Dynamical and rheological characterization of 2D architectures at the air/water interface

This Thesis reports the characterization of the mechanical response and of the internal microscopic dynamics of 2D Langmuir films, performed and interpreted within the theoretical framework provided by fluctuation-response function theorems. The mechanical shear modulus of Langmuir films has been measured by means of the Interfacial Shear Rheology technique (ISR). X-ray photon correlation spectroscopy (XPCS) experiments were performed to characterize the slow microscopic dynamics of samples either at the air/water interface or transferred on solid substrate. Two kind of systems have been investigated: polymeric films made of a photosensitive azobenzene-polyacrylate (PA4), and films of gold nanoparticles (GNPs) forming a 2D gel. Langmuir films of PA4 present an elastic shear response, which can be controlled at will by suitable illumination, induceing photoisomerisation of the azobenzene and therefore a transformation from an elastic to a viscous film. This process is accompanied by an acceleration, of more than one order of magnitude, of the internal dynamics, without a change in any thermodynamic parameter (temperature or surface pressure). Also, back isomerization with blue light acts on the system as an “optical quench”. All this allows the polymer to be brought out of equilibrium- and its dynamics being studied- in a novel and unconventional way. GNPs films are characterized by an heterogeneous morphology, with a foam-like structure at low concentration, and a Levy-distribution of sizes, which is deemed to be at the origin of the elastic, gel-like mechanical response observed. The slow dynamics of the films, observed by XPCS on the spatial scale of hundreds of nanometers, has an hyper-diffusive character which has been found in many arrested systems. Dynamical heterogeneities have been observed and characterized by means of higher order correlation functions -in particular, four times correlation functions have been experimentally accessed for the first time in an XPCS experiment in the course of this work. The GNPs 2D gel dynamics is characterized by two distinct time scales; both fast and slow motions are confined to the surface plane (2D) and have similar dependencies on the exchanged momentum, and on the coverage fraction. The degree of heterogeneity increases with concentration; at the same time, it seems that the motion involves a hierarchy of spatial scales. These results are compatible with theoretical results and simulations of the dynamics of colloidal gels.