In this work we have investigated the active motion of self-propelled colloids confined at the air-water interface. The two dimensional motion of micron-sized Silica-Platinum Janus colloids has been experimentally measured by particle tracking video-microscopy under increasing concentration of the catalytic fuel, i.e. H2O2. Comparing to the motion in bulk, a dramatic enhancement of both the persistence length of trajectories and the speed has been observed. We measured long range directional trajectories at the water surface with persistence length l =ca. 140 µm.The interplay of colloid’s self-propulsion, due to an asymmetric catalytic reaction occurring on the colloid, and interfacial frictions controls the enhancement of the directional movement. The slowing down of the rotational diffusion at the interface, also measured experimentally, plays a pivotal role in the control and enhancement of active motion.

Surface states in semiconducting materials are usually fragile with respect to disorder and perturbations. However, there are systems in which surface states become robust because of the non-trivial topology of the band structure. A well-known example is the integer quantum Hall effect in which magnetic field splits conduction band of 2D carriers into discrete energy levels (Landau levels) inducing specific edge channels of quantized conductance. But recently, another type of topological phase was identified in materials with energy band inversion and strong spin orbit interaction. It is called the topological insulator phase. The most interesting property of 2D topological insulators is the presence of spin polarized counter-propagating edge channels of quantized conductance, protected from back scattering. The topologically nontrivial state was first realized in HgTe-based quantum wells.

Recently, a type of topological invariance was predicted in materials with band inversion (semiconductor with a gap between the upper p-type and lower s-type energy bands) due to strong spin-orbit coupling . In this case, one speaks of topological insulators.2 This kind of topologically protected surface state was first demonstrated to exist in two-dimensional HgTe/CdTe quantum wells (QWs).3,4 In this work, we report on a Terahertz magneto-photoconductivity study of inverted band structure HgTe-based Field Effect Transistor (FET) : i) We observe a resonance that could be attributed to topological phase transition related to anticrossing of the zero-mode Landau levels at a critical value of the magnetic field5. ii) We also observe unknown resonances in the direct band structure regime which are shifting to higher magnetic field values with increasing gate voltage. These resonances cannot be attributed to magnetoplasmons and may be linked with impurity resonant transitions. iii) We finally observe the cyclotron resonance line giving electron effective mass m* = 0.03 m0.

We study the fingering instability of the interface between two miscible fluids, a colloidal suspension and its own solvent [1]. The temporal evolution of the interface in a Hele-Shaw cell is found to be governed by the competition between the nonlinear viscosity of the suspension and an off-equilibrium, effective surface tension $\Gamme_e$. By studying suspensions in a wide range of volume fractions, $\Phi_C$, we show that $\Gamma_e ∼ \Phi_C^2$, in agreement with Korteweg’s theory for miscible fluids. The surface tension exhibits an anomalous increase with particle size, which we account for using entropy arguments. [1] Truzzolillo et al, Phys. Rev. Lett 112, 128303 (2014).

We describe an extension to current data analysis schemes of Taylor dispersion data that allows size polydispersity to be quantified for an arbitrary sample [1], thereby significantly enhancing the potentiality of this technique. The method is based on a cumulant development similar to that used for the analysis of dynamic light scattering data. Specific challenges posed by the cumulant analysis of Taylor dispersion data are discussed, and practical ways to address them are proposed. We successfully test this new method by analyzing both simulated and real experimental data for solutions of moderately polydisperse polymers and polymer mixtures. [1] L. Cipelletti, J.-P. Biron, M. Martin, H. Cottet, Anal. Chem 2014.

Combined Resonant Raman spectroscopy, high resolution transmission electron spectroscopy and electron diffraction experiments on the same suspended (free-standing) individual carbon nanotubes is the ultimate method to determine unambiguously the intrinsic features of phonons in these nano systems. By using this approach, the effect of coupling between nanotubes on the phonons of two carbon-based nanostructures, namely: dimers of single-walled carbon nanotubes and double-walled carbon nanotubes (DWNT), is investigated. Both these systems provide unique models for studying the role of the coupling at the nanoscale.

Combined resonant Raman spectroscopy, high resolution transmission electron spectroscopy (HRTEM) and electron diffraction experiments on the same suspended (free-standing) individual carbon nanotubes is an efficient method to determine unambiguously the intrinsic features of phonons in these nano systems. In this talk, a special attention is focusing on multi–walled carbon nanotubes, mainly on double-walled carbon nanotubes (DWNT), because they provide a unique model system for studying the role of the coupling between the layers on the phonons.