The large scale density fluctuations of the ordered state in active matter systems are usually characterized by studying the ``giant number fluctuations'' of particles in any finite volume, as compared to the expectations from the central limit theorem. However, in ordering systems, the fluctuations in density ordering are often captured through their structure functions deviating from Porod law. We ask: (1) Do active matter systems in general violate Porod law? (2) Are the two characterizations of large fluctuations, namely Porod law violation and giant number fluctuations, related to each other? In this work, we study the relationship between giant number fluctuations and structure functions, for different models of active matter as well as other non-equilibrium systems.

We study the propagation of sound through a two-dimensional granular medium consisting of photoelastic disks, which are packed into different crystalline and disordered structures. Acoustic sensors placed at the boundaries of the system capture the acoustic signal produced by a local and well-controlled mechanical excitation. By compressing the system we find that the speed of the ballistic part of the acoustic wave behaves as a power law of the applied force with both exponent and prefactor sensitive to the internal geometry of the contact network. This information, which we are able to link to the force-deformation relation of single grains under different contact geometries, provides enough information to reveal the structure of the granular medium

Conceptual mechanism of amplification of phonons by phonons on the basis of nonlinear band-gap transmission (supratransmission) phenomenon is presented. As an example the system of weakly coupled chains of anharmonic oscillators is considered. One (source) chain is driven harmonically by boundary with a frequency located in the upper band close to the band edge of the ladder system. Amplification happens when a second (gate) chain is driven by a small signal in the counter phase and with the same frequency as first chain. If the total driving of both chains overcome the band-gap transmission threshold the large amplitude band-gap soliton emerges and amplification scenario is realized. The mechanism is interpreted as nonlinear superposition of evanescent and propagating nonlinear modes manifesting in single or double soliton generation working in band-gap or band-pass regimes, respectively. The results could be straightforwardly generalized for all-optical or all-magnonic contexts and has all the promises for logic gate operation.

Mechanosensitive (MS) channels are ion channels located on the cellular membrane of some organisms, which play a very important role on protecting the cell against osmotic shocks. Due to specific patterns of deformation that MS channels create on the membrane, they can present attractive or repulsive interactions with neighbouring channels and this gives rise to important collective effects. In this talk, I will discuss the clustering of MS channels in E. coli through a modified lattice gas model, trying to understand how it can affect the response of these channels to increases in membrane tension. Results for the dynamical behaviour of the response of the channels to osmotic shocks will be presented and compared to biological relevant data.

We study the collective effects in collections of active particles in confined geometries with reflecting (specular) boundary conditions. Our analysis reveals various remarkable macroscopic features induced by the existence of a confinement. First, the often-observed ``boundary-following'' effect is a collective effect that requires particles to exhibit a velocity alignment mechanism. In the presence of a ferromagnetic alignment, particles self-organize in such a way that they start to rotate either clockwise or counterclockwise. For nematic alignment and below the critical point, half of the particles rotate clockwise while the other half counterclockwise. Interestingly, close to the critical point we observe that rotational order competes with metastable macroscopic structures, which we call bridges. We show that for both alignment mechanisms, the onset of orientational order is associated to a density instability by which the reflecting walls act effectively as attractive.

Abstract:
We study the behaviour of a semi-dilute solution of polymers in micro channels, where a main flow of continuous flow rate cross a perpendicular oscillatory flow. We found that at high oscillatory flow rates the flow in the main channel exhibits turbulence. We show that this behaviour depends on both main and oscillatory flow rate. The effect off both shear thinning and elasticity on the nature of the flow are also investigated by comparing our results to numerical simulations of purely shear-thinning fluid.


Abstract (fr) :
Nous avons étudié la réponse de l’écoulement laminaire d’une solution semi-dilué de polymères à une perturbation sinusoïdale dans des micro-canaux. Dans notre montage deux écoulements, un écoulement principal constant et un écoulement sinusoïdal se croisent. Lorsque l’amplitude des oscillations devient suffisamment grande, l’écoulement principal perd sont caractère laminaire et devient turbulent. Notre investigation a montré que cette transition est dépendante du débit des deux écoulements. Nous avons aussi étudié l’effet des caractères rhéofluidifiants et élastiques de notre fluide en comparant nos résultats expérimentaux à des simulations numériques réalisées sur des modèles purement rhéofluidifiants.