Luminescence Dynamics of Single Self-Assembled Chains of Förster (FRET)-Coupled CdSe Nanoplatelets.

Self-assembled linear chains of CdSe nanoplatelets are known to exhibit highly efficient Förster resonant energy transfer (FRET) leading to fast exciton diffusion between platelets. Here, we compare the luminescence decay dynamics of single nanoplatelets, clusters of a few platelets, and self-assembled chains. As the number of stacked platelets is increased, we show that the luminescence decay becomes faster, which can be interpreted as the FRET-mediated effect of quenchers: excitons may diffuse to nearby quenchers so that their decay rate is increased. On the other hand, a minor slow decay component is also observed for single platelets, corresponding to trapping-detrapping mechanisms in nearby trap states. The contribution of the slow component is enhanced for the platelet chains. This is consistent with a FRET-mediated trapping mechanism where the excitons would diffuse from platelet to platelet until they reach a trap state. Finally, we develop toy models for the FRET-mediated quenching and trapping effects on the decay curves and analyze the relevant parameters.

Reversible strain alignment and reshuffling of nanoplatelet stacks confined in a lamellar block copolymer matrix.

We designed a nanocomposite consisting of CdSe nanoplatelets dispersed in the form of short stacks in the polybutadiene domains of a polystyrene-polybutadiene-polystyrene (SBS) thermoplastic elastomer matrix. Under strain, the material displays reversible, macroscopic anisotropic properties, e.g. the fluorescence signal. We present here a structural study of the composite under stretching, by in situ high-resolution X-ray scattering using synchrotron radiation. Modelling the scattering signal allows us to monitor the evolution of both the matrix and the platelets under strain. In particular, we show that the strain "reshuffles" the platelet stacks, which tilt their long axis from parallel to the plane of the microstructure lamellae at rest to perpendicular to this plane at high strain, at the same time breaking into smaller pieces, more easily accommodated in the soft butadiene domains. This reshuffling is fully reversed after strain relaxation. Moreover, it can be prevented by adding free oleic acid, which reinforces the interactions between the platelets in the stacks.

Strain-controlled fluorescence polarization in a CdSe nanoplatelet-block copolymer composite.

By dispersing semi-conducting CdSe nanoplatelets within a styrene-butadiene-styrene block copolymer matrix we form homogeneous fluorescent hybrid films. Reversible orientation control of the nanoplatelets is simply achieved through stretching the film, leading to tuneable fluorescence anisotropy. Such adjustable polarization effects are useful for modulating the optical response in composite materials.

Microfluidics and complex fluids.

In this paper, we describe four experimental studies we carried out over the last four years in the MMN lab, regarding the dynamical behaviour of complex fluids in microfluidic systems. The topics are: (1) Polymer breakup in microfluidic systems. (2) Flows of polymer solutions in microchannels close to a smooth wall. (3) Shear banding flows in microchannels (rheology, instabilities). (4) Flows of concentrated solutions of microgel particles through microchannels. Depending on the situation, we exploit the duality low Reynolds numbers/high Weissenberg numbers (for instance, by working at high shear rates without generating turbulence), use visualization windows naturally offered by the microfluidic environment or take advantage of the integration of various functionalities on the chip. In all cases, new information, hardly accessible to non-miniaturized approaches, could be obtained by using microfluidic technology.

Boosting migration of large particles by solute contrasts.

Brownian diffusion is a keystone concept in a large variety of domains, from physics, chemistry to biology. Diffusive transport controls situations as diverse as reaction-diffusion processes in biology and chemistry, Brownian ratchet processes, dispersion in microfluidic devices or even double-diffusive instability and salt-fingering phenomena in the context of ocean mixing. Although these examples span a broad range of length scales, diffusive transport becomes increasingly inefficient for larger particles. Applications, for example, in microfluidics, usually have recourse to alternative driving methods involving external sources to induce and control migration. Here, we demonstrate experimentally a strongly enhanced migration of large particles, achieved by slaving their dynamics to that of a fast carrier species, a dilute salt. The underlying fast salt diffusion leads to an apparent diffusive-like dynamics of the large particles, which is up to two orders of magnitude faster than their natural 'bare' diffusion. Moreover both spreading and focusing of the particle assembly can be achieved on demand. A model description shows a remarkable quantitative agreement with all measured data. Applications of this process are illustrated in microfluidics for filtering and concentrating operations, as well as in conjunction with standard hydrodynamic focusing. In a wider perspective, this mechanism can affect a broad range of scales and phenomena, from biological transport to the dispersion of sediments and pollutants in oceanographic situations.