231 research outputs found
The relationship between fragility, configurational entropy and the potential energy landscape of glass forming liquids
Glass is a microscopically disordered, solid form of matter that results when
a fluid is cooled or compressed in such a fashion that it does not crystallise.
Almost all types of materials are capable of glass formation -- polymers, metal
alloys, and molten salts, to name a few. Given such diversity, organising
principles which systematise data concerning glass formation are invaluable.
One such principle is the classification of glass formers according to their
fragility\cite{fragility}. Fragility measures the rapidity with which a
liquid's properties such as viscosity change as the glassy state is approached.
Although the relationship between features of the energy landscape of a glass
former, its configurational entropy and fragility have been analysed previously
(e. g.,\cite{speedyfr}), an understanding of the origins of fragility in these
features is far from being well established. Results for a model liquid, whose
fragility depends on its bulk density, are presented in this letter. Analysis
of the relationship between fragility and quantitative measures of the energy
landscape (the complicated dependence of energy on configuration) reveal that
the fragility depends on changes in the vibrational properties of individual
energy basins, in addition to the total number of such basins present, and
their spread in energy. A thermodynamic expression for fragility is derived,
which is in quantitative agreement with {\it kinetic} fragilities obtained from
the liquid's diffusivity.Comment: 8 pages, 3 figure
Construction of the free energy landscape by the density functional theory
On the basis of the density functional theory, we give a clear definition of
the free energy landscape. To show the usefulness of the definition, we
construct the free energy landscape for rearrangement of atoms in an FCC
crystal of hard spheres. In this description, the cooperatively rearranging
region (CRR) is clealy related to the hard spheres involved in the saddle
between two adjacent basins. A new concept of the simultaneously rearranging
region (SRR) emerges naturally as spheres defined by the difference between two
adjacent basins. We show that the SRR and the CRR can be determined explicitly
from the free energylandscape.Comment: 11 pages, 3 figures, submitted to J. Chem. Phy
A thermodynamic unification of jamming
Fragile materials ranging from sand to fire-retardant to toothpaste are able
to exhibit both solid and fluid-like properties across the jamming transition.
Unlike ordinary fusion, systems of grains, foams and colloids jam and cease to
flow under conditions that still remain unknown. Here we quantify jamming via a
thermodynamic approach by accounting for the structural ageing and the
shear-induced compressibility of dry sand. Specifically, the jamming threshold
is defined using a non-thermal temperature that measures the 'fluffiness' of a
granular mixture. The thermodynamic model, casted in terms of pressure,
temperature and free-volume, also successfully predicts the entropic data of
five molecular glasses. Notably, the predicted configurational entropy avoids
the Kauzmann paradox entirely. Without any free parameters, the proposed
equation-of-state also governs the mechanism of shear-banding and the
associated features of shear-softening and thickness-invariance.Comment: 16 pgs double spaced. 4 figure
DEM of triaxial tests on crushable sand
This paper presents simulations of high-pressure triaxial shear tests on a crushable sand. The discrete element method is used, featuring a large number of particles and avoiding the use of agglomerates. The triaxial model features a flexible membrane, therefore allowing realistic deformation, and a simple breakage mechanism is implemented using the octahedral shear stress induced in the particles. The simulations show that particle crushing is essential to replicate the realistic behaviour of sand (in particular the volumetric contraction) in high-pressure shear tests. The general effects of crushing during shear are explored, including its effects on critical states, and the influence of particle strength and confining pressure on the degree of crushing are discussed
Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
We consider a system of coupled classical harmonic oscillators with spatially
fluctuating nearest-neighbor force constants on a simple cubic lattice. The
model is solved both by numerically diagonalizing the Hamiltonian and by
applying the single-bond coherent potential approximation. The results for the
density of states are in excellent agreement with each other. As
the degree of disorder is increased the system becomes unstable due to the
presence of negative force constants. If the system is near the borderline of
stability a low-frequency peak appears in the reduced density of states
as a precursor of the instability. We argue that this peak
is the analogon of the "boson peak", observed in structural glasses. By means
of the level distance statistics we show that the peak is not associated with
localized states
Nitrogen uptake and internal recycling in Zostera marina exposed to oyster farming: eelgrass potential as a natural biofilter
Oyster farming in estuaries and coastal lagoons frequently overlaps with the distribution of seagrass meadows, yet there are few studies on how this aquaculture practice affects seagrass physiology. We compared in situ nitrogen uptake and the productivity of Zostera marina shoots growing near off-bottom longlines and at a site not affected by oyster farming in San Quintin Bay, a coastal lagoon in Baja California, Mexico. We used benthic chambers to measure leaf NH4 (+) uptake capacities by pulse labeling with (NH4)-N-15 (+) and plant photosynthesis and respiration. The internal N-15 resorption/recycling was measured in shoots 2 weeks after incubations. The natural isotopic composition of eelgrass tissues and vegetative descriptors were also examined. Plants growing at the oyster farming site showed a higher leaf NH4 (+) uptake rate (33.1 mmol NH4 (+) m(-2) day(-1)) relative to those not exposed to oyster cultures (25.6 mmol NH4 (+) m(-2) day(-1)). We calculated that an eelgrass meadow of 15-16 ha (which represents only about 3-4 % of the subtidal eelgrass meadow cover in the western arm of the lagoon) can potentially incorporate the total amount of NH4 (+) excreted by oysters (similar to 5.2 x 10(6) mmol NH4 (+) day(-1)). This highlights the potential of eelgrass to act as a natural biofilter for the NH4 (+) produced by oyster farming. Shoots exposed to oysters were more efficient in re-utilizing the internal N-15 into the growth of new leaf tissues or to translocate it to belowground tissues. Photosynthetic rates were greater in shoots exposed to oysters, which is consistent with higher NH4 (+) uptake and less negative delta C-13 values. Vegetative production (shoot size, leaf growth) was also higher in these shoots. Aboveground/belowground biomass ratio was lower in eelgrass beds not directly influenced by oyster farms, likely related to the higher investment in belowground biomass to incorporate sedimentary nutrients
DEM of triaxial tests on crushable cemented sand
Using the discrete element method, triaxial simulations of cemented sand consisting of crushable particles are presented. The triaxial model used features a flexible membrane, allowing realistic deformation to occur, and cementation is modelled using inter-particle bonds. The effects of particle crushing are explored, as is the influence of cementation on the behaviour of the soil. An insight to the effects that cementation has on the degree of crushing is presented
Rapid ecological change in the coastal zone of Lake Baikal (East Siberia): Is the site of the world\u27s greatest freshwater biodiversity in danger?
Ecological degradation of the benthic littoral zone is an emerging, urgent problem at Lake Baikal (East Siberia), the most species-rich lake on Earth. Within the last five years, multiple changes have occurred in the nearshore benthos where most of the lake\u27s endemic species reside. These changes include proliferation of benthic algae, deaths of snails and endemic sponges, large coastal wash-ups of dead benthic algae and macrophytes, blooms of toxin-producing benthic cyanobacteria, and inputs of industrial contaminants into parts of the lake. Some changes, such as massive coastal accumulations of benthic algae, are currently shared with the Laurentian Great Lakes (LGLs); however, the drivers of these changes differ between Lake Baikal and the LGLs. Coastal eutrophication from inputs of untreated sewage is causing problems at multiple sites in Lake Baikal, whereas in the LGLs, invasive dreissenid mussels redirect pelagic nutrients to the littoral substrate. At other locations in Lake Baikal, ecological degradation may have different causes including water level fluctuations and the input of toxic industrial contaminants. Importantly, the recent deterioration of the benthic littoral zone in both Lake Baikal and the LGLs has occurred while little change has occurred offshore. This highlights the necessity of monitoring both the littoral and pelagic zones of large lakes for assessing ecosystem health, change and conservation
Apelin Deficiency Accelerates the Progression of Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the selective loss of motor neurons. Recent studies have implicated that chronic hypoxia and insufficient vascular endothelial growth factor (VEGF)-dependent neuroprotection may lead to the degeneration of motor neurons in ALS. Expression of apelin, an endogenous ligand for the G protein-coupled receptor APJ, is regulated by hypoxia. In addition, recent reports suggest that apelin protects neurons against glutamate-induced excitotoxicity. Here, we examined whether apelin is an endogenous neuroprotective factor using SOD1G93A mouse model of ALS. In mouse CNS tissues, the highest expressions of both apelin and APJ mRNAs were detected in spinal cord. APJ immunoreactivity was observed in neuronal cell bodies located in gray matter of spinal cord. Although apelin mRNA expression in the spinal cord of wild-type mice was not changed from 4 to 18 weeks age, that of SOD1G93A mice was reduced along with the paralytic phenotype. In addition, double mutant apelin-deficient and SOD1G93A displayed the disease phenotypes earlier than SOD1G93A littermates. Immunohistochemical observation revealed that the number of motor neurons was decreased and microglia were activated in the spinal cord of the double mutant mice, indicating that apelin deficiency pathologically accelerated the progression of ALS. Furthermore, we showed that apelin enhanced the protective effect of VEGF on H2O2-induced neuronal death in primary neurons. These results suggest that apelin/APJ system in the spinal cord has a neuroprotective effect against the pathogenesis of ALS
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