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 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.

Graphene is a key material for nanoelectronics. Nevertheless, its zero gap makes it unsuitable for applications needing semiconductors with sizeable energy gaps. Therefore, a lot of efforts are being made to develop materials with non-zero gap compatible with the hexagonal lattice of graphene. In this framework, small pieces of graphene such as graphene nanoribbons and quantum dots have a lot of assets. Thanks to the so-called bottom-up synthesis a precise control of the size, shape and edges of these objects is achievable [1]. This opens the way to a precise tailoring of their physical properties.

In this presentation, I will first present the different research topics of the group. Then I will show our recent results on the optical properties of both graphene nanoribbons and quantum dots synthesized by bottom-up chemistry [2-4].


References:

[1] R. Rieger and K. Müllen, "Forever young: polycyclic aromatic hydrocarbons as model cases for structural and optical studies", J. Phys. Org. Chem. 23, 315-325 (2010).

[2] Shen Zhao et al, “Optical Investigation of On-Surface Synthesized Armchair Graphene Nanoribbons”, Physica Status Solidi b 254, 1700223 (2017)

[3] Shen Zhao et al, “Fluorescence from graphene nanoribbons of well-defined structure” Carbon 119, 235 (2017)

[4] Shen Zhao et al, “Single photon emission from graphene quantum dots at room temperature”, in preparation

Describing three bodies moving under known mutual forces is difficult, especially in quantum mechanics. On the other hand, the two-body problem is very simple. A new method uses the two-body solution to solve this three-body problem.

We present a general approach for the solution of the three-body problem for a general interaction and apply it to the case of the Coulomb interaction. This approach is exact, simple, and fast. It makes use of integral equations derived from the consideration of the scattering properties of the system. In particular, this makes full use of the solution of the two-body problem, the interaction appearing only through the corresponding known T matrix. In the case of the Coulomb potential, we make use of a very convenient expression for the T matrix obtained by Schwinger. As a check, we apply this approach to the well-known problem of the helium atom ground state and obtain a perfect numerical agreement with the known result for the ground-state energy. The wave function is directly obtained from the corresponding solution. We expect our method to be, in particular, quite useful for the trion problem in semiconductors.

Sub-wavelength localization of single nano-emitters allows super-resolution imaging and subtle probing of spatio-temporal nano-environments [1]. Based on this general feature, I will present different types of nano-emitters used to study the complexity of the brain. Following a general introduction presenting the current state-of the art in this domain, I will introduce an original nanoparticle delivery method in the brain of live animals which made possible single quantum dot tracking in intact brain slices [2]. I will then introduce a recent strategy based on phase imaging which pushes the performance of 3D single particle tracking and 3D super-resolution microscopy deep into biological tissues [3]. Interestingly, for deep tissue imaging, single walled carbon nanotubes, which bear optical resonances in the near infrared, nanoscale dimensions and high aspect ratios, are particularly promising [4,5]. I will show that nanotubes diffusing in the brain extracellular space can be recorded at the single nanotube level. Analysis of their specific movements provides super-resolved maps of tissue architecture which can be modulated upon biochemical digestion of the brain extracellular matrix in live animals [6] or in neurodegenerative brains. If time permits, I will finally discuss the possibility to use sp3-defect functionalized nanotubes for single particle tracking [7].


[1] A. G. Godin, et al Biophys. J. 107 (2014) 1777
[2] Varela et al, Nat. Comm. 7 (2016) 10947
[3] Bon et al Nat. Meth. (2018) accepted
[4] N. Fakhri, et al Science 330, (2010) 1804.
[5] N. Danné, et al ACS Nano 5, 2, (2018) 359-364
[6] A. G. Godin, et al Nat. Nanotechnol. 12 (2017) 238
[7] N. Danné, et al submitted

Condensed He-4 is a fascinating system because it exhibits quantum properties at the macroscopic scale. The most famous of these is probably the transition of the liquid into a so called superfluid state when cooled down below 2.17 K at vapor pressure. I will present and discuss experiments investigating the metastable phases of liquid and solid He-4. Metastable solid He-4 is interesting because it is a potential candidate for a hypothetical (not yet discovered) supersolid state of matter. Metastable liquid He-4 is interesting because due to its extreme purity, it is a model system to study homogeneous cavitation. The metastable states are produced by focusing an acoustic wave in the bulk of the solid (liquid). We measure the local density within the wave thanks to an optical time-resolved interferometric method. In both metastable liquid and solid He-4, we have observed instabilities at densities which are not compatible with theoretical predictions. In the particular case of metastable liquid He-4, our density measurement of its cavitation threshold especially questions the validity of a well theoretically established equation of state of the metastable liquid. It then appears necessary to try to measure the equation of state experimentally which could be done using a stimulated Brillouin scattering technique.

Magnetic susceptibility is a physical quantity that plays an important role in magnetic resonance imaging. Often considered to be a nuisance, it offers rather impressive opportunities to tailor image contrast. Starting out with susceptibility weighted imaging, which is exquisitely sensitive to venous blood, hemorrhage and iron storage, the field has more recently moved to quantitative susceptibility mapping (QSM). QSM represents a novel quantitative contrast of an intrinsic physical tissue property that reflects the reaction of tissue to the static main magnetic field. The talk will trace some of the historical developments and will introduce the technical underpinnings of susceptibility-based imagng as well as present some applications.

Not many ingredients are needed for a quantum system to turn complex, with the helium atom as the arguably most elementary example. Much as for classically chaotic systems, accurate predictions of the long-time dynamics, or an exhaustive description of the very structure of such a system’s state turns quickly prohibitive as the size of state space is increased. This has been a concern of theory for long, and recently receives renewed interest as also quantum optics labs turn “complex”, where a stunning level of control over elementary constituents now makes contact with the exponential proliferation of alternative transition amplitudes. To characterise and to certify such systems, let alone to actively control them, we need to identify robust, coarse-grained quantifiers which allow to discriminate their distinctive features, on the basis of experimentally accessible and scalable, yet necessarily incomplete information. We will have a closer look at some exemplary certification problems, with a special emphasis on indistinguishability as a specific source of complexity in many-particle quantum dynamics.