80 research outputs found
Shear and longitudinal viscosity of non-ionic C12E8 aqueous solutions
We present measurements of the steady shear viscosity, the longitudinal
elastic modulus and the ultrasonic absorption in the one-phase isotropic liquid
region of the nonionic surfactant C12E8 aqueous solutions. The overall results
support the presence of two separated intervals of concentration corresponding
to different structural properties. In the surfactant-rich region the
temperature dependence of the steady shear viscosity follows an equation
characteristic of glass-like systems. The ultrasonic absorption spectra show
unambiguous evidence of viscoelastic behaviour described by a Cole-Cole
relaxation formula. In the water-rich region the behaviour of the measured
quantities are more complex and reflect the presence of dispersed aggregates
whose size increases with temperature and concentration. An additional low
frequency contribution is also observed, which is ascribed to the exchange of
water molecules and/or surfactant monomers between the aggregates and the bulk
solvent region.Comment: 23 Pages, 7 Figures, 1 Table, submitted to J. Phys. Chem B, accepted
for publicatio
A Bayesian analysis of neutron spin echo data on polymer coated gold nanoparticles in aqueous solutions
We present a neutron spin echo study (NSE) of the nanosecond dynamics of
polyethylene glycol (PEG) functionalised nanosized gold particles dissolved in
DO at two temperatures and two different PEG molecular weights. The
analysis of the NSE data was performed by applying a Bayesian approach to the
description of time correlation function decays in terms of exponential terms,
recently proved to be theoretically rigorous. This approach, which addresses in
a direct way the fundamental issue of model choice in any dynamical analysis,
provides here a guide to the most statistically supported way to follow the
decay of the Intermediate Scattering Functions I(Q, t) by basing on statistical
grounds the choice of the number of terms required for the description of the
nanosecond dynamics of the studied systems. Then, the presented analysis avoids
from the start resorting to a pre-selected framework and can be considered as
model free. By comparing the results of PEG coated nanoparticles with those
obtained in PEG2000 solutions, we were able to disentangle the translational
diffusion of the nanoparticles from the internal dynamics of the polymer
grafted to them, and to show that the polymer corona relaxation follows a pure
exponential decay in agreement with the behavior predicted by coarse grained
molecular dynamics simulations and theoretical models. This methodology has one
further advantage: in the presence of a complex dynamical scenario I(Q,t) is
often described in terms of the Kohlrausch-Williams-Watts function that can
implicitly represent a distribution of relaxation times. By choosing to
describe the I(Q,t) as a sum of exponential functions and with the support of
the Bayesian approach, we can explicitly determine when a finer-structure
analysis of the dynamical complexity of the system exists according to the
available data without the risk of overparametrisation
Interpreting the Terahertz Spectrum of Complex Materials: The Unique Contribution of the Bayesian Analysis
In the last few decades, experimental studies of the terahertz spectrum of density
fluctuations have considerably improved our knowledge of the mesoscopic dynamics of disordered
materials, which also have imposed new demands on the data modelling and interpretation. Indeed,
lineshape analyses are no longer limited to the phenomenological observation of inelastic features, as
in the pioneering stage of Neutron or X-ray spectroscopy, rather aiming at the extraction from their
shape of physically relevant quantities, as sound velocity and damping, relaxation times, or other
transport coefficients. In this effort, researchers need to face both inherent and practical obstacles,
respectively stemming from the highly damped nature of terahertz modes and the limited energy
resolution, accessible kinematic region and statistical accuracy of the typical experimental outcome.
To properly address these challenges, a global reconsideration of the lineshape modelling and the
enforcement of evidence-based probabilistic inference is becoming crucial. Particularly compelling
is the possibility of implementing Bayesian inference methods, which we illustrated here through
an in-depth discussion of some results recently obtained in the analysis of Neutron and X-ray
scattering results
Neutron spin echo monitoring of segmental-like diffusion of water confined in the cores of carbon nanotubes
An unexpected formal similarity to the segmental dynamics of entangled polymers is found for the subdiffusional behavior of water axially confined inside single-wall carbon nanotubes of average diameter d = 1.4 nm
Solid Surface Structure Affects Liquid Order at the Polystyrene/SAM Interface
We present a combined x-ray and neutron reflectivity study characterizing the
interface between polystyrene (PS) and silanized surfaces. Motivated by the
large difference in slip velocity of PS on top of dodecyl-trichlorosilane (DTS)
and octadecyl-trichlorosilane (OTS) found in previous studies, these two
systems were chosen for the present investigation. The results reveal the
molecular conformation of PS on silanized silicon. Differences in the molecular
tilt of OTS and DTS are replicated by the adjacent phenyl rings of the PS. We
discuss our findings in terms of a potential link between the microscopic
interfacial structure and dynamic properties of polymeric liquids at
interfaces
The damping of terahertz acoustic modes in aqueous nanoparticle suspensions
In this work, we investigate the possibility of controlling the acoustic damping in a liquid when nanoparticles are suspended in it. To shed light on this topic, we performed Inelastic X-Ray Scattering (IXS) measurements of the terahertz collective dynamics of aqueous suspensions of nanospheres of various materials, size, and relative concentration, either charged or neutral. A Bayesian analysis of measured spectra indicates that the damping of the two acoustic modes of water increases upon nanoparticle immersion. This effect seems particularly pronounced for the longitudinal acoustic mode, which, whenever visible at all, rapidly damps off when increasing the exchanged wavevector. Results also indicate that the observed effect strongly depends on the material the immersed nanoparticles are made of
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