2,298 research outputs found
Combining rare events techniques: phase change in Si nanoparticles
We introduce a combined Restrained MD/Parallel Tempering approach to study
the difference in free energy as a function of a set of collective variables
between two states in presence of unknown slow degrees of freedom.
We applied this method to study the relative stability of the amorphous vs
crystalline nanoparticles of size ranging between 0.8 and 1.8 nm as a function
of the temperature. We found that, at variance with bulk systems, at low T
small nanoparticles are amorphous and undergo an amorphous-to-crystalline phase
transition at higher T. On the contrary, large nanoparticles recover the
bulk-like behavior: crystalline at low and amorphous at high T
Order-disorder phase change in embedded Si nano-particles
We investigated the relative stability of the amorphous vs crystalline
nanoparticles of size ranging between 0.8 and 1.8 nm. We found that, at
variance from bulk systems, at low T small nanoparticles are amorphous and they
undergo to an amorphous-to-crystalline phase transition at high T. On the
contrary, large nanoparticles recover the bulk-like behavior: crystalline at
low T and amorphous at high T. We also investigated the structure of
crystalline nanoparticles, providing evidence that they are formed by an
ordered core surrounded by a disordered periphery. Furthermore, we also provide
evidence that the details of the structure of the crystalline core depend on
the size of the nanoparticleComment: 8 pages, 5 figure
Collapse of superhydrophobicity on nanopillared surfaces
The mechanism of the collapse of the superhydrophobic state is elucidated for
submerged nanoscale textures forming a three-dimensional interconnected vapor
domain. This key issue for the design of nanotextures poses significant
simulation challenges as it is characterized by diverse time and length scales.
State-of-the-art atomistic rare events simulations are applied for overcoming
the long time scales connected with the large free energy barriers. In such
interconnected surface cavities wetting starts with the formation of a liquid
finger between two pillars. This break of symmetry induces a more gentle bend
in the rest of the liquid-vapor interface, which triggers the wetting of the
neighboring pillars. This collective mechanism, involving the wetting of
several pillars at the same time, could not be captured by previous atomistic
simulations using surface models comprising a small number of pillars (often
just one). Atomistic results are interpreted in terms of a sharp-interface
continuum model which suggests that line tension, condensation, and other
nanoscale phenomena play a minor role in the simulated conditions
Unraveling the Salvinia paradox: design principles for submerged superhydrophobicity
The complex structure of the Salvinia molesta is investigated via rare event
molecular dynamics simulations. Results show that a hydrophilic/hydrophobic
patterning together with a re-entrant geometry control the free energy barriers
for bubble nucleation and for the Cassie-Wenzel transition. This natural
paradigm is translated into simple macroscopic design criteria for engineering
robust superhydrophobicity in submerged applications
The Influence of Silicon Nanoclusters on the Optical Properties of a-SiNx Samples: A Theoretical Study
By means of ab-initio calculations we investigate the optical properties of
pure a-SiN samples, with , and samples embedding silicon
nanoclusters (NCs) of diameter nm. In the pure samples
the optical absorption gap and the radiative recombination rate vary according
to the concentration of Si-N bonds. In the presence of NCs the radiative rate
of the samples is barely affected, indicating that the intense
photoluminescence of experimental samples is mostly due to the matrix itself
rather than to the NCs. Besides, we evidence an important role of Si-N-Si bonds
at the NC/matrix interface in the observed photoluminescence trend
An observable for vacancy characterization and diffusion in crystals
To locate the position and characterize the dynamics of a vacancy in a
crystal, we propose to represent it by the ground state density of a quantum
probe quasi-particle for the Hamiltonian associated to the potential energy
field generated by the atoms in the sample. In this description, the h^2/2mu
coefficient of the kinetic energy term is a tunable parameter controlling the
density localization in the regions of relevant minima of the potential energy
field. Based on this description, we derive a set of collective variables that
we use in rare event simulations to identify some of the vacancy diffusion
paths in a 2D crystal. Our simulations reveal, in addition to the simple and
expected nearest neighbor hopping path, a collective migration mechanism of the
vacancy. This mechanism involves several lattice sites and produces a long
range migration of the vacancy. Finally, we also observed a vacancy induced
crystal reorientation process
Relaxation of a steep density gradient in a simple fluid: comparison between atomistic and continuum modeling
We compare dynamical nonequilibrium molecular dynamics and continuum
simulations of the dynamics of relaxation of a fluid system characterized by a
non uniform density profile. Results match quite well as long as the
lengthscale of density nonuniformities are greater than the molecular scale (10
times the molecular size). In presence of molecular scale features some of the
continuum fields (e.g. density and momentum) are in good agreement with
atomistic counterparts, but are smoother. On the contrary, other fields, such
at the temperature field, present very large difference with respect to
reference (atomistic) ones. This is due to the limited accuracy of some of the
empirical relations used in continuum models, the equation of state of the
fluid in the example considered
La condizione di insularità nell’Unione Europea: accessibilità e incidenza del trasporto marittimo
Questo paper esamina gli aspetti relativi all’accessibilità delle isole, così come definite dall’U.E., in
ambito Europeo. L’accessibilità, nell’Unione Europea (in accordo con lo studio ESPON "Atlas" del 2006)
è stata legata al concetto di “cuore” del territorio Europeo e di “periferia”; in questo modo l’ubicazione
geografica e la distanza fisica sono divenuti i parametri significativi in relazione all’accessibilità in
termini di infrastrutture e di sistema di trasporti.
L’obiettivo del seguente studio è quello di indagare in che modo l’insularità possa essere analizzata,
caratterizzata e misurata in relazione alle peculiarità endogene ed ai requisiti strutturali e funzionali del
sistema dei collegamenti ed in che modo questa misura possa garantire un confronto quantitativo,
semplice da interpretare, dell’accessibilità con realtà e regioni della terraferma, anche periferiche.
In particolare, vengono proposti una serie di indicatori che descrivono l’accessibilità delle isole in
riferimento al sistema dei trasporti marittimi, attraverso la specificazione di una serie di attributi
dell’accessibilità che fanno riferimento ai parametri di lontananza (distanza reale), isolamento e
discontinuità geografica (frequenza e tempi di attesa), parametri che caratterizzano le realtà insulari
Mechanism of the Cassie-Wenzel transition via the atomistic and continuum string methods
The string method is a general and flexible strategy to compute the most
probable transition path for an activated process (rare event). We apply here
the atomistic string method in the density field to the Cassie-Wenzel
transition, a central problem in the field of superhydrophobicity. We discuss
in detail the mechanism of wetting of a submerged hydrophobic cavity of
nanometer size and its dependence on the geometry of the cavity. Furthermore,
we discuss the algorithmic analogies between the string method and CREaM
[Giacomello et al., Phys. Rev. Lett. 109, 226102 (2012)], a method inspired by
the string that allows for a faster and simpler computation of the mechanism
and of the free-energy profiles of the wetting process. This approach is
general and can be employed in mesoscale and macroscopic calculations
Liquid intrusion in and extrusion from non-wettable nanopores for technological applications
In this article, we review some recent theoretical results about intrusion and extrusion of non-wetting liquids in and out of cavities of nanotextured surfaces and nanoporous materials. Nanoscale confinement allows these processes to happen at conditions which significantly differ from bulk phase coexistence. In particular, the pressure at which a liquid penetrates in and exits from cavities is of interest for many technological applications such as energy storage, dissipation, and conversion, materials with negative compressibility, ion channels, liquid chromatography, and more. Notwithstanding its technological interest, intrusion/extrusion processes are difficult to understand and control solely via experiments: the missing step is often a simple theory capable of providing a microscopic interpretation of the results, e.g., of liquid porosimetry or other techniques used in the field, especially in the case of complex nanoporous media. In this context, simulations can help shedding light on the relation between the morphology of pores, the chemical composition of the solids and liquids, and the thermodynamics and kinetics of intrusion and extrusion. Indeed, the intrusion/extrusion kinetics is determined by the presence of free energy barriers and special approaches, the so-called rare event techniques, must be used to study these processes. Usually, rare event techniques are employed to investigate processes occurring in relatively simple molecular systems, while intrusion/extrusion concerns the collective dynamics of hundreds to thousands of degrees of freedom, the molecules of a liquid entering in or exiting from a cavity, which, from the methodological point of view, is itself a challenge
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