26,911 research outputs found
Structural transformations in porous glasses under mechanical loading. II. Compression
The role of porous structure and glass density in response to compressive
deformation of amorphous materials is investigated via molecular dynamics
simulations. The disordered, porous structures were prepared by quenching a
high-temperature binary mixture below the glass transition into the phase
coexistence region. With decreasing average glass density, the pore morphology
in quiescent samples varies from a random distribution of compact voids to a
porous network embedded in a continuous glass phase. We find that during
compressive loading at constant volume, the porous structure is linearly
transformed in the elastic regime and the elastic modulus follows a power-law
increase as a function of the average glass density. Upon further compression,
pores deform significantly and coalesce into large voids leading to formation
of domains with nearly homogeneous glass phase, which provides an enhanced
resistance to deformation at high strain.Comment: 25 pages, 12 figure
On the influence of the intermolecular potential on the wetting properties of water on silica surfaces
We study the wetting properties of water on silica surfaces using molecular
dynamics (MD) simulations. To describe the intermolecular interaction between
water and silica atoms, two types of interaction potential models are used: the
standard Br\'odka and Zerda (BZ) model, and the Gulmen and Thompson (GT) model.
We perform an in-depth analysis of the influence of the choice of the potential
on the arrangement of the water molecules in partially filled pores and on top
of silica slabs. We find that at moderate pore filling ratios, the GT silica
surface is completely wetted by water molecules, which agrees well with
experimental findings, while the commonly used BZ surface is less hydrophilic
and is only partially wetted. We interpret our simulation results using an
analytical calculation of the phase diagram of water in partially filled pores.
Moreover, an evaluation of the contact angle of the water droplet on top of the
silica slab reveals that the interaction becomes more hydrophilic with
increasing slab thickness and saturates around 2.5-3 nm, in agreement with the
experimentally found value. Our analysis also shows that the hydroaffinity of
the surface is mainly determined by the electrostatic interaction, but that the
van der Waals interaction nevertheless is strong enough that it can turn a
hydrophobic surface into a hydrophilic surface.Comment: Article: 9 pages, 7 Figures. There is also a supplementary
information file: 2 pages, 3 Figure
Freezing point depression and freeze-thaw damage by nano-fuidic salt trapping
A remarkable variety of organisms and wet materials are able to endure
temperatures far below the freezing point of bulk water. Cryo-tolerance in
biology is usually attributed to "anti-freeze" proteins, and yet massive
supercooling (C) is also possible in porous media containing only
simple aqueous electrolytes. For concrete pavements, the common wisdom is that
freeze-thaw damage results from the expansion of water upon freezing, but this
cannot explain the large pressures (~MPa) required to damage concrete,
the observed correlation between pavement damage and de-icing salts, or the
damage of cement paste loaded with benzene (which contracts upon freezing). In
this Letter, we propose a different mechanism -- nanofluidic salt trapping --
which can explain the observations, using simple mathematical models of
dissolved ions confined to thin liquid films between growing ice and charged
surfaces. Although trapped salt lowers the freezing point, ice nucleation in
charged pores causes enormous disjoining pressures via the rejected ions, until
their removal by precipitation or surface adsorption at a lower temperatures
releases the pressure and allows complete freezing. The theory is able to
predict the non-monotonic salt-concentration dependence of freeze-thaw damage
in concreter and provides a general framework to understand the origins of
cryo-tolerance.Comment: 5 figure
Mechanism of cellular uptake of genotoxic silica nanoparticles.
Mechanisms for cellular uptake of nanoparticles have important implications for nanoparticulate drug delivery and toxicity. We have explored the mechanism of uptake of amorphous silica nanoparticles of 14 nm diameter, which agglomerate in culture medium to hydrodynamic diameters around 500 nm. In HT29, HaCat and A549 cells, cytotoxicity was observed at nanoparticle concentrations ≥ 1 μg/ml, but DNA damage was evident at 0.1 μg/ml and above. Transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy confirmed entry of the silica particles into A549 cells exposed to 10 μg/ml of nanoparticles. The particles were observed in the cytoplasm but not within membrane bound vesicles or in the nucleus. TEM of cells exposed to nanoparticles at 4°C for 30 minutes showed particles enter cells when activity is low, suggesting a passive mode of entry. Plasma lipid membrane models identified physical interactions between the membrane and the silica NPs. Quartz crystal microbalance experiments on tethered bilayer lipid membrane systems show that the nanoparticles strongly bind to lipid membranes, forming an adherent monolayer on the membrane. Leakage assays on large unilamellar vesicles (400 nm diameter) indicate that binding of the silica NPs transiently disrupts the vesicles which rapidly self-seal. We suggest that an adhesive interaction between silica nanoparticles and lipid membranes could cause passive cellular uptake of the particles
Mechanical properties of mesoporous ceria nanoarchitectures
Architectural constructs are engineered to impart desirable mechanical properties facilitating bridges spanning a thousand meters and buildings nearly 1 km in height. However, do the same 'engineering-rules' translate to the nanoscale, where the architectural features are less than 0.0001 mm in size? Here, we calculate the mechanical properties of a porous ceramic functional material, ceria, as a function of its nanoarchitecture using molecular dynamics simulation and predict its yield strength to be almost two orders of magnitude higher than the parent bulk material. In particular, we generate models of nanoporous ceria with either a hexagonal or cubic array of one-dimensional pores and simulate their responses to mechanical load. We find that the mechanical properties are critically dependent upon the orientation between the crystal structure (symmetry, direction) and the pore structure (symmetry, direction). This journal i
Recent advances in the formation of phase inversion membranes made from amorphous or semi-crystalline polymers
Structural characteristics in membranes formed by diffusion induced phase separation processes are discussed. Established theories on membrane formation from ternary systems can be extended to describe the effects of high or low molecular weight additives. A mechanism for the formation of nodular structures in the top layer of ultrafiltration membranes is presented. In the last part structures arising from polymer crystallization during immersion precipitation are discussed
Electrokinetic flow of aqueous electrolyte in amorphous silica nanotubes
We study the pressure-driven flow of aqueous NaCl in amorphous silica nanotubes using nonequilibrium molecular dynamics simulations featuring both polarizable and non-polarizable molecular models. Different pressures, electrolyte concentrations and pore sizes are examined. Our results indicate a flow that deviates considerably from the predictions of Poiseuille fluid mechanics. Due to preferential adsorption of the different ionic species by surface SiO! or SiOH groups, we find that a significant electric current is generated, but with opposite polarities using polarizable vs. fixed charge models for water and ions, emphasizing the need for careful parameterization in such complex systems. We also examine the influence of partial deprotonation of the silica surface, and we find that much more current is generated in a dehydrogenated nanopore, even though the overall efficiency remains low. These findings indicate that different methods of nanopore preparation, which can produce a range of surface properties, should be examined more closely in the related experimental methods to generate electrokinetic current
IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models
Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of
the interstellar medium. In dense molecular clouds, it is also present in the
solid-phase as a constituent of the mixed water-dominated ices covering dust
grains. Its presence in the solid-phase is inferred from its infrared (IR)
signals. In experimental observations of solid CO/water mixed samples, its IR
frequency splits into two components, giving rise to a blue- and a redshifted
band. However, in astronomical observations, the former has never been
observed. Several attempts have been carried out to explain this peculiar
behaviour, but the question still remains open. In this work, we resorted to
pure quantum mechanical simulations in order to shed some light on this
problem. We adopted different periodic models simulating the CO/HO ice
system, such as single and multiple CO adsorption on water ice surfaces, CO
entrapped into water cages and proper CO:HO mixed ices. We also simulated
pure solid CO. The detailed analysis of our data revealed how the quadrupolar
character of CO and the dispersive forces with water ice determine the
energetic of the CO/HO ice interaction, as well as the CO spectroscopic
behaviour. Our data suggest that the blueshifted peak can be assigned to CO
interacting {\it via} the C atom with dangling H atoms of the water ice, while
the redshifted one can actually be the result of CO involved in different
reciprocal interactions with the water matrix. We also provide a possible
explanation for the lack of the blueshifted peak in astronomical spectra. Our
aim is not to provide a full account of the various interstellar ices, but
rather to elucidate the sensitivity of the CO spectral features to different
water ice environments.Comment: MNRAS, accepte
Molecular Dynamics in grafted layers of poly(dimethylsiloxane) (PDMS)
Dielectric relaxation spectroscopy 10^-1 Hz to 10^6 Hz) is employed to study
the molecular dynamics of poly(dimethylsiloxane) (PDMS, Mw=1.7 10^5 g/mol and
Mw=9.6 10^4 g/mol as grafted films with thicknesses d below and above the
radius of gyration Rg. For d smaller than Rg the molecular dynamics becomes
faster by up to three orders of magnitude with respect to the bulk resulting in
a pronounced decrease of the Vogel temperature T0 and hence the calorimetric
glass transition temperature Tg. For d larger than Rg the molecular dynamics is
comparable to that of the bulk melt. The results are interpreted in terms of a
chain confinement effect and compared with the findings for low molecular eight
glass forming liquids contained in nanoporous glasses and zeolites.
Crystallization effects - well known for PDMS - are observed for films of
thicknesses above and below Rg.Comment: 20 pages, 4 figure
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