6,969 research outputs found
A rapid prototyping/artificial intelligence approach to space station-era information management and access
Applications of rapid prototyping and Artificial Intelligence techniques to problems associated with Space Station-era information management systems are described. In particular, the work is centered on issues related to: (1) intelligent man-machine interfaces applied to scientific data user support, and (2) the requirement that intelligent information management systems (IIMS) be able to efficiently process metadata updates concerning types of data handled. The advanced IIMS represents functional capabilities driven almost entirely by the needs of potential users. Space Station-era scientific data projected to be generated is likely to be significantly greater than data currently processed and analyzed. Information about scientific data must be presented clearly, concisely, and with support features to allow users at all levels of expertise efficient and cost-effective data access. Additionally, mechanisms for allowing more efficient IIMS metadata update processes must be addressed. The work reported covers the following IIMS design aspects: IIMS data and metadata modeling, including the automatic updating of IIMS-contained metadata, IIMS user-system interface considerations, including significant problems associated with remote access, user profiles, and on-line tutorial capabilities, and development of an IIMS query and browse facility, including the capability to deal with spatial information. A working prototype has been developed and is being enhanced
Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening
A simple theory for glassy polymeric mechanical response which accounts for
large scale chain relaxation is presented. It captures the crossover from
perfect-plastic response to strong strain hardening as the degree of
polymerization increases, without invoking entanglements. By relating
hardening to interactions on the scale of monomers and chain segments, we
correctly predict its magnitude. Strain activated relaxation arising from the
need to maintain constant chain contour length reduces the dependence of
the characteristic relaxation time by a factor during
active deformation at strain rate . This prediction is consistent
with results from recent experiments and simulations, and we suggest how it may
be further tested experimentally.Comment: The theoretical treatment of the mechanical response has been
significantly revised, and the arguments for coherent relaxation during
active deformation made more transparen
Rods are less fragile than spheres: Structural relaxation in dense liquids composed of anisotropic particles
We perform extensive molecular dynamics simulations of dense liquids composed
of bidisperse dimer- and ellipse-shaped particles in 2D that interact via
repulsive contact forces. We measure the structural relaxation times obtained
from the long-time decay of the self-part of the intermediate scattering
function for the translational and rotational degrees of freedom (DOF) as a
function of packing fraction \phi, temperature T, and aspect ratio \alpha. We
are able to collapse the \phi and T-dependent structural relaxation times for
disks, and dimers and ellipses over a wide range of \alpha, onto a universal
scaling function {\cal F}_{\pm}(|\phi-\phi_0|,T,\alpha), which is similar to
that employed in previous studies of dense liquids composed of purely repulsive
spherical particles in 3D. {\cal F_{\pm}} for both the translational and
rotational DOF are characterized by the \alpha-dependent scaling exponents \mu
and \delta and packing fraction \phi_0(\alpha) that signals the crossover in
the scaling form {\cal F}_{\pm} from hard-particle dynamics to super-Arrhenius
behavior for each aspect ratio. We find that the fragility at \phi_0,
m(\phi_0), decreases monotonically with increasing aspect ratio for both
ellipses and dimers. Moreover, the results for the slow dynamics of dense
liquids composed of dimer- and ellipse-shaped particles are qualitatively the
same, despite the fact that zero-temperature static packings of dimers are
isostatic, while static packings of ellipses are hypostatic.Comment: 10 pages, 17 figures, and 1 tabl
Reversible plasticity in amorphous materials
A fundamental assumption in our understanding of material rheology is that
when microscopic deformations are reversible, the material responds elastically
to external loads. Plasticity, i.e. dissipative and irreversible macroscopic
changes in a material, is assumed to be the consequence of irreversible
microscopic events. Here we show direct evidence for reversible plastic events
at the microscopic scale in both experiments and simulations of two-dimensional
foam. In the simulations, we demonstrate a link between reversible plastic
rearrangement events and pathways in the potential energy landscape of the
system. These findings represent a fundamental change in our understanding of
materials--microscopic reversibility does not necessarily imply elasticity.Comment: Revised pape
Geometrical families of mechanically stable granular packings
We enumerate and classify nearly all of the possible mechanically stable (MS)
packings of bidipserse mixtures of frictionless disks in small sheared systems.
We find that MS packings form continuous geometrical families, where each
family is defined by its particular network of particle contacts. We also
monitor the dynamics of MS packings along geometrical families by applying
quasistatic simple shear strain at zero pressure. For small numbers of
particles (N < 16), we find that the dynamics is deterministic and highly
contracting. That is, if the system is initialized in a MS packing at a given
shear strain, it will quickly lock into a periodic orbit at subsequent shear
strain, and therefore sample only a very small fraction of the possible MS
packings in steady state. In studies with N>16, we observe an increase in the
period and random splittings of the trajectories caused by bifurcations in
configuration space. We argue that the ratio of the splitting and contraction
rates in large systems will determine the distribution of MS-packing
geometrical families visited in steady-state. This work is part of our
long-term research program to develop a master-equation formalism to describe
macroscopic slowly driven granular systems in terms of collections of small
subsystems.Comment: 18 pages, 23 figures, 5 table
Evaluation of a Novel Viral Load Based Surveillance Strategy for Prevention of Cytomegalovirus (CMV) Disease in Hematopoietic Cell Transplantation
Characterization and cloning of fasciclin I and fasciclin II glycoproteins in the grasshopper
Monoclonal antibodies were previously used to identify two glycoproteins, called fasciclin I and II (70 and 95 kDa, respectively), which are expressed on different subsets of axon fascicles in the grasshopper (Schistocerca americana) embryo. Here the monoclonal antibodies were used to purify these two membrane-associated glycoproteins for further characterization. Fasciclin II appears to be an integral membrane protein, where fasciclin I is an extrinsic membrane protein. The amino acid sequences of the amino terminus and fragments of both proteins were determined. Using synthetic oligonucleotide probes and antibody screening, we isolated genomic and cDNA clones. Partial DNA sequences of these clones indicate that they encode fasciclins I and II
Measurements of the Yield Stress in Frictionless Granular Systems
We perform extensive molecular dynamics simulations of 2D frictionless
granular materials to determine whether these systems can be characterized by a
single static yield shear stress. We consider boundary-driven planar shear at
constant volume and either constant shear force or constant shear velocity.
Under steady flow conditions, these two ensembles give similar results for the
average shear stress versus shear velocity. However, near jamming it is
possible that the shear stress required to initiate shear flow can differ
substantially from the shear stress required to maintain flow. We perform
several measurements of the shear stress near the initiation and cessation of
flow. At fixed shear velocity, we measure the average shear stress
in the limit of zero shear velocity. At fixed shear force, we
measure the minimum shear stress required to maintain steady flow
at long times. We find that in finite-size systems ,
which implies that there is a jump discontinuity in the shear velocity from
zero to a finite value when these systems begin flowing at constant shear
force. However, our simulations show that the difference , and thus the discontinuity in the shear velocity, tend to zero in
the infinite system size limit. Thus, our results indicate that in the large
system limit, frictionless granular systems are characterized by a single
static yield shear stress. We also monitor the short-time response of these
systems to applied shear and show that the packing fraction of the system and
shape of the velocity profile can strongly influence whether or not the shear
stress at short times overshoots the long-time average value.Comment: 7 pages and 6 figure
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Discovery of high-entropy ceramics via machine learning
AbstractAlthough high-entropy materials are attracting considerable interest due to a combination of useful properties and promising applications, predicting their formation remains a hindrance for rational discovery of new systems. Experimental approaches are based on physical intuition and/or expensive trial and error strategies. Most computational methods rely on the availability of sufficient experimental data and computational power. Machine learning (ML) applied to materials science can accelerate development and reduce costs. In this study, we propose an ML method, leveraging thermodynamic and compositional attributes of a given material for predicting the synthesizability (i.e., entropy-forming ability) of disordered metal carbides. The relative importance of the thermodynamic and compositional features for the predictions are then explored. The approach’s suitability is demonstrated by comparing values calculated with density functional theory to ML predictions. Finally, the model is employed to predict the entropy-forming ability of 70 new compositions; several predictions are validated by additional density functional theory calculations and experimental synthesis, corroborating the effectiveness in exploring vast compositional spaces in a high-throughput manner. Importantly, seven compositions are selected specifically, because they contain all three of the Group VI elements (Cr, Mo, and W), which do not form room temperature-stable rock-salt monocarbides. Incorporating the Group VI elements into the rock-salt structure provides further opportunity for tuning the electronic structure and potentially material performance
Genetic Dissection of Structural and Functional Components of Synaptic Plasticity. III. CREB Is Necessary for Presynaptic Functional Plasticity
AbstractIncreased cAMP (in dunce mutants) leads to an increase in the structure and function of the Drosophila neuromuscular junction. Synaptic Fasciclin II (Fas II) controls this structural plasticity, but does not alter synaptic function. Here, we show that CREB, the cAMP response element–binding protein, acts in parallel with Fas II to cause an increase in synaptic strength. Expression of the CREB repressor (dCREB2-b) in the dunce mutant blocks functional but not structural plasticity. Expression of the CREB activator (dCREB2-a) increases synaptic strength only in FasII mutants that increase bouton number. This CREB-mediated increase in synaptic strength is due to increased presynaptic transmitter release. Expression of dCREB2-a in a FasII mutant background genetically reconstitutes this cAMP-dependent plasticity. Thus, cAMP initiates parallel changes in CREB and Fas II to achieve long-term synaptic enhancement
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