Gene therapy is based on nucleic acid delivery to pathogenic cells in order to modulate their gene expression. The most used non viral vectors are lipid-based nanoaggregates, which are safer than viral carriers and have been shown to assemble easily with both DNA and RNA. However, the transfection efficiency of non viral carriers still needs to be improved before intensive practise in clinical trials can be implemented. For this purpose the in depth characterization of the complexes formed by nucleic acids and their transporters is of great relevance. In particular, information on the structure and assembly mechanism can be useful to improve our general knowledge of the se artificial transfection agents. In this paper the complexation mechanism of short interfering RNA and DNA molecules (siRNA and siDNA, respectively) with cationic micelles are investigated by combining Small Angle X-Ray Scattering experiments and Molecular Dynamics simulations. Micelles were obtained by Gemini surfactants with different spacer length (12-3-12, 12-6-12). The siRNA and siDNA used were double strand molecules characterized by the same length and homologous sequence, in order to perform close comparison. We showed that complexes appear in solution immediately after mixing and, therefore, the investigation of complex formation requires fast experimental techniques, such as stopped-flow synchrotron SAXS. The obtained systems had internal arrangement constituted by layers of squeezed micelles alternating the nucleic acids. Both SAXS and MD analysis allowed to evaluate the mean size of complexes in the range of few nanometers, with looser and less ordered stacking for the DNA containing aggregates.

Small-angle scattering is a powerful and popular technique for the characterization of the average structure of dense nanoparticle assemblies and aggregates. It is usually limited to not too big assemblies due to the limited q-range, and low enough concentrations to avoid interactions. A straightforward and quantitative analysis of the generally available scattered intensity – even for large assemblies, at high concentrations – at intermediate-q is detailed. It is based on the similarity in local structure between infinitely large homogeneous assemblies of hard spheres or moderately sticky hard spheres with any finite-sized nanoparticle assembly dominated by hard sphere interactions. The method provides information on the local volume fraction of particles. The approach is then extended to polydispersities up to 40%, using numerical simulations of hard spheres and mildly sticky hard spheres. As a result, a simple relationship between the observed structure factor minimum – termed the correlation hole – and the local volume fraction on the scale of neighboring particles, which is also linked to the coordination number, is given. This relationship shall be useful as a simple and efficient tool for the structural analysis of arbitrary aggregated colloidal systems. Finally, recent examples of filler structure in model and industrial polymer nanocomposites will be reviewed.

Polymer nanocomposites are mixtures of nanoparticles and polymer chains, where the filler particles are usually added to enhance mechanical properties, e.g. of car tire materials. Performance depends to a great extent on the structure of the filler, i.e. its dispersion state in the polymer matrix, which itself depends on mixing protocols, but also on the thermodynamics of the system. The latter can be tuned by performing either chemical surface modifications of the nanoparticles, or by playing with the mass and/or grafting properties of the polymer chains. Some straightforward ways to obtain information from small-angle scattering on the filler structure of nanocomposites will be presented. In particular, we recently developed a quantitative analysis of the scattered intensity in the intermediate q range. It is based on the similarity in local structure between infinitely large homogeneous assemblies of hard spheres or moderately sticky hard spheres with any finite-sized nanoparticle assembly dominated by hard sphere interactions. This method provides direct information on the local volume fraction of particles. It will then be applied to the impact of small molecules on the silica structure in nanocomposites close to industrial applications.Finally, a second issue for the mechanical response of nanocomposites relates to the polymer dynamics, which may also be governed by the filler dispersion. We will present first quasi-elastic neutron scattering results showing the impact of the aggregation state on the segmental dynamics of the same samples by means of incoherent neutron spin-echo.

In a first part I will focus on the destabilization of dilute oil-in-water emulsion-based liquid sheets expanding in air. A sheet results from the collision of a single tear on a small solid target ; it disintegrates through the nucleation and growth of holes that perforate the sheet. We have developed an optical technique that allows the determination of the time and space-resolved thickness of the sheet to gain a, understanding of the physical mechanisms f the perforation events This bursting based-liquid sheet destabilization is at the origin of emulsion-based anti-drift formulations are developed for agricultural spray. In a second part, I will investigate freely expanding sheets formed by ultra soft spherical gel beads of elastic but also liquid droplets with various surface tensions, and simple viscoelastic fluids (Maxwell fluids), produced by impacting them on a silicon wafer covered with a thin layer of liquid nitrogen that suppresses viscous dissipation by an inverse Leidenfrost effect. The experiments reveal a universal behaviour of the impact dynamics with impact velocity, for both solids and liquids, and even viscoelastic fluids, that we have rationalized .