The dynamics of colloidal particles at the interface between two fluids plays a central role in micro-rheology, encapsulation, emulsification, biofilms formation and water remediation. Moreover, this subject is also challenging from a theoretical point of view because of the complexity of hydrodynamics at the interface and of the role of the contact line. Despite this great interest, the behavior of a single particle at a fluid interface was never directly characterized. In this thesis, we study the Brownian motion of micrometric spherical silica beads and anisotropic polystyrene spheroids at a flat air-water interface. We fully characterize and control all the experimentally relevant parameters. The bead contact angle is finely tuned in the range 30-140° by surface treatments and measured in situ by a homemade Vertical Scanning Interferometer. The spheroid aspect ratio varies in the range 1 – 10 by stretching of commercial beads. The translational and the rotational dynamics are followed by particle tracking. Counter-intuitively, and against all hydrodynamic models, the diffusion is much slower than expected. To explain this extra dissipation we devised a model considering the contribution of thermally activated fluctuations of the interface at the triple line. Such fluctuations couple with the lateral movement of the particle via random forces that add to the ones due to the shocks of surroundings molecules. Fluctuation-dissipation theorem allows obtaining the extra friction associated to this additional mechanism. The fitting values of the total friction are discussed in term of the typical scales of particle surface heterogeneities and of surface capillary waves.

"Thé au cube" est la pause café scientifique de l'axe PTh. L'orateur y expose de manière interactive et accessible au théoriciens non-spécialistes, ses intérêts scientifiques du moment. N'oubliez pas de ramener vos tasses et autres mugs préférés.

I will explain how to compute the Hilbert series that
counts chiral operators parametrising the Coulomb branch of the
moduli space of 3d N=4 supersymmetric field theories. The formula has
applications to physics, such as to the study of infrared dualities
in three dimensions and of backgrounds with eight supercharges, as
well as to mathematics, where it provides a new way to characterise
hyperKähler cones as algebraic varieties. I will then show how the
Coulomb branch of certain 3d N=4 generalised quiver gauge theories
can be used to compute the Hilbert series of the moduli space
of k G-instantons, for any simple Lie group G.
The construction is alternative to the ADHM construction
that is only available for classical groups G.

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.

On a coarse-grained level, colloids often interact via so-called ultrasoft potentials, which assume at short interparticle distances values in the order of a few k$_{\rm B}T$; thus, these particles are able to overlap at the cost of a relatively small energy penalty. Under equilibrium conditions such ultrasoft particles are able to form aggregates (clusters) of overlapping particles which can then either form a disordered or an ordered cluster phase. In the latter case, these aggregates populate the positions of a regular fcc or bcc lattice. Cluster crystals display rather unconventional properties, such as a density-independent lattice constant [1]. Also under out-of-equilibrium conditions, ultrasoft systems show unexpected features. Exposing a cluster crystal to shear leads – with increasing shear rate $\dot \gamma$ – to the following novel response-scenario: for small $\dot \gamma$-values the crystal melts; then gradually strings parallel to the flow direction form which are arranged in a hexagonal grid in the gradientvorticity plane. Upon further increasing $\dot \gamma$ this lattice eventually melts [2]. Exposing a cluster crystal to Poiseuille flow the emergence of a quantized flow pattern is observed where the height and the width of the fluid stream display well-defined plateaus, indicating a successive fluidization of crystal layers adjacent to the channel walls [3].
[1] B.M. Mladek et al., Phys. Rev. Lett. 96, 045701 (2006).
[2] A. Nikoubashman et al., Phys. Rev. Lett. 107, 068302 (2011).
[3] A. Nikoubashman et al., Soft Matter 8, 4121 (2012).

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.