures at varying compositions. Structural properties of chains are found in agreement with theoretical predictions as well as consistent with experimental data. Most interestingly, in water-rich mixtures we observe a substantial amount of the good solvent molecules trapped inside polymer globules, with an excess amount located at the polymer surface acting like a protective layer between the hydrophobic polymer and the water-rich medium. After gaining knowledge on the single-chain level, we performed coarse-grained MD simulations of FNP, where we focus on the fabrication of amphiphilic Janus NPs. These particles consist of two distinct domains, i.e. hydrophilic and hydrophobic ones and are promising candidates as emulsion stabilizers. We develop a computational model to describe the self-assembly of a blend of two homopolymers and one type of amphiphilic block copolymer (BCP). We investigate the design space and guide the experiments to produce colloids of the desired morphology through MD simulations. We determine the optimal process parameters for the formation of the amphiphilic Janus particles, such as the BCP concentration and the volume fraction of the hydrophilic block in a BCP chain. Finally, we study the interfacial behavior of the fabricated colloids primarily amphiphilic Janus and hydrophobic polymer particles. We place the colloids at the interface between two immiscible liquids and compute the interfacial tension of the system. Our study reveals, that both the shape of the colloid at the interface as well as its interaction with surrounding fluids are important in predicting particle performance as colloidal surfactants.

Tuesday, June 23, 11:00

**Kevin Jiguet-Covex**

## Escape problem in a single active particle model

In equilibrium problems, the transition between metastable states is governed by a deceptively simple law : the mean escape time scales in an Arrhenius way, as e^

ΔU/T, with ΔU the energy barrier and T the temperature. This holds independently of the details of the potential (in the small temperature limit). This so-called ’Kramers escape problem’ is more complex to understand in out-of-equilibrium dynamics, where the details of the potential start to matter.To shed light on this problem, we study a simple non-equilibrium dynamics : the one-dimensional Active Ornstein-Uhlenbeck Particle (AOUP), described by a Langevin dynamics where the noise is correlated in time on a timescale τ. For τ>0 the problem is non-equilibrium. Using path-integral techniques, we develop a perturbative scheme in powers of τ in order to compute the mean escape time from a smooth potential well. We show that, surprisingly, the optimal path of escape can be degenerate as soon as τ>0. τ. Furthermore, we have indications that at least in a particular case, the convergence radius could be finite, thus leading to a phase transition. We discuss the physical consequences and the relation to the large-τ limit.

Tuesday, July 07, 11:00

**Nicolas Cuny (PSM, LIPhy)**

## From microstructure to rheology of dense suspensions

Dense suspensions concern a large spectrum of materials made of solid particles immersed in a liquid phase. Although the nature of solid particles can be very different, dense suspensions share some common rheological properties under shear stress such as rheofluidification (under jamming) or Herschel-Bulkley law between stress and shear-rate (below jamming). Establishing an evolution equation of the stress tensor as a response to a given deformation is of great importance in a way to describe the rheology of such systems. Most attempts to get such a constitutive model are purely phenomenological ones, with equations build on mathematical considerations to exhibit the empirical laws people are interested in (see for example the work of P. Saramito and O. Ozenda). In this PhD work, we’re interested in getting such a constitutive model starting from the particle dynamics. For this, we use tools from statistical physics to establish an equation on correlation functions, that describe the structure of the system, getting then a constitutive equation on the stress tensor by using the link between the pair correlation function and the stress tensor. In this talk, I’ll detail the main ideas allowing us to come from microscopical particle dynamics equation to a macroscopic equation linking stress to external deformation. I’ll then briefly discuss some simple deductions on the rheology of our system from this equation and I’ll finally present future perspectives for this ongoing work.

Thursday, July 16, 11:00

**Robin Botrel (PSM, LIPhy)**

## Effect of Special Relativity in the Liquid-Gas Transition in Mercury

Mercury stands out from its neighbours in the periodic table and shows remarquable physical and chemical behaviour. As a heavy element, its core electrons are prone to relativistic effects, as was shown earlier in the depletion of the melting point. In this internship we examined the case of the liquid-vapor transition. Using Monte Carlo simulations the phase diagram of models was determined from relativistic and non-relativistic models of mercury. The Gibbs ensemble method was employed to alleviate the difficulties of simulating phase coexistence directly. Preliminary results using pair potentials confirm the strong effects of relativity on the phase diagram of mercury.

Thursday, October 01, 11:00

**Olivier Coquand (DLR Köln)**

## Rheology of granular liquids

Granular liquids are ubiquitous around us, from the physics of geological phenomena to the food processing industry. However, the description of their behaviour remains a challenge for fundamental physics. One of the most successful of such approaches is the so-called "µ(I)" law, a phenomenological law that describes with good accuracy experimental and numerical results, but still lacks theoretical support.

In this seminar, I will present our recent works on the subject. From a set of fundamental equations, we managed to explain the µ(I) law from the competition between different time scales associated with fundamental processes within the granular flow. This shed a new light on the physics of these systems, and represents a first step towards the establishment of a complete theoretical framework to describe the physics of dense granular flows.

Thursday, October 08, 11:00

**Alex Erlich (LIPhy)**

## Collective dynamics of actively crawling cells interacting through hydrodynamics and chemotaxis

Cells can move actively and respond to forces by a complex mechanism in

which the cytoskeleton is reorganised (repolarisation). In recent

experiments it was shown that collisions of two cells can result in four

types of motion : the cells either reverse velocity, form a train, stall

motion on approach, or bypass each other. In a recent advance it was

shown by Recho et al. [1] that repolarization in response to a

mechanical force can be triggered by a rearrangement of molecular

motors, an explanation that requires no biochemical signalling. This

model has been successfully reduced to an active particle model [2],

where a cell is described as a dimensionless particle, with degrees of

freedom position and polarity. This remarkable simplification preserves

the experimentally observed types of motion in cell collision, thus

forming a highly promising building block for a tissue theory based on

active particles. We explore the interaction of cells in the active

particles model through hydrodynamic as well as chemotactic interaction.

In the former case, the cells move actively in a fluid and interact with

each other mechanically by stressing the fluid. We find remarkably 3D

dynamics in which cells form clusters, reverse velocity, or oscillate

around each other in helical fashion. In the chemotactic case, particles

exhibit different but equally interesting dynamics, including Hopf

bifurcations and limit cycles. To understand the complex dynamics of the

active particle model, we focus on the case of two particles on a one

dimensional track, which emulates the physiological case of motility

along the fibres of the extracellular matrix and cell crawling inside a

capillary. For both the hydrodynamic and chemotactic cases, we study in

detail the simplest canonical cases on a 1D track, and discuss more

complex cases (which we capture numerically) and their possible

implications for 3D dynamics.

Keywords : biological physics, cell crawling, active particle, dynamical

systems, bifurcation analysis, collective behavior, hydrodynamic

interaction, chemotaxis

[1] Recho P, Putelat T, Truskinovsky L. Force-induced repolarization of

an active crawler. New J Phys. 2019 ;21 : 033015.

[2] Recho P, Putelat T, Truskinovsky L. Active gel segment behaving as

an active particle. Phys Rev E. 2019 ;100 : 062403.

Thursday, November 19, 11:00

**Kirsten Martens (PSM)**

## A coarse-grained stochastic lattice approach based on microscopic insights for the steady state and transient dynamics of sheared disordered solids

In this short presentation I will present a framework to study the mechanical response of athermal amorphous solids via a coupling of mesoscale and microscopic models. Using measurements of coarse grained quantities from simulations of dense disordered particulate systems, we present a coherent elasto-plastic model approach for deformation and flow of yield stress materials. For a given set of parameters, this model allows to match consistently transient and steady state features of driven disordered systems under both applied shear-rate and creep protocols.

Thursday, November 26, 11:00

**Irene Adroher-Benítez**

## Interactions involved in the permeation and distribution of ions and biomolecules inside charged microgels

Ionic microgels are colloidal particles of gel dispersed in a solvent, formed by cross-linked polyelectrolyte chains. They can swell or shrink in response to a wide variety of stimuli such as temperature, pH or salt concentration. This feature is an advantage for a wide number of biotechnological applications, such as the design of drug transport and delivery systems. With this aim, my research activity at the University of Granada was mostly dedicated to studying the permeation of ions and other solutes inside microgels particles. In this seminar I will describe the theoretical method based on the Ornstein-Zernike integral formalism that we used to analyze the effect of the microgel-counterion interactions on the microgel effective charge and swelling behaviour.

**2019**

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**