746 research outputs found
Local inversion-symmetry breaking controls the boson peak in glasses and crystals
It is well known that amorphous solids display a phonon spectrum where the
Debye law at low frequency melds into an anomalous excess-mode
peak (the boson peak) before entering a quasi-localized regime at higher
frequencies dominated by scattering. The microscopic origin of the boson peak
has remained elusive despite various attempts to put it in a clear connection
with structural disorder at the atomic/molecular level. Using numerical
calculations on model systems, we show that the microscopic origin of the boson
peak is directly controlled by the local breaking of center-inversion symmetry.
In particular, we find that both the boson peak and the nonaffine softening of
the material display a strong positive correlation with a new order parameter
describing the local inversion symmetry of the lattice. The standard
bond-orientational order parameter, instead, is shown to be a poor correlator
and cannot explain the boson peak in randomly-cut crystals with perfect
bond-orientational order. Our results bring a unifying understanding of the
boson peak anomaly for model glasses and defective crystals in terms of a
universal local symmetry-breaking principle of the lattice.This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by the American Physical Society
A COLREGs-Compliant Decision Support Tool to Prevent Collisions at Sea
Groundings and collisions still represent the highest percentage of marine accidents despite the current attention on Maritime Education and Training and the improvement of sensor capability. Most of the time, a collision is caused by a human error with consequences ranging from moderate to severe, with a substantial impact on both environment and life safeguarded at sea. In this paper, a brief statistical data regarding human element as a root cause of marine incidents together with collision regulations misunderstanding is presented as a background chapter. Furthermore, the present work discusses a decision support system architecture to suggest an appropriate action when the risk of a potential collision is detected. The proposed architecture system is based on various modules integrated with proper sensor input data regarding the surrounding navigation area. As a result, the tool can support the Officers of Watch in the decision‐making process providing an early suggestion in compliance with the COLlision REGulations. The proposed system is intended to be used onboard independently from the degree of automation of the ship, and it is based on AIS, which is mandatory, making it widely applicable. The proper use of the system can considerably reduce the number of collisions, as demonstrated by the obtained results
Shear-driven solidification of dilute colloidal suspensions
We show that the shear-induced solidification of dilute charge-stabilized
(DLVO) colloids is due to the interplay between the shear-induced formation and
breakage of large non-Brownian clusters. While their size is limited by
breakage, their number density increases with the shearing-time. Upon flow
cessation, the dense packing of clusters interconnects into a rigid state by
means of grainy bonds, each involving a large number of primary colloidal
bonds. The emerging picture of shear-driven solidification in dilute colloidal
suspensions combines the gelation of Brownian systems with the jamming of
athermal systems
Nonmonotonic dependence of polymer glass mechanical response on chain bending stiffness
We investigate the mechanical properties of amorphous polymers by means of coarse-grained simulations
and nona ne lattice dynamics theory. A small increase of polymer chain bending sti ness
leads rst to softening of the material, while hardening happens only upon further strengthening
of the backbones. This nonmonotonic variation of the storage modulus G0 with bending sti ness is
caused by a competition between additional resistance to deformation o ered by sti er backbones
and decreased density of the material due to a necessary decrease in monomer-monomer coordination.
This counter-intuitive nding suggests that the strength of polymer glasses may in some
circumstances be enhanced by softening the bending of constituent chains.CN acknowledges the Maudslay-Butler Research Fellowship at Pembroke College, Cambridge for financial support; VVP and AZ acknowledge financial support from the US Army Research Laboratory under grant nr. W911NF 16-2-0091
Nonequilibrium free energy of colloidal glasses under shear
The free energy of hard-sphere systems provides a direct link between the particle-scale structure and macroscopic thermodynamic properties. Here, we employ this framework to investigate the shear-induced structure of a colloidal glass, and link it to its macroscopic mechanical and thermodynamic state. We measure the nonequilibrium free energy under shear from the free volumes of the particles, and monitor its evolution with the applied strain. Unlike crystals, for which the elastic energy increases quadratically with strain due to affine particle displacements, for glasses the free energy decreases due to non-affine displacements and dissipation, reflecting the ability of the glass to reach deeper free-energy minima. We model this decrease using the nonaffine shear modulus and a standard viscous dissipative term. Our model and measurements allow us to disentangle the complex contributions of affine and nonaffine particle displacements in the transient shear deformation of glasses
Quantifying the Reversible Association of Thermosensitive Nanoparticles
Under many conditions, biomolecules and nanoparticles associate by means of
attractive bonds, due to hydrophobic attraction. Extracting the microscopic
association or dissociation rates from experimental data is complicated by the
dissociation events and by the sensitivity of the binding force to temperature
(T). Here we introduce a theoretical model that combined with light-scattering
experiments allows us to quantify these rates and the reversible binding energy
as a function of T. We apply this method to the reversible aggregation of
thermoresponsive polystyrene/poly(N-isopropylacrylamide) core-shell
nanoparticles, as a model system for biomolecules. We find that the binding
energy changes sharply with T, and relate this remarkable switchable behavior
to the hydrophobic-hydrophilic transition of the thermosensitive nanoparticles
The relation between stretched-exponential relaxation and the vibrational density of states in glassy disordered systems
Amorphous solids or glasses are known to exhibit stretched-exponential decay over broad time intervals in several of their macroscopic observables: intermediate scattering function, dielectric relaxation modulus, time-dependent elastic modulus, etc. This behaviour is prominent especially near the glass transition. In this Letter we show, on the example of dielectric relaxation, that stretched-exponential relaxation is intimately related to the peculiar lattice dynamics of glasses. By reformulating the Lorentz model of dielectric matter in a more general form, we express the dielectric response as a function of the vibrational density of states (DOS) for a random assembly of spherical particles interacting harmonically with their nearest-neighbours. Surprisingly we find that near the glass transition for this system (which coincides with the Maxwell rigidity transition in this model), the dielectric relaxation is perfectly consistent with stretched-exponential behaviour with Kohlrausch exponents 0.56<β<0.65, which is the range where exponents are measured in most experimental systems. Crucially, the root cause of stretched-exponential relaxation can be traced back to soft modes (boson-peak) in the DOS
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