26 research outputs found
Surface viscosity in simple liquids
The response of Newtonian liquids to small perturbations is usually
considered to be fully described by homogeneous transport coefficients like
shear and dilatational viscosity. However, the presence of strong density
gradients at the liquid/vapor boundary of fluids hints at the possible
existence of an inhomogeneous viscosity. Here, we show that a surface viscosity
emerges from the collective dynamics of interfacial layers in molecular
simulations of simple liquids. We estimate the surface viscosity to be 8-16
times smaller than that of the bulk fluid at the thermodynamic point
considered. This result can have important implications for reactions at liquid
surfaces in atmospheric chemistry and catalysis
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
Ice phonon spectra and Bayes inference: a gateway to a new understanding of terahertz sound propagation in water
Understanding how molecules engage in collective motions in a liquid where a
network of bonds exists has both fundamental and applied relevance. On the one
hand, it can elucidate the ``ordering" role of long-range correlations in an
otherwise strongly dissipative system; on the other hand, it can inspire new
avenues to control such order to implement sound manipulation. Water represents
an ideal investigation case to unfold these general aspects and, across the
decades, it has been the focus of thorough scrutiny. Despite this investigative
effort, the spectrum of terahertz density fluctuations of water largely remains
a puzzle for Condensed Matter physicists. To unravel it, we compare previous
scattering measurements of water spectra with new ones on ice. Thanks to the
unique asset of Bayesian inference, we draw a more detailed portrayal of the
phonon response of ice. The comparison with the one of liquid water challenges
the current understanding of density fluctuations in water, or more in general,
of any networked liquid.Comment: 30 pages, 9 figure
The hydrogen-bond collective dynamics in liquid methanol
The relatively simple molecular structure of hydrogen-bonded (HB) systems is often belied by their exceptionally complex thermodynamic and microscopic behaviour. For this reason, after a thorough experimental, computational and theoretical scrutiny, the dynamics of molecules in HB systems still eludes a comprehensive understanding. Aiming at shedding some insight into this topic, we jointly used neutron Brillouin scattering and molecular dynamics simulations to probe the dynamics of a prototypical hydrogen-bonded alcohol, liquid methanol. The comparison with the most thoroughly investigated HB system, liquid water, pinpoints common behaviours of their THz microscopic dynamics, thereby providing additional information on the role of HB dynamics in these two systems. This study demonstrates that the dynamic behaviour of methanol is much richer than what so far known, and prompts us to establish striking analogies with the features of liquid and supercooled water. In particular, based on the strong differences between the structural properties of the two systems, our results suggest that the assignment of some dynamical properties to the tetrahedral character of water structure should be questioned. We finally highlight the similarities between the characteristic decay times of the time correlation function, as obtained from our data and the mean lifetime of hydrogen bond known in literature
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
Switching off hydrogen-bond-driven excitation modes in liquid methanol
Abstract Hydrogen bonding plays an essential role on intermolecular forces, and consequently on the thermodynamics of materials defined by this elusive bonding character. It determines the property of a vital liquid as water as well as many processes crucial for life. The longstanding controversy on the nature of the hydrogen bond (HB) can be settled by looking at the effect of a vanishing HB interaction on the microscopic properties of a given hydrogen-bonded fluid. This task suits the capabilities of computer simulations techniques, which allow to easily switch off HB interactions. We then use molecular dynamics to study the microscopic properties of methanol, a prototypical HB liquid. Fundamental aspects of the dynamics of methanol at room temperature were contextualised only very recently and its rich dynamics was found to have striking analogies with that of water. The lower temperature (200 K) considered in the present study led us to observe that the molecular centre-of-mass dynamics is dominated by four modes. Most importantly, the computational ability to switch on and off hydrogen bonds permitted us to identify which, among these modes, have a pure HB-origin. This clarifies the role of hydrogen bonds in liquid dynamics, disclosing new research opportunities and unexplored interpretation schemes
Neutron Brillouin scattering and ab initio simulation study of the collective dynamics of liquid silver
Producción CientÃficaWe present a thorough investigation of the collective dynamics of liquid Ag combining neutron Brillouin scattering and ab initio molecular dynamics (AIMD) determinations of the dynamic structure factor S(Q,ω). The main scope of this work is not only to provide experimental results for some important dynamical properties of this liquid metal in the wave-vector range 4<Q<16nm−1, but also to inquire about the scarce detectability of shear waves apparently characterizing two elements of group IB, differently from other metals. In fact, as in the case of Au, a transverse-like dynamics is not deducible from the experimental S(Q,ω) of Ag, despite the indisputable quality of the neutron data collected on the BRISP spectrometer at the Institut Laue Langevin in Grenoble. However, the significant agreement between experiment and AIMD calculations allowed for an in-depth study of the simulated S(Q,ω) in a Q range overlapping and extending the experimental one. A multimode analysis, already proven very successful in the description of various dynamical properties of fluid systems, is shown to be extremely effective also to analyze the intermediate scattering function predicted by AIMD at the various Q values, and eventually enables a reliable determination of both longitudinal and transverse branches in the dispersion curve of this liquid. Throughout the paper we highlight the importance of referring to theoretically well-founded models for S(Q,ω) and of imposing physical constraints in a fit-based analysis: These ensure that the used models obey fundamental properties of the dynamic structure factor.Ministero dell’Istruzione dell’Università e della Ricerca Italiano (grant PRIN2017-2017Z55KCW)Ministerio de EconomÃa, Industria y Competitividad (project PGC2018-093745-B-I00)Junta de Castilla y León (project VA124G18