64 research outputs found
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
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