Orsolya Czakkel

Beyond simple exponential correlation functions and equilibrium dynamics in x-ray photon correlation spectroscopy

We discuss the complex dynamics in condensed matter studied with x-ray photon correlation spectroscopy (XPCS) in which non-exponential correlation functions and dispersion relations deviating from the simple diffusion law are observed. Results are presented for two systems whose dynamics are characterized by compressed, faster-than-exponential correlation functions associated with hyper-diffusive motion. In the first case, the microscopic response of an aerogel following sectioning is investigated. In the second, the out-of-equilibrium dynamics in a dense colloidal gel recovering from shear is analyzed. In both cases, the dynamics, which can be associated with relaxation of internal stress, exhibits ageing. Included in the analyses are calculations of two-time correlation functions and the variance of the instantaneous degree of correlation, yielding the dynamical susceptibility.

Effect of molybdenum on the structure formation of resorcinol–formaldehyde hydrogel studied by coherent x-ray scattering

A detailed study on the effect of Mo on the gelation process of resorcinol-formaldehyde systems is presented. The evolution of the system was followed by x-ray photon correlation spectroscopy, which allows in situ investigation of the dynamics as well as of the structural evolution in non-equilibrium processes. The Mo was introduced into the system after a pre-polymerization period (PP), the effect of which was also examined. Our results show that the presence of Mo substantially modifies the gelation process by favoring the growth of large compact clusters with weak bonds between them. However, this effect can be reduced by increasing PP.

Ionic Liquids: evidence of the viscosity scale-dependence

Ionic Liquids (ILs) are a specific class of molecular electrolytes characterized by the total absence of co-solvent. Due to their remarkable chemical and electrochemical stability, they are prime candidates for the development of safe and sustainable energy storage systems. The competition between electrostatic and van der Waals interactions leads to a property original for pure liquids: they self-organize in fluctuating nanometric aggregates. So far, this transient structuration has escaped to direct clear-cut experimental assessment. Here, we focus on a imidazolium based IL and use particle-probe rheology to (i) catch this phenomenon and (ii) highlight an unexpected consequence: the self-diffusion coefficient of the cation shows a one order of magnitude difference depending whether it is inferred at the nanometric or at the microscopic scale. As this quantity partly drives the ionic conductivity, such a peculiar property represents a strong limiting factor to the performances of ILs-based batteries.