Cooperative adsorption of gases by porous frameworks permits more efficient uptake and removal than does the more usual non-cooperative (Langmuir-type) adsorption. Cooperativity, signaled by a step-like isotherm, is usually attributed to a phase transition of the framework. However, the class of metal-organic frameworks mmen-M2(dobpdc) exhibit cooperative adsorption of CO2 but show no evidence of a phase transition. Here we show how cooperativity emerges in these frameworks in the absence of a phase transition. We use a combination of quantum and statistical mechanics to show that cooperativity results from a sharp but finite increase, with pressure, of the mean length of chains of CO2 molecules that polymerize within the framework. Our study provides microscopic understanding of the emergent features of cooperative binding, including the position, slope and height of the isotherm step, and indicates how to optimize gas storage and separation in these materials.

In this talk, we introduce our research activity towards new architecture based on 2D materials such
as graphene and MoS2. First, we show that graphene or monolayer MoS2 can be rolled up into
nanoscroll structure, distinct from nanotube or 2D atomic layers. Properties of rolled-up graphene
and MoS2 were investigated. Secondly, we present the effect of laser irradiation on 2D monolayers.
We observed the swelling-up of the monolayer from the SiO2 substrate upon laser illumination. The
mismatch in the thermal expansion between the substrate and 2D layers can result in the structural
deformation. Employing this method, one can induce structural deformation in a desired pattern.
We demonstrate the patterning of periodic ripples in monolayers by using laser irradiation. The new
architecture and the strain engineering might provide a useful tool with enhancing the performance
of electronic and optoelectronic devices based on 2D materials.

Mixing viscous fluids is difficult. The question addressed here is how the presence of rigid particles in a viscous shear flow accelerates this process. This information is crucial for instance to predict the transport of drugs or oxygen in blood flow or the transport of nutrients inside biological cells undergoing cytoplasmic-streaming. Here, high-resolution measurements of the fluid phase velocity field are performed in a model sheared suspension to reconstruct the stretching histories of material lines within the interstitial fluid. We found that the presence of particles changes the very nature of the flow kinematics: stretching in a pure shear flow is linear, it becomes exponential and widely distributed (converging to a log-normal) in the presence of particles. These trends can be well captured by multiplicative stretching process based on the distribution of local shear-rates and their persistence time. This complete description of the flow kinematics allows us to make predictions for the mixing times and show how, and in which limit, the presence of particles accelerates mixing within sheared suspensions.

We investigated the kinetics of a polymer collapse due to the formation of irreversible crosslinks between its monomers. We use the contact probability as a function of the monomeric distance as a scale dependent order parameter and show in simulations the emergence of a cooperative pearling instability. This produces a sharp conformational transition in time, inducing a crossover between short and long distance behaviors due to the formation of pearls. The size of pearls and the transition time depends on the equilibrium dynamics of the polymer and the rate at which cross links are formed. We finally confirm experimentally the existance of this transition using a chromsome confomation capture experiment.

Among the many models aimed to describe the emergence of directional motion in active particles, an important class relies on the use of effective, velocity-dependent forces which, when coupled to an asymmetric potential, give rise to directed transport. A common feature of such models is that velocity-dependent forces are anti-dissipative in a range of velocities, which is consistent with a locally negative effective friction coefficient. In this talk I will present a new model for chemotactic response which does not rely on the introduction of a chemotactic potential, but in a nonequilibrium quasi-Onsagerian mesoscopic chemo-mechanical coupling. When sufficiently coarsegrained, our model can be reduced to an effective description in terms of a velocity dependent force. Remarkably, such force leads to directed motion allowing the motor even to carry a cargo, in spite of the effective friction coefficient being strictly possitive, i.e., dissipative, for all velocities. I will discuss on the mechanics and thermodynamics of such a motor, and clarify that the origin of directed motion in this apparently dissipative system resides on the breakdown of the time-reversal symmetry of the effective friction coefficient.

We will present recent results in light scattering and transport in disordered media in which unexpected behaviors show up. In light transport (diffusion), we will discuss an invariance property that has been demonstrated recently, showing that the averaged path length in the scattering process is independent on the microstructure of the medium. In the context of imaging through opaque scattering media, we will show that a spatial correlation between the reflected and transmitted speckle patterns exists, and persists even in the multiple scattering regime. The mutual information connecting the transmitted and reflected speckles can be estimated, providing an interesting connection between concepts of information theory and mesoscopic physics. Finally, we will discuss the influence of spatial correlations in the disorder. In the case of hyperuniform disorder (a specific class of correlated disorder) we will show that correlations can induce transparency even in the multiple scattering regime. These results bring new perspectives for the design of disordered materials with specific photonic properties, and for the control of light transport in complex media.

Self-assembly and aggregation of soft condensed matter like proteins, colloids or polymers into
poorly connected and weakly elastic solids is very common and ubiquitous in nature. Phase separa-
tion, spinodal decomposition as well as externally driven self-assembly or aggregation often lead to
gels, which display diverse structures and solid-like mechanical features. The structural complexity of
soft gels entails a versatile mechanical response that allows for large deformations, controlled elastic
recovery and toughness in the same material. A limit to exploiting the potential of such materials
is the insufficient fundamental understanding of the microstructural origin of the bulk mechanical
properties. Investigating how the mechanical response depend on the material microstructure will
provide a new rationale, which would ultimately lead to several applications, ranging from improving
the performance of batteries (colloidal gels), designing smart composites that can prevent the cascade
of catastrophic events and can be used in anti-seismic buildings, and many with important biological
function, such as new scaffolds for tissue engineering.
In the first part of my talk, using numerical simulations of a minimal model, I will present the link
between the topology of the network and the non-linear rheological response: our analysis elucidates
how the network connectivity alone could be used to modify the gel mechanics at large strains, from
strain-softening to hardening and even to a brittle response. Then I will show the relevance of our
model to understand the mechanics of F-actin cytoskeleton. Our study helps to clarify, the mechanism by which mutations cause podocyte dysfunction and progressive kidney disease in humans.

A time-reversal invariant topological insulator in two dimensions, also known as a quantum spin Hall insulator (QSHI) is a new state of matter characterized by an insulating bulk and spin-polarized gapless helical states at the sample edges. So far, evidence for the helical edge mode in QSHI, particularly quantized transport, has been limited to two semiconductor systems – inverted HgTe/CdHgTe quantum wells (QWs) and broken-gap InAs/GaSb QW bilayers. In my talk, I review recent theoretical and experimental results on single and double HgTe QWs, obtained in our group in Montpellier. In the first part, I will focus on phase transitions in single HgTe QWs induced by temperature and present the picture of the edge states in QSHI phase beyond the Bernevig-Hughes-Zhang model. In the second part, I report theoretical study of phase transitions in double HgTe QWs consisting of two HgTe layers separated by tunneling CdHgTe barrier. Particularly, I show an existence of specific “bilayer-graphene” metal and trivial insulator phases, arising in the inverted double HgTe QWs. Finally, motivated by recent observation of quantum spin Hall effect at temperatures up 100 K in 1T′-WTe2 monolayers with a band-gap of 45 meV (Science 359, 76 (2018)), I will briefly discuss a possibility of realization of high-temperature QSHI in multi-layer InAs/GaSb QWs with temperature stable band gap.