Field induced anisotropic cooperativity in a magnetic colloidal glass

The translational dynamics in a repulsive colloidal glass-former is probed by time-resolved X-ray Photon Correlation Spectroscopy. In this dense dispersion of charge-stabilized and magnetic nanoparticles, the interaction potential can be tuned, from quasi-isotropic to anisotropic by applying an external magnetic field. Structural and dynamical anisotropies are reported on interparticle lengthscales associated with highly anisotropic cooperativity, almost two orders of magnitude larger in the field direction than in the perpendicular direction and in zero field

Compressed correlation functions and fast aging dynamics in metallic glasses

We present x-ray photon correlation spectroscopy measurements of the atomic dynamics in a Zr67Ni33 metallic glass, well below its glass transition temperature. We find that the decay of the density fluctuations can be well described by compressed, thus faster than exponential, correlation functions which can be modeled by the well-known Kohlrausch-Williams-Watts function with a shape exponent {\beta} larger than one. This parameter is furthermore found to be independent of both waiting time and wave-vector, leading to the possibility to rescale all the correlation functions to a single master curve. The dynamics in the glassy state is additionally characterized by different aging regimes which persist in the deep glassy state. These features seem to be universal in metallic glasses and suggest a non diffusive nature of the dynamics. This universality is supported by the possibility of describing the fast increase of the structural relaxation time with waiting time using a unique model function, independently of the microscopic details of the system.Comment: 7 pages, 4 figures. To be published in J. Chem. Phy

Dynamics of nanoparticles in a supercooled liquid

The dynamic properties of nanoparticles suspended in a supercooled glass forming liquid are studied by x-ray photon correlation spectroscopy. While at high temperatures the particles undergo Brownian motion the measurements closer to the glass transition indicate hyperdiffusive behavior. In this state the dynamics is independent of the local structural arrangement of nanoparticles, suggesting a cooperative behavior governed by the near-vitreous solvent

Comparing the atomic and macroscopic aging dynamics in an amorphous and partially crystalline Zr44Ti11Ni10Cu10Be25 bulk metallic glass

This article has been published in a revised form in Journal of Materials Research, https://doi.org/10.1557/jmr.2017.187. This version is free to view and download for private research and study only. Not for re-distribution, re-sale or use in derivative works. © Materials Research Society.Several recent X-ray photon correlation spectroscopy works have reported an anomalous atomic dynamics in hyperquenched metallic glasses. Here, we compare and contrast these microscopic dynamics with that found in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass, prepared with a cooling rate some 6 orders of magnitude lower. In both cases, structural relaxation in the glass is governed by internal stresses, giving rise to highly compressed density correlation functions. Differently from the fast aging reported in previous studies, here the atomic dynamics displays a slow linear atomic-level aging, while not affecting the shape parameter. Traditional macroscopic phenomenological models fail to capture the temperature dependence of the microscopic structural relaxation time, suggesting a length scale dependence of the aging. Interestingly, the dynamics does not seem to be affected by the presence of a low percentage of frozen nanocrystals and displays a temperature dependence similar to that observed in macroscopic viscosity measurements.Peer ReviewedPostprint (author's final draft

Dynamics in shear flow studied by X-ray Photon Correlation Spectroscopy

X-ray photon correlation spectroscopy was used to measure the diffusive dynamics of colloidal particles in a shear flow. The results presented here show how the intensity autocorrelation functions measure both the diffusive dynamics of the particles and their flow-induced, convective motion. However, in the limit of low flow/shear rates, it is possible to obtain the diffusive component of the dynamics, which makes the method suitable for the study of the dynamical properties of a large class of complex soft-matter and biological fluids. An important benefit of this experimental strategy over more traditional X-ray methods is the minimization of X-ray induced beam damage. While the method can be applied also for photon correlation spectroscopy in the visible domain, our analysis shows that the experimental conditions under which it is possible to measure the diffusive dynamics are easier to achieve at higher q values (with X-rays).Comment: 9 pages, 7 figures, to appear in Eur. Phys. J.

Controlling the dynamics of a bidimensional gel above and below its percolation transition

The morphology and the microscopic internal dynamics of a bidimensional gel formed by spontaneous aggregation of gold nanoparticles confined at the water surface are investigated by a suite of techniques, including grazing-incidence x-ray photon correlation spectroscopy (GI-XPCS). The range of concentrations studied spans across the percolation transition for the formation of the gel. The dynamical features observed by GI-XPCS are interpreted in view of the results of microscopical imaging; an intrinsic link between the mechanical modulus and internal dynamics is demonstrated for all the concentrations. Our work presents, to the best of our knowledge, the first example of a transition from stretched to compressed correlation function actively controlled by quasistatically varying the relevant thermodynamic variable. Moreover, by applying a model proposed time ago by Duri and Cipelletti [A. Duri and L. Cipelletti, Europhys. Lett. 76, 972 (2006)] we are able to build a novel master curve for the shape parameter, whose scaling factor allows us to quantify a 'long time displacement length'. This characteristic length is shown to converge, as the concentration is increased, to the 'short time localization length' determined by pseudo Debye-Waller analysis of the initial contrast. Finally, the intrinsic dynamics of the system are then compared with that induced by means of a delicate mechanical perturbation applied to the interface

Revealing the fast atomic motion of network glasses

Still very little is known on the relaxation dynamics of glasses at the microscopic level due to the lack of experiments and theories. It is commonly believed that glasses are in a dynamical arrested state, with relaxation times too large to be observed on human time scales. Here we provide the experimental evidence that glasses display fast atomic rearrangements within a few minutes, even in the deep glassy state. Following the evolution of the structural relaxation in a sodium silicate glass, we find that this fast dynamics is accompanied by the absence of any detectable aging, suggesting a decoupling of the relaxation time and the viscosity in the glass. The relaxation time is strongly affected by the network structure with a marked increase at the mesoscopic scale associated with the ion-conducting pathways. Our results modify the conception of the glassy state and asks for a new microscopic theory

Low-frequency elastic behavior of a supercooled liquid

By X-ray photon correlation spectroscopy we quantify the influence of elasticity and viscosity on the capillary wave (CW) surface dynamics of a supercooled liquid. To fit the data a novel model combining Maxwell-Debye and Voigt-Kelvin viscoelasticity is derived yielding a saturation of relaxation rates at high q as well as an offset in the CW dispersion relation. Diffuse X-ray scattering confirms the result and data taken on the surface of supercooled polypropylene glycol (PPG-4000) evidence a low-frequency elastic plateau of the storage modulus. A possible connection between the observed solid-like response and the supercooled state is discussed