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

### A venir

Tuesday, October 05, 11:00

**Jiting Tian**

## 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.

### Séminaires passés

**2021**

Thursday, January 14, 11:00

**Karel Proesmans (University 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 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.

**2020**

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.

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**

**2018**